Development of Solution Processed Thin Film …...Stefan Langner, Leona Wendt, Yugal Agarwal, Varun Sharma, Frank Fecher and Fu Yang for having a wonderful time during my PhD. I would

Development of Solution Processed Thin Film Barriers for Encapsulating

Thin Film Electronics

Entwicklung von lösungsprozessierten Dünnschichtbarrieren für die Verpackung von

Dünnschichtelektronik

Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg

zur

Erlangung des Grades

DOKTOR-INGENIEUR (Dr.-Ing.)

vorgelegt von

M.Eng. Iftikhar Ahmed Channa

aus Naushahro Feroze, Pakistan

Als Dissertation genehmigt

von der Technischen Fakultät der

Friedrich-Alexander-Universität Erlangen-Nürnberg

Tag der mündlichen Prüfung: 13 Dezember 2019

Vorsitzender des Promotionsorgans: Prof. Dr.-Ing. habil. Andreas Paul Fröba

Gutachter: Prof. Dr. Christoph J. Brabec

Prof. Dr. Josef Breu

i

This thesis is dedicated to my

Teachers, Family, Friends and Colleagues

ii

ACKNOWLEDGMENTS

Taking this opportunity, I wish to say thank Prof. Dr. Christoph J. Brabec for accepting and

providing me the great opportunity to perform a Ph.D. in his group and introducing me to

a nice world of photovoltaics. It has been a great pleasure to work under his supervision.

Secondly, I would like to address to my group leader and mentor Dr. Hans-Joachim

Egelhaaf, who has been a true inspiration throughout the time. His open-minded look to

my research activities, perfect suggestions, new ideas, and meaningful guidance,

encouraged me to make a nice and very interesting scientific work. I highly appreciate that

Dr. Egelhaaf always had a time for the discussions on my results.

I owe a great deal of appreciation for Dr. Andreas Distler for teaching me research

methodologies and healthy discussions on barrier performance and lifetime of organic solar

cells. I would also like to thank Dr. Edda Stern for her constant support and help during

early stages of my PhD that gave me a smooth start.

Special thanks to Mr. Benedikt Scharfe for providing glass flakes and sharing tricks for the

processing of the glass flake filled films. I am very grateful to Dr. Karen Forberich and Dr.

Benjamin Lipovsek for their support in performing optical simulations on the layers filled

with glass flakes.

I am particularly grateful to my colleague Eric Tam for performing SEM cross sections of

the layers which was a tricky and time consuming job, and he did that nicely for me. I am

also thankful to my other colleagues Atif Makhdoom, Taimoor Ahmed, Arne Riecke,

Dongju Jang, Sarmad Feroze, Philipp Maisch, Peter Kubis, Felix Hoga, Michael Wagner,

Stefan Langner, Leona Wendt, Yugal Agarwal, Varun Sharma, Frank Fecher and Fu Yang

for having a wonderful time during my PhD. I would also like to thank Dieter Schmidt for

helping me out with his technical skills in handling hardware. A special thanks to wonderful

ladies at ZAE and iMEET Astrid Kidzun, Nidia Gawehns, Anja Kottlowski, Irina Döhrer,

Madeleine Heyder and Claudia Koch for extending their support whenever I needed,

especially in handling formalities and document translations.

I thank my friends, Syed Qurban Ali, Hassan Sohaib, Jamal uddin, Ayaz Mahmood, Asmat

Soomro, Khalid Rasheed, Abdul Latif, Saleem Raza and Laraib Sarfraz for their constant

iii

support during my stay in Germany specially Yaseen Memon (your wonderful cooking

skills can never be forgotten).

Special thanks to the Higher Education commission Pakistan and German Academic

Exchange Service (DAAD) for their financial support and ZAE for allowing me to work in

its laboratories and providing me a nice research environment.

I take pride to express thanks and love for my family for their endless support throughout

my life that provided me confidence and courage. Whatever I have achieved, is due to their

care, prayers and untiring efforts. I also pay my special gratitude to my brothers specially

Muhammad Nawaz and Sister Noor Jehan for their unconditional help, prayers and love.

Later in the day after work, nothing was more jubilant than time spent with my children

(Aiza and Muhammad Yaqub). You both are most nearest to my heart.

My acknowledgement would be incomplete without thanking my wife. She has been the

biggest source of my strength. Her care and unwavering love provided me courage and

confidence to meet every challenge.

iv

SUMMARY

Recently, organic solar cells (OSCs) with efficiencies of 17% have been demonstrated,

which brings organic photovoltaics in the same league as inorganic thin film technologies.

OSCs require encapsulation by transparent and high quality barrier materials to achieve

decent lifetimes without compromising performance. The most common practice for

encapsulation of the OSCs is the lamination between barrier sheets using adhesives. This

lamination process adds extra processing steps and thus increases overall processing cost

and limits the throughput. Many attempts have thus been made to create coated barriers

with quality comparable to those processed from vacuum assisted techniques. Direct

application of coated barriers on top of OSCs will not only minimize cost but also maximize

throughput as direct coating processes can be performed with roll-to-roll methods.

Therefore, the goal of this work is the development of materials and processes for the

encapsulation of organic thin films electronics by direct coating.

The thesis is subdivided into six chapters. The main objective of Chapter 1 is to express

the motivation towards the research direction. In this chapter, also the challenges and

hurdles faced by coated barriers are discussed. Chapter 2 describes the theoretical

background of diffusion and permeability and introduces industrial units for measuring the

barrier quality. Various factors are also defined briefly which influence the barrier quality.

This chapter gives theoretical details of the barriers based on filler platelets and describes

various theoretical models for predicting the barrier quality from the platelet properties

such as size, shape, concentration and orientation. Finally basics about organic solar cells

and the working principle along with degradation mechanism are described in this chapter.

Chapter 3 describes the state of the art of coated barriers. Silica layers processed from the

polymer class of polysilazanes are also described in detail, along with the methods of

processing. Finally, miscellaneous materials like ORMOCERS and fluoropolymers are also

discussed in this chapter. Chapter 4 is devoted to experimental details describing all of the

raw materials and processing methods used in the work. In Chapter 5 results on

experimental data discussed. This chapter is further divided into three parts. Part I provides

results obtained from the investigations on filler based barriers using clay as filler. Clay

based barriers were prepared as a well characterized reference system. In Part II, the novel

concept of barriers based on PVB films filled with glass flakes was investigated. To this

end, barriers were prepared from glass flakes of different aspect ratios and different loading

v

concentrations to systematically study the effect of aspect ratio and loading concentration

on barrier quality and optical transmission. It was found that the glass flakes are distributed

homogeneously in the PVB film, with an almost perfect orientation of the platelets’ long

axes parallel to the film surface. In this way, barrier films with optical transmission values

of > 85% and moisture permeation values of ~0.14 g.m-2.day-1 were obtained with glass

flakes having an aspect ratio of 2000 at a loading concentration of 25 vol%.The barrier

properties persisted even after 20,000 cycles of bending at a radius of 3 cm. The WVTR

values measured for different aspect ratios and different loadings were shown to be in

reasonable accordance with the predictions of the Bharadwaj model. The haze of the glass

flake filled PVB films, which, according to optical simulations, is mainly due to surface

roughness of the films, was reduced by coating a smoothing layer on top. The lifetime of

organic solar cells (OSCs) increased from few hours to beyond 150 h under damp heat

conditions without any loss in efficiency, when the devices were encapsulated with the

glass flake based barrier films. Part III describes the results obtained on barrier films based

on perhydropolysilazane (PHPS). Two methods were used to cure PHPS, namely curing by

exposure to damp heat and curing by irradiation with deep UV. Curing with deep UV in

addition with heat is found to be the quickest way to cure PHPS completely. FTIR has been

used to find the end point of curing which can subsequently be used to predict the barrier

properties of cured PHPS layers. Prepared barrier films show water vapor transmission

rates (WVTR) of <10-2 g m-2day-1 (40oC / 85%RH) and oxygen transmission rates (OTR)

of <10-2 cm3m-2 day-1 bar-1 at ambient conditions maintaining optical transmission of >90%

in visible region. Flexibility of the resulting barrier films is improved by coating a barrier

stack of several thin PHPS layers alternating with organic polymer interlayers. These stacks

show an increase of WVTR values by less than 10% after 3000 bending cycles. Direct

coating of the PHPS films on top of organic solar cells enhances the device lifetime in damp

heat conditions from few hours to around 700 hours. Chapter 6 gives the conclusions and

provides an outlook on the possible impact of the developments of this thesis on the roll-

to-roll production of printed opto-electronics.

vi

ZUSAMMENFASSUNG

Kürzlich wurden organische Solarzellen (OSCs) mit Wirkungsgraden von 17%

demonstriert, was die organische Photovoltaik in die gleiche Liga wie anorganische

Dünnschichttechnologien bringt. OSCs benötigen eine Verkapselung mit transparenten und

hochwertigen Barrierematerialien, um eine lange Lebensdauer zu erreichen, ohne die

Leistung zu beeinträchtigen. Die gebräuchlichste Verkapselung von OSCs ist die

Laminierung zwischen zwei Barrierefolien mittels Klebstoff. Dieser Laminierprozess

erfordert zusätzliche Verarbeitungsschritte und erhöht so die Gesamtverarbeitungskosten

und begrenzt den Durchsatz. Es wurden daher viele Versuche unternommen, gedruckte

Barrieren mit einer Qualität zu schaffen, die mit der von vakuumunterstützten Techniken

vergleichbar ist. Die direkte Anwendung von gedruckten Barrieren auf OSCs minimiert

nicht nur die Kosten, sondern maximiert auch den Durchsatz, da direkte

Beschichtungsprozesse mit Rolle-zu-Rolle-Verfahren durchgeführt werden können.

Ziel dieser Arbeit ist daher die Entwicklung von Materialien und Verfahren zur

Verkapselung von organischer Dünnschichtelektronik durch Direktbeschichtung.

Die Arbeit ist in sechs Kapitel unterteilt. Das Hauptziel von Kapitel 1 ist es, die Motivation

und Zielsetzung dieser Forschungsarbeit zu beschreiben. In diesem Kapitel werden auch

die Herausforderungen für gedruckte Barrieren diskutiert. Kapitel 2 beschreibt den

theoretischen Hintergrund von Diffusion und Permeabilität und stellt industrielle Geräte

zur Messung der Barrierequalität vor. Darüber hinaus werden kurz verschiedene Faktoren

definiert, die die Barrierequalität beeinflussen. Dieses Kapitel enthält zudem theoretische

Details zu Barrieren auf Basis von Füllplättchen und beschreibt verschiedene theoretische

Modelle zur Vorhersage der Barrierequalität aus den Eigenschaften der Plättchen wie

Größe, Form, Konzentration und Ausrichtung. Schließlich werden in diesem Kapitel die

Grundlagen organischer Solarzellen und deren Funktionsprinzip sowie

Degradationsmechanismen beschrieben. Kapitel 3 beschreibt den Stand der Technik von

gedruckten Barrieren. Aus der Polymerklasse der Polysilazane prozessierte

Kieselsäureschichten werden ebenso wie die Verarbeitungsmethoden ausführlich

beschrieben. Schließlich werden in diesem Kapitel auch verschiedene Materialien wie

ORMOCERS und Fluorpolymere behandelt. Kapitel 4 widmet sich experimentellen

Details, die alle Materialien und Verarbeitungsmethoden beschreiben, die in der Arbeit

verwendet werden. In Kapitel 5 werden Ergebnisse zu experimentellen Daten diskutiert.

vii

Dieses Kapitel ist in drei Teile gegliedert. Teil I enthält Ergebnisse aus den Untersuchungen

zu Füllstoffbarrieren mit Ton als Füllstoff. Auf Ton basierende Barrieren wurden als gut

charakterisiertes Referenzsystem hergestellt. In Teil II wurde ein neuartiges Konzept mit

Barrieren auf Basis von PVB-Folien mit Glasflocken untersucht. Zu diesem Zweck wurden

Barrieren aus Glasflocken mit unterschiedlichen Seitenverhältnissen und Konzentrationen

hergestellt, um den Einfluss von Seitenverhältnis und Konzentration auf die

Barrierequalität und optische Transmission systematisch zu untersuchen. Es wurde

festgestellt, dass die Glasflocken homogen in der PVB-Folie verteilt sind, mit einer nahezu

perfekten Ausrichtung der Längsachse der Plättchen parallel zur Folienoberfläche. Auf

diese Weise wurden Barriereschichten mit optischen Transmissionswerten > 85% und

Feuchtepermeationswerten von ~0,14 g.m-2.day-1 mit Glasflocken mit einem

Aspektverhältnis von 2000 bei einer Beladungskonzentration von 25 Vol% erhalten, wobei

die Barriereeigenschaften auch nach 20.000 Biegezyklen bei einem Radius von 3 cm

erhalten blieben. Die WVTR-Werte, die für verschiedene Seitenverhältnisse und

unterschiedliche Belastungen gemessen wurden, stimmten gut mit den Vorhersagen des

Bharadwaj-Modells überein. Die Trübung der glaslamellengefüllten PVB-Folien, die nach

optischen Simulationen hauptsächlich auf die Oberflächenrauheit der Folien

zurückzuführen ist, wurde durch die Beschichtung einer Glättungsschicht reduziert. Die

Lebensdauer von organischen Solarzellen (OSCs) stieg von wenigen Stunden auf über 150

Stunden unter feuchten Wärmebedingungen ohne Effizienzverlust, wenn die Zellen mit

Barrierefolien auf Glasflockenbasis verkapselt wurden. Teil III beschreibt die Ergebnisse

von Barriereschichten auf Basis von Perhydropolysilazan (PHPS). Zwei Methoden wurden

zur Aushärtung von PHPS verwendet, nämlich die Aushärtung durch Behandlung mit

feuchter Hitze und die Aushärtung durch Bestrahlung mit tiefem UV. Die Aushärtung mit

tiefem UV und zusätzlich mit Wärme ist der schnellste Weg PHPS vollständig auszuhärten.

FTIR wurde verwendet, um den Zeitpunkt der vollständigen Aushärtung zu finden, mit

dem anschließend die Barriereeigenschaften der ausgehärteten PHPS-Schichten ermittelt

werden können. Die Barrierefolien zeigen Wasserdampfdurchlässigkeitsraten (WVTR)

von <10-2 g m-2day-1 (40oC / 85%RH) und Sauerstoffdurchlässigkeitsraten (OTR) von <10-

2 cm3m-2 day-1 bar-1 bei Umgebungsbedingungen, die eine optische Transmission von >90%

im sichtbaren Bereich aufweisen. Die Flexibilität der resultierenden Barrierefolien wird

verbessert, indem ein Stapel aus mehreren dünnen PHPS-Schichten im Wechsel mit

organischen Polymerfolien beschichtet wird. Diese Stapel zeigen eine Erhöhung der

WVTR-Werte um weniger als 10% nach 3000 Biegezyklen. Die direkte Beschichtung der

viii

PHPS-Schichten auf organischen Solarzellen erhöht die Lebensdauer der Zellen in feuchter

Hitze von wenigen Stunden auf etwa 700 Stunden. Kapitel 6 enthält die

Schlussfolgerungen und einen Ausblick auf die möglichen Auswirkungen der in dieser

Arbeit entwickelten Verkapselungstechnologien für die Rolle-zu-Rolle-Fertigung

gedruckter Opto-Elektronik.

TABLE OF CONTENTS

1

TABLE OF CONTENTS

ACKNOWLEDGMENTS ................................................................................................... ii

SUMMARY .................................................................................................................... iv

ZUSAMMENFASSUNG ................................................................................................ vi

TABLE OF CONTENTS ................................................................................................. 1

.............................................................................................................................. 5

MOTIVATION AND CONCEPT ................................................................................... 5

.............................................................................................................................. 8

THEORETICAL BACKGROUND ................................................................................. 8

2.1 Theoretical background of diffusion through barriers ....................................... 9

2.2 Permeation rates .............................................................................................. 10

2.2.1 Temperature dependence: ........................................................................ 11

2.3 Factors affecting Permeability ......................................................................... 13

2.3.1 Coefficient of diffusion (D) ..................................................................... 14

2.3.2 Coefficient of solubility (H) ..................................................................... 14

2.3.3 Surface coverage () ................................................................................ 14

2.3.4 Tortuosity (τ) ............................................................................................ 15

2.4 Modeling and simulation of barrier characteristics of filled polymers ........... 15

2.4.1 Overview of the various models............................................................... 17

2.5 Working principle and degradation of organic solar cells ............................... 24

2.5.1 Working principle .................................................................................... 24

2.5.2 Current density voltage characteristics .................................................... 26

2.6 Degradation mechanism of Organic Solar Cells ............................................. 27

............................................................................................................................ 32

STATE OF THE ART.................................................................................................... 32

3.1 Bulk Polymers ...................................................................................................... 33

TABLE OF CONTENTS

2

3.2 Increasing tortuous path ........................................................................................ 35

3.2.1 Clay based barriers ................................................................................... 35

3.2.2 Graphene based barriers ........................................................................... 38

3.2.3 Getter materials ........................................................................................ 42

3.3 Barriers based on impermeable coatings .............................................................. 43

3.3.1 Polysilazane .............................................................................................. 43

3.3.1.1 Thermal curing ........................................................................................ 45

3.3.1.2 Curing in the presence of catalyst ........................................................... 46

3.3.1.3 Deep UV curing ..................................................................................... 47

3.3.1.4 Combined methods .................................................................................. 48

3.3.1.5 Barrier performance ................................................................................ 49

3.3.2 ORMOCERS ............................................................................................ 53

3.4 Reducing solubility ............................................................................................... 56

............................................................................................................................ 60

EXPERIMENTAL ......................................................................................................... 60

4.1 Materials .......................................................................................................... 61

4.2 Processing ........................................................................................................ 62

4.3 Preparation of OSCs ........................................................................................ 64

4.3.1 Encapsulation of the OSC devices ........................................................... 64

4.4 Characterization ............................................................................................... 65

4.4.1 Barrier quality .......................................................................................... 65

4.4.1.1 Water vapor transmission rate (wvtr) ................................................... 65

4.4.1.2 Oxygen transmission rate (OTR) .......................................................... 67

4.4.2 Spectroscopic analysis.............................................................................. 68

4.4.3 Bending of the barrier layers .................................................................... 68

4.4.4 Degradation test........................................................................................ 68

4.4.4.1 Optical measurements ........................................................................... 68

TABLE OF CONTENTS

3

4.4.4.2 Damp heat degradation ......................................................................... 68

4.4.4.3 Degradation under sun .......................................................................... 68

4.4.4.4 Electrical measurements ....................................................................... 69

4.4.4.5 SEM images .......................................................................................... 69

4.4.4.6 Optical micrographs ............................................................................. 69

............................................................................................................................ 70

RESULTS AND DISCUSSION .................................................................................... 70

5.1 Filler based barriers: Clay and glass flakes ..................................................... 71

5.1.1 Clay based barriers ................................................................................... 71

5.1.1.1 IR analysis of nanocomposites ............................................................. 72

5.1.1.2 Surface morphology ............................................................................. 72

5.1.1.3 Transparency and haze of the Nanocomposites ................................... 73

5.1.1.4 Moisture permeability........................................................................... 74

5.1.1.5 Validation of the experimental data...................................................... 77

5.1.1.6 Bendability............................................................................................ 78

5.1.1.7 Conclusion ............................................................................................ 79

5.1.2 Glass flakes based barriers ....................................................................... 81

5.1.2.1 Surface roughness of the layers ............................................................ 83

5.1.2.2 Transparency of the layers .................................................................... 83

5.1.2.3 Influence of bulk scattering: ................................................................. 86

5.1.2.4 Influence of the Surface roughness: ..................................................... 89

5.1.2.5 Barrier performance of glass flakes ...................................................... 91

5.1.2.6 Oxygen permeability ............................................................................ 94

5.1.2.7 Bendability............................................................................................ 95

5.1.2.8 Encapsulation of organic solar cells ..................................................... 96

5.1.2.9 Photo bleaching of P3HT ..................................................................... 96

5.1.2.10 Lifetime under damp heat ................................................................... 97

TABLE OF CONTENTS

4

5.1.2.11 Lifetime under irradiation by sun simulator ....................................... 98

5.1.2.12 Conclusion .......................................................................................... 99

5.2 Polysilazane ................................................................................................... 101

5.2.1 Optimizing the curing method for PHPS ............................................... 101

5.2.2 Curing by the combination of heat and deep UV at distance of 5 mm: . 103

5.2.3 Correlation of WVTR with Infrared peak ratios .................................... 104

5.2.4 Correlation of WVTR with IR peak (damp heat) ................................... 107

5.2.5 Optimization of the PHPS conversion rate ............................................ 109

5.2.6 Hydrophobic nature of the PHPS film: .................................................. 111

5.2.7 Flexibility / bendability of PHPS-based barriers.................................... 111

5.2.8 Protection of organic electronic devices by PHPS-based barriers ......... 117

5.2.9 Encapsulation of OSCs by direct deposition of PHPS ........................... 121

5.2.10 Intermediate layer of ZnO to avoid delamination .................................. 124

5.2.11 Lifetime tests .......................................................................................... 125

5.2.12 Investigation on device failure in sun test: ............................................. 126

5.2.13 Conclusion .............................................................................................. 127

.......................................................................................................................... 129

CONCLUSION ............................................................................................................ 129

Biblography .................................................................................................................. 130

List of Tables .................................................................................................................... a

List of Figures .................................................................................................................. b

List of Abbreviations, symbols and constants ................................................................... i

MOTIVATION AND CONCEPT

5


Organic electronics, namely organic light emitting diodes (OLEDs) [1], [2], organic

solar cells (OSCs) [3], [4], and organic field effect transistors (OFETs) [5] have opened up

new chances, attributable to their intrinsic characteristic, for example, light weight,

mechanical adaptability and semitransparency [6]. This is especially true for organic solar

cells as these properties make them the perfect choice for mobile chargers and building

integration [7]. Very recently, organic photovoltaics (OPV) have experienced a boost as

power conversion efficiencies (PCEs) of ~17% have been reported, which brings OPV into

the same league as inorganic thin films technologies [8]. Another intriguing characteristic

of organic electronics is the printability, which makes high throughput roll-to-roll (R2R)

processing possible with the use flexible substrates and thus production at low cost [9]. So

as to be feasible in the market, such products should not only offer high efficiencies and

low cost but also adequately long lifetimes. As the degradation of unencapsulated organic

devices is mainly caused by moisture and oxygen, their lifetime can be extended

significantly by encapsulation with appropriate barrier materials [10].

Figure 1.1 shows an overview of the requirements to packaging in different fields of

application. Figure 1.2 serves to visualize the challenge of preparing adequate barriers by

showing the amounts of water diffusing through football ground sized barriers of different

WVTR values over the time period of one month.

While food and pharmaceutical products can be packaged in barriers having water vapor

transmission rate (WVTR) values between 100 -101 g.m-2.day-1 by utilizing common

polymers [11]–[14], OLEDs are highly sensitive to moisture and oxygen and hence require

ultra-high barriers (WVTR ~10-6 g.m-2.day-1) for their encapsulation [15], [16], which can

only be achieved with metal oxide coatings like SiOx [14], SiNx [17], and Al2O3 [18] etc.,

deposited from the gas phase (ALD, CVD, PVD).

In a study carried out by Hauch et al., it was shown that for OSCs barrier materials

having WVTRs of around 10-3 g m-2 day-1 @ 25oC/40%RH can protect the device to provide

lifetimes of 3-5 years [11]. These WVTR values do not require vacuum processed barriers,

but are within the reach of solution processed barriers [19]–[21]. From the point of view of

printed flexible opto-electronics as appealing products for textile or building integration, it


6

is important that the encapsulation does not affect the respective properties of the devices.

Thus, the encapsulating material should be optically transparent, flexible, light weight and

cost efficient [16]. As encapsulation of OSCs in barrier films adds an extra lamination step

to the manufacturing process and also makes the resulting modules much heavier and less

flexible, direct coating of the barrier on top of the devices would be very beneficial in terms

of both, costs and quality of the modules. Additionally, directly coated barrier layers can

also enhance the compatibility of encapsulation with roll-to-roll manufacturing, thus

increasing the throughput, reducing the processing steps and minimizing the overall

processing cost. Coatable barriers can thus be a promising alternative to vacuum assisted

vapor deposition techniques for obtaining medium quality barriers at reasonable cost.

Interesting applications for coated barriers are either short to medium life time devices, e.g.

mobile phone chargers, or robust products such as inorganic PV modules or luminescence

down shifting foils for retrofitting solar power plants. Finally, in some cases, e.g. for

devices of arbitrary 3-dimensional shape, coating of barriers is the only possible way of

applying barriers.

The goal of this thesis is thus the development of solution processed barriers for the

R2R compatible encapsulation of flexible OSCs, providing a combination of high barrier

quality (WVTR < 10-2 g.m-2.day-1), transparency (~90% in a range of 400 nm to 1000 nm)

and flexibility (several thousand bending cycles at a bending radius < 5 cm) [16]. Following

Fick’s 1st law of diffusion as a guideline, two approaches towards coatable barriers will be

used, namely enhancing tortuosity by filling glass flakes into PVB films and reducing

accessible area by silica coatings obtained by UV curing of perhydropolysilazanes.


7

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

10-5

10-4

10-3

10-2

10-1

100

101

102

103

Encapsulation

for OSCs

BetterExcellent Poor

OT

R (

cm

3/m

2.d

ay

.ba

r)

WVTR (g/m2.day)

Bulk

polymers

Food /

Pharmaceutical

packaging

Encapsulation

for OLEDs

PVD / CVD / ALD

single / multilayers

Solution processed

Filler based/

PHPS based

PET,

PEN

Good

Metal coated

polymers

Figure 1.1. Water vapor transmission rates (WVTR) and oxygen transmission rates (OTR)

of bulk polymers, food packaging, as well as of solution and vacuum processed high quality

barriers. (Reproduced from [22] with the permission from Elsevier, with modifications)

Figure 1.2. Illustration of the amounts of water transmitted through barrier films of the

size of a football field (5000 m2) over a period of 1 month at the WVTR values given (in

g.m-2.day-1). Data extracted from [23].

THEORETICAL BACKGROUND

8


This chapter presents the theoretical background of diffusion through barriers. Later in this

chapter, the theoretical background of organic solar cells and their degradation due to

oxygen and humidity is given.


9

2.1 Theoretical background of diffusion through barriers

Transfer of gases or vapors through membranes due to concentration gradients

usually occurs by the means of diffusion. Diffusion is defined as a flow of species from one

place to another as a result of difference in chemical potential of the flowing species [24].

In the field of packaging materials for organic electronics, we are mainly concerned with

the diffusion of oxygen and moisture, as these are the most detrimental substances to the

different layers of the devices.

The amount of substance n passing through unit area A per unit time t is termed flux

[24]–[27]:

J = 1

𝐴lim⧍t→0

∆𝑛

∆𝑡 =

𝑑𝑛

𝐴∙𝑑𝑡

Eq. 1

In the case of transport by diffusion, referring to Fick’s first law, the flux (J) is obtained as:

𝐽 =1

𝐴

𝑑𝑛

𝑑𝑡= −𝐷

𝜕𝑐

𝜕𝑥

Eq. 2

Where D is the coefficient of diffusion and ∂c

∂x is the concentration gradient normal to the

membrane surface.

In the steady state, i.e. at J=constant, the diffusion flux through a membrane of thickness l,

with C1 and C2 being the concentrations of the diffusant at opposite sides of the membrane,

becomes

J = 𝐷(𝐶2−𝐶1)

𝑙 Eq. 3

Assuming that the coefficient of diffusion is not a function of concentration, i.e., D≠f(c).

Substituting J from relation Eq. 2 we obtain

⧍𝑛

𝐴∆𝑡= 𝐷

(𝐶2 − 𝐶1)

𝑙

Eq. 4

And

⧍𝑛= D (𝐶2−𝐶1)

𝑙 𝐴∆𝑡

Eq. 5

If the partial pressure p of the diffusant outside of the membrane is sufficiently low, the

equilibrium concentration C of the diffusant inside the membrane is given by Henry’s law:


10

C = H* ƿ Eq. 6

Where H is the solubility coefficient of the diffusing gas, called Henry’s constant, which is

defined as the concentration at a certain partial pressure of that gas. The SI unit of H is

mol/(m3 Pa).

By substituting C in relation Eq. 5 by the partial pressure of the diffusant, we obtain:

⧍𝑛 =D 𝐻(ƿ2−ƿ1)

𝑙 𝐴∆𝑡

Eq. 7

The permeability (P) of the diffusing species is given by the relation:

P = DH = ⧍𝑛∙ 𝑙

𝐴∙∆𝑡∙(ƿ2−ƿ1)

Eq. 8

Therefore, permeability depends only on material constants, namely the coefficient of

diffusion and the solubility of the permeant in the membrane material, respectively. These

properties vary as the function of various materials’ properties like morphology, cohesive

energy and volume etc. [28].

2.2 Permeation rates

Transmission rates are given as the amount of material diffusing through unit area

membrane in unit time, i.e., it depends on both material constants (D and H) and

experimentally variable parameters (l and p):

∆𝑛

𝐴∙∆𝑡=

𝐷∙𝐻

𝑙∙ ∆𝑝

Eq. 9

The water vapor transmission rate it is usually given in mass units (𝑔

𝑚2∙𝑑𝑎𝑦), rather than in

molar units:

𝑊𝑉𝑇𝑅 =∆𝑚

𝐴 ∙ ∆𝑡 =

𝐷 ∙ 𝑆𝐻2𝑂𝑚

𝑙∙ 𝑃𝐻

𝐻2𝑂

(𝑇) ∙ ∆𝑟ℎ Eq. 10

𝑆𝐻2𝑂𝑚 denotes the solubility of water in the membrane material in units of mass per volume

and pressure. 𝑃𝐻𝐻2𝑂

(T) denotes the saturation partial pressure of water at the given


11

temperature T (Figure 2.1) and ∆𝑟ℎ denotes the difference of relative humidity across the

barrier.

Figure 2.1: Relation of water vapor pressure vs temperature (Data taken from Dortmund

data bank, licensed by CC BY 3.0).

The oxygen transmission rate (OTR), typical unit 𝑐𝑚3

𝑚2∙𝑏𝑎𝑟∙𝑑𝑎𝑦, is given as the volume ∆𝑉𝑂2

0 of

oxygen, reduced to normal conditions, passing through the unit area A of a membrane of

thickness l per unit time at a given oxygen partial pressure difference of ∆𝑃𝑂2

𝑂𝑇𝑅 =∆𝑉𝑂2

0

𝐴 ∙ ∆𝑃𝑂2∙ ∆𝑡

=𝐷 ∙ 𝑆𝑂2

𝑉

𝑙

Eq. 11

Where 𝑆𝑂2

𝑉 denotes the solubility of oxygen in the membrane material in units of volume

oxygen, reduced to normal conditions, per membrane volume and pressure.

2.2.1 Temperature dependence:

The Arrhenius equation is the most straight forward way for analyzing the effects of

temperature on gas permeability. In general, Arrhenius relation works decently well, over

moderate temperature ranges, to simulate the temperature dependences of diffusion,

solubility, saturation vapor pressure, and thus of permeation.

The relation of diffusion coefficients of permeating gases to temperature is given as Eq. 12,

where 𝐷0 is the diffusion coefficient of the permeating gas and 𝐸𝑑 its activation energy,

R is the universal gas constant and T is temperature. The diffusion coefficient D ideally

follows an Arrhenius relationship [29], [30].


12

𝐷 = 𝐷0 𝑒𝑥𝑝 (−𝐸𝑑

𝑅𝑇) Eq. 12

This relation is illustrated in Figure 2.2(a, c), where the permeation (WVTR and OTR)

dependence of biaxially oriented polypropylene (BOPP) and biaxially oriented polyvinyl

alcohol (BOPVA) on temperature are shown [31]. Solubility coefficients of permeating

gases are also temperature dependent and usually described well by the Arrhenius relation

(Eq. 13) [30], [32].

𝑆 = 𝑆0 𝑒𝑥𝑝 (−∆𝐻𝑠

𝑅𝑇) Eq. 13

Where 𝑆 is the solubility, 𝑆0 is solubility coefficient of permeating gas and 𝐻𝑠 is apparent

heat of solution. For the solubility of gases in liquids and polymers around room

temperature, ∆𝐻𝑠 < 0, so that solubility decreases with increasing temperature.

It follows for the temperature dependence of permeability[33], [34].

𝑃 = 𝐷 ∙ 𝑆 = 𝐷0 ∙ 𝑆0 ∙ 𝑒−(𝐸𝑑+∆𝐻𝑠) 𝑅𝑇⁄ = 𝑃0 ∙ 𝑒−𝐸𝑝 𝑅𝑇⁄

Usually, |𝐸𝑑| > |∆𝐻𝑠|Figure 2.2

It should be noted that permeation of gases is generally affected by the presence of other

gases. Figure 2.2 (b,d) shows the dependence of permeation of oxygen and water on relative

humidity for BOPP and BOPVA membranes. PVA being a water soluble polymer, it shows

more moisture permeation with increasing relative humidity on one hand, in contrast to the

hydrophobic polymer BOPP, while on the other hand, it shows a reduced oxygen

permeation rate due to increasing –OH intermolecular forces and the resulting lower

oxygen solubility [31].


13

a)

b)

c)

d)

Figure 2.2: Moisture permeation of biaxially oriented polypropylene and biaxially

oriented PVA, a) moisture permeation dependence on temperature at 50% RH, b)

moisture permeation dependence on relative humidity (RH%) at 23oC, c) OTR values of

biaxially oriented PVA at different temperatures at 50% RH and d) OTR values of

biaxially oriented PVA at different relative humidity (RH%) at 23˚C (Copied from [31]

licensed bb CC BY 4.0) .

2.3 Factors affecting Permeability

Rearranging Eq. 9 yields

∆𝑛

∆𝑡=

𝐷 ∙ 𝐻 ∙ 𝐴𝑒𝑓𝑓

𝑙𝑒𝑓𝑓∙ ∆𝑝 Eq. 14

Which states that permeation rates are proportional to diffusion coefficient D, solubility H,

the surface area of the membrane which is actually accessible to the permeant Aeff, and the


14

actual length leff of the path the permeant has to take to arrive at the opposite end of the

membrane. Relating the actual surface area to the geometric one by the surface coverage

and relating the actual path length to the geometric one by the tortuosity factor τ, we obtain

∆𝑛

∆𝑡=

𝐷 ∙ 𝐻 ∙ 𝐴𝑔𝑒𝑜𝑚 ∙ (1 − 𝜃)

𝑙𝑔𝑒𝑜𝑚 ∙ 𝜏∙ ∆𝑝 Eq. 15

Which yields the equation for transmission rates when normalized to the geometric area of

the barrier

∆𝑛

𝐴𝑔𝑒𝑜𝑚 ∙ ∆𝑡=

𝑫 ∙ 𝑯 ∙ (1 − 𝜽)

𝑙𝑔𝑒𝑜𝑚 ∙ 𝝉∙ ∆𝑝 Eq. 16

Eq. 16 serves as a design rule of barriers. There are thus four levers which serve to control

barrier properties: D, H, , and τ.

2.3.1 Coefficient of diffusion (D)

The diffusion coefficient is usually decreased by decreasing the Free Volume of polymers,

e.g., by cross linking of the polymer chains, or by enhancing the crystallinity of a material,

e.g., by decreasing the defect density of the barrier material [35], [36].

2.3.2 Coefficient of solubility (H)

Solubility is defined as a measure of the amount of solute that is dissolved in the membrane

at equilibrium. Solubility of gas in rubbery polymers is well characterized in terms of

Henry’s law of solubility, Eq. 6. This expression is effective for gasses with low molecular

weight and at low pressures [36]. For glassy polymers, the solubility of gases is more

accurately described by the Langmuir isotherm [37]. The deployment of materials having

low oxygen and moisture solubilities can be used as effective packaging materials [31].

Solubility of oxygen decreases with increasing polarity of the membrane materials, the

solubility of water decreases with decreasing polarity and decreasing tendency for

hydrogen bonding.

2.3.3 Surface coverage ()

The effective surface of a membrane which is actually exposed to diffusing molecules is

reduced by covering the surface by an impermeable barrier, such as a (defect free) metal

oxide layer [18], [38], [39].


15

2.3.4 Tortuosity (τ)

The effective path length that a diffusing molecule must take from one side of the barrier

to another relates to both, the thickness of the barrier film and its internal structure [40].

Increasing thickness would result in decreased permeability as the molecules will take long

time for the diffusion. But constant increase in the thickness is not the solution for

packaging. Increase in thickness may post certain disadvantages like additional weight and

relatively less mechanical flexibility. Furthermore, thicker films are also not compatible for

roll-to-roll production. Therefore, in order to not go beyond certain thickness requirement

for roll-to-roll processing, the effective path length is increased by creating a zig zag path

for diffusing molecules by adding impermeable platelets [41]. Due to zig zag (tortuous)

path diffusing molecules take longer time and overall permeability is decreased [42],

[43].This is further explained in the following section.

2.4 Modeling and simulation of barrier characteristics of filled polymers

As described earlier in Eq. 8, permeability is the product of the diffusion coefficient D and

solubility H. In the binary system, containing the polymer along with the nano-fillers, the

gas solubility in the nanocomposite is expressed as:

𝐻 = (1 − Φ)𝐻𝑜 Eq. 17

Where H0 is the solubility of the gas in the unfilled polymer and Φ is the filler volume

fraction. Eq. 17 assumes that the local properties of the matrix are not affected by the

presence of the nanostructures. In such a system, where fillers are assumed impermeable,

the diffusing molecules have to go through a more tortuous path to leave the coating [44].

Thus, the effective path is enhanced and the path length in the nanocomposite is given by:

𝑙𝑒𝑓𝑓 = 𝑙𝑔𝑒𝑜𝑚 ∙ 𝜏 Eq. 18

where lgeom is the thickness of the pure polymer film and 𝜏 is the tortuosity factor. Formally,

the increase of the effective diffusion path can also be expressed by the decrease of the

diffusion coefficient

𝐷 =𝐷0

𝜏

Eq. 19


16

By combining Eq. 17 and Eq. 19, it is possible to define the relative permeability as a

function of the filler volume fraction and the tortuosity factor:

𝑃

𝑃0=

1 − Φ

𝜏

Eq. 20

where P0 is the permeability coefficient of the pure polymer and P that of the

nanocomposite.

Each of the different models that are presented in the following sections proposes an

expression for the tortuosity factor 𝜏 and thus for the relative permeability. The tortuosity

factor 𝜏 depends on several geometrical parameters such as the volume fraction of nano-

fillers Φ, their aspect ratio 𝛼 and the aspect ratio of pores and slits across adjacent fillers in

the similar horizontal plane σ. To define these parameters, we first consider a repeating unit

cell as seen in Figure 2.3(ii), where each of the rectangular plates, representing the filler

particles with dimensions w, t, and l, is filled into one of the polymer unit cells, having

finite width W, thickness T and length L [45], [46].

The volume fractions in two and three dimensions, respectively, are defined as follows

[46]:

Φ2𝐷 =𝑤𝑡

𝑊𝑇

Eq. 21

Φ3𝐷 =𝑤𝑡𝑙

𝑊𝑇𝐿

Eq. 22


17

Figure 2.3: Schematic diagram a film (i) without fillers offering no hindrance, (ii) film

filled with regularly arranged platelets perpendicular to the direction of diffusion, creating

a tortuous path. (Reproduced from [45] with permission from Elsevier).

2.4.1 Overview of the various models

Modeling of the barrier characteristics depends on the distribution and arrangement of the

fillers within the matrix. These can further be classified in three different classes as

described below.

a) Regularly distributed and perpendicularly oriented fillers

Most of the models in literature developed earlier describing diffusion in a composite

material are based on 2D systems in which the filler particles have a rectangular shape,

resembling ribbons of infinite length with a finite width (w) and thickness (t) [47]. These

models assume that the fillers are arranged regularly in the polymer matrix and

perpendicularly with respect to the diffusion direction, as can be seen in Figure 2.3. Some

of these models are presented in Table 1. First, Nielsen [48] proposed a simple permeability

model (Eq. 23) for such a system which is based on the idea that diffusing molecules need

to pass by a longer path in order to exit the film as seen in Figure 2.3. This model is widely

used but is only applicable in the dilute regime (𝛼Φ ≪ 1).Wakeham and Mason [49]

suggested a new model (Eq. 24) that is based on perforated laminae in which they found

the resistance to diffusion was created by the need of the penetrant to enter into the narrow

perforations. Similarly, Aris [50] developed an analogous model (Eq. 25) where the

diffusion resistance is mainly due to aspect ratio of pores (σ) and interaction of molecules


18

with pores which is termed as necking. Eq. 25 consists of four terms, where the first term

is 1 representing the case when the volume fraction of flakes is zero. The second term

represents the contribution due to the tortuous path taken by diffusing molecules around

the flakes. The third term characterizes the resistance due to the constraints between the

flakes. The last term is attributed to the diffusing of permeant through the pores and flakes

[51], [52]. Cussler et al., [43] neglected the third and fourth term and proposed a simpler

model (Eq. 26) for narrower pores (σ ≪ 1). This model predicts a quick decrease of the

relative permeability already at small values of Φ [43], [52]. Moggridge et al. [53] modified

Cussler et al.’s model by multiplying the second term by ½, which was their estimate for

the miss-alignment of the ribbon-like flakes. Additionally, they developed another model

for hexagonal flakes (Eq. 28). From this model, it can be noted that a change in the

geometry of the flakes affects the effectiveness of the barrier. In this case, hexagonal flakes

are less effective compared to ribbon-like flakes.

Table 1: Some characteristics of the models developed to study regularly distributed and

perpendicularly oriented fillers in polymer nanocomposites. 𝛼 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜, 𝛷 =

𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙𝑒𝑟, 𝜎 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑙𝑖𝑡𝑠.

Model

(Year)

Particle

type Tortuosity factor

Equation

#

Nielsen[48]

(1968) Ribbons 1 +

𝛼Φ

2 Eq. 23

Wakeham

and

Mason[49]

(1979)

Perforated

laminae 1 +

𝛼2Φ2

4(1 − Φ)+

𝛼Φ

2𝜎+ 2(1 − Φ) ln [

1 − Φ

2𝜎Φ] Eq. 24

Aris[50]

(1986) Ribbons

1 +𝛼2Φ2

4(1 − Φ)+

𝛼Φ

2𝜎

+2𝛼Φ

𝜋(1 − Φ)ln [

𝜋𝛼2Φ

4𝜎(1 − Φ)]

Eq. 25

Cussler et

al.[43]

(1988)

Ribbons 1 +𝛼2Φ2

4(1 − Φ) Eq. 26

Moggridge

et al.[53]

(2003)

Ribbons 1 +𝛼2Φ2

8(1 − Φ) Eq. 27

Hexagonal

flakes 1 +

𝛼2Φ2

54(1 − Φ) Eq. 28


19

b) Randomly distributed and perpendicularly oriented fillers

To describe the random distribution of fillers that are oriented perpendicularly to the

diffusion direction, Brydges et al. [54] proposed a model (Eq. 30) where they use the

stacking parameter 𝛾 defined by:

𝛾 =𝑥

𝑙 Eq. 29

where x and l are represented in Figure 2.3. This parameter takes into account the deviation

from ideally positioned flakes by expressing the horizontal offset of each particle with

regard to the one below it [55].

When 𝛾 = 1 2⁄ , the ribbons in one layer are centered to the gaps of the layer under it.

Cussler et al.[43] derived a similar expression for such systems (Eq. 31), however instead

of the factor 𝛾(1 − 𝛾) , they introduced 𝜇 , a combined geometrical factor which

characterizes the randomness of the porous media. Models from the beginning of the 21st

century focused on more realistic representations of the geometry of the fillers; particles

are not any longer considered as infinite ribbons but instead they have finite width, length,

and thickness (3D systems)[46] . Such a model was presented in Fredrickson and Bicerano

[56], where they examined the effective diffusion in composites containing randomly

placed disks having high aspect ratios and derived Eq. 32. To calculate the aspect ratio of

the disks, they used the radius of the disk instead of the width. Gusev and Lusti [57]

obtained Eq. 33 for a similar 3D system. They obtained it using a finite-element method

and fitting their results to an exponential function. A more recent model was proposed by

Minelli et al.[58] in which computational fluid dynamics (CFD) was used to solve the mass

transport problem in the layers filled with flakes and evaluation of the reduction in

permeability. From their study, they were able to derive the models presented in Eq. 34 and

Eq. 35 where 𝑟 is described by Eq. 36. This model incorporates many filler geometries and

can be used to study the enhancement of barrier properties resulting from the addition of

disks as well as rectangular, hexagonal, and octagonal flakes to a polymer matrix.


20

Table 2: Some characteristics of the models developed to study randomly distributed and

perpendicularly oriented fillers in polymer nanocomposites. 𝛼 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜, 𝛷 =

𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙𝑒𝑟, 𝛾 = 𝑠𝑡𝑎𝑐𝑘𝑖𝑛𝑔 𝑝𝑎𝑟𝑎𝑚𝑒𝑡𝑒𝑟, 𝜇 = 𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐𝑎𝑙 𝑓𝑎𝑐𝑡𝑜𝑟 .

Model

(Year)

Particle


Equation

#

Brydges et al. [54]

(1975)

Rectangular

flakes 1 +

𝛼2Φ2

(1 − Φ)𝛾(1 − 𝛾) Eq. 30

Cussler et al. [43]

(1988) Ribbons 1 +

𝜇 𝛼2Φ2

(1 − Φ) Eq. 31

Fredrickson and

Bicerano [56]

(1999)

Disks

(1

2 + [(2 − √2)𝜋𝛼Φ/4 ln(𝛼2)]

+1

2 + [(2 + √2)𝜋𝛼Φ/4 ln(𝛼2)]

)

−2

Eq. 32

Gusev and Lusti

[57]

(2001)

Disks 𝑒𝑥𝑝 [−𝛼Φ

3.47]0.71

Eq. 33

Minelli et al. [58]

(2011)

Disks and

rectangular,

hexagonal

or

octagonal

flakes

𝑟 ≤ 1

∶ (𝛼 + 2)2Φ

2𝛼+

(𝛼 + 2)4Φ2

4(𝛼2 − 𝛼Φ(𝛼 + 2))

+2(𝛼 + 2)2Φ

𝜋𝛼ln (

4

𝜋(

𝛼

Φ(𝛼 + 2)− 1))

Eq. 34

𝑟 ≥ 1

∶ 1 +Φ(𝛼 + 2)

2

+2(𝛼 + 2)2Φ

𝜋𝛼ln (

𝛼 + 2

𝜋)

Eq. 35

Minelli et al. [58]

(2011) 𝑟 =

2(𝛼 − Φ(𝛼 + 2))

(𝛼 + 2)2Φ Eq. 36


21

c) Randomly oriented fillers

The barrier properties are best, i.e., the tortuosity factor 𝜏 is at its highest, when the nano-

fillers are oriented perpendicularly with respect to the diffusion direction [55]. However,

in reality, this cannot always be achieved and, thus, the need for models that take the filler

orientation into account is imperative. Bharadwaj [42] introduced an order parameter S in

order to quantify the non-uniform orientation of the particles. This parameter is expressed

by:

𝑆 =1

2⟨3 cos2 𝜃 − 1⟩

Eq. 37

where 𝜃 represents the angle between the diffusion direction, usually identical with the

normal vector to the membrane surface, and S is calculated by averaging over all particles.

Figure 2.4: The order parameter S for three different cases; when all filler particles are

parallel to the diffusion direction (S=-1/2), when they are perpendicularly oriented (S=1)

and when they are randomly oriented (S=0). (Reproduced with modifications from [42]

with permission from American Chemical Society (ACS))

This parameter was used by Bharadwaj to modify Nielsen’s equation in order to take into

account the orientation of the flakes and the new model that he proposed is given by Eq.

38.

𝑃𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒

𝑃𝑝𝑜𝑙𝑦𝑚𝑒𝑟=

1 − Φ

1 + (𝛼2 ∙ Φ ∙ (

23) ∙ (𝑆 +

12))

Eq. 38

Lu and Mai [41], [47] developed a model that uses Bharadwaj’s order parameter to

approximate critical volume fraction Φ𝑐 of platelets/ ribbons of clay corresponding to

lowest gas permeability in exfoliated nanocomposites. They determined that the

permeation of gas molecules in clay based nanocomposites is an aspect ratio-controlled

mechanism and it is at its lowest when the volume fraction satisfies Eq. 39. In this equation,


22

p𝑐 takes the value of 0.38 and 0.72 for two-dimensional and three-dimensional models

respectively [45], [47].

Φ𝑐 = (3

2𝑆 + 1) (

1

𝛼) p𝑐

Eq. 39

Maksimov et al. [59] developed an empirical relation given by Eq. 40 to predict the

permeation in nanocomposites with randomly oriented rectangular flakes in 3D. In their

equation, 𝑃∥ refers to the permeability of the composite calculated by Nielsen’s relation.

𝑃 =1

3(𝑃∥ + 2𝑃0(1 − Φ))

Eq. 40

Recently, Greco and Maffezzoli [60] derived a new geometrical model for arbitrarily

oriented lamellae based on 2D and 3D simulation results. This model predicts the

permeability in polymers filled with impermeable fillers by calculating normalized path

length and probability of collision between the diffusing particle and the lamellar surface

[60]. For simulation of the diffusion, a finite-element (FE) method was used and the

simulations were performed for different filler concentrations, aspect ratios as well as

different orientation angles. It was found that all the data fit on a single curve indicating

that the normalized diffusion coefficient 𝐷𝑛𝑜𝑟𝑚 depends on the normalized path length

𝐿𝑛𝑜𝑟𝑚, which is a function of the nanocomposite morphology that is a combination of

aspect ratio, volume fraction and orientation) [60]; as given in Eq. 41 and Eq. 42.

𝐷𝑛𝑜𝑟𝑚 = (1

𝐿𝑛𝑜𝑟𝑚)4

Eq. 41

𝐿𝑛𝑜𝑟𝑚 = 1 +𝛼Φ

2cos 𝜃 (1 − sin 𝜃)

Eq. 42

Where, 𝐷𝑛𝑜𝑟𝑚 is the ratio between diffusion coefficients of simulated nanocomposite and

polymeric matrix. Additionally, in their paper, they showed that their new 3D model fits

better to the finite element calculations than the 2D Bharadwaj model.


23

d) Accounting for additional influencing parameters

So far, the presented models have considered mainly some of the fillers parameters such as

their aspect ratio, volume fraction, stacking position, and orientation. In this section,

additional models are discussed that include further influencing parameters like the

polydispersity of the fillers and their thickness, [44], [53] the polymer chain immobility,

the existence of an interfacial region [40], and aggregation of the fillers in the matrix [61],

[62].

Table 3: Some characteristics of the models presented in this sub-section.

Model

(Year)

Particle


Equation

#

Lape et al.

[62]

(2004)

Ribbons [1 +2

3(Φ

𝑡∑𝑛𝑖𝑤𝑖)∑𝑛𝑖𝑤𝑖

2]2

Eq. 43

Xu et al.

[63]

(2006)

Ribbons 𝜉 [1 +𝛼𝑡

2√Φ(t + b)−3/2] Eq. 44

Sorrentino

et al. [64]

(2006)

Ribbons (1 − Φ) + Φ(

𝑤 + 2𝑡𝑤 𝑠𝑖𝑛𝜃 + 2𝑡 𝑐𝑜𝑠𝜃)

2

1 + 𝛽Φ

Eq. 45

Picard et

al.[44]

(2007)

Ribbons (1 +Φ

3

∑𝑛𝑖 (𝑤𝑖

𝑡𝑖)2

∑𝑛𝑖𝑤𝑖

𝑡𝑖

)

2

Eq. 46

Nazarenko

et al.[61]

(2007)

Disks 1 +Φ𝛼

2𝑁 Eq. 47

𝑛𝑖 is number of flakes in size category, 𝑤𝑖 is half of the flake length, 𝜉 is polymer chain segment

immobility, b is face to face distance between ribbons, 𝛽 is volume and diffusion function of

ribbons as described in Eq. 48 and N refers to number of layers in layer stack.

Lape et al. [62] proposed a model (Eq. 43) that deals with flakes having a size distribution

in a system where they are randomly dispersed with an infinite length. They assumed the

flakes to have a discrete distribution of widths and a constant thickness t. In Eq. 43, 𝑛𝑖


24

represents the number of flakes having a specific width and 𝑤𝑖 represents half of the flake

width[45], [65]. In their study, Lape et al. [62] deduced an unexpected result; they found

that the barrier properties of polydispersed flakes are superior to that of monodispersed

ones with the same average size. Picard et al. [44] modified Lape et al.’s model in order to

also take into account the polydispersity of the thickness of the flakes. The relation

proposed by them is described in Eq. 46. Consequently, Picard et al. [44] also account for

the presence of aggregation in the system since it includes the polydispersity of the filler’s

thickness. Similarly, Nazarenko et al. [61] modified Nielsen’s model in order to consider

the effect of layer aggregation on the barrier properties. This model is presented in Eq. 47

where N represents the number of layers stacked together [55]. When N=1, the layers are

well dispersed and thus they are in an exfoliated state. However, when N increases, the

barrier properties are less efficient, leading to a bad quality barrier [55].

Another parameter influencing barrier properties was analysed by Sorrentino et al. [40].

They proposed a model (Eq. 45) that includes the effect of the presence of an interfacial

region between the polymer matrix and the inorganic flakes. This was done by the

introduction of the 𝛽 parameter, which is calculated by:

𝛽 =𝑉𝑠𝐷𝑠

Φ𝐷0−

𝑉𝑠 + Φ

Φ

Eq. 48

where 𝐷𝑠 and 𝑉𝑠 are the diffusion coefficient and the volume fraction of the interface

region, respectively, and 𝐷0 is the diffusion coefficient of the unfilled matrix.

2.5 Working principle and degradation of organic solar cells

2.5.1 Working principle

As shown in Figure 2.5, a typical organic solar cell has two electrodes, at least one of which

is semitransparent. This permits light radiations to interact with active layer. The active

layer uses the energy of the radiations (photons) to create charge carriers. A layer between

electrode and active layer i.e. usually PEDOT:PSS (Poly(3,4-ethylenedioxythiophene)

polystyrene sulfonate) to achieve selectivity of the contacts and to support charge extraction

[66], [67]. Two cell architectures are possible by making variations in the sequence of the

layers. One is termed as ‘normal structure’ and other is called ‘inverted structure’. In normal

structure holes (positive charges) are obtained through the bottom electrode, while negative


25

charges leave the device through the top electrode. In the case of inverted structure, the

device polarity is reversed [3], [4], [68].

Figure 2.5: a) Schematic structure of a typical organic solar cell showing a glass or PET:

Polyethylene terephthalate (substrate), Indium tin oxide: ITO (bottom electrode), ZnO:

Zinc oxide (electron extraction layer), blend of P3HT:PCBM (active layer), PEDOT:PSS:

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (hole extraction layer) and

Silver:Ag (top electrode) b) Energy band diagram of a normal cell structure and c) Energy

band diagram of an inverted cell structure. Data extracted from [69].

In contrast to a typical inorganic silicon based photovoltaic, a typical organic solar cell has

an active layer of semiconductors which is of organic nature. In such organic

semiconductors, carbon atoms having single and double alternating bonds are the core

responsible for initiation of the conductivity [70]. Because of the sp2 hybridization, the

overlapping pz electron wave functions allow delocalization of the corresponding electrons,

which enables transportation of the charge. HOMO (bonding π orbital) is defined as highest

occupied molecular orbital, LUMO (antibonding π* orbital) is referred as lowest

unoccupied molecular orbital. Opening between bonding (π) and antibonding (π*) states

pertaining to Peierls distortion is called as band gap (𝐸𝑔) [71]. By irradiation with light in

the range of visible region, electronic conversion between HOMO and LUMO can be

stimulated [72].


26

The working principle of an OSC device involves six basic steps [67]. These basis steps

are briefly described below.

Absorption of light and exciton formation: A light ray transmits from the semitransparent

electrode and incites the shift of an electron from HOMO to LUMO.

This shift results in a stimulated state, which is described as an electron-hole pair on the

same molecule (‘exciton’). Pertaining to their relatively petite distance and the feeble

relative permittivity of compounds of 𝜀 ≈ 2-4 of organic nature, electron and hole possess

sturdily binding of Coulomb [73].

Exciton diffusion: The exciton of neutral charge exciton distribute throughout the layer

until it decays or bumps into an interface. When exciton reaches at interfaces, it detaches

and gets into separate charges.

Charge carrier separation: To incapacitate the binding of electron and hole, a material

having inferior LUMO is employed. This low LUMO material forms a second phase and

delivers a propitious energy level for the electron. On the boundary of donor and acceptor

within photoactive layer, the electron is shifted between phases and as a result, holes and

electrons are separated. If the event of absorption takes place at donor / acceptor phase

boundaries, charge carriers are generated at once which is within the femtosecond time

scale, thus excluding the earlier mentioned exciton diffusion process.

Movement of carriers to the electrodes: In general, holes and free electrons move by

means of hopping from molecule to molecule unless encountered by electrodes. Electrons

travel in the phase formed by the acceptor molecules, while holes travel in the phase formed

by the donor molecules. This requires phase separation to an extent that continuous

percolation paths are formed.

Charge collection: After charges have crossed the phase boundary barriers, they are

collected at an electrode and hence photocurrent is generated.

2.5.2 Current density voltage characteristics

For the characterization of a solar cell, current density-voltage (J-V) curves are measured

under illumination. The jV-curve can be obtained by treating the solar cell in terms of the

so called one diode model, which corresponds to the equivalent circuit described in Figure

2.6. From a typical curve, the useful information is extracted in terms of power conversion

efficiency (PCE), short circuit current density (Jsc), open circuit current (Voc) and fill factor

(FF).


27

Figure 2.6: Equivalent circuit of an organic solar cell (one diode model) (Reproduced from

[74] with permission from Elsevier)

• The power conversion efficiency (𝑷𝑪𝑬) is the most important parameter and defines the

actual maximum electric power obtained divided by the radiation power which the device

is exposed to. It is calculated from the jV curve by

𝑃𝐶𝐸 = 𝑗𝑠𝑐 ∙ 𝑉𝑜𝑐 ∙ 𝐹𝐹

𝑃𝑙𝑖𝑔ℎ𝑡,𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡

• The short circuit current density (𝑱𝑺𝑪) is the current across the solar cell when the voltage

through the cell is nil. It corresponds to the generation and collection of light-generated

carriers at electrode and thus depends on the number of incoming light rays as well as on

the results of each of the six basic steps described earlier in the Working principle.

• The open circuit voltage (𝑽𝐎𝐂)

The open-circuit voltage, 𝑉OC, is the highest potential difference obtained from a solar cell,

and this occurs at zero current [75]. In a heterojunction solar cell, 𝑉OC is described as a

formation of a charge-transfer complex between the blend of materials in active layer. Each

phase goes through charge transfer when irradiated with light. The charge transfer of

excitonic state is lower than pristine state and hence is occupied at the interface between

the donor and acceptor[75], [76]. The energy of the charge transfer in excitonic state is

equal to the difference of the donor’s HOMO and the acceptor’s LUMO level [77].

• The fill factor (𝑭𝑭) is the ratio of the maximum obtainable power, divided by the product

of 𝑉OC and compared to 𝑉𝑂𝐶 and the 𝐹𝐹 is a more delicate sign for voltage dependent

recombination processes within the solar cell device.

2.6 Degradation mechanism of Organic Solar Cells

As compared to materials used in inorganic solar cells, most organic matter utilized in

organic solar cells, have the susceptibility to degrade chemically by oxygen, moisture and


28

ultraviolet rays interactions (as shown in Figure 2.7) [6], [78]–[80]. Such materials are also

prone to morphological instabilities at relatively higher temperatures [81]–[83]. The

degradation within OSC can start in a localized manner or at the interfaces of individual

layers and thus leading to loss of overall cell performance [84], [85].

Figure 2.7: A schematic diagram of few processes responsible for degradation in OSC with

P3HT:PCBM as photoactive layer, (Reproduced from [80] with permission from Elsevier).

Degradation of solar cell in chemical or physical manner may affect different phenomenon

such as absorption of photons, dissociation and transportation of charge towards electrode.

The degradation leaves a negative impact on aforementioned phenomenon and as a result

number of charges collected at electrode decrease, which ultimately deteriorates short

circuit current (𝐽𝑆𝐶) [73], [80]. Changes in the work function of electrodes or levels of

HOMO and LUMO can leave a significant negative impact on open circuit current (𝑉𝑂𝐶).

[67]. Since, the fill factor (FF) is responsible for provides information on the quality of the

charge extraction, therefore any changes in recombination losses / formation of space

charges due to instable transport could results in FF loses [86]. Therefore, the fill factor is

mainly impelled by parallel and series resistances, thus can be co-related with the

modulation in the JV-curve [73], [80], [86].

Some of the possible degradation mechanisms of organic solar cells reported in literature

are mentioned below:


29

The main channels for diffusion of oxygen and moisture in to encapsulated OSC devices

are either microscopic pinholes present in the encapsulation or through edges of lamination

via glues [87], [88]. Molecules continue diffusing within the layer until they reach electrode

[89]. These diffused moisture and oxygen molecules modify inner surface of the electrode

by chemical reactions such as voids formation or patches of insulation, and ultimately

causing reduction in the electrode/photoactive layer charge transfer which is usually

referred as photo-bleaching [6], [76], [87]. This rate of photo-bleaching and permeability

of oxygen of a barrier film of thickness d to oxygen at partial pressure p(O2) can be

calculated from the rate of photobleaching of a P3HT film underneath the barrier when the

photobleaching reaction is diffusion controlled. The reaction rate is defined as the number

of moles, n, of thiophene rings being oxidized per unit time t and unit area A, ∆𝑛

𝐴∙∆𝑡, of the

P3HT film underneath the barrier. Assuming the consumption of three to five moles of

molecular oxygen per mole of thiophene rings, depending on the stage of the reaction [90],

we obtain from the resulting oxygen flux J(O2) Eq. 49. The reaction rate is obtained from

the rate of absorbance loss, inserting 𝜀 = 8000 𝑐𝑚2 𝑚𝑚𝑜𝑙−1 thiophene rings for the molar

extinction coefficient at the absorption maximum. This provides the permeability and the

OTR of the barrier Eq. 50 & Eq. 51):

𝑃(𝑂2) = 𝐽(𝑂2) ∙𝑑

∆𝑝(𝑂2)=

5∙∆𝑛

𝐴∙∆𝑡∙

𝑑

∆𝑝(𝑂2) Eq. 49

𝑃(𝑂2) =5 ∙ ∆𝐸

𝜀 ∙ ∆𝑡∙

𝑑

∆𝑝(𝑂2)

Eq. 50

𝑂𝑇𝑅 =5 ∙ ∆𝐸

𝜀 ∙ ∆𝑡∙

1

∆𝑝(𝑂2)

Eq. 51

Mechanical delamination can also take place if the device goes through extended exposure

to diffusing gasses and simultaneous mechanical stresses [91].

Metal electrodes used for collecting electrons having low work function are susceptible to

oxidation as compared to hole collecting metal electrodes which usually have high work

function [3], [66], [89]. Like OLEDs, there is always a chance of diffusion of silver

electrode into the active layer which may create shorting issues [66]. As mentioned earlier

organic materials of OSC are sensitive to moisture and oxygen especially under

illumination and elevated temperature which are termed as photo and thermal oxidation

[79], [92]. In one of the study it was stated that degradation in OSC can either be reversible


30

or irreversible depending on the types of degradation i.e. limited exposure to either oxygen

or moisture [79]. However, long term device exposure to oxygen / water under illumination

can generate defects and can cause loss of absorption density which will lead to irreversible

degradation. [79]. Doping of active layer with p or n type dopants can significantly alter

the device performance [87], [93]. Aggregation within the blend of active layer may block

the path of charge carriers which may result in substantial decrease in in device

performance [94]. Additional hindrances to charge carriers may also come from the

impurities, dopants or during handling / processing of the active layer .[87], [93].

Furthermore, the use of highly hydrophilic materials like PEDOT:PSS can also

significantly accelerate degradation under humid conditions [6], [66], [95]. Additionally,

PEDOT:PSS loses conductivity when exposed to UV radiation which ultimately causes

degradation in OSC device [95].

Table 3: Degradation of the OSC parameters and their possible effect on the device

performance as described by Grossiord et al,. [86]

Parameters affecting PCE and key factors

determining them

Possible causes

Fill factor (FF) –

- Charge transportation and

recombination process

- Weakening of charge transportation in

active layers (P3HT;PCBM) or hole

transport layer (PEDOT:PSS)

- Modification in charge recombination

mechanism

- Generation of shorts or shunts.

Open circuit current (VOC) –

- Differences in levels of HOMO (donor)

LUMO (acceptor)

- recombination process

- Active layer or electrode interface

reduction

- Modification of effective band gap in

blend of photoactive materials.

- Modification in work function of

electrode

- generation of shorts

Short circuit current (JSC) – - Degradation of the conjugation of the

photoactive polymer


31

- Efficient absorption of light (thickness

of active layer, band gap, molecular

architecture),

- Efficient dissociation of exciton

(matching level of HOMO (acceptor)

and LUMO (Donor), morphology of

blend in active layer),

- Efficient carrier transportation as well

as collection (path towards electrodes

i.e., donor goes to electrode that collects

hole and acceptor goes to electron that

collects electron,

- Crystallinity of active layer,

- Architecture of the device

- Deterioration of the optical transparency

of the layers laying between light

illumination and active layer

- Deterioration of interface between donor

and acceptor or increase in blend

domains above the diffusion path length

of the excitons

- Loss of percolating paths due to blend

reorganization

- Deterioration of interface between active

layer and electrode

- Formation of crack in active layer

- Delamination of electrode or active layer

- Deterioration of the mobility of charge

carriers due to degradation of materials.

STATE OF THE ART

32

STATE OF THE ART

This chapter gives the state of the art in the field of coated barriers. It is categorized

according to the parameters on which permeability depends, i.e., coefficient of solubility,

thickness (increasing path of diffusion), coefficient of diffusion and effective area.

To control permeability, the use of polymers having low solubility of diffusing gases can

be beneficial. Therefore in this chapter, different polymers are described which are cost

effective and abundantly available.

Later in the chapter filler barriers are discussed which are based on increasing the diffusion

path called as tortuosity factor, this is done by filling polymers with impermeable inorganic

platelets (nanoclay, graphene oxide, glass flakes etc.,) having some aspect ratio. The use of

platelets will increase the diffusing path, thereby referred as tortuous path. Various

theoretical models are discussed which predict the barrier quality of filler polymers.

In order to decrease permeation, control over the coefficient of diffusion is necessary, hence

polymers like SiO2 are effective materials and can decrease permeability by exhibiting low

coefficient of diffusion. The perhydropolysilaze simply termed as polysilazane or PHPS is

an inorganic polymer which yields SiO2 network after curing. The resulting SiO2 layer acts

as an excellent diffusion barrier against water and oxygen. Various curing mechanism of

PHPS are discussed in this chapter.

ORMOCERS (Organically modified ceramics) are a class of materials having tailored

properties. The choice of organic or inorganic parts depends on the application.

ORMOCERS with tailored properties can effectively be used in packaging industry not

only as barrier in itself but also as the planarization layer. Such type of materials have

effectively been utilized to decouple surface defects and hence decreasing effective area.

Fluoropolymers due to their hydrophobic nature, can be effectively used in the packaging

industry. CYTOP, a class of fluoropolymer is described in this chapter, has not only the

barrier effect but also leave a smoother surface finish.

The use of scavengers has also been beneficial for the packaging industry as they can absorb

moisture and can keep the product safe. Such kinds of barriers are also discussed in this

chapter.

STATE OF THE ART

33

3.1 Bulk Polymers

Polymers are one of the most important classes of packaging materials, along with metallic,

ceramic (glass), and cellulosic materials (paper and cardboard) [96]. Because of the light

weight and cost effectiveness, along with a variety of other favorable physical and chemical

characteristics, polymers are the most commonly used materials in the field of packaging

[97]. As per an estimation, polymers carry around 40% of the market share in the food

packaging industry [65]. One of the basic beneficial factor of the polymers is their simple

processing by roll-to-roll techniques. Although polymeric materials have revolutionized

the packaging sector and exhibit many merits over their counterparts, their permeability to

small molecules and environmental gases is a serious disadvantage, which limits their

applicability in encapsulating organic electronic devices (OEDs) [98]. Figure 3.1 compares

the permeability to oxygen and water vapor for different polymeric materials.

The barrier quality in terms of OTR of most of the common bulk polymers like PET, PAN

etc. ranges between 10 𝑐𝑚3 ∙ 𝑚−2 ∙ 𝑑𝑎𝑦−1 ∙ 𝑏𝑎𝑟−1 to 1 𝑐𝑚3 ∙ 𝑚−2 ∙ 𝑑𝑎𝑦−1 ∙ 𝑏𝑎𝑟−1 and

are usually considered sufficient for packaging food [24], [35], [99].

100010010

1

0.10.01

10000

1000

100

10

1

0.1

OT

R (

cm

3/m

2.d

ay.b

ar)

@ 2

3oC

/ 5

0 %

RH

WVTR (g/m2.day)

@ 23oC / 85% RH

0.01

LCP

CelluloseEVOH

PVDC

PEN PAN

PET

PVC-UPLA

BOPP

PE-HD

PVB

PA 6

PS

COC

PC

PVC-P

PP

PE-LD

Figure 3.1: OTR and WVTR of different bulk polymers normalized to 100 µm thickness.

[56] PE-LD =polyethylene low density, PE-HD= polyethylene high density,

PP=polypropylene, PS=polystyrene, bopp=biaxially oriented polypropylene,

PLA=polylactic acid, PVC=polyvinyl chloride, PA6=polyamide 6, LCP=liquid crystalline

polymer, EVOH=ethyl vinyl alcohol, PAN= polyacrylonitrile, PEN= polyethylene

napthalene, PET= polyethylene terephthalate, PVDC= polyvinylidene chloride, PC=

polycarbonate, PVC-P=polyvinyl chloride-plasticized, PVC-U= polyvinyl chloride-

unplasticized, PVB= Polyvinhyla butyral. (Reproduced from [100] licensed by CC BY 3.0).

STATE OF THE ART

34

The permeation to a particular molecule of gas can be influenced by the existence of others.

For example, ethylene vinyl alcohol (EVOH) offers high hindrance to the permeation of

oxygen at relatively low humidity conditions. However, in exceptionally wet conditions

(e.g., above 75% RH), its oxygen permeation rate is enhanced by almost one order of

magnitude. The reason for this is the swelling of polymer which happens due to the

presence of water [101].

Polymers in combination with other materials or multilayer stack systems are commonly

employed. For instance, to obtain a good quality oxygen barrier in a highly wet

environment, materials like EVOH or PVA, which show a high sensitivity to water, but

exhibit a very low permeation to oxygen, can be placed between two layers of a

hydrophobic polymer like polyethylene (PE) [100] [102]. Mixing of one polymer into

another is also an effective approach to get desired barrier characteristics that cannot be

obtained with a single polymer [103]–[105]. With polymer mixing [104] and creating

multilayer by lamination of different polymeric films [106], packaging materials with

relatively low gas permeations can be produced, however, these methods unfortunately

have high production costs because of the use of special and expensive glues that

complicate the manufacturing process. Recycling of such barriers films is also a

challenging work. Therefore, there is still a great interest in the polymer industry to generate

monolayer coatings with improved mechanical and barrier properties [24], [106].

The barrier properties required for organic electronic devices (OEDs) are clearly

substantially beyond those bulk polymers [107] and there is no pristine polymer that shows

all the required gas barrier and mechanical characteristics for such applications [7].

Nevertheless, as a temporary protection of inorganic PV against moisture and oxygen; poly

(vinyl butyral), ethylene vinyl acetate, polyolefins, ionomers and thermoplastic

polyurethanes have been reported in the literature [31]. Therefore, in order to fulfil the

barrier requirements for OED encapsulation different strategies based on Eq. 16 are used

to improve the barrier properties of polymers, which include used of fillers and creating

impermeable coatings [108]. The later are usually deposited by sputtering or ALD, often in

multilayer structures with polymers. These achieve excellent values e.g. Barix [13].

However, these barriers are expensive and have other disadvantages like the processing of

such barrier require special vacuum systems which are not only complex and require high

level of maintenance, moreover super flexibility is still a challenge for such barriers.

Therefore, solution processable barriers based on polymer films have been developed and

are described in the following sections.

STATE OF THE ART

35

3.2 Increasing tortuous path

As pointed out above, barrier layers consisting of neat polymer films do not have the

capacity to hinder the permeability of unwanted substances to the extent required for

protecting OEDs [109], [110]. According to Eq. 16 a possible strategy of decreasing the

permeability consists in increasing the path that permeant molecules have to follow through

the coating in order to reach the other side of the membrane [110], [111]. The addition of

obstacles (Figure 2.3) is thought to create a tortuous path for diffusing molecules [45], [65].

By virtue of their improved barrier characteristics, polymers filled with particles have

gained much importance and have been studied extensively in literature [45], [110]. The

addition of the particles usually does not affect coatability and thus processing remains

simple. The type of materials used for this application are two-phase systems consisting of

a polymeric matrix with dispersed inorganic nanoparticles such as clay minerals, metal

oxides, graphene etc., which are impermeable to molecular species [45], [46], [58], [112].

The nature of the fillers and their degree of compatibility govern the permeation

characteristics. If the filler is compatible with the polymer, it can easily fill the voids present

inside the polymer, thus resulting in a tortuous path that makes the diffusion path for

permeants longer, which in turn improves the permeation resistance [113]. According to

Eq. 16, the degree of tortuosity depends on the aspect ratio and orientation of the filler

particles as well as on the volume fraction of the filler [45], [65], [111]. If the filler is

incompatible with the polymeric system, instead of filling up the free volume sites, the filler

will agglomerate and result in the formation of voids, which increases the free volume sites

and reduces the permeation resistance [114], [115]. Several material systems have been

developed which will be described in the following.

3.2.1 Clay based barriers

The most widely used class of inorganic fillers in polymers for the production of packaging

is clay. [70], [94] This frequent use of clay as the filler is favored by its availability in

abundance, easy processing, non-toxic nature and relatively high aspect ratios. The clays

belong to a 2:1 phyllosilicates family,[70] which have sheet like structures.

Montmorillonite (MMT) [93], hectorite, and vermiculite are some of the examples of the

family and most commonly used fillers in polymer clay composites [70], [92], [95]. The

clay being hydrophilic in nature usually requires water as a dispersing agent. The

performance of MMT clay as a filler in polymer matrix usually depends on its aspect ratio

STATE OF THE ART

36

along with its surface compatibility [96]. In order to increase the compatibility of the clay

with the polymer matrix, quaternary ammonium salts can be used as surfactants, which

develop strong interactions between clay and the polymer chains of the matrix. This

interaction plays a vital role in improving barrier characteristics [97], [98]. In general, clays

exhibit three types of morphologies in the composites which are based on the degree of the

dispersion of the clay, these are: aggregated, intercalated and exfoliated [99]. When clay is

in the aggregated structure its tactoids are nicely dispersed in the matrix, but the single

platelets within the tactoids are not delaminated and still remain at their original positions

as shown in Figure 3.2.

Figure 3.2 Schematic representation of clay morphology when mixed with polymers.

(Reproduced from [116] with permission from Elsevier)

In case of the intercalated structure, the clay platelets to some extent are delaminated from

the tactoids, and thus these platelets can diffuse and distribute through the polymer chains.

In the exfoliated condition, the clay platelets are completely delaminated from the tactoids

and single layered platelets disperse homogeneously within the matrix. The most desired

state is the completely exfoliated structure because it has the capability to offer excellent

barrier characteristics and also imparts maximum resistance against mechanical and

STATE OF THE ART

37

thermal deteriorations at relatively very low clay contents [117]. However, complete

exfoliation state is very hard to achieve and hence polymer nanocomposites have clays in

states either intercalated or semi exfoliated [45], [118]. Using MMT clay, Gaume et al.,

[119] produced a barrier film comprising of PVA and clay. OSCs based on P3HT:PCBM

photoactive layers were laminated between the barrier films, and their lifetimes were

compared to those obtained for OSCs packaged in PET (a common polymer used as a

substrate) and PVA coated PET.

Figure 3.3: Lifetime of organic solar cells tested under irradiation with a solar simulator

(AM 1.5G, 30 oC, ambient RH 30-40%): normalized power conversion efficiency (PCE)

of OSCs encapsulated with PET film, PVA coated PET film, and PET coated with PVA-

MMt 5 wt% nanocomposite (Copied from [119] with permission from Elsevier).

Encapsulated solar cells were irradiated with a solar simulator. The use of clay-based

barriers seems to result in slightly improved lifetime as compared to PET. Laminated OSCs

lost 20% of the initial power conversion efficiency (PCE) within 20 hours of testing time

and 80% of its initial performance in around 70 h (as shown in Figure 3.3).

This improved lifetime of the OSC was due to improved barrier performance of the

PVA/clay composite and exhibited an improvement factor for OTR and WVTR of 6.9 and

2.6, respectively as compared with pristine PVA. This is not an ideal lifetime performance,

but clearly shows the potential of the clay based barriers.

STATE OF THE ART

38

In one of the studies on clay, Yano et al., fabricated a comoposite of clay and polyimide

with four different clays which included synthetic mica, montmorillonite, hectorite and

saponite [120]. It was noticed that the improvements in permeation characteristics rely on

the size (aspect ratio) of the platelets. Mica showed the most impressive effect and

enhanced barrier properties by the factor of ~10 with clay concentration of just 2 wt% in

nanocomposites. In other studies carrier out by Messersmith and Giannelis [121] reported

79% reduction in moisture permeation with 4.8 vol% of mica /poly (Ɛ- caprolactone) (PCL)

nanocomposites. In another study, Zhou et al. in 2016, studied the permeability of

poly(butelene succinate) (PBS)/clay nanocomposites [122]. They reported that if the

content of clay exceeds a certain amount, it exhibits a greater aspect ratio and more regular

dispersion in PBS matrix which eventually results in a large decrease in gas permeation

properties. Recently, modified synthetic clay (smectites) is reported to have ultra high

barrier characteristics [123]. The modification of the clay was performed by osmotic

swelling method, which yielded ultra-high aspect ratio platelets of around ~20,000. Such

clay platelets were used with PVA matrix [123], [124] and resulting composite barrier

exhibited the OTR and WVTR of 0.11 cm3 m−2 day−1 bar−1 (23oC/ 90%RH) and 0.18 g m−2

day−1 (23oC/ 90%RH), respectively, for a coating of 0.42 μm. In another study regarding

such a clay, a thin layer polymer/clay composite having thickness of 21.4 μm showed

extremely high barrier characteristics i.e. OTR < 0.0005 cm3 m−2 day−1 bar−1 and WVTR

of 0.0007 g m−2 day−1 at testing conditions of 23 °C and 50% RH. Even in the most

challenging environments (38 °C and 90% RH), values as low as 0.24 cm3 m−2 day−1 bar−1

and 0.003 g m−2 day−1 were found for OTR and WVTR, respectively [110], [124]. These

recent developments in clay modification generating very high aspect ratio of over 20000

and producing extremely high gas barrier from solutions make clay a wonder material and

ideally suited for encapsulation of organic electronic devices.

3.2.2 Graphene based barriers

Graphene is a single layered sp2 hybridised atom of carbon arranged in a two dimensional

lattice [45], [125]. It is usually fabricated by the exfoliation of graphite nano-sheets [126].

The theoretical specific surface area of graphene sheet is 2630-2956 m2/g with a large

aspect ratio exceeding = 2000 [125], [127]. One of the most beneficial use of graphene

is its application in producing polymer nanocomposite where graphene is used as a filling

agent. However, this application has some limitations like solubility and uniform dispersion

STATE OF THE ART

39

in polymers [45], [127]. This is because of graphene has insignificant solubility in most of

the conventional solvents [116], [128]. Additionally, existence of wan der Walls interaction

between large surface area of graphene platelets causes substantial aggregations when

incorporated in polymeric matrix [129]. Tseng et al., [130] fabricated polyimide/graphene

oxide (GO) nanocomposite with various GO loading. It was observed that the WVTR is

reduced by 83% with the addition of only 0.001 wt%. It was reported that the increase in

filler content from 0.001 wt% to 0.01 wt% linearly reduced WVTR. This result is in contrast

to polyimide/montmorillonite nanocomposites, in which an amount of around 8 wt.% was

needed to reduce WVTR to same extent [120] . In Figure 3.4, the effects of contents of

graphene and nanoclay on water vapour transmission rate of polyimide nanocomposite are

shown. Nanocomposite of polyimide containing only 0.5 wt.% of graphene reduced water

vapour transmission rate by 88% and nanocomposite with clay of the same content reduced

WVTR by 63% as competed to pristine polyimide. The different of barrier performance of

the nanocomposite can be attributed to significantly higher aspect ratio of graphene offering

large resistance to permeating molecules as compared to clay [131].

Figure 3.4: WVTR (g/m2.h) of composites of polyaniline / graphene and polyaniline / clay

as function of graphene loading. (Copied from [45] with permission from Elsevier).

Chen et al. in 2014 prepared a gas barrier nanocomposite comprising of poly(vinyl alcohol)

(PVA) and graphene oxide and reported reduced gas permeabilities. The reported OTR was

<0.005 cm3/m2.day.bar at graphene oxide loadings of only 0.07 vol%. Isothermal

recrystallization of the composite introduced PVA crystals, which acted as barriers along

STATE OF THE ART

40

with graphene oxide sheets. The resulting barrier properties of the composite were 1-8

orders of magnitude better compared with polymer/inorganic composite coatings [132].

The O2 relative permeability of the film showed almost the exact fitting to the Cussler

model Figure 3.5. At lower GO contents in PVA (referred as bled of PVA and GO), the

experimental data are in close accordance with the Cussler curve with ɑ =500, whereas the

data approach the curve with ɑ= 2000 at higher concentrations. The deviation of the data

from Cussler’s model for ɑ=2000 is due to GO agglomeration because of higher loading

concentrations which reduces the surface area and effective aspect ratio of graphene oxide

(GO) platelets. In contrast to blend of PVA/graphene oxide, hybrid PVA/graphene oxide

(0.07 vol% of GO and PVA crystallization), effectively reduced oxygen permeability. This

reduction in relative permeability fits the curve for ɑ=5000 [132].

Figure 3.5: Relative oxygen permeability for PVA, mixture of PVA/GO coating and hybrid

PVA/GO for 0.07 vol% layer in comparison to predictive permeation curves proposed by

three models ( i.e. Nielsen, modified Nielsen and Cussler) for different aspect ratios (𝛼)

(Copied from [132] with permission from Elsevier,

Kim et al. (2014) developed solution processed barriers for encapsulating OSC using

opaque layers of reduced graphene oxide (rGO) [133]. Inverted OSC devices based on poly

[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-

benzothiadiazole)]:[6,6]-phenyl-C71 butyric acid methyl ester ( PCDTBT:PC70BM) active

layers were encapsulated by two methods, i.e. direct coating rGO (without polymer matrix)

STATE OF THE ART

41

on device by spin coating and lamination of barrier (rGO without polymer matrix coated

on PEN substrate) using an adhesive. By coating devices directly, the lifetime was

improved by a factor of 50, while for lamination, devices maintained 43% efficiency even

after exposure to 100% humidity (at room temperature) for 240 h [133]. These observations

suggest that graphene oxide based nanocomposites have a great potential for developing

coated barriers but still are not suitable for encapsulation OSC because of lacking of

transparency.

Figure 3.6 gives summarized permeation reduction (%) of polymeric composites (Pc) as

compared to pristine polymer (Pp) which is calculated as a function of various filler types

(Eq. 52) against transparency:

Permeation reduction (%) = 𝑃𝑝−𝑃𝑐

𝑃𝑝 x 100

Eq. 52

It is observed that the graphene based barriers have the potential to be used a quality barrier

and can also maintain high transparency. Clay as discussed before is the best available filler

that not only can fulfil barrier requirements but can also exhibit high transparency. Boron

nitride, Molybdenum(IV)sulphide (MoS2) and glass flakes have also been reported in the

literate as a potential barrier fillers but no significant barrier improvements are reported

[115], [134], [135],

30405060708090100

10

20

30

40

50

60

70

80

90

100

Tra

nsp

are

ncy (

%)

(GO + recrystallization, 0.07 vol%)

(Expanded graphite, 4 wt%)

Boron Nitride, 0.01 vol%

(RGO, 4 wt%)

(Cloisite B30, 5 wt%)

(Cloisite Na+, 5 wt%)

(GO, 1 wt%)

(Al2O

3 grafted GO

(Synthetic clay (Smectic)

Reduction in permeation (%)

Carbon based

Clay based

Boron Nitride

Figure 3.6: Transparency vs reduction in permeation for different filler types and loadings

in polymer matrices.

STATE OF THE ART

42

3.2.3 Getter materials

Getters are usually termed as scavengers. Such materials are used to attract and capture the

unwanted substances. Getters are used to capture substances like solid, liquid and gases,

especially oxygen, hydrogen and moisture [136]. The incorporation of getters can be highly

beneficial for the field of packaging and can help keeping the product safe from unwanted

contaminations [137]. The getters are generally used as fillers within a permeable

polymeric material, however in this case the fillers will not generate a tortuous path but

will capture the unwanted species like moisture and prevent their diffusion to the other side

of the membrane, hence decreasing the permeation rate.

Water getters are usually inorganic compounds that form hydrates when exposed to

moisture. Zeolites and other minerals are most commonly employed getter materials [12],

[80]. The getters are uniformly distributed in a polymeric matrix and films from such

materials can be used as effective moisture barriers. Getters may require thermal activation,

as they absorb water during storage or transportation. This thermal activation will dehydrate

the getters and bring back their full capacity to absorb water. Wu et al (2010) used beta

type of zeolite nanoparticles in the packaging of OLED to keep the environment dry [12].

Beta zeolite nanoparticles were synthesized from a clear solution of tetraethyl orthosilicate

(TEOS), aluminum iso-propoxide (AIP), tetraethylammonium hydroxide (TEAOH), and

water at a molar ratio of 1 TEOS : 0.04 AIP : 0.36 TEAOH : 25 H2O. A semi-transparent

solution was prepared which was diluted and centrifuged at high rotational speeds (23000

rpm). With this process nanoparticles of zeolite having crystalline nature were separated

from amorphous part. The nano-zelolite particles were mixed with acrylic resins and a

nanocomposite was thus prepared. The coated layer exhibited a good transparency of

around 85% in the visible region. The water absorption of acrylic/zeolite films was

analyzed and the performance was compared with films of acrylic without nanoparticles. It

was observed that layers with ~10 wt% zeolite nanoparticles show improved water

adoption capacity as compared to films of pristine acrylic. It was reported that the specific

capacity of the embedded zeolite was slightly lower than that of silane modified zeolite

powder preheated at 200 oC. Thus, the acrylic matrix reduces the moisture adsorption

capacity of the zeolite only to a small extent. Based on these results, the authors claim that

a WVTR of about 10-3 g/m2.day can be achieved with a film thickness of 130 µm having

40 wt% loading of zeolite without compromising the transparency [12].

STATE OF THE ART

43

3.3 Barriers based on impermeable coatings

Barriers based on impermeable coatings deposited on polymeric substrates rely on the

reduction of unprotected polymer surface according to Eq. 2.14. These coatings generally

consist of metal or semimetal oxides, namely Al2O3 and SiO2. Especially silica coatings

with a small coefficient of diffusion have attained much importance in many fields,

including corrosion [138]–[140], erosion [141], [142], wear [143], electrical insulation

[144], anti-oxidation [140], packaging [145]–[147] etc. These types of thin coatings are

usually prepared by vacuum assisted methods such as chemical vapor deposition [148],

[149], ion beam assisted deposition [150], or flame hydrolysis deposition [151]. In order to

enhance barrier quality, multilayer stacks are formed, decoupling defect growth by

depositing organic polymer layers between the metal oxide layers. In this way, barriers with

WVTR values below 10-5 gm-2day-1 have been created and such barrier coatings are referred

as Barix. Ultrathin multilayers of metal oxides (SiO2, SiNx, Al2O3) are deposited on plastic

substrate as the main barrier layers. Polymers are used as alternating interlayers between

metal oxides layers [13], [152]–[155]. The polymeric interlayers not only serve as

planarization layer allowing defect free growth of metal oxide on them but also enhance

overall flexibility [13]. Many efforts have been made to create coatings having barrier

effects comparable to Barix. Methods like sol-gel [140], [142], liquid phase deposition

[156], and electrophoretic deposition [157], [158] have been reported. The relatively low

density of the films obtained through these coating techniques causes high permeability

due to the availability of free volume within the film. In order to obtain dense silica

coatings, sol gel reactions are followed by thermal treatments at several hundred degrees

to remove the organic parts [159], [160]. Therefore, due to the boundary condition of

temperature, it is very hard to prepare dense silica coatings on polymeric substrates

applicable to packaging materials [161]. One of the solutions to this matter of the concern

is the use of liquid precursor avoiding high temperature to produce dense silica coatings.

3.3.1 Polysilazane

Recently, polysilazanes have been reported in the literature as an alternative route for the

production of dense, homogeneous, and defect free silica films [162].

Perhydropolysilazanes (PHPS) are inorganic polymers that consist of silicon and nitrogen

atoms in their backbone (-SiH2-NH-) and have been used extensively to produce silica

coatings [163]. With specified conditions PHPS yields a dense and homogenous SiO2

STATE OF THE ART

44

structure, which is the main reason for their good barrier properties [164]. This makes

PHPS unique materials that can be used in a variety of applications, especially in the

semiconductor industry, OLED displays, and packaging [143], [158], [165]. One of the

advantages of PHPS based coatings is their lower susceptibility to crack formation and

shrinkage. This is because its molecular weight rises during the conversion of PHPS to

silica, owing to its reaction with air and moisture [21], [164], [166], which results in volume

expansion. During the curing process of sol-gel layers, in contrast, alcohol or water is

released, resulting in a lower molecular weight and consequent shrinkage [167].

The concept of polysilazanes was first introduced by Krüger and Rochow in 1964 [168].

They formed polysilazane by a reaction of chlorosilanes with ammonia which generated

tetrameric cyclosilazanes. This product was further treated at high temperature with a

catalyst and formed a polysilazane with high molecular weight. Complete cured PHPS

remains optically clear and transparent and yield a very smooth surface as can be seen in

Figure 3.7 [169].

Figure 3.7: Transmission and appearance of cured PHPS films, a) showing the transparent

appearance, b) bendable transparent cured PHPS coating and c) transmission spectra of

PET film and different types of SiO2 coatings.(Copied from [169] licensed by CC BY 4.0)

There are different methods used for complete curing of Polysilazane to yield silica. These

methods can be summarized as:

STATE OF THE ART

45

a) Thermal curing

b) Curing in the presence of catalyst

c) Deep UV curing

d) Combination of the methods.

3.3.1.1 Thermal curing

The most common method to obtain a dense and stable SiOx network from PHPS is the

hydrolysis of Si-NH bonds and the subsequent formation of Si-O bonds at high

temperatures [170]–[174]. At low temperatures this process proceeds slowly. Complete

conversion to SiO2 requires treatment at temperatures of ~1000oC for several hours [175].

Matsuo et al. were the first to report that PHPS can be converted to silica thin films with a

density of 2.1 g cm-3 and a refractive index of 1.45, similar to silica glass, at relatively lower

temperatures (300 oC – 350 oC) by adding catalysts that promote oxidation or hydrolysis

[172]. Bauer et al., worked on the curing kinetics of PHPS at low temperature and

concluded that moisture and temperature have significant effects on the PHPS curing

mechanism [174]. At relatively low temperature or nearly ambient conditions the curing of

PHPS happens in two steps as shown in scheme 1. At first hydrolysis takes place and

silanols are generated by eliminating hydrogen and ammonia. . In the next step silanols go

through a polycondensation process, resulting in silica and water. It should also be noted

that the crosslinking of the silanols starts before all the Si-N bonds are hydrolyzed [173].

Scheme 1. Perhydropolysilazane (PHPS) curing mechanism at low temperature in the

presence of moisture:. a) Hydrolysis, b) polycondensation [173].

Zhang et al. 2015 conducted a work to explain the conversion of PHPS to silica via

thermogravimetric analysis (TGA) [166], [176]. It was concluded that PHPS exhibits a

minor loss of weight of 1.2% between 70 oC and 180 oC. This loss in weight is due to

evaporation of entrapped solvent and to some extent to the deterioration of chemical

compounds including N-H and Si-H bonds. From temperatures of around 200 oC and above

STATE OF THE ART

46

PHPS weight gain reaches 105.4% at 510 oC, followed by loss in weight. At the end the

weight gain reaches 103.8% at 1000oC. The weight gain below 510 oC can be assigned to

the oxidization of Si-H bonds as well as to the replacement of residual N atoms with O

atoms. Loss of weight above 510 oC is mostly due to the condensation reaction of the Si-

OH groups. This suggests that the volume variation of PHPS is negligible while heating

which results in a defect free and highly compact coating [166].

3.3.1.2 Curing in the presence of catalyst

For various applications, the use of high temperature is not always recommended for curing

PHPS. Therefore, different alternative methods for curing PHPS have been reported. These

include; exposure to ammonia atmosphere, reaction with water by catalytic action of amine

in the baking step, and exposure to hydrogen peroxide vapor.

a) Exposure to ammonia

Some authors have reported that the curing of PHPS can be promoted by exposing it to

gaseous ammonia. Kubo et al., [139] reported that the exposure of PHPS to ammonia gas

for 6 hours resulted in fully converted silica which resembles the amorphous silica

structure. Morlier et al., [173] reported that the curing of PHPS can be carried out by

exposing PHPS to either aqueous or gaseous ammonia. Both, aqueous and gaseous

ammonia have significant influence on the transformation of PHPS in terms of conversion

rate. However, immersion in ammonia solution seems to have a dominant effect and leads

to faster conversion. Immersing PHPS in water alone is not a sufficient condition to

significantly accelerate conversion. Exposing the PHPS coating to ammonia vapors for

elongated time duration, i.e. 24 h, does not promote total cross linking of silanols. Ammonia

catalyst makes the conversion rate faster but still leaves an incomplete conversion because

it promotes exclusively the first reaction step, i.e. hydrolysis. Morlier et al., 2012 [173]

assumed that the ammonia acts either as nucleophilic agent or as a base, in the latter case,

the catalyst effect could be attributed to the pH of the solution.SiOH polycondensation is

only promoted by elevated temperatures; therefore, post curing of 30 min at 150 oC is

preferred.

b) Exposure to peroxide catalyst

A further method for curing PHPS is its exposure to aqueous hydrogen peroxide [139].

Hydrogen peroxide acts as a catalyst and strongly accelerates SiH and SiN bond scission

[139] [173]. A study performed by Morlier et al., shows that the conversion of SiN and

STATE OF THE ART

47

SiH is achieved 10 minutes after immersion in a hydrogen peroxide solution. However, the

layers obtained via this method showed highly hydroxylated silica. While curing with

peroxide catalyst, two competitive reactions are assumed to occur in the presence of HO.

radicals and water: SiN and SiH bonds can either be gradually hydrolyzed by moisture or

rather be quickly dissociated with the formation of radicals by HO. species, as can be

observed from Figure 15(c). The radicals formed as result of this bond scission can

recombine with other hydroxyl radicals and lead to the formation of silanol species [165].

a)

b)

c)

Figure 3.8: FTIR spectra of uncured PHPS and PHPS cured with different methods.

a) IR spectra of uncured PHPS (solid line) and IR spectra of PHPS curing at 180 °C

under moisturized atmosphere for 300 min (dashed line), b) IR spectra of uncured PHPS

(solid line), PHPS cured after exposure to ammonia vapor for 60 minutes (dashed line),

c) IR spectra of uncured PHPS (solid line) and PHPS cured by submerging into 20%

aqueous hydrogen peroxide solution for 10 minutes. (Copied from [165] with

permission from Elsevier)

3.3.1.3 Deep UV curing

Prager et al., [164], reported for the first time a water free vacuum ultra violet triggered

process of converting PHPS into SiOx in the presence of O2. This UV curing process was

a break through because by the use of this method conversion of silazanes was possible

without using harsh conditions like pyrolysis or hydrolysis [164]. This opened a vast range

of applications for polysilazanes, including OSC encapsulation [165]. Since then, this

method has been adopted by various investigators [19], [155], [175], [177]. Xenon excimer

lamps are generally used as high power vacuum ultraviolet light sources that can efficiently

emit radiation with a wavelength of 172 nm [164], [174][164], [174]. Exposure of oxygen

to deep UV light generates excited atomic oxygen (O(1D)) and ozone (O3) [164]. Since

oxygen has a high absorption coefficient with respect to VUV light with a wavelength of

STATE OF THE ART

48

172 nm, VUV irradiation can generate reactive oxygen species very efficiently [178], [179].

Prager et al. in his study reported deep UV irradiation from various sources at wavelengths

ranging between 172 and 222 nm. The study was carried out under controlled atmosphere

because oxygen absorbs such high energy photos to form free radicals and ozone [165],

[178]. Free radicals of oxygen (generated as a result of interaction with deep UV light) react

with PHPS and conversion to SiO2 starts from top surface and progresses inwards [19].

Additionally, water generates hydroxyl radicals upon absorbing VUV light. Due to the

extremely highly oxidizing nature of atomic oxygen and hydroxyl radicals, they react with

polysilazane, thus forming silica [164], [174], [180].

−(SiH2 − NH)𝑛 − + 𝑛𝑥O. → 𝑛SiO𝑥 + 𝑛NH3 Eq. 53

The conversion of PHPS to silica under VUV light may in principle proceed via two

different mechanisms. One mechanism involves the dissociation of bonds in PHPS, which

is directly caused by VUV irradiation, while the other process involves oxidative reaction

involving O(1D) and O3. Nagnuma et al., [180] reported that oxidation reactions were

dominant as compared with dissociation of bonds by VUV irradiation. Prager et al., [164]

and Kobayashi et al., [161] strongly suggested that the dissociation of bonds is much faster

than the oxidation reaction. Therefore, VUV light should effectively reach the coating in

order to efficiently convert PHPS to silica. The following Eq. 54 has been used by

Kobayashi (2013) to study the effect of irradiation dose on the conversion mechanism of

PHPS to silica.

𝐼

𝐼0= 𝑒𝑥𝑝 (−Ɛ ×

𝐶𝑂2

100 ×

𝑑

10)

Eq. 54

Where 𝑐𝑂2 , d, and I/I0 are the oxygen concentration of the atmosphere (%), irradiation

distance, and VUV light transmittance, respectively. The absorption coefficient of O2 for

172 nm VUV light is = 15 cm-1. This equation clearly demonstrates that the transmittance

of VUV light increases as 𝐶𝑂2 and d become smaller.

3.3.1.4 Combined methods

In order to convert PHPS not only at milder conditions but also at faster pace, various

studies have been carried out. One study used dipping of PHPS into water or aqueous H2O2

and simultaneous irradiation with light of 405 nm wavelength and observed that PHPS

cures relatively fast and at lower temperature [174]. Similar studies also used dipping of

STATE OF THE ART

49

PHPS into NH4OH and observed conversion of PHPS into dense silica [138], [158]. Jung

et al., in 2010 used dipping in combination with heating the bath to 80 oC for ~60 minutes

and observed a faster way of conversion, which yielded PHPS of better quality in terms of

density and hydrophobicity [172]. It was observed that the conversion rate of PHPS was

still low. In order to further reduce the curing time, Ohisi et al., [169] reported the heating

of the PHPS coating at simultaneous irradation with deep UV, thus successfully reducing

the curing time down to ~20 minutes. In this study, it was reported that emission at 172 nm

has a photon energy of 166 kcal/mol and the polysilazane bonds like Si-N, Si-H, and N-H

have bond energies 105, 71 and 92 kcal/mol, respectively. The energy of the deep UV light

breaks the bonds and SiO2 can be formed by singlet oxygen (O(1D)) reaction with PHPS.

Introducing slightly higher temperature during vaccum UV exposure accelerating the

diffusion of oxygen radicals into the coated film, which promotes the reaction of singlet

oxygen with PHPS and enhances the conversion rate. In order to further investigate this

phenomenon, a PHPS film was treated in different ways which included heating it with

different temperatures like 80 oC, 100 oC, 200 oC and 300 oC. Simultaneously, other PHPS

films were treated with deep UV irradiation and heated at different temperatures. It was

observed via FTIR analysis that increasing temperature during UV irradiation, enhances

curing rate of polysilazane. Uncured polysilazane shows characteristic peaks at 830 cm-1,

2200 cm-1, and 3400 cm-1 which refer to Si-N, Si-H and N-H, respectively Figure 3.8. PHPS

cured with vacuum UV irradiation and simultaneous heating shows rapid decrease in the

peak intensities of Si-N, Si-H and N-H and formation of new peaks of Si-O-Si at 450 cm-1

and 1050 cm-1. This gave evidence that the combination of heat and irradiation is the fastest

way of curing PHPS. In comparison to other curing methods like pyrolysis, which not only

depends on the film thickness but also on the range of temperature and may still have

incomplete transformation. In contrast, the combination of temperature and VUV

irradiation is the fastest and have high chances of complete transformation of PHPS into

SiO2[169].

3.3.1.5 Barrier performance

Various coating techniques, curing methods, and strategies have been reported in the

literature to improve the barrier characteristics of Polysilazane derived coatings. It is

reported that the barrier performance of Polysilazane against moisture and oxygen mainly

depends on the completeness of the curing process of the PHPS coating, thickness of the

layer, number of defects present in the layer and number of coating layers.

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Prager et al. reported an oxygen barrier improvement factor of 400 with a single 150 nm

deposit of PHPS on a PET substrate.

Ohishi 2003 et al., [181] developed a thin PHPS layer by curing it at 140 oC for 20 min in

90 % RH atmosphere on PET already coated with ITO and reduced its moisture and oxygen

barrier properties down to 0.15 g/m2.day and 0.02 cm3/m2.day.bar, respectively. In another

study in 2014, PHPS layers were cured with low pressure mercury lamps (HgLP) at a

radiation of 185 nm wavelength under nitrogen having low concentration of O2 [165]. It

was reported that the complete transformation of PHPS to silica depends on the thickness

of the film. As the films gets thicker, the curing with deep UV becomes difficult. The curing

starts from the surface and proceeds into the depth of the film. As soon as the surface is

cured, it becomes harder for the oxygen radicals to diffuse and hence leave uncured PHPS

underneath. This uncured PHPS is a poor barrier and offers channels of diffusion and thus

results in poorer barrier properties. Thick PHPS layers are not only difficult to cure but also

exhibit brittle nature and are highly susceptible to cracking which leads to deteriorate the

barrier properties [182].

Ohishi et al., 2017 [169] reported that photo-heat treated films at 150oC exhibit WVTR of

<0.02 g.m-2.day-1. Kobayashi et al., 2013 [161] studied silica coatings fabricated by VUV

irradiation of PHPS films under various conditions. It was reported that silica coatings

prepared with lower oxygen concentration and shorter irradiation distance show the lowest

WVTR values (Table 4), indicating that the direct dissociation of bonds by the VUV light

plays a key role in the formation of dense silica coating exhibiting high gas barrier

properties with respect to water vapor.

Table 4: Irradiation condition and WVTR of each sample

Samples Oxygen

concentration %

Irradiation

distance (mm)

WVTR

g. m-2.day-1

1 <5 2.6 0.721

2 19 2.6 1.88

3 <5 13.4 1.91

4 19 13.4 2.74

In order to develop superior gas barrier coatings, it is very important to reduce the size and

number of defects on and beneath the surface. It was reported that the silica coatings

prepared from PHPS via hydrolysis by water possessed a relatively small number of SiOH

groups, which might ultimately serve as defects. Since curing with VUV does not involve

water, the number of SiOH in VUV cured coatings can be minimum and, thus, it can be

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51

expected that the siloxane network would be fully developed with dense and homogenous

silica [164], [183]. Therefore, it is suggested that the silica coating obtained via VUV

irradiation in combination with temperature is advantageous in fast curing along with layer

quality as this method promotes defect free growth and eliminates the chances of both

macro and molecular size defects. Table 5 below gives barrier quality of PHPS coated on

both sides of polyimide substrate cured with temperature and combination of simultaneous

VUV irradiation at different temperatures. The barrier quality obtained with combined

method (VUV+temperature) is superior than curing at higher temperatures without VUV

irradiation.

Preparing multilayers of PHPS is another effective method to enhance barrier performance.

Barrier characteristics against oxygen <0.1 cm3.m2.day.bar and <0.02 g.m-2.day-1 for double

layers have been reported [169]. Ohishi et al. 2017 prepared a stack of two PHPS layers

and subsequently cured each layer by deep UV irradiation. The value of water vapor

permeability for a two layered stack (Figure 3.9) at 25oC calculated from the Arrhenius plot

showed an extremely low value of 4.9 x 10-4 g/m2.day and the activation energy was 236

kJ/mol.

Table 5: WVTR of PHPS coated on polyimide substrate on both sides via spin coating and

cured via VUV irradiation at different temperatures for 20 minutes (data extracted from

[162]).

Treatment type Thickness

(nm)

WVTR

(g/m2.day)

Polyimide (substrate) without PHPS ~300 143

heat treatment at 150oC ~300 92.8

Heat treatment at 200 oC ~300 40.9



VUV+ Heat treatment at 80 oC ~300 15.3

VUV + Heat treatment at 100 oC ~300 3.42

VUV+ Heat treatment at 120 oC ~300 1.84

VUV + Heat treatment at 150 oC ~300 0.17

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On one hand, increasing the number of layers may enhance barrier properties but, on the

other hand induce brittleness and chances of defects within the film [184]. Adding an

organic layer between SiO2 layers seems to be advantageous. It does not only enhance the

flexibility of the coating but also decouples the fracture growth and surface defects [19],

[21], [184]. Such barrier materials may fulfill OPV requirements as shown by Burrows et

al., [13]. However, Graff et al., [19], [185] studied multilayer structures and stated that

molecules need to permeate through the defects of inorganic layer and diffuse horizontally

in the organic layers until they encounter another defect in the next inorganic layer, hence

generating a tortuous path and decreasing permeability. Permeation below 10-4 g/m2.day

cannot be obtained with such multilayers stacks as the presence of nano-sized defects is

unavoidable [13], [185]. Morlier et al., [182] achieved moisture permeation rates of 2x10-2

g/m2.day for a multilayer structure.

Figure 3.9: Temperature dependence of water vapor transmission rates of the polysilazane

derived SiO2 coatings (2 coates) (Copied from [169] licensed by CC BY 4.0).

Morlier et al., for the protection of organic solar cells, modified the structure of the barrier

and produced stack of 5 barrier layers on PET, consisting of one PVA layer sandwiched

between two PHPS layers on each side (PET/PHPS/PHPS/PVA/PHPS/PHPS) and

laminated OSC (P3HT:PCBM) device with this barrier. The performance of this

encapsulation film was compared with bare PET and a commercial barrier (Figure 3.10). It

was observed that devices encapsulated with PET degraded faster because PET is a

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53

relatively poor barrier against moisture and oxygen. In contrast, the devices encapsulated

with PHPS or the commercial barrier remained stable. The loss of ~30% in PCE is observed

for devices encapsulated with the commercial barrier and PHPS based barrier coated on

both sides of PET, whereas devices encapsulated with PHPS based barrier having a 5-layer

structure remained stable and only a minor degradation was observed over a period of ~400

h. This degradation is exclusively due to the side ingression of oxygen through the

adhesive. This suggests that PHPS has a potential to be used as an encapsulation for the

OSCs [182].

Figure 3.10:Performance of organic solar cells in terms of normalized power conversion

efficiency (PCE) and normalized short circuit current (Jsc) during the exposure to AM 1.5,

10 0 0 W m –2 light soaking, encapsulated with (a) PET having thickness of 50 μ m, (b) a

commercial barrier, (c) PHPS based barrier having one PHPS (250 nm) coat on both sides

of PET, (d) PHPS based barrier, having 5 layer structure (PET/PHPS250 nm/PHPS250

nm/PVA1 μ m /PHPS250 nm/PHPS250 nm) on one side of PET and (e) un-encapsulated OSC

device degraded under irradiation in glovebox. (Copied from [182] with permission from

Elsevier).

3.3.2 ORMOCERS

ORMOCER system (organically modified ceramics) are hybrid type composite materials

and mainly consist of three parts i.e. organic, inorganic and polysiloxane [186]. All three

STATE OF THE ART

54

parts have different roles to play to create certain functionality of an ORMOCER. The

organic part is responsible for cross linking, polarity and optical behavior. Inorganic part

withstands against thermal, mechanical and chemical deteriorations and elasticity is

generated by the polysiloxane parts [108], [186]. Variety of properties can be obtained by

tailoring ORMOCER structure [187], [188]. Unlike conventional composites,

ORMOCERS are processed from alkoxysilanes and hence have inorganic center having

silicon and oxygen [189], [190]. In order to not to destroy the organics, ORMOCER

structures are usually processed at low temperatures. That means the potential of molecular

chemistry to tailor structures can be used advantageously for ORMOCER processing.

Generally the inorganic backbone is synthesized by the sol-gel process and the organic part

(-R) having reasonable molar mass are added by crosslinking polymerization [188]–[191].

The type functional group “-R” of course determines the material properties to a great deal,

different groups as alkyl or unsubstituted aryl (e.g. –C4H2n+1, – C6H5) functional groups

(e.g. –NH2, -COOH, other chelate ligands, -CH, -SH, -CN) along with polymerizing groups

(e.g. epoxy, methacryl, vinyl and other olefins) can be used as organic part in ORMOCERS.

Therefore, a wide range of materials can be developed according to the need of specific

applications.

Table 6) [30], [159]. E.g., the ligands can impart hydrophilic or hydrophobic properties to

the matrix or crosslink it, which changes the permeation properties of small molecules [16],

[192], [193] [190],[194].

Table 6: Bifunctional silanes R’ (CH2)nSi(OR)3, few functional organic groups R’ for

producing an organic network and functionalization of the matrix. Data extracted from

[192].

n R’ effect

3

Network former

Density

Elasticity

Rigidity

Thermally or

Photochemically curable

0

3

3

3

STATE OF THE ART

55

3

2 -CH3 Network modifier

Density

Hydrophobic

Hydrophilic

Oleo-phobic

Better adhesion

3 -SH

3 -NH2 + -NR3

3 -(CF2)5CF3

As the barrier properties of ORMOCER layers are almost as same as for commercial PET

i.e. around 4 g.m-2.day-1 [195], which is not sufficient for packaging opto-electronic

devices. Thus Fraunhofer ISC developed a new class of barrier called POLO using

ORMOCER as interlayers in combination with vapor deposited SiOx [196]. POLO barrier

has the same structure as that of Barix i.e. several metal oxide layers having organic

interlayers as shown in Figure 3.11. In POLO, ORMOCERS are used as interlayers to

smoothen the surface to allow homogenous growth of evaporated layer. Moisture

permeation values as low as 10-4 g.m-2.day-1 have been reported [39], [197].

Figure 3.11: Schematic diagram for roll-to-roll production of ORMOCER/inorganic oxide

hybrid barrier films. (Re drawn from [167]).

Miesbauer et al., [170] conducted a study on the role of substrate on the performance of

Fraunhofer’s developed POLO barrier and concluded that PET may not be suitable as

encapsulation material for long term outdoor applications, since polyester films like PET

are degraded by UV radiation and humidity in combination with high temperature.

Therefore, they suggested the use of fluoropolymers, e.g. ethylene tetrafluoroethylene

(ETFE) [2] [171] that show excellent resistance against UV radiation and also against

STATE OF THE ART

56

weathering and, therefore, qualify well as flexible substrates instead of PET for

encapsulation of photovoltaic devices [170].

The use of ORMOCER lacquer is not limited to create planarization layers but can also be

used as an adhesive and sealant [198] [192]. A combination of smoothening and sealing

effect of ORMOCER can be a suitable choice for cost effective short term encapsulation of

solar cells avoiding classical lamination processes

3.4 Reducing solubility

One way for enhancing barrier properties of the coatings is to incorporate hydrophobicity.

Hydrophobicity may not exactly be an intrinsic property of a polymer but is more of its

surface property. One study concluded that although hydrophobic surfaces are water-

repelling, they do not repel water vapor [199]. Condensation of water vapor will take place,

thus resulting in higher permeation followed by internal wetting. For example

Polydimethylsiloxane (PDMS) is a hydrophobic material but still has high moisture

permeability [199], [200]. This effect is not suitable for OPV’s. However, roughening of

the polymer substrate by lithography followed by coating with an encapsulant will help

improve its barrier values. Hence, efficient methods and techniques must be found out to

improve these limitations [16], [200]. In an study, in order to fabricate a highly hydrophobic

surface, a modified fluoroalkylsilanes were applied on a Poly(methyl methacrylate) (

PMMA) substrate and cured, which resulted in a hydrophobic PMMA surface with contact

angles increased from 60° to 110° [201].

Fluorinated polymers which are commonly called as fluoropolymers. By using

fluoropolymers in organic electronic device packaging, lifetime performance can be

substantially enhanced [16]. The characteristics of fluoropolymers make them a suitable

solution for the applications that require barrier against the diffusion of moisture, oxygen,

bases, acids, and most importantly an ability to significantly reduce mechanical wear and

friction. Additionally, the easy processing and coatability of the fluoropolymers is also

advantageous for their use in the packaging industry. For creating commercially viable

encapsulation structures for flexible OPV devices, a commercial polymer CytopTM was

reported [16].

CytopTM is colorless, transparent, amorphous, and can be deposited using conventional thin

film deposition techniques such as spin coating, doctor blading etc. CytopTM is

STATE OF THE ART

57

commercially available as a high viscosity resin and its chemical structure is shown in

Figure 3.12.

Figure 3.12:(a) Chemical structure for Cytop TM (b) Spin-coated Cytop film on glass

substrate under atomic force microscope (Copied from [202] with permission from AIP

Publishing).

J.B. Chae, et al. (2014) [203]) studied the hydrophobic nature of the Cytop materials. The

contact angles of diionized water droplets on Cytop layers thicker than 3 nm maintained an

angle of approximately 110°; the contact angles of the droplets on Cytop layers thinner than

3 nm abruptly dropped and decreased as the layer thickness decreased Figure 3.13. On the

basis of the result the author optimized the layer thickness, suitable for having a highly

hydrophobic nature.

STATE OF THE ART

58

Figure 3.13: Characterization of hydrophobicity in terms of water droplet contact angles

and thickness values of films with respect to weight percentages (wt%) of Cytop in solution.

Images below droplets are measured by atomic force microscopy (AFM) (Copied from

[203] with permission from Elsevier).

In order to achieve a coatable barrier materials [204] carried out the study on Cytop and

used it as an encapsulation for an OPV device. AFM (Atomic Force Microscopy) analysis

shows that CytopTM yields very smooth films using spin coating with a root mean square

roughness of 3.8 Å [204].

Work done by Jimmy Granstrom et al., [202] showed that Cytop can be used an organic

interlayer in multilayer structure to produce ultra-high barrier to sufficiently increase the

lifetime of organic light emitting diodes (OLEDs), where Cytop provides the part of the

barrier. Results of calcium degradation indicated that the multilayer of metal oxides having

CytopTM interlayer (see inset of Figure 3.14 type B) are more likely to reach the oxygen

and moisture transmission rates (<10-2 g.m-2.day-1) needed for 10,000 hour lifetimes [202],

[205]. The authors concluded that the thicker Cytop has better planarization effect but is

likely to cause cracking in ALD processed Al2O3 layer due to elastic mis-match. This

cracking can either be decrease with inserting additional layer in the stack or precisely

controlling the parameters to alter the Cytop thickness. Jimmy Granstrom et al., altered

Cytop thickness and also deposited a compressively stressed SiNx layer between Cytop and

Al2O3. With the use of SiNx in between cytop and Al2O3 crack generation in ALD layer

can be avoided completely. This enables the free choice of cytop thickness ranging from

STATE OF THE ART

59

40 nm to 4300 nm without affecting the barrier performance. Figure 3.14 shows

degradation mechanism of calcium encapsulated with multilayers of Al2O3 and Cytop a)

without SiNx and b) with SiNx which are referred as type a and type b respectively in the

Figure 3.14.

Figure 3.14: Calcium degradation mechanism, Type A Ca films as a function of time for

varying CYTOP film thicknesses and Ttype-B Ca films as a function of time for varying

CYTOP film thicknesses having SiNx interlayer (Copied from [205] with permission from

AIP Publishing).

In order to make use of Cytop without using evaporated metal oxide, Grandstrom et al.,

[206] carried out a work in which they used Cytop as a coated barrier and encapsulated

Calcium directly with it. The coated calcium was exposed to water vapors and degradation

was analyzed by optical calcium degradation test. Lifetime of Ca film was found to be

around 200 minutes when coated with 200 nm of Cytop layer. The Ca lifetime of 200

minutes is equivalent to WVTR of a commercial PET film having thickness of 100 µm

thick in the same conditions. A coated Cytop layer 50 times thinner than PET yielded

almost equivalent Ca lifetime as compared to 100 µm thick layer. Ca degradation indicated

that the deterioration started from the defects (pin holes) within the coated layer.

Controlling surface and internal defects and increasing layer thickness may increase the

overall lifetime of Ca [206].

This indicates the potential of Cytop as an intermediate barrier and can be used a temporary

protection of OSCs.

EXPERIMENTAL

60

EXPERIMENTAL

This chapter introduces the details of all of the raw materials used in the work, all materials

used for producing barrier layers including organic and inorganic polymers, inorganic

fillers and substrates.

This chapter also gives details on the methods adopted for the processing of the films

including solution preparation, film coating and drying. Pristine polymers and their

composites with fillers were coated on substrates (PET and glass) and later peeled off for

getting a free standing layers. Uniform distribution of the fillers within matrix is very

essential for creating effective gas barriers. It was a real challenge to prepare uniform layer

from glass flakes filled polymer. The main reason was a high viscosity of the solution. The

coating parameters, processing and coating optimization of the polysilazane based layers is

discussed later in the chapter. This chapter also includes the details of OSC structure,

materials and processing.

Finally, short descriptions of the characterization techniques are given.

Part of this chapter has been published in:

I.A Channa, A. Distler, M. Zaiser, C.J. Brabec, H.-J. Egelhaaf, Thin Film Encapsulation of

Organic Solar Cells by Direct Deposition of Polysilazanes from Solution, Advanced Energy

Materials (2019) In this section parts authored by I.A. Channa are reproduced in subchapter 2.5.2.

Additionally, through author ownership declaration, a permission was granted from all co-authors for

utilization of the whole content of publication as part of this thesis.

EXPERIMENTAL

61

4.1 Materials

All of the materials used in this work were commercially purchased and their details are

mentioned in the table 8 below:

Table 7: Materials used in the experiments

Materials Trade name Supplier Used for

Materials for Barrier preparation

Organic polymers

Polyvinyl alcohol (PVA) - Sigma Aldrich

Chemie GmbH

Polymeric matrix

for fillers

Polyvinyl butyral (PVB) Butvar-98 Sigma Aldrich

Chemie GmbH

Polymeric matrix

for fillers

Polyvinylidene fluoride

(PVDF)

- Thermo Fisher

GmbH

Interlayer for

barrier

Ethylene vinyl acetate

(EVA)

- Honeywell

International Inc.

Interlayer for

barrier

Inorganic polymer

Perhydropolysilazane

(PHPS)

Polysilazane durXtreme GmbH Barrier layer

Fillers

MMT Na+ clay

CLOISITE-

Na+

BYK ltd.

Glass flakes - Eckart GmbH

Solvents

De-ionized water - Solvent for PVA

Dimethyl sulfoxide

(DMSO)

- Thermo Fisher

GmbH

Solvent for

PVA/PVDF

Benzyl alcohol (BA) - Solvent for PVB

Di-n-butyl ether - For dilution of

PHPS

Adhesives

DELO Katiobond

LP655

DELO Industrie

Klebstoffe GmbH &

Co. KGaA

Barrier lamination

and interlayer

barrier

Rolic RPL-521 Rolic Technologies

Ltd. Switzerland

Interlayer for

barrier

Materials for Organic solar cells

ZnO N10 Avantama,

Switzerland

Hole Inject layer

P3HT - OPVIUS GmbH Semiconductor

https://www.byk.com/en/additives/additives-by-name/cloisite-na.php




EXPERIMENTAL

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PC60BM - OPVIUS GmbH Semiconductor

PEDOT:PSS HTL Solar HERAEUS Hole transport

layers

Substrates

Polyethylene (PE) Common

Commercial grade

packaging

Substrate for FTIR

of PHPS coatings

PET Melinex

ST504

DuPont Teijin

Films UK Ltd

Substrate for barrier

coating

ITO sputtered glass 15-20

Ω/sq

- Weidner Glas

GmbH

Substrate for OSC

Commercial barrier

Mitsubishi - VIEW-BARRIER,

VD-K3DA

Reference barrier

4.2 Processing

All of the solutions were processed with doctor blade (ZAA 2300- manufactured by

Zehntner Testing Instruments, Switzerland). The details of the layers are given below.

4.2.1 Filler based barrier films

Mainly two types of filler materials were used that include, MMT nanoclay and glass flakes

having different aspect ratios.

4.2.1.1 Clay based barriers

Dissolution of 10 wt% PVA in de-ionized water was carried out and the solution was stirred

continuously on a hot plate at 90oC for 3-4 hours (until the solution became homogenous).

After the complete dissolution of PVA, clay (Cloisite MMT-Na+ nanoclay powder) was

mixed to the solution with concentrations of 2 vol% to 10 vol% and stirred overnight at

60oC. This mixture was ultra-sonicated for 10 minutes right before the coating process.

For coatings glass substrates (5 minutes ultrasonically cleaned by each isopropanol and

acetone bath) were used. The layers were prepared by means of the doctor blade. The

coating speed was set to 5 𝑚𝑚. 𝑠−1, and the temperature on the doctor blade surface was

maintained at 30oC. As soon as the layers were processed with doctor blade, they were

positioned in an oven at 80oC for a few hours drying. The films were peeled off from the

substrate and free standing were obtained. Further characterizations including

measurements of barrier properties were carried out on free standing films.

EXPERIMENTAL

63

4.2.1.2 Glass flakes based

Polyvinyl butyral (PVB) was dissolved in benzyal alcohol with 30 wt% concentrtion. The

solution was stirrered at 80oC on hot plate for few hours. As sson as the solution became

homogenized glass flakes were blended in the solution at different volume fractions (0

vol%, 5 vol%, 10 vol%, 15 vol%, 20 vol%, 25 vol%, 30 vol%). Dispersion of glass flakes

in this mixture is very important, but due to 30wt % solution of PVB, the viscosity is very

high, therefore; continous gentle mixing is very essential because vigorious or hard mixing

can break the glass flakes that will change the aspect ratios. Therefore, manual and gentle

mixing procudure was adopted. Once, the glass flakes were properly dispersed in the PVB

matrix, the solution was exposed to vacuum for at least 15-30 mins to extract entrapped air

from the solution. The coatings on PET substrate were processed with low coating speeds,

i.e., 5 𝑚𝑚 ∙ 𝑠−1, and the blade gap was varied between 500 – 2000 µm. After the coating,

the samples were positioned in an oven at 80 oC for complete evaporation of the solvent.

The layers were peeled off from the subrates and characterizations were carried out on free

standing layers.

4.2.2 Polysilazane based barriers films

A cleaned PET was used as a substrate for Perhydropolysilazane (PHPS 20 wt% in di-butyl

ether) based barriers films. The cleaning of the substrate plays a vital role in PHPS based

thin films. A films processed on an improper cleaned substrate may result in bad barrier.

Therefore, extra care has to be take while cleaning PET substrates.

The PHPS solution was diluted in di-butyl ether before the processing of the barrier films.

Parameter related to PHPS coatings are summarized in Table 8.

Table 8: Coating Parameters for coating PHPS layers from an amount of 70 µL on PET

substrate, subsequently cured with deep UV irradiation in combination with temperature.

Parameters

Substrate PET (125 µm)

Amount of materials (PHPS) 50 – 70 µm

Dilution ratio 1:1 – 1:6 (PHPS: solvent)

Coating speed 1 – 30 mm/s

Blade gap 20 – 400 µm

Curing method Heat / vacuum VU / combination

Curing time Depends on curing method

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4.3 Preparation of OSCs

Bulk heterojunction organic photovoltaic cells having inverted structure,

glass/ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag were produced and subsequently either

laminated with either polymer filled barrier and PET coated PHPS based barrier films or

directly coated with solution of PHPS onto the top electrode. For preparation of the devices

glass substrates coated with laser structured indium tin oxide (ITO) (sheet resistance of

21 Ω/) were used. The processing of all layers was carried out by blade coating in ambient

conditions except the top silver (Ag) electrode. After coating, the ZnO layer was annealed

for 5 min at 120 oC in air. The blend of P3HT:PCBM [with a ratio of 1:0.8 (wt/wt)] was

coated via doctor blading from o-xylene:1-methylnaphthalene (19:1, v/v) solution. A layer

of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was prepared

from the HTL Solar diluted with 1:1 ratio in water. After deposition of all layers, the silver

top electrode was deposited by thermal evaporation in ultrahigh vacuum (10-4 Pa) through

a mask to define an OSC active area of 0.1 cm2. The layout of the solar cell device and

schematic diagram of working cell is shown in Figure 4.1. The thickness of the evaporated

Ag film was kept as small as 50 nm in order to minimize its gas barrier effect. Initial current

voltage characteristics and power conversion efficiencies were measured in inert

atmosphere.

Figure 4.1: Details of the OSC device, a) Layout of the complete OSC device, b) schematic

diagram of the working cell

4.3.1 Encapsulation of the OSC devices

After the initial measurement, the OSC device was encapsulated by two methods: a)

lamination with polymer filled barriers and PET coated with PHPS films by means of a

commercial epoxy based adhesive (DELO Katiobond LP655) which was subsequently

EXPERIMENTAL

65

cured with UVA ~400 nm light for 2 minutes (a shown in Figure 4.2a, b) direct coating of

PHPS based barrier from solution via doctor blading on top of solar cell (as shown in Figure

4.2b).

Figure 4.2: Schematic diagrams of the encapsulated solar cells, a) solar cells encapsulated

with traditional lamination of the barrier films using epoxy as an adhesive, b) directly

coated solar cell.

4.4 Characterization

Prepared films were characterized via various different techniques mainly, barrier quality,

spectroscopy, bendability etc., and applied to solar cells as barrier layers and lifetime of the

encapsulated solar cells were monitored. The descriptions of these characterization

techniques are given in the following sections.

4.4.1 Barrier quality

The films were characterized in terms of barrier quality and for that water vapor

transmission rate (WVTR) and oxygen transmission rate (OTR) were measured.

4.4.1.1 Water vapor transmission rate (wvtr)

Measurement of water vapor transmission rate was carried out with two methods, one is

using cup in compliance with ASTM E-96 standard and other is using commercial testing

equipment called as SYSTECH 7002.

a) Cup method

Standard Aluminum cup as shown in Figure 4.3 having diameter of 6.35 cm which

compiles with ASTM standard E-96 [3] purchased from Thwing-Albert Instrument

Company (Germany) was used. This test can be carried out in two approaches; the first

approach is filling the cup with water up to ¾ of its capacity with distilled water and then

sealing the cup with the barrier layer as in Figure 4.3a. The cup filled with water is placed

EXPERIMENTAL

66

in the controlled conditions and weight loss of the water is monitored with time. Leakage

is considered as the primary error while performing this kind of experiments. In this case,

there will be 100 %RH inside the cup and controlled atmosphere of set %RH will be

outside. This change will cause the moisture to transmit from inside to outside hence loss

of water can be observed. In second approach the cup is filled with a desiccant (Calcium

chloride) Figure 4.3b. The cup was then place inside a humidity chamber with controlled

temperature and relative humidity conditions. In this case, there will be nearly 0 %RH

inside the cup because of calcium chloride (a desiccant material) and controlled atmosphere

of set %RH will be outside. This change will cause the moisture to transmit from outside

of the cup to inside, hence weight gain can be observed. The weight of whole assembly

(cup plus barrier layer) was measured before placing it inside the controlled environment

and every 24 hours afterwards, with first approach i.e. water method the weight of the

assembly decreased and in second approach using desiccant, the weight of the assembly

increased. The straight slope of change of the rate of the weight was then used to calculate

water vapor transmission rate and permeability.

Figure 4.3: a) Schematic diagram showing cup test using water b) cup test using

desiccants, c) Aluminum cup according to ASTM standard E96, b) SYSTECH 7002 method

Moisture permeability measurements were also performed using an M7002 water vapor

permeation analyzer (SYSTECH Illinois, UK) as shown in Figure 4.4, having lower

detection limits of 0.02 g m-2day-1 or 0.002 g m-2day-1, depending on the size of the sample

with temperature and humidity range of 5 oC to 50 oC and 20 to 90 %RH respectively. This

device is equipped with a sensitive P2O5 sensor for accurate measurement of vapor

transmission with exact temperature and humidity. The measurement method complies

with ASTM F-1249*, ISO 15105-2, ISO 15106-3 and DIN 53122-2.

EXPERIMENTAL

67

a)

b)

Figure 4.4: a) photograph of the WVTR device (SYSTECH 7002), b) schematic view of

the permeation cell of SYSTECH 7002 device showing the flow of the dry and wet

nitrogen through the cell chamber.

4.4.1.2 Oxygen transmission rate (OTR)

Oxygen permeation rate was analyzed by using a permeation chamber fitted with an optical

oxygen sensing spot PSt9 (Manufactured by PreSens Precision Sensing GmbH) with a

detection limit of 0.1 cm3 m-2 day-1 bar-1. Samples were carefully mounted between the two

chambers of the device and nitrogen gas was flushed inside both of the permeation cells for

15 minutes and leakage rate was measured. Then oxygen was flushed in bottom chamber

(as shown in Figure 4.5) for half a minute and then increase in oxygen fraction in upper

chamber was measured constantly during a time interval for a few days, this data was

further used to calculate oxygen transmission rate (OTR) and permeability.

Gas outletGas inlet

Gas outletFilm sample

Gas inlet

Optical fiberOxygen sensor

Oxygen

Nitrogen

Figure 4.5: Schematic view of the oxygen permeation cell

EXPERIMENTAL

68

4.4.2 Spectroscopic analysis

A Perkin Elmer Lambda 950 double beam spectrometer including a 150 mm integrating

sphere with a photomultiplier and an InGaAs detector was used for the measurements

including total and diffuse transmittance and reflectance of pristine polymer and polymers

containing fillers. For measurement, the samples were positioned in the transmission and

reflection ports of the sphere in a way that the rougher side faced the illumination.

IR spectra were recorded in ATR mode with a Fourier transform infrared (FTIR)

spectrophotometer (Bruker ALPHA-P) FTIR operating with OPUS 7.2 software. Spectra

were obtained using 128 scan summations at 4 cm-1 resolution.

4.4.3 Bending of the barrier layers

Bending tests were performed by using an in-house made cyclic bend tester having one end

fixed, other end moves linearly back and forth, thus cycling the barrier film in a customized

bending radius. For each test at least three samples having size of 3 x 10 cm2 were used.

Bent films were cut from the middle for further characterizations.

4.4.4 Degradation test

The degradation tests were perform by below mentioned methods.

4.4.4.1 Optical measurements

UV/vis absorption spectra of single photoactive layers (P3HT) were recorded with

Shimadzu UV-1800 spectrophotometer. A customized sample holder was used to make

sure that the beam hits the same spot when remounting the sample after various time

interval during degradation test.

4.4.4.2 Damp heat degradation

The coated and laminated OSC devices were placed in an artificial weathering chamber

(ESPEC LHL-114), with the pre-set condition of 40oC and 85%RH. Accordingly,

degradation tests were performed under the same conditions that were used to measure the

barrier characteristics of the films.

4.4.4.3 Degradation under sun

To check degradation induced by light, coated and laminated devices were placed under

constant illumination in ambient air in the compartment of a SUNTEST XXL+ sun

simulator (Atlas Materials Testing Technology GmbH) equipped with daylight filter. The

EXPERIMENTAL

69

light source is a Xenon lamp with an illumination intensity pre-set to 60 W m-² in the range

of 300-400 nm. The compartment temperature was controlled at 65 °C.

4.4.4.4 Electrical measurements

Current-voltage characteristics and power conversion efficiencies of the solar cells were

measured during the ageing experiments by an LOT solar simulator (Class AAA) at 1000

W m-². For this purpose, the solar cells were taken out of the respective ageing chambers

and put back after the measurement.

4.4.4.5 SEM images

Images of the cross section were obtained by a JEOL scanning electron microscopy (SEM)

JSM-7610F using a secondary electron image detector. For this purpose, SEM was operated

at 2 kV accelerating voltage, in a low probe current mode of ~65 nA, maintaining a distance

of 6 mm. Backscattered as well as secondary electrons and a combination of both was

detected for the creation of an image.

IB-19500CP polisher was used to prepare samples for SEM cross section analysis.

4.4.4.6 Optical micrographs

For microscopic analysis, two optical microscopes were utilized. One of them is an MX51

manufactured by Olympus Co. which can perform imaging in various modes including

bright filed, dark field and polarized. Other microscope is a μsurf confocal manufactured

by NanoFocus AG. This microscope was used for the surface characterization such as

roughness as well as thickness measurements.

RESULTS AND DISCUSSION

70


This chapter presents the development of solution-based barrier coatings for the protection

of organic solar cells against oxygen and moisture. It is divided into two subchapters. The

first one describes the development of barriers based on clay and glass flakes. The second

one describes the development of barriers based on impermeable silica coatings from

perhydropolysilazanes.

Part of this chapter has been published in:

I.A Channa, A. Distler, M. Zaiser, C.J. Brabec, H.-J. Egelhaaf, Thin Film Encapsulation of

Organic Solar Cells by Direct Deposition of Polysilazanes from Solution, Advanced Energy

Materials (2019)

In this chapter only parts authored by I.A. Channa are reproduced in section 5.2.

Additionally, through author ownership declaration, a permission was granted from all co-authors for

utilization of the whole content of publication as part of this thesis.


71

5.1 Filler based barriers: Clay and glass flakes

In this chapter barriers based on impermeable fillers (i.e. increasing the tortuous path) are

discussed. The addition of fillers with high aspect ratios (α) incorporated at sufficient

concentrations creates hurdles for diffusing molecules, thus compelling them to take longer

routes. This results in extra time for diffusion and hence improved barrier characteristics.

This chapter is mainly divided into two sections, first section is related to the use of clay as

a filler and the second section deals with fillers based on glass flakes.

In the first section, work on the clay filler particles is described. The clay as filler was used

to start the work to develop decent barriers. For this purpose, polyvinyl alcohol (PVA)

polymer (a water soluble) matrix was used. As clay is of hydrophilic nature, it requires

water for its uniform distribution. The barrier characteristics of PVA against moisture were

improved with addition of clay and WVTR of 2.8 g.m-2.day-1 was achieved which

corresponds to an improvement of 86% as compared to PVA without clay. The barrier layer

maintained transparency in the visible region.

In the second section, barriers based on glass flakes fillers are discussed. In order to develop

high quality barriers, glass flakes having different aspect ratios of α = 200, α = 400 and α

= 2000 were incorporated into the polymer matrix. The use of glass flakes showed

significant improvement in the barrier quality and moisture permeation rates as low as 0.04

g.m-2.day-1 were achieved. OSCs were encapsulated with such barrier films and accelerated

lifetime tests in damp heat (40oC/ 85%RH) were performed. The results showed significant

stability improvement of OSCs encapsulated with glass flakes based barrier as compared

to barriers prepared from polymers without glass flakes. The OSC showed almost no loss

in efficiency for a period of 150 h, whereas the OSC with neat polymer encapsulation died

within less than 24 h. This suggests that the glass flakes have a high potential in the

packaging industry.

5.1.1 Clay based barriers

In order to start the development of barrier films which comply with all of the requirements

of OSC encapsulation, various materials were screened for their applicability, including

boron nitride, graphene, montmorillonite (MMT) and mica clay (See appendix). Due to its


72

attractive aspect ratio, uniform dispensability, and transparency along with sufficient

barrier quality, MMT-Na+ clay was chosen as potential filler material.

5.1.1.1 IR analysis of nanocomposites

FTIR spectra were recorded on free standing films of PVA/MMT Na+ clay composite as a

supporting data to prove the existence of clay in the matrix. The FT-IR spectra of pure PVA

and its nanocomposite in the 1500–500 cm-1 spectral range are shown in Figure 5.1.

500600700800900100011001200130014001500

500600700800900100011001200130014001500

Tra

nsm

ittance

[%

]

Wavenumber (cm-1)

PVOH

6 vol% clay

Figure 5.1: FT-IR transmission spectra of PVA films and its composites with clay

concentration of 6 vol. % in the range of 1500–500 cm-1.

Pristine PVA shows no significant characteristic peak near 1030 cm-1 and with the addition

of the MMT clay, a peak near to 1030 cm-1 appears. This peak can be associated to the

stretching vibration of the Si–O bond, representing the clay, which contains silicates as the

main component. Therefore, the FTIR spectra confirm the presence of the nano-clay within

the PVA matrix. This result is in accordance with the work done by Gaume et al. [119].

5.1.1.2 Surface morphology

Figure 5.2 (a, b and c) shows the surface morphologies of PVA/MMT-NA+ clay layers

having 2 vol%, 4 vol% and 10 vol%, respectively. The micrographs reveal no detrimental

or damaging surface defects. Addition of the clay to the PVA matrix does not create any

surface defects that could affect the overall barrier quality of the layers. The small black

dots represent agglomerated clay particles. It will be shown later that the agglomeration of

the particles has almost negligible effect on the barrier quality.


73

a): 2 vol. % clay

b): vol. % clay

c): 6 vol. % clay

Figure 5.2: Optical micrographs of the PVA-clay nanocomposite, where PVA contains

a) 2 vol%, b) 4 vol%, and c) 6 vol% MMT-Na+ nanoclay, respectively.

5.1.1.3 Transparency and haze of the Nanocomposites

UV-vis spectroscopic analysis reveals that pristine PVA exhibits a transparency of 92.5%

(Figure 5.3a). However, a slight decrease in total transmittance to ~90% is observed when

MMT-Na+ clay is added to PVA ranging from 2 to 6 vol%. The negligible decrease in

transparency at a wavelength of 600 nm is shown in Figure 5.3b. PVA has a refractive

index of 1.5 and MMT-Na+ clay has a refractive index of 1.52, which explains the small

decrease in total transmittance. In order to characterize the optical properties of the barrier

further, diffuse transmission measurements of the layers were carried out. Diffuse

transmission and the increase in diffuse transmission at 600 nm is shown in Figure 5.3 (c,d).

The pure PVA shows a diffuse transmittance of around 2% and PVA with 6 vol. % clay

exhibits diffuse transmittance of around 8% at 600 nm wavelength. Haze can be due to

different reasons. Surface roughness or agglomeration of the clay particles at the surface or

within the film can cause scattering of the light. In this case, the increase in haze is

associated to surface roughness, as the layer with 6 vol% clay shows a relatively rough

surface as compared to layers with low concentration clay (Figure 5.2).

a) b)


74

300 450 600 750 9000

20

40

60

80

100T

ota

l tr

ansm

isttance (

%)

Wavelength (nm)

Pristine PVA

2 wt% clay in PVA

4 wt% clay in PVA

6 wt% clay in PVA

0 2 4 686

88

90

92

94

96

98

100

To

tal T

ran

sm

itta

nce

(%

) @

60

0 n

m

Clay in PVA (wt%) c)

Pristine PVA

2 wt% clay in PVA

4 wt% clay in PVA

6 wt% clay in PVA

300 450 600 750 9000

20

40

60

80

100

Diffu

se T

ransm

itta

nce (

%)

Wavelength (nm)

d)

0 2 4 6

2

4

6

8

10

Diffu

sed tra

nsm

itta

nce (

%)

@ 6

00 n

m

Clay wt % in PVA

Figure 5.3: UV-vis spectra of PVA and its composites with different clay concentrations

coated on glass substrates, a) Total transmittance spectra of PVA and its composites

with clay, b) total transmittance at 600 nm as a function of clay content, c) diffuse

transmittance spectra of PVA and its composites with clay, d) diffuse transmittance at

600 nm as a function of clay content.

5.1.1.4 Moisture permeability

Being water soluble, PVA still shows a decent barrier to water vapors and maintains low

moisture permeability. The PVA layer with a thickness of about 25 µm shows moisture

permeation of 90 g.m-2.day-1. This permeation rate is further decreased by increasing the

thickness of the layers. The effect of layer thickness and its effect on moisture permeation

is shown in Figure 5.4 and subsequent values are mentioned in Table 9.


75

0 1 2 3 4 5 6 7 8 9 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

We

igh

t lo

ss (

g)

Time (days)

PVA (25 µm)

PVA (50 µm)

PVA (75 µm)

PVA (100 µm)

Figure 5.4: Weight loss of water from a cup sealed with PVA barriers of different

thicknesses vs time at 40oC / 65%RH (moisture permeation calculated from the slope

according to ASTM-E96).

Table 9: Moisture permeation of the PVA layer having different thickness values.

Thickness of the layers

(µm)

Moisture permeation

(g.m-2.day-1)

25 90 ± 5.2

50 47 ± 4.2

75 29 ± 3.4

100 20.5 ± 2.5

The increase in thickness creates longer paths and moisture molecule take longer time to

diffuse from one side to another, hence moisture permeation is decreased. The thickest layer

with thickness of 100 µm exhibited moisture permeation rate of 20 g.m-2.day-1. This infers

to ~77% improvement in barrier quality. According to Eq. 13, the blocking effect of the

layers is linearly proportional to the thickness of the PVA layer; as can be seen in Figure

5.5.


76

0 10 20 30 40 50 60 70 80 90 1000.00

0.01

0.02

0.03

0.04

0.05

1/W

VT

R (

m2.d

ay/g

)

Thickness (µm)

1/WVTR

Linear fit

Figure 5.5: Blocking effect of PVOH layers having different thicknesses.

The WVTR values of PVA films are significantly reduced upon adding MMT-NA+

nanoclay. Table 10 gives the values of moisture transmission rate and permeability which

are obtained from the cup measurements of water vapor transmission rate for PVA films

containing different volume fractions of nanoclay particles, as shown in Figure 5.6. At 40oC

and 65 %RH, the 10 vol% clay layer shows the lowest moisture permeability, which is 2.8

g.m-2.day-1. This means that the addition of clay of up to 10 vol% reduces the permeation

of moisture through PVOH down by 86%, whereas reduction by 84%, 80%, 71% and 49%

was observed for 8 vol%, 6 vol%, 4 vol% and 2 vol% clay, respectively (Table 10).

Table 10: Calculated moisture permeation values of PVOH and its composites at

conditions 40oC / 65%RH; film thickness of 100 µm in all cases except PET (125 µm).

Material WVTR [g.m-2.day-1] Permeability [g.cm.m-2.day-1]

PET 5 0.062

PVA 20.5 0.205

2 vol % clay 10 0.1

4 vol % clay 6 0.06

6 vol % clay 4 0.04

8 vol % clay 3.2 0.032

10 vol % clay 2.8 0.028


77

0 2 4 6 8 100.0

0.2

0.4

0.6

0.8

1.0W

eig

ht lo

ss (

g)

Time (days)

Pristine PVA

2 wt % clay

4 wt % clay

6 wt % clay

8 wt % clay

10 wt % clay

Figure 5.6: Water weight loss vs time from cups sealed with films of PVA and its composites

with MMt clay at 40oC / 65%RH.

5.1.1.5 Validation of the experimental data

In order to validate the permeation data for the nanocomposite, the experimental values

were compared to the theoretical permeation model proposed by Bharadwaj et al. (Eq. 41).

[42] This model calculates the tortuosity factor considering volume fraction of the

particles, their aspect ratio along with the order parameter based on the orientation of the

particles within the polymer matrix as well as the thickness of the layers. The aspect ratio

of α = 500 for MMT-Na+ clay is used in this work as suggested by Manias and Gaume

[119], [207] for semi-exfoliated structures. The calculations based on this aspect ratio fit

quite well with Bharadwaj’s model. The order parameter of S = 0 as suggested by Gaume

[119] corresponds to random orientation of the clay particles. The comparison of

experimental data to Bhardwaj’s theoretical model is shown in

Figure 5.7. The experimental data match with the theoretical calculations for S = 0 and

hence confirm that the nanoclay particles in the PVOH films do not have a preferential

orientation.


78

0.00 0.02 0.04 0.06 0.08 0.100.0

0.2

0.4

0.6

0.8

1.0R

ela

tive

Pe

rme

ab

ility (

Pc / P

p)

Clay content (wt %)

Bharadwaj's model (S=0)

Experimental

Figure 5.7: Evolution of relative permeability of PVA films with increasing content of

nanoclay particles. (Triangles – experimental data. DDotted line - calculated data)

according to the Bharadwaj model for an aspect ratio of α = 500 and order parameters of

S = 0.

5.1.1.6 Bendability

The PVA/ clay nanocomposites were subjected to bending cycles at a bending radius of 5

cm to check if they lose barrier quality (

Figure 5.8). The nanocomposite films show a decrease of the water barrier properties by

only 10% of the initial value even after 10K bending cycles. This suggests that after 10K

bending cycles the nano-clay platelets are still firmly connected to the PVA matrix.


79

0 2000 4000 6000 8000 100000.0

0.2

0.4

0.6

0.8

1.01

/WV

TR

[N

orm

alize

d]

Bending cycles (No.)

PVA

2 %

4 %

6 %

8 %

10 %

Figure 5.8: Reciprocal WVTR of PVA and its MMT-Na+ clay composite with loading

concentrations of 0-10 volume %.vs number of bending cycles with bending radius of 5 cm.

Each layer has a thickness of 100 µm.

5.1.1.7 Conclusion

A promising decrease in moisture permeability has been achieved by adding nanoclay into

the matrix of PVA. The moisture barrier has been deposited from solution. Processing of

barrier layers from solution is an easy, environmentally friendly and economical method.

The nanocomposites were tested for barrier quality using ASTM E96 standard.

Nanocomposites having nanoclay contents of 2 vol%, 4 vol%, 6 vol%, 8 vol% and 10 vol%

show reduced moisture permeation by 49%, 71%, 80%, 84% and 86% respectively, as

compared to the pure polymer. They exhibit high transparencies of ~92% in the white light

region. The addition of nanoclay seems to have an almost negligible effect on transparency

except for light scattering by agglomerated nanoclay particles at the surface. The haze

caused by this scattering was found to be not more than 8% in the visible region, even for

the highest loading of 10 vol%. The resulting polymer/nanoclay composites also show


80

excellent flexibility, maintaining their barrier quality even after 10,000 bending cycles.

These properties make PVA/clay nano-composites good candidates for packaging

materials. However, even composites with 10 vol% nanoclay show WVTR values no

smaller than 2 𝑔 ∙ 𝑚−2 ∙ 𝑑−1. These values are appropriate for food packaging, but are not

sufficient for the application to organic solar cells encapsulation, where WVTR values of

less than 10-2 g.m-2.day-1 are required. According to the Bharadwaj model, the barrier

properties can be enhanced by two parameters. Increasing the orientation factor from S = 0

to S = 1 will enhance the barrier effect by a factor of almost three (for high aspect ratios).

However, due to the small lateral extension of the Montmorillonite particles of few

nanometers, the particles are not aligned by the coating process. An increase of the aspect

ratio from 500 to, e.g., 2000 will increase the barrier effect by another factor of around four.

Thus, WVTR values of an order of magnitude less than for Montmorillonite clay particles

can potentially be achieved. Enhancing the filler loading and increasing the thickness of

the barrier films will provide another order of magnitude. In the following chapter, this

approach will be demonstrated by using glass flakes as filler particles with high aspect

ratios.


81

5.1.2 Glass flakes based barriers

In this section, glass flakes of different aspect ratios are used as filler particles, instead of

clay to produce flexible barriers with enhanced barrier properties. Glass flakes have been

chosen as candidates for filler particles because they offer several favorable properties with

respect to their use in transparent and flexible barriers. They are transparent in the visible

range of the spectrum. They offer large aspect ratios (AR) and they are easily aligned

parallel to the film surface during the coating process, due to their large lateral extension

which is on the same order as the film thickness. Due to their particulate nature, the

resulting barrier should be resilient towards bending.

Three different types of glass flakes are investigated. The different types of glass flakes

have almost the same lateral extension of around 50 – 300 µm, but different thicknesses of

about 1 µm down to 0.1 µm, hence the aspect ratio (α ) ranges from 200 to 2000 (Figure

5.9). The barrier films are prepared by inserting the glass flakes into PVB matrices as

described in chapter 4. PVB is chosen as the matrix polymer due to its flexible nature and

compatibility with glass flakes in terms of good adhesion of the filler particles with the

matrix and relatively good refractive index matching (n(550 nm) = 1.525) and PVB (n(550

nm) = 1.49) [208], [209].

a)

b)

c)

Figure 5.9: Optical micrographs of glass flakes. (a) glass flakes with thickness ~ 1 µm,

(b) glass flakes with thickness ~0.5 µm, (c) glass flakes with thickness of ~0.1 µm


82

The top view of the coated layers is shown in Figure 5.10a. The film contain 15 vol% of

glass flakes. The top view of the layers suggests that the flakes are oriented preferentially

parallel to the film surface. SEM cross section image and EDX image mapping of Si of

corresponding PVB films are shown in Figure 5.10(b,c). The SEM cross section clearly

shows that the flakes are preferentially oriented parallel to the film and this is further

confirmed by the EDX mapping of SiO2.

a)

b)

c)

Figure 5.10: Micrographs of PVB films containing 15 vol% glass flakes of AR = 400. a)

Optical micrograph (top view) (b) SEM cross section of a semi-polished PVB/glass

flakes composite filmc) EDX image of the highlighted section (fully polished) of the

PVB/glass flakes layer with Si mapping.


83

5.1.2.1 Surface roughness of the layers

Confocal images of the layer of PVB containing 25 vol% of glass flakes are shown in Figure

5.11 (a-c). The figures show the surface morphological scan of the layers having areas of

1.5 x 1.5 mm2. The surface of the layer containing flakes of A.R~200, shows a highly rough

surface (Ra ~ 120 nm). Similarly, the layers with flakes of A.R~400 and A.R~2000 also

exhibited rough surfaces i.e. Ra ~100 nm and Ra ~120 nm respectively (Figure 5.11 (a-c)).

This surface roughness in the layers can be attributed to the large lateral extension of the

glass flakes (200-300 µm), together with the high volume fraction of glass flakes. As soon

as the solvent starts to evaporate, the PVB matrix contracts around the flakes and as a result

leaves behind substantial surface roughness.

a)

b)

c)

Figure 5.11: Confocal micrograph showing the surface of the PVB filled with 25 vol%

of the glass flakes, a) glass flakes with A.R~200, b) glass flakes with A.R~400, and c)

glass flakes with A.R~2000.

5.1.2.2 Transparency of the layers

Figure 5.12 shows the optical measurement of a pristine PVB layer having a thickness of

~70 µm. PVB shows a total transmission of ~93% throughout the visible range of the

spectrum, whereas the diffuse transmission increases continuously from 5% at 800 nm to

12% at 400 nm, due to scattering at crystalline domains (Figure 5.12 a). Total reflectance

of ~7% is observed in the visible region of the spectrum (Figure 5.12 b). The diffuse

reflectance shown by the PVB layer increases steadily from 3% at 800 nm to 7% at 250

nm. The sum of total transmittance and total reflectance of PVB layer is 100%. It means

that there is no measurable absorption by the PVB film above 400 nm.

0.0 0.5 1.0 1.5 mm

mm

0.0

0.5

1.0

0

50

100

150

200

250

0.0 0.5 1.0 1.5 mm

mm

0.0

0.5

1.0

0

50

100

150

200

250 0.0 0.5 1.0 1.5 mm

mm

0.0

0.5

1.0

0

50

100

150

200

250

nm nm nm


84

a)

300 400 500 600 700 8000

20

40

60

80

100

TotalTransmittance

Tra

nsm

itta

nce (

%)

Wavelentgth (nm)

Diffused Transmission

b)

300 400 500 600 700 8000

20

40

60

80

100

Diffused reflectanceReflecta

nce (

%)

Wavelentgth (nm)

Total reflectance

Figure 5.12: Spectra of a pristine PVB film having a thickness of ~ 70 µm, a) Total

transmission (open triangles in black), diffuse transmission (full triangles in red), b)

Total reflectance (full square in black) and diffuse reflectance (black squares).

Subsequently, transmission (total and diffuse) along with reflection (total and diffuse)

measurements were carried out for the PVB layers containing glass flakes. The addition of

glass flakes leads to cutoff around 330 nm. The results are shown in Figure 5.13 a-f. PVB

layers filled with glass flakes having aspect ratios of 200 and 400, at concentrations of 5

vol%, 15 vol% and 25 vol%, respectively, show total transmittance values of around 90%,

whereas for AR = 2000, the total transmission decreases from 89% to 85% with increasing

volume fraction of glass flakes. Total reflection is between 8% and 9% for all samples, so

that absorption in the visible range is smaller than 6% for all samples. Only below 350 nm,

there is significant absorption by the glass flakes.

In contrast to total transmission, diffuse transmittance values vary significantly at different

concentrations of the flakes. For glass flakes of α = 200, the diffuse transmittance remains

around 5% and 10% for concentrations of 5 vol% and 15 vol%, respectively, but for 25

vol% the diffuse transmittance inreases to around 30% at 550 nm (Figure 5.13a). Nearly

the same behaviour is observed for glass flakes of α = 400, i.e., diffuse transmittance

remains around 10% for 5 vol% and 15 vol% and increases to ~30% for 25 vol% (Figure

5.13b). For glass flakes of α = 2000, almost all of the glass flake loadings show a high

diffuse transmittance of around 33% (Figure 5.13c). Diffuse reflectance in all cases remains

around 6%.


85

a)

300 400 500 600 700 8000

20

40

60

80

100T

ransm

itta

nce (

%)

Wavelength (%)

Total transmission

5 vol. %

15 vol. %

25 vol. %

Diffused transmission

5 vol. %

15 vol. %

25 vol. %

b)

300 400 500 600 700 8000

20

40

60

80

100

Diffuse transmission

5 vol. %

15 vol. %

25 vol. %

Tra

nsm

itta

nce (

%)

Wavelength (nm)

Total transmission

5 vol. %

15 vol. %

25 vol. %

c)

300 400 500 600 700 8000

20

40

60

80

100

Tra

nsm

itta

nce (

%)

Wavelength (nm)

Total transmission

5 vol. %

15 vol. %

25 vol. %

Diffuse transmission

5 vol. %

15 vol. %

25 vol. %

d)

300 400 500 600 700 8000

20

40

60

80

100

Reflecta

nce (

%)

Wavelentgth (nm)

Total Reflectance

5 vol. %

15 vol. %

25 vol. %

Diffuse Reflectance

5 vol. %

15 vol. %

25 vol. %

e)

300 400 500 600 700 8000

20

40

60

80

100R

eflecta

nce (

%)

Wavelentgth (nm)

Total Reflectance

5 vol. %

15 vol. %

25 vol. %

Diffuse Reflectance

5 vol. %

15 vol. %

25 vol. %

f)

300 400 500 600 700 8000

20

40

60

80

100

Re

fle

cta

nce

(%

)

Wavelentgth (nm)

Total Reflectance

5 vol. %

15 vol. %

25 vol. %

Diffuse Reflectance

5 vol. %

15 vol. %

25 vol. %

Figure 5.13: Transmittance and reflectance spectra of PVB containing glass flakes. a)

Total transmittance and diffuse transmittance spectra of PVB filled with 5-15 vol% of

glass flakes (A.R = 200), b) Total transmittance and diffuse transmittance spectra of PVB

filled with 5-15 vol% of glass flakes (A.R = 400), c) Total transmittance and diffuse

transmittance spectra of PVB filled with 5-15 vol% of glass flakes (A.R = 2000), d) Total

reflectance and diffuse reflectance spectra of PVB filled with 5-15 vol% of glass flakes

(A.R = 200), e) Total reflectance and diffuse reflectance spectra of PVB filled with 5-15

vol% of glass flakes of A.R = 400, and f) Total reflectance and diffuse reflectance spectra

of PVB filled with 5-15 vol% of glass flakes of A.R = 2000.

In summary, glass flakes are ideally suited as filler particles for PVB films, as the total

transmittance of the film remains around 90% even at high glass loading and thus is hardly

reduced with respect to pristine PVB. This is mainly due to the almost perfect refractive

index matching of sold lime glass (n(550 nm) = 1.52) and PVB (n(550 nm) = 1.48). Both,

total and diffuse reflectance hardly vary with the type of glass flake or the glass loading.

However, diffuse transmittance is significant, even for low glass loadings and increases

further with increasing glass loading of the film. Diffuse transmittance may depend on

several parameters, which include light scattering by the glass particles in the bulk of the

film, surface roughness of the film, and light scattering by gas bubbles which are trapped

in the polymer matrix. In the following, we will analyze the different possible causes for


86

the diffuse part of the transmitted light. The increase in the diffuse transmittance can be

due to different reasons, which include, surface roughness, orientation of the glass flakes,

mis-match of the refractive indices of the flakes and polymer matrix etc.

5.1.2.3 Influence of bulk scattering:

In order to quantify the effect of light scattering of the glass particles in the PVB matrix on

the optical characteristics of the filled PVB layers, optical simulations were performed. For

the optical simulations, an optical model for simulating light scattering and propagation in

polymer filled with particles was developed by Dr. Benjamin Lipovšek from the University

of Ljubljana, Slovenia [210]. The detailed description of the model can be found in

Benjamil et. al., 2015 [210]. The model for particle filled polymers was used by integrating

it in the optical simulator CROWM (Combined Ray Optics/ Wave Optics Model)[210],

[211]. CROWM is based on the combination of three-dimensional ray tracing and transfer

matrix methods to analyze light propagation in thick and thin layers respectively. For

simulations a simple geometry of the flakes is considered – the surfaces are assumed to be

plane-parallel and perfectly smooth.

In a first step, the optical transmittance at 550 nm of a single glass flake of refractive index

n = 1.525 [212] inside a PVB matrix of n = 1.49 [213] is simulated for different tilting

angles of the glass flake with respect to the film surface (Figure 5.14). (Up to tilt angles of

70°, 99 % of light is transmitted through the flake, retaining the specular direction of

propagation. Only above tilt angles of 70°, reflectance is large enough so that the reflected

(scattered) light becomes „visible“ as a loss in the transmitted light.

In a second step, the simulation is performed on PVB films filled with glass flakes at

volume concentrations of 5 to 30%. Three different cases were simulated which include a)

(A) variation of the tilt angle (θ) (surface normals of all glass flakes parallel, i.e., no rotation

around vertical axis of the film) and particle volume concentration (PVC), (B) Fixed tilt,

random rotation (ϕ) around the vertical axis and (C) Random tilt, random rotation around

the vertical axis. The schematic diagrams of simulated cases are shown in Figure 5.15.


87

Figure 5.14: Simulated direct/total transmission of a single glass flake filled in PVB matrix

at a wavelength of 550 nm for different polarizations of the incident light.

Figure 5.15: Simulated tilt (θ) and rotation angles of flakes (ϕ), for conditions a)

variation of the tilt angle of the flakes (0≤ 𝜃 ≤ 180𝑜) and fix rotation, b) Fixed tilt,

rotation around the vertical axis (0≤ 𝜙 ≤ 360𝑜) and c) Random tilt and random

rotation around the vertical axis, where is a vector normal to flake surface.


88

In case A (Figure 5.15a) , the layers show high transmission of the light (> 90%) for all tilt

angles up to 70°. Significant scattering, i.e., haze, takes place only for tilt angles above 60o.

At 80° tilt, total transmission reaches a minimum of 85% for PVC = 25%, while diffuse

transmission (haze) reaches its maximum of 30%. At 90° tilt angle, the incident light

completely „misses“ the flakes, this is why the haze drops to zero in these simulations. In

case B (Figure 5.15b), for glass flakes with fixed tilt and random rotation around the vertical

axis, qualitatively the same behaviour as in case (A) is observed(Figure 5.15b). However,

at 80° tilt angle of the glass flakes, total transmission is only reduced to 88% while haze

reaches a value of 70% for PVC = 25%. In the most realistic case C (Figure 5.15c), the

effects of the tilt angle range (±θ) and particle volume concentration (PVC) are simulated.

The effect of tilting is negligible for angles below 70°. Above 70°, total transmittance

experiences a minute drop from 92% to 91% and haze reaches a maximum of 30% for the

tilt angle range of 90° and a PVC = 25% (Figure 5.16).

Figure 5.16: Simulated transmission (total and diffuse) (@550 nm) of PVB films filled

with glass flakes at different particle volume concentrations havingRandom tilt, random

rotation around the vertical axis.


89

From these simulations, it is to be concluded that light scattering due to the inclusion of

glass flakes into the PVB film does not lead to a significant reduction of total

transmission, which is in perfect accordance with the experimental data. However, the

haze of between 5% and 30% observed in experiment (Figure 5.13) is not reproduced

by the simulations, as the average tilt angle of the glass flakes in the PVB films is less

than 10° for 80% of flake content, even for the sample with the relatively low PVC

(Figure 5.10c). In the following, causes for significant haze other than bulk scattering are

investigated.

5.1.2.4 Influence of the Surface roughness:

From the confocal micrographs (Figure 5.11) it is evident that the layers exhibit

considerable surface roughness (Ra) on the order of 100 nm. This rough surface can be a

potential reason for the high value of diffuse transmission. Therefore, in order to reduce

the surface roughness, an additional UV cured epoxy layer was deposited on both sides

of the glass flakes filled PVB layers. The epoxy was selected because of its easy and

solvent free processing nature. If other polymers are coated on PVB, de-wetting and re-

dissolution of the PVB are observed. Additionally, the refractive index of the epoxy is n

= 1.51, which ideally matches the glass flakes (1.52).

The results of transmittance (total and diffuse) and reflectance (total and diffuse)

measurements of the epoxy coated PVB/glass flakes composite are shown in Figure

5.17(a-f). The epoxy adhesive absorbs in the wavelength range of 300-410 nm, which

leads to a significant drop of transmittance in the UV part of the spectrum, but does not

show measurable absorption in the visible part of the spectrum. Total reflection is slightly

enhanced with respect to PVB/glass composites without epoxy layers to around 10 to

12%, which leads to a slight drop of total transmittance to values between 83% and 89%.

The behaviour of the epoxy coated composite films is summarized in Figure 5.18(a-c)

which shows the dependence of the diffuse transmission at 550 nm on glass loading,

before and after epoxy coating onto both sides of the PVB/glass composite films. For the

flakes with A.R = 200 and 400, about 5% reduction in haze is observed for PVC = 5

vol% and 15 vol%, while about 15% reduction is observed for PVC = 25 vol%. For the

flakes with A.R = 2000, about 15%, 10% and 11% reduction in diffuse transmission is

observed for PVC = 5vol%, 15 vol% and 25 vol%, respectively.


90

These observations suggest that at least part of diffuse transmittance is caused by the

surface roughness. However, the additional epoxy layers do not only reduce the diffuse

part of transmittance, but at the same time also reduce total transmission, due to

absorption of UV light and enhanced reflectance at the surfaces of the epoxy layers.

As diffuse transmission is beneficial for solar cells [214] and the reduction of total

transmittance is detrimental, smoothing layers will not be employed for the following

experiments.

a)

300 400 500 600 700 8000

20

40

60

80

100

Diffuse Transmission

5 vol. %

15 vol. %

25 vol. %

Total Transmssion

5 vol. %

15 vol. %

25 vol. %

Tra

nsm

itta

nce

(%

)

Wavelength (nm)

AR ~ 200

b)

300 400 500 600 700 8000

20

40

60

80

100AR ~ 400

Tra

nsm

itta

nce (

%)

Wavelength (nm)

Total Transmssion

5 vol. %

15 vol. %

25 vol. %


5 vol. %

15 vol. %

25 vol. %

c)

300 400 500 600 700 8000

20

40

60

80

100

Tra

nsm

itta

nce

(%

)

Wavelength (nm)

AR ~ 2000

Total Transmssion

5 vol. %

15 vol. %

25 vol. %


5 vol. %

15 vol. %

25 vol. %

d)

300 400 500 600 700 8000

20

40

60

80

100AR ~ 2000

Reflecta

nce (

%)

Wavelength (nm)

Total Reflectance

5 vol. %

15 vol. %

25 vol. %

Diffuse Reflectance

5 vol. %

15 vol. %

25 vol. %

e)

300 400 500 600 700 8000

20

40

60

80

100AR ~ 400

Re

fle

cta

nce

(%

)

Wavelength (nm)

Total Reflectance

5 vol. %

15 vol. %

25 vol. %

Diffuse Reflectance

5 vol. %

15 vol. %

25 vol. %

f)

300 400 500 600 700 8000

20

40

60

80

100

Reflecta

nce (

%)

Wavelength (nm)

Total Reflectance

5 vol. %

15 vol. %

25 vol. %

Diffuse Reflectance

5 vol. %

15 vol. %

25 vol. %

AR ~ 2000

Figure 5.17: Transmission and reflectance spectra of PVB containing glass flakes after

coating epoxy on both sides of PVB filled with flakes. a) Total transmission and diffuse

transmission spectra of PVB filled with 5-15 vol% of 200 aspect ratio glass flakes, b)

Total transmission and diffuse transmission spectra of PVB filled with 5-15 vol% of

400 aspect ratio glass flakes, c) Total transmission and diffuse transmission spectra of

PVB filled with 5-15 vol% of 2000 aspect ratio glass flakes, d) Total reflectance and

diffuse reflectance spectra of PVB filled with 5-15 vol% of 200 aspect ratio glass

flakes, e) Total reflectance and diffuse reflectance spectra of PVB filled with 5-15 vol%

of 400 aspect ratio glass and f) Total reflectance and diffuse reflectance spectra of

PVB filled with 5-15 vol% of 2000 aspect ratio glass flakes.


91

a)

0 5 10 15 20 25 300

5

10

15

20

25

30

35

40

45

50

Diffu

sed T

ransm

issio

n (

%)

@ 5

50 n

m

Vol (%)

A.R ~200 Before

A.R ~200 After

b)

0 5 10 15 20 25 300

5

10

15

20

25

30

35

40

45

50

Diffu

sed T

ransm

issio

n (

%)

@ 5

50 n

m

Vol (%)

AR 400 before

AR 400 After

c)

0 5 10 15 20 25 300

5

10

15

20

25

30

35

40

45

50

Diffu

sed T

ransm

issio

n (

%) @

550 n

m

Vol (%)

AR 2000 Before

AR 2000 After

Figure 5.18: Dependence of diffuse transmission of PVB films @ 550 nm on glass flake

loading before (black squares) and after (red dots) coating epoxy on both sides of the

film, a) layers of flakes with α ~200 b) layers of flakes with α ~400, and c) layers of

flakes with α ~2000.

5.1.2.5 Barrier performance of glass flakes

Pristine PVB films (70 µm) show water vapor transmission rates (WVTR) of 65 g.m-2.day-

1 (@40 °C/85% RH).Upon addition of glass flakes, the WVTR values are reduced

significantly, due to the increasing tortuous path length (Tables. 12 -14). The WVTR

decreases with both, increasing α and growing volume fraction of the glass flakes. The

smallest WVTR value of 0.14 g.m-2.day-1, corresponding to a permeability of 3e-3 g.cm.m-

2.day-1, is achieved for glass flakes with α ~2000 at a volume fraction of 25% (Tab. 12).

This corresponds to a reduction of the permeability with respect to the value of pristine

PVB by a factor of 150, which is termed the barrier improvement factor (BIF).

𝐵𝐼𝐹 = 𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑦𝑡(𝑃𝑉𝐵)

𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦(𝑃𝐵𝑉+𝑔𝑙𝑎𝑠𝑠 𝑓𝑙𝑎𝑘𝑒𝑠) Eq. 55

Table 11 show the development of the BIF with increasing volume fraction of the glass

flakes and the comparison of the experimental BIF to Bhardwaj’s theoretical model for

different aspect ratios (200-2000) are shown in Figure 5.19. BIF of 30, 56.2 and 150 have

been achieved for 200, 400 and 2000 aspect ratio (ɑ) glass flakes respectively for the layers

containing 25 vol% of the flakes. The experimental data is almost in close matching with

Bhardwaj’s BIF except for the higher loadings for large aspect ratio flakes. The reason for

this large difference in BIF is defects within the layer filled with flakes of α ~2000 as shown

in Figure 5.20. These defects act as the diffusion path for the permeating gas and increase


92

permeability. The causes of these defect can be either un-dissolved polymer or improper

stacking of the glass flakes within the layer.

Bharadwaj (S=1, a=200)

Experimental (A.R~200)





0 5 10 15 20 250

50

100

150

200

250B

IF o

f co

mp

osite

s

Concentration (vol %)

Figure 5.19: Experimental barrier improvement factor of the barrier (PVB/glass flakes

composites) compared with BIF of composited according to Bhardwaj’s model.

Experimental BIF of flakes with α ~200 (closed black circle), Bhardwaj’s simulated

(dotted black line), experimental BIF of flakes with α ~400 (blue closed triangle) and

Bhardwaj simulated (dotted blue line) and experimental BIF of flakes with α ~2000 (dark

yellow square) and Bhardwaj simulated (dotted dark yellow line). vs the glass flakes

volume concentration in the PVB layers.

Figure 5.20: SEM cross section image of PVB filled with 25 vol% of glass flakes of α

~2000, highlighted area shows the defects within the layer.


93

Table 11: WVTR (@40oC/85 % RH) of PVB films filled with different volume

concentrations of glass flakes of α ~ 200.

G. flakes vol%

(α ~200)

Film thickness

(µm)

WVTR

g.m-2. day-1

Permeability

g.cm.m-2.day-1

BIF

0 70 65 0.45 1

5 150 10 0.1 4.5

10 260 3.5 0.05 9

15 390 1.6 0.03 15

20 210 1.1 0.02 22.5

25 180 0.8 0.015 30

30 212 0.3 0.013 34.6

Table 12: WVTR (@40oC/85 % RH) of PVB films filled with different concentration of glass

flakes with α ~400 aspect ratio glass flakes with different concentrations.

G. flakes vol%

(α ~400)

Film thickness

(µm)

WVTR

g.m-2. day-1

Permeability

g.cm.m-2.day-1

BIF

0 70 65 0.45 1

5 78 9 0.07 6.4

10 140 1.7 0.023 19.5

15 310 0.7 0.021 21.4

20 140 0.9 0.012 37.5

25 240 0.34 0.008 56.2

30 140 0.33 0.004 112

Table 13: WVTR (@40oC/85 % RH) of PVB films filled with different concentrations of

glass flakes with α ~2000.

G. flakes vol%

(α ~2000)

Film thickness

(µm)

WVTR

g.m-2. day-1

Permeability

g.cm.m-2.day-1

BIF

0 70 65 0.45 1

5 78 2.1 0.017 26.4

15 150 0.24 0.0035 128.6

25 160 0.14 0.003 150


94

5.1.2.6 Oxygen permeability

Neat PVB films show OTR values of 110 cm3 m-2 day-1 bar-1. The incorporation of glass

flakes reduces the OTR significantly (Table 14). The PVB film filled with 5 vol%, 15 vol%

and 25 vol% of glass flakes with α = 2000 exhibits OTR values of 4.8 cm3.m-2.day-1.bar-1,

0.45 cm3.m-2.day-1.bar-1, and 0.35 cm3.m-2.day-1.bar-1, respectively. The lowest

permeability of 𝑃 = 7 ∙ 10−3 𝑐𝑚3 ∙ 𝑚𝑚 ∙ 𝑚−2 ∙ 𝑑𝑎𝑦−1 ∙ 𝑏𝑎𝑟−1 is obtained for 25 vol% ,

which corresponds to a BIF of 140 (Table 14).

Table 14: OTR of PVB films filled with different concentrations of glass flakes with α =

2000.

G. flakes vol%

(α ~2000)

Film

Thickness

(µm)

OTR

cm3.m-2. day-1.bar-1

Permeability

cm3.cm. m-2. day-1.bar-1

BIF

0 90 110 0.98 1

5 80 4.8 0.04 24.8

15 190 0.45 0.008 122

25 210 0.35 0.007 140

0 5 10 15 20 250

20

40

60

80

100

120

140

160

BIF

Vol. Fraction

Oxygen permeability

moisture permeability

Figure 5.21: Barrier improvement factor of the PVB film filled with glass flakes (α ~2000)

against oxygen (red dots) and moisture (Black Square) vs the volume fraction of glass

flakes.


95

The BIF with respect to oxygen permeation shows, within the accuracy limits of the

experiment, the same dependence on the volume fraction of glass flakes as the BIF in the

case of humidity permeation (Figure 5.21). This is in accordance with the tortuous path

model, which assumes that the improvement of the barrier with increasing filler content

does not depend on the nature of the diffusant, as long as it does neither permeate nor

interact with the filler particles. Oxygen permeability can thus also be predicted by the

Bhardwaj’s model.

5.1.2.7 Bendability

Flexibility is one of the key requirements for water and oxygen barriers for flexible OSCs.

Therefore, the barriers based on glass flakes were subjected to bending with a bending

radius of 3 cm (for details see chapter 4). For this purpose, the PVB film containing 15

vol% glass flakes with α = 2000 was selected. The WVTR value of the layer equals 0.24

g.m-2.day-1 before and after 20,000 bending cycles (Figure 5.22). Furthermore, no visible

damage to the film is observable after bending. Obviously, the glass flakes always return

to their initial position after bending, neither loosing adhesion to the PVB matrix nor

damaging it.

0 5000 10000 15000 200000.0

0.2

0.4

WV

TR

(g.m

-2.d

ay

-1)


Figure 5.22: WVTR of PVB film with 15 vol% glass flakes (α ~ 2000) vs. number of bending

cycles.


96

5.1.2.8 Encapsulation of organic solar cells

In order to test the barriers based on glass flakes under operational conditions, they are used

for the encapsulation of organic solar cells (OSCs). For this purpose, PVB films filled with

15% v/v glass flakes of α = 2000 are laminated on top of the solar cells (for details see

chapter 4) with a UV curable adhesive. The solar cells thus encapsulated are subsequently

subjected to accelerated lifetime tests, namely damp heat tests and irradiation by a sun

simulator.

5.1.2.9 Photo bleaching of P3HT

J(O2) , P(O2) and OTR of a PVB/glass flakes barrier film of thickness 210 µm to oxygen at

partial pressure p(O2)0.2 bar can be assessed from the rate of photobleaching of a P3HT

film underneath the barrier by using Eq. 52, Eq. 53 and Eq. 54 when the photobleaching

reaction is diffusion controlled.

a)

400 450 500 550 600 650 7000.0

0.2

0.4

0.6

Abs.

Wavelength (nm)

b)

400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

Abs.

Wavelength nm.

c)

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d A

bso

rba

nce

@ 5

25

nm

Time (h)

PVB/Glass flakes

PVB

Figure 5.23: UV–vis spectra of P3HT films on glass encapsulated with a) a PVB layer

and b) a PVB filled with 25 vol% of α ~2000 glass flakes during exposure to the light of

a sun simulator in ambient air at 65 C. c) Normalized absorbance loss at 525 nm of

P3HT films encapsulated with plain PVB, PVB/glass flakes filled barrier.

Using Eq. 52, Eq. 53 and Eq. 54 equations, and inserting the bleaching rate

∆𝐸 ∆𝑡 = 0.03 𝑑𝑎𝑦−1⁄ obtained from Figure 5.23(c) as well as the oxygen partial pressure

difference ∆𝑝(𝑂2) = 0.2 𝑏𝑎𝑟, and assuming the consumption of five moles of molecular

oxygen per mole thiophene rings bleached [6], [79], the OTR value of ~0.5 cm3.m-2.day-

1.bar-1 for PVB/glass flakes layer is obtained, which is in close accordance with the OTR

value of 0.45 𝑐𝑚3 ∙ 𝑚−2 ∙ 𝑑𝑎𝑦−1 ∙ 𝑏𝑎𝑟−1 measured with the commercial OTR device.


97

5.1.2.10 Lifetime under damp heat

For the damp heat degradation tests, the organic solar cells were laminated with (1) PVB

barrier films and (2) PVB/ glass flakes composite barrier films. Encapsulated solar cells

were placed in a climate chamber with controlled test conditions of 40 oC and 85% relative

humidity (RH). These conditions were chosen to be compatible with those at which the

WVTR were measured. Figure 5.24 provides the degradation data of the two samples

within 168 hours of exposure to damp heat conditions.

0 40 80 120 1600

2

4

6

8

10

12

15 % glass flakes

PVB

Jsc (m

A/c

m2)

Time (h)

0 40 80 120 1600

10

20

30

40

50

60

70

15 % glass flakes

PVB

FF

(%

)

Time (h)

0 40 80 120 1600.0

0.2

0.4

0.6

0.8

15 % glass flakes

PVB

Voc (

V)

Time (h)

0 40 80 120 1600

1

2

3

4

15 % glass flakes

PVB

PC

E (

%)

Time (h)

Figure 5.24: Damp heat degradation test (40 °C/85% RH) of P3HT:PCBM based devices

encapsulated with pristine PVB films and PVB films filled with 15% v/v glass flakes (α

= 2000). Jsc: short circuit current. Voc: open circuit current. FF: fill factor, PCE:.

Power conversion efficiency.

It can be seen from the Figure 5.24 that a dramatic loss of PCE is observed for the device

encapsulated with pristine PVB film. The device loses PCE, fill factor, Jsc and Voc and

thus dies within 24 hours. This effect is known to occur upon ingress of water into the

packaging and a subsequent damage to the interface between the active layer and the

PEDOT:PSS hole extraction layer. In contrast to the sample encapsulated with pristine PVB


98

film, the device encapsulated with the PVB/glass flakes composite film does not show any

degradation during the testing period. According to its WVTR value of 0.24 g.m-2.day-1,

the barrier film has only transmitted 0.0001 g.m-2 of water into the device during the

exposure period, which corresponds to a film of liquid water of around <0.1 µm in

thickness. This is much less than the capacity of the adhesive film used for lamination of

~5 µm and thus not sufficient to damage the active layer/PEDOT:PSS interface [215].

5.1.2.11 Lifetime under irradiation by sun simulator

For the sun degradation test, OSCs encapsulated with PVB / glass flakes composite films

and, for reference purposes with pristine PVB films as well as with commercial Mitsubishi

films were subjected to the irradiation by a sun simulator (1000 W m-² @ 65 °C black body

temperature). Figure 5.25 shows the photovoltaic key parameters (Jsc, FF, Voc and PCE) of

the encapsulated devices subjected to sun test. Devices with Mitsubishi barrier show no

degradation at all after 160 hours of irradiation, which confirms that P3HT:PCBM based

solar cells do not degrade due to thermally induced processes, e.g., morphology changes,

at 65 °C . The slight increase in PCE is due to the increase in Jsc which is caused by the

post annealing effect at the elevated temperature in the sun tester [92], [216], [217]. The

device encapsulated with pristine PVB film, degrades rapidly in both Jsc and FF, and thus

reaches 80% of its initial PCE within less than 20 hours, which infers that the PVB is not a

good barrier against oxygen and hence loses PCE. The device encapsulated with the

PVB/glass flakes composite film has degraded to 80 % of its initial PCE only after 160

hours of the test. The loss in PCE is caused by loss of Jsc, whereas fill factor and Voc remain

stable. This indicates, that degradation is due to the diffusion of oxygen through the barrier

and its subsequent photo-induced reaction with the active layer [6], [218]. This is supported

by the fact that the amount of oxygen which has diffuse through the barrier within 160

hours at 65 °C is sufficient to photo-oxidize about 1% of the thiophene rings in the active

layer, which is the damage at which P3HT:PCBM cells have been reported to loose around

20% of their initial performance [218].

Further support comes from the observation that no degradation in damp heat is observed

(see Figure 5.25). This can be explained by the effect the effect of temperature as in damp

heat conditions the temperature is only 40 °C, i.e., the diffusion rate of oxygen is only a

fifth to a tenth of that in the sun soaker and hence devices degrade faster.


99

0 40 80 120 1600

2

4

6

8

10

12

Mitsubishi

15 vol% (2000 GF)

PVB

Jsc (m

A/c

m2)

Time (h)

0 40 80 120 1600

10

20

30

40

50

60

70

Mitsubishi

15 % glass flakes

PVB

FF

(%

)

Time (h)

0 40 80 120 1600.0

0.2

0.4

0.6

0.8

Voc (

V)

Time (h)

Mitsubishi

15 vol% (2000 GF)

PVB

0 40 80 120 1600

1

2

3

4

Mitsubishi

15 vol% (2000 GF)

PVB

PC

E (

%)

Time (h)

Figure 5.25: Sun degradation test of P3HT:PCBM based solar cells encapsulated in

three different barriers: Mitsubishi barrier film, pristine PVB film, and PVB film filled

with 15% v/v glass flakes (α = 2000). Jsc: short circuit current. Voc: open circuit current.

FF: fill factor, PCE: power conversion efficiency.

5.1.2.12 Conclusion

For the first time, transparent and flexible barriers for organic electronic devices have been

prepared on the basis of glass flakes. WVTR values of as little as 0.14 g.m-2.day-1 have

been achieved, by maximizing the aspect ratio and by aligning the glass flakes with the film

surface, thus maximizing the order parameter of the Bhardwaj equation. It has been shown

that the permeation depends on the volume content of the glass flakes. At volume fractions

of ~15%, the alignment of the flakes is almost perfect. At higher volume fractions of around

25%, the alignment is almost perfect but the layers contain defects. BIF values of around

140 have been achieved against both moisture and oxygen with ~2000 aspect ratio glass

flakes. WVTR is improved by the factor of 150, while the layers maintained the total

transmission of around 90% along with slight increase in the diffuse transmission.


100

The bending results show that the barrier quality remains unchanged even after 20000

bending cycles, which proves the good adhesion of the PVB matrix to the glass flakes.

Application of such barriers as an encapsulation for the OSC also resulted in increase of

the lifetime of the devices. In the case of the sun irradiation test, the lifetime of OSCs was

extended from few hours to beyond 160 h, whereas in the case of damp heat tests, almost

no degradation is observed. Hence, based on these results, barrier layers comprising glass

flakes as fillers in polymeric matrix have a high potential to be used as the encapsulation

for the thin film organic solar cells.


101

5.2 Polysilazane

Perhydropolysilazane (PHPS) is the trade name of a solution of perhydropolysilazane in

di-n-butyl ether. PHPS is an inorganic polymer composed of Si-N, Si-H and N-H bonds.

Proper curing of PHPS leads to formation of silica. Generally SiO2 films deposited from

vacuum assisted techniques show low co-efficient of diffusion of gas molecules. In this

work, PHPS is used as an alternative to get SiO2 networks processed from solution avoiding

expensive techniques. Therefore, processing of PHPS needs optimization before it can be

used as an effective coated barrier for OSCs. Therefore, in this chapter development and

optimization of an in-line encapsulation method for printed electronics based on the

deposition of the barrier material PHPS directly on the device from solution and subsequent

curing by VUV irradiation are discussed. In a first step, we will describe how the properties

of PHPS-based barriers on top of PET films can be optimized with respect to WVTR- and

OTR-values, flexibility, and processing speed. The resulting barrier films are subsequently

tested as encapsulation materials for organic solar cells. Following the design rules thus

developed, PHPS/polymer sandwich layers are then coated directly on P3HT:PCBM based

solar cells by roll-to-roll compatible methods. The protective effect of the directly coated

barrier on the solar cells is finally investigated in damp heat and under irradiation by a sun

simulator.

The successful demonstration of direct coating of PHPS/polymer sandwich layers on top

of organic devices does not only enable higher throughput and lower material consumption

in the roll-to-roll production of printed electronics, it also opens the way for printing

organic electronics onto 3D objects, which cannot be protected otherwise, because

encapsulation by lamination of barrier films is not possible in this case.

5.2.1 Optimizing the curing method for PHPS

In order to make PHPS compatible to be used as the inline encapsulation for OSCs, the

PHPS curing mechanism needs to be optimized. Hence, PHPS films are irradiated with

deep UV (~172 nm peak wavelength), as this method of curing is reported frequently in the

literature [164], [165], [169], [183]. Therefore, for this purpose, PHPS films coated on PET

substrates are optimized with respect to irradiation distance, composition of the ambient

atmosphere and temperature during VUV irradiation. The conversion mechanism of the

PHPS film coated on polyethylene substrate is monitored by recording FTIR spectra. Figure

5.26 shows IR spectra of PHPS cured at different irradiation distances between sample


102

surface and lamp. PHPS was cured at three distances, namely 100 mm, 30 mm and 5 mm.

All the layers had the same thickness (~ 500 nm) and were exposed to deep UV light for

the same amount of time (~25 minutes). The characteristic peaks in un-cured films appear

at 3400 cm-1 (N-H), 2150 cm-1 (Si-H), and 850 cm-1 (Si-N-Si). In fully cured films, all of

these peaks have disappeared, while additional peaks at 450 cm-1 (Si-O-Si) and 1050 cm-1

(Si-O-Si) appear. The curve representing 100 mm distance in Figure 5.26 shows peaks at

3400 cm-1, 2150 cm-1, 1050 cm-1, 850 cm-1 and 450 cm-1 which means, the uncured PHPS

still exists in the film, because the peak intensity at 850 cm-1 (Si-N-Si) is still higher than

the peak at 1050 cm-1, while in the curve representing 30 mm distance, the peaks

representing NH and SiH bonds at 3400 cm-1 and 2150 cm-1, respectively, are almost gone.

However, the peak at 850 cm-1 is still larger than that at 1050 cm-1, which infers that the

transformation is still incomplete and Si-N-Si still exists in the coating. The spectrum

representing 5 mm distance shows almost no NH and SiH peaks and the peak at 1050 cm-1

is dominant over the 850 cm-1, which suggest that PHPS has been transformed more or less

completely into SiO2 network as the IR spectra does not show any existence of Si-N-Si, Si-

H and NH bonds. Therefore, it can be concluded from the results that the curing with VUV

at smaller distances cures PHPS faster. This is due to the fact that the intensity of the deep

UV increases and forms more and more oxygen radicals near the sample surface that can

react and diffuse through the PHPS film and form SiO2 network. The conversion of PHPS

to SiO is not as significant in 30 mm and 100 mm cured layers compared with 5 mm, hence

the distance of 5 mm is most suitable for curing PHPS.


103

8001600240032004000

5 mm distance

30 mm distance

Wave number (cm-1)

Abro

bance

As deposited

100 mm distance

Figure 5.26:FTIR spectra of a 500 nm thick PHPS film coated on PE substrate cured with

deep UV light for ~25 minutes with irradiation distance of 100 mm (red curve), 30 mm

(blue curve) and 5 mm (purple curve).

5.2.2 Curing by the combination of heat and deep UV at distance of 5 mm:

The combination of curing methods, i.e. of thermal and deep UV curing, at the distance of

5 mm is the most suitable way for curing PHPS as shown in Figure 5.27. This is because

the oxygen radicals generated by deep UV diffuse faster to and within the PHPS layer. Also

the reaction itself is accelerated by increasing temperature. As a result, for thin PHPS films

(< 500 nm) the curing is achieved in ~15 minutes.


104

8001600240032004000

Un-cured PHPS

100 oC + VUV

Wavenumber (cm-1)

Abso

rbance

Figure 5.27: : FTIR spectra of PHPS film (500 nm thick) cured with the combination of

heat (100oC) and irradiation with 172 nm wavelength light at a distance of 5 mm for 15

minutes.

5.2.3 Correlation of WVTR with Infrared peak ratios

The conversion of PHPS films to silica has been reported by exposure to either damp heat

or deep UV (172 nm) irradiation [161], [162], [164], [174], [177]. According to Prager et

al.[164], the faster of the two methods is curing PHPS by deep ultraviolet irradiation, which

can be further accelerated at increased temperature [169]. This makes curing by deep UV

the most favorable candidate for roll-to-roll printing of barrier layers. In order to ensure

reproducible quality of PHPS-based barriers even under varying conditions, a quantitative

endpoint control of the curing process in the roll-to-roll production line is required. Precise

endpoint control also allows minimizing the photon dose used for curing PHPS, which

reduces the photo-damage to the active materials when coating barriers directly on devices.

Therefore, we establish a quantitative correlation between the achieved values of water

vapor transmission rates (WVTR) of the resulting barriers and the peak ratios of infrared

vibrations related to the conversion of PHPS to silica. Using peak ratios instead of


105

intensities of single peaks offers several advantages, e.g., the independence of equipment

characteristics and of absolute film thicknesses. To this end, 500 nm thick PHPS layers are

coated on poly(ethyleneterephthalate) (PET) and polyethylene (PE) substrates for moisture

permeation and Fourier Transform Infrared (FTIR) spectroscopic analysis, respectively,

and irradiated subsequently by deep UV light (wavelength of ~172 nm) at a distance of

around 5 mm at room temperature conditions. The temporal development of the FTIR

spectra during the conversion of PHPS is shown in Figure 5.28 a. In agreement with

literature, the peaks corresponding to the N-H (3400 cm-1), Si-H (2150 cm-1), and Si-N (830

cm-1) stretching vibrations decrease within few minutes of curing. Concomitantly, the peaks

assigned to the Si-O bending and stretching vibrations near 450 cm-1 and 1050 cm-1,

respectively, increase. No significant signal at 3200 cm-1 is observed, indicating a

negligible concentration of OH-groups in the final film. The ratio of the peaks at 830 cm-1

and 1050 cm-1 is plotted in Figure 5.28 b. This peak ratio, I(1050 cm-1)/I(830 cm-1), is the

most appropriate one for the quantification of the reaction progress, whereas the peaks at

450 cm-1 and 3400 cm-1 are less appropriate for quantitative evaluation, due to their low

intensities. The peak ratios suggest that after 16 minutes of irradiation most of the

polysilazane has been transformed to silica. As the curing continues, the conversion process

further proceeds, however, at a much slower rate. Figure 5.28 b demonstrates nicely that

with progressing conversion of polysilazane to SiO2, i.e., with increasing peak ratios, the

barrier quality in terms of WVTR values improves continuously. The closest correlation of

WVTR values and peak ratios is obtained for the peak ratio 1050/830, which indicates full

conversion at a ratio of around 2.2, corresponding to WVTR values of around 0.05 g m-2

day-1 for 500 nm thick films (equivalent to a permeability of 2.5 x10-6 g cm m2 day-1, in

accordance with literature) [183]. FTIR peak ratios thus represent a precise and reliable

tool for determining PHPS barrier qualities, thus avoiding time consuming water vapor

transmission measurements.


106

a)

b)

Figure 5.28. a) FTIR spectra of a 500 nm thick PHPS film, cured by deep UV light for the

times specified in the figure. The corresponding WVTR values are given next to the spectra.

b) FTIR peak ratios (1050 cm-1/(830 cm-1, open circles) of Figure 5.28 (a), correlated with

the corresponding WVTR values (full squares) at different curing times (Published in

reference [184] and reproduced with permission from John Wiley and Sons).

40080012001600200024002800320036004000

40080012001600200024002800320036004000

Abs

orba

nce

2 min

Wave number (cm-1)

0.06 g/m2.day16 min

0.1 g/m2.day10 min

0.8 g/m2.day6 min

2.1 g/m2.day4 min

3.5 g/m2.day

4 g/m2.dayun-cured PHPS

20 min 0.05 g/m2.day

0 5 10 15 200.0

0.5

1.0

1.5

2.0

2.5 1050/830

1/WVTR

Irradiation time (min)

Peak r

atio

0

5

10

15

20

1/W

VT

R (

m2

. day/g

)


107

5.2.4 Correlation of WVTR with IR peak (damp heat)

As mentioned in the previous section about correlation of WVTR and PHPS curing with deep

UV and a nice relationship is obtained which can suggest the WVTR properties of the cured

films. Similarly we tried to develop correleationship of WVTR of the films cured with damp

heat conditions. To develop such a relationship, the PHPS films with thickness of ~800 nm

were exposed to damp heat (65oC, 85%RH) and subsequently monitored with IR and

measuring WVTR (The time during the WVTR measurement (@40oC, 90%RH) is

excluded). FTIR spectra of coating of PHPS and their corresponding WVTR are shown in

Figure 5.29 a,b. The results show that the conversion of PHPS proceeds relatively slower

under these conditions. The characteristic peaks in un-cured film are 3400 cm-1 (N-H), 2170

cm-1 (Si-H), and 830 cm-1 (Si-N-Si). The peaks near 3400 cm-1 (N-H) and 2170 cm-1 (Si-H)

are almost gone after 60 minutes, at the same time, absorption peaks based on a siloxane

bond (-Si-O-Si-) near 450 cm-1 and 1050 cm-1 appeared and increased. These peaks indicate

the formation of SiO2. The spectrum for the exposure time of 60 minutes showed absorption

peaks based on Polysilazane 830 cm-1 (Si-N-Si), indicating that Polysilazane existed and that

the transformation to SiO2 was still incomplete. With increasing exposure time (120 – 180

minutes), the polysilazane-based peaks are reduced, indicating the conversion progressed.

Un-transformed PHPS exhibits poor barrier properties, as can be seen from the WVTR value

of as deposited layer (4 g/m2.day), which corresponds to the permeation value for the PET

substrate. As the exposure time to damp heat increases, the curing continues and polysilazane

transformation proceeds to completion. The trend of conversion of the PHPS is nearly as

same as that of deep UV cured but the barrier behavior is slightly different. However, the

shape of the spectra is slightly different as compared to deep UV with much more sharp peak

at 1050 cm-1 (Figure 5.29a). In the case of damp heat, lowest permeation is obtained with

exposure time of 300 minutes which is 0.09 g/m2.day which corresponds to the peak ratio,

I(1050 cm-1)/I(830 cm-1) of 4.5, which is higher as compared to deep UV curing. With this

higher peak ratio, the corresponding WVTR values for damp heat cured films are clearly

inferior. The possible reason for this can be the interstitial site generated by the diffuse water

while exposure of the damp heat conditions, that served as the permeation paths for the water

molecules. Based on these results, curing of PHPS with deep UV is much better not only in

terms of barrier quality but also in curing time.


108

a)

40080012001600200024002800320036004000

0.09 g/m2.day

0.2 g/m2.day

0.5 g/m2.day

1.6 g/m2.day

2.2 g/m2.day

300 min

200 min

120 min

60 min

Wave number (cm-1)

Abs

orba

nce

un-cured PHPS

30 min

4 g/m2.day

b)

0 50 100 150 200 250 3000

1

2

3

4

5 1050/830

1/WVTR

Curing time (min)

Peak r

atio

0

2

4

6

8

10

12

1/W

VT

R (

m2.d

ay.g

-1)

Figure 5.29: a) FTIR spectra of a 800 nm thick PHPS film, cured by an exposure to damp

heat @ 65oC/85% RH for the times specified in the figure. The corresponding WVTR values

are given next to the spectra. b) FTIR peak ratios (1050 cm-1/(830 cm-1, open circles) of

Figure 5.29a, correlated with the corresponding WVTR values (full squares) at different

curing times.


109

5.2.5 Optimization of the PHPS conversion rate

PHPS (20 wt%) in di-butyl ether when coated by blading at a gap of 50 µm and speed of

10 mm. sec-1, yields a relatively thick films of about 2-3 µm. At this thickness, curing of

PHPS takes relatively long. Therefore, to get optimum thickness, for which the curing time

matches with the roll-to-roll processing, different parameters were varied and the results

are shown in Table 15. An amount of 70 µl of PHPS was coated on PET substrate at

different coating speeds. The blade gap was always maintained at 50 µm. It is observed that

the curing time depends on the final thickness of the film. The thicker the films, the longer

it will take to cure them (the time mentioned in the table is the time when Si-H peak has

vanished completely in the FTIR spectra).

Table 15: Parameters for coating 70 µl PHPS on PET substrate

Coating speed

(mm.s-1)

Dilution

(ratio)

Wet layer

thickness

(µm)

Final

thickness

(nm)

Curing method Curing

time

(min)

1 mm/s 1:6 50 ~ 70 DUV+Temp 1-2

1 mm/s 1:5 50 ~ 100 DUV+Temp 2-3

1 mm/s 1:1 50 ~ 500 DUV+Temp ~10-20

5 mm/s 1:1 50 ~ 700 DUV+Temp ~25-35

10 mm/s 1:1 50 ~ 1200 DUV+Temp >50

15 mm/s 1:1 50 ~ 1500 DUV+Temp >60

20 mm/s 1:1 50 ~ 1600 DUV+Temp >80

30 mm/s 1:1 50 ~ 2500 DUV+Temp >100

As the PHPS layers get thicker, they need more time to be cured completely, due to the low

diffusion rate of the UV-generated oxygen atoms through the upper parts of the PHPS films,

especially after conversion to SiO2 [21]. However, in roll-to-roll coating of barrier films

minimizing the time for curing PHPS is essential. Therefore, we attempt to reduce the

curing time by creating stacks of ultrathin layers. Figure 5.30 shows the FTIR spectra of

two different 400 nm thick PHPS layers which were both cured for a total of 12 min. The

first film (red data) was coated in a single step and subsequently cured by deep UV light

for 12 min. The second film (black data) represents a stack of four subsequently coated thin

PHPS layers on top of each other, each individual layer being 100 nm thick and cured for

three minutes before coating the following one. After 12 min of curing, the film prepared

in a single curing step still contains unreacted PHPS, as obvious from the peak ratio of


110

I(1050 cm-1)/I(830 cm-1) ≈ 1.5. In contrast, the 4-layer stack displays a peak ratio of I(1050

cm-1)/I(830 cm-1) ≈ 2, which indicates that the layer is almost completely transformed.

The values of WVTR of the single layer film and the multilayer stack after 12 min of curing

are 0.15 g m-2 day-1 and < 0.02 g m-2 day-1 (the limit of our measurement setup),

respectively, in accordance with the observed FTIR peak ratios. After 20 min of curing,

also the single layer film reaches an FTIR peak ratio of I(1050 cm-1)/I(830 cm-1) ≈ 2 and a

WVTR of < 0.02 g m-2day-1, demonstrating that in principle also thick PHPS layers can be

fully converted, albeit at much longer reaction times, due to the much lower diffusion

coefficients of the UV generated oxygen atoms at later stages [165].

Figure 5.30. FTIR spectra of a 400 nm thick PHPS film (red curve) and of a stack of four

100 nm thick PHPS layers (black curve), both cured with deep UV light for a total of 12

minutes. (Published in reference [184] and reproduced with permission from John Wiley

and Sons).

40080012001600200024002800320036004000

0.15 g.m-2.day

-1

Wave number (cm-1)

1 thick coat

4 thin coats

<0.02 g.m-2.day

-1

~100 nm ~100 nm

~100 nm

~ 400 nm

~100 nm


111

5.2.6 Hydrophobic nature of the PHPS film:

PHPS as coated on PET substrate in uncured form shows a slightly hydrophobic nature wih

the contact angle of 92.8o but somehow looses it hydrophobic nature when cured with deep

UV but still maintains slightl hydrophocity 88.3o (Figure 5.31). This hydrophic nature of

the PHPS is also favourable for its application as encapsulation of OSCs.

a)

c)

b)

d)

Figure 5.31: Hydrophobic nature of the PHPS, a) droplets of water on un-cured PHPS

surface, b) contact angle (C.A) of 92.8o on un-cured PHPS, c) C.A of 88.3o of PHPS after

curing and d) contact angle of PET substrate 80o.

5.2.7 Flexibility / bendability of PHPS-based barriers

For the encapsulation of flexible devices, barrier materials also need to be flexible without

being damaged. However, due to their chemical nature, fully cured PHPS films are brittle.

Thus, increasing the thickness of the PHPS layers, on one hand, reduces the initial WVTR,

but on the other hand leads to rapid damage of the films in bending tests. Figure 5.33 shows

the comparison of the WVTR values of PHPS layers of different thicknesses with

increasing number of bending cycles. Films with thicknesses of 270 nm gradually lose their

barrier properties, which after 150 bending cycles finally leads to worse values than for

PHPS (Before curing)

C.A = 92.8o

C.A = 80o

PHPS (after curing)

C.A = 88.3o

PET


112

films of only 170 nm. Calcium tests demonstrate that this is due to tensile cracks which

form perpendicularly to the curvature of the substrate (Figure 5.32).

Figure 5.32: Optical calcium test of PHPS barrier (270 nm) on PET film (d = 125 µm)

after 50 bending cycles (bending radius of 3 cm). The PET/PHPS barrier film has been

laminated (using UV curable adhesive) on a calcium film of 200 nm in thickness,

evaporated on a glass slide. Testing conditions: 65oC / 85 %RH. Images taken after a) 1 h,

b) 2 h, c) 5 h, d) 8 h and e) 10 h (Published in reference [184] and reproduced with

permission from John Wiley and Sons).

Figure 5.33. Reciprocal WVTR vs. number of bending cycles for stacks of different

numbers of PHPS layers on PET film ( d = 125 µm) at a bending radius of 3 cm

(Published in reference [184] and reproduced with permission from John Wiley and

Sons)..

0 25 50 75 100 125 150

0

1

2

3

4

5

6

1/W

VT

R (

m2.d

ay/g

)


70 nm

170 nm

270 nm

un-coated PET


113

It has been shown before that the barrier properties of PHPS films can be enhanced by

building multilayer stacks of alternating layers of PHPS and organic polymers [21], [182].

In order to make use of this effect for enhancing the bendability of the barrier films, we

deposited multilayer stacks of two thin PHPS layers (ca. 170 nm) alternating with two

polymer films of around 5 – 10 µm thickness on PET substrates (see inset of Figure 5.36).

As obvious from Figure 5.33, cured PHPS films of ~170 nm in thickness should hardly be

affected by bending. For the deposition of the organic interlayers, different polymer

formulations were tested such as a) solutions of the polymers polyvinyl alcohol (PVA),

polyvinylidene fluoride (PVDF), and polyvinyl butyral (PVB) as well as b) solvent-free

epoxy and acrylic adhesives. The adhesives were investigated because of their enhanced

adhesion to the PHPS films with respect to the non-crosslinked polymer films, in order to

explore the possibility of reduced delamination during the bending tests. In addition, the

acrylic adhesive formulation acts as a UV filter (Figure 5.34), which otherwise would have

to be added as an extra layer to the packaging of devices. The resulting initial WVTR values

are 0.14 g m-2day-1, 0.16 g m-2day-1, 0.12 g m-2day-1, 0.10 g m-2day-1and 0.11 g m-2day-1 for

PVA, PVB, PVDF, epoxy adhesive and acrylic adhesive, respectively. The oxygen

transmission rates (OTR) of these multilayer stacks are below the detection limit of our

setup of 0.01 cm3 m-2 day-1 bar-1. It is interesting to note that the WVTR of two 170 nm

thick PHPS films on top of each other, i.e., without an intermediate polymer layer, is about

five times higher, i.e., 0.6 g m-2 d-1 than in the case with the polymer interlayer. This

difference cannot be explained by the additional barrier effect of the polymer layers, as

these showed no measurable reduction of the WVTR when coated on top of PET films,

which is also in accordance with the observation that the WVTR values of the multilayer

stacks are approximately the same for all polymer interlayers. The beneficial effect of the

intermediate polymer layer is either caused by the decoupling of defects in the two PHPS

layers, or, more probably, by their planarization effect, which enables a more defect free

growth of the top PHPS layer [19].

Bending tests were performed on PHPS/interlayer sandwich barriers deposited on PET

substrate with overall sample thickness 140d m. The details of the failure mechanism

is described in Channa et al.[184].


114

Figure 5.34: Ttransmission spectra of PET film and of PHPS/Acrylic/PHPS barrier coated

on PET film

Two types of behavior were observed (Figure 5.36). For all polymer interlayers deposited

from solution and for the UV cured acrylic adhesive, the initial WVTR values remain

almost constant during the first 1000 bending cycles and increase by only around 20% even

after 3000 cycles.

The same behavior is also observed for non-crosslinked polymers (PVDF and PVB)

interlayers, where the barrier films remain stable for two hundred bending cycles (Figure

5.35). In contrast, for the epoxy based adhesive, an increase of the WVTR by 60% after

3000 bending cycles is observed.

400 600 800

0

20

40

60

80

100T

ransm

issio

n (

%)

Wavelength (nm)

PET

PHPS/Acrylic/PHPS on PET


115

Figure 5.35: Normalized reciprocal WVTR of sandwich coatings with alternating PHPS

and organic layers (see inset) vs. the number of bending cycles with a bending radius of 3

cm. Each PHPS layer has a thickness of ~170 nm, while the organic layers are ~ 5-10 um

thick. For the organic interlayers, two non-cross linked polymers, namely PVDF (black

squares) and PVB (red circles) were used. (Published in reference [184] and reproduced

with permission from John Wiley and Sons).

We interpret the observed behavior in terms of internal imperfections within the layers,

which may propagate to form tensile cracks that deteriorate the WVTR. The critical size of

such flaws can be estimated from fracture mechanics considerations which relate tensile

stress , critical flaw size a , and fracture toughness IcK . We use the relation with

Ic 0.30K = MPa m1/2 for the fatigue threshold of silica [219], [220]. With a characteristic

fracture process zone size 0 200a nm as deduced from fracture surface observations on

silica [221], we arrive at a characteristic critical flaw size a = 0.9 μm for a characteristic

tensile stress of 160 MPa. Using the idea that the characteristic size of fabrication induced

flaws is of the order of the layer thickness (i.e., thinner layers contain smaller flaws and are


116

therefore mechanically more stable) this explains why single layers with thicknesses well

below 1 μm become flaw insensitive as seen in Figure 5.33.

Turning to multilayer structures, the main effect of the interlayers, which are both

elastically softer and much thicker than the PHPS layers, consists in mechanically

decoupling the cured PHPS layers. The PHPS layers now essentially deform in parallel and

flaws in different layers do not appreciably interact. Accordingly, the flaw insensitivity of

the single layers shown in Figure 5.33 transfers to the multilayer architecture and the films

maintain their barrier quality during mechanical cycling. The exception are epoxy

interlayers, which because of the brittle nature of the epoxy may themselves be prone to

fracture during cyclic loading and then induce failure of the PHPS barrier layers.

0 500 1000 1500 2000 2500 30000

2

4

6

8

10

1/W

VT

R (

m2day

1/g

)


Epoxy adhesive

Acrylic adhesive

PHPSorganic layer

PHPS

organic layer

Figure 5.36: Reciprocal WVTR of sandwich coatings with alternating PHPS and organic

layers (see inset) vs. the number of bending cycles with a bending radius of 3 cm. Each

PHPS layer has a thickness of ~170 nm, while the organic layers are ~ 5-10 µm thick. For

the organic interlayers, two UV-curable adhesives on epoxy (black squares) and acrylic

basis (red circles) were used (Published in reference [184] and reproduced with

permission from John Wiley and Sons).


117

5.2.8 Protection of organic electronic devices by PHPS-based barriers

In order to assess the quality of the barriers described above under real conditions, the

protection of organic solar cells by PHPS/polymer/PHPS sandwich layers on PET against

photo-oxidation and damp heat was evaluated.

First, the protection of P3HT films against photo-oxidation by encapsulation in

PHPS/polymer/PHPS sandwich films was measured and compared with commercially

available barriers. For this purpose, P3HT was coated on glass substrates and subsequently

encapsulated with different barrier foils under nitrogen atmosphere. P3HT was chosen as

the probe material because of its well-characterized behavior towards photo-oxidation[85].

For encapsulation, a PHPS-based barrier film on PET (PET/170 nm PHPS/5 µm acrylic

adhesive/170 nm PHPS/), a glass slide, a Mitsubishi barrier film, or a plain PET film were

rim-encapsulated on top of the glass substrate using a UV-cured epoxy glue in a way that

the adhesive did not cover the P3HT layer (Figure 5.37).

Figure 5.37: Schematic view of encapsulation of P3HT films on glass, rim encapsulated

with PHPS based sandwich barrier on PET film. (Published in reference [184] and

reproduced with permission from John Wiley and Sons).

The samples were illuminated in ambient atmosphere with the light of a sun simulator from

the glass substrate side. The rate of photo-oxidation was quantified by recording UV/Vis

spectra at different time intervals and analyzing the loss of absorbance of the P3HT film at

the wavelength of 525 nm (Figure 5.38).

Adhesive

Glass

AdhesiveP3HT

PET

PHPS

Acrylic

PHPS


118

a)

b)

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8P3HT Encapsulated with

PHPS/Acrylic/PHPS

Abs.

Wavelength (nm)

c)

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

Abs.

Wavelength (nm)

Encapsulated with Mitsubishi

d)

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8Encapsulated with glass

Abs.

Wavelength (nm) e)

PET

Mitsubishi

Glass

PHPS/Acrylic/PHPS

0 300 600 900

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d A

bso

rba

nce

@ 5

25

nm

Time (h)

Figure 5.38: UV/vis spectra of P3HT films on glass encapsulated with a) a plain PET,

b) a PHPS based barrier, c) Mitsubishi ( a commercial barrier) and d) glass during

exposure to the light of a sun simulator in ambient air at 65oC. The increase of absorption

at 400 nm in (b) and (c) is due to the yellowing of the PET substrates. e) Normalized

absorbance loss at 525 nm of P3HT films encapsulated with plain PET, PHPS based

barrier, Mitisubishi and glass on glass. (Published in reference [184] and reproduced

with permission from John Wiley and Sons).

400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8A

bs.

Wavelength (nm)

Sun irradiation

T= 504 h

P3HT encapsulated with PET


119

As can be seen from Figure 5.38, that the P3HT film encapsulated with plain PET lost 60%

of its initial optical density (OD) within the first 600 h, as expected from the poor barrier

properties of PET. Samples encapsulated with glass slides or Mitsubishi barrier foils did

not show any degradation over the duration of the experiment of 960 hours. P3HT protected

by the PHPS based barrier lost less than 1% of its initial optical density during the time

span of 960 h. From the bleaching kinetics, the oxygen permeability of the barriers can be

estimated from equations Eq. 52, Eq. 53 and Eq. 54[90], [222]. Assuming the consumption

of five moles of molecular oxygen per mole thiophene rings bleached, this drop in

absorbance corresponds to OTR values of ≈ 8 cm3 m-2 day-1 bar-1 for the plain PET film

and of less than 0.1 cm3 m-2 day-1 bar-1 for the PHPS/polymer/PHPS sandwich film, which

is in accordance with the measurements performed with the commercial OTR device.

Damp heat tests of the PHPS/polymer/PHPS sandwich barrier are conducted on

encapsulated P3HT:PCBM based organic solar cells (OSCs) of inverted architecture,

whose degradation behavior is the most studied one of all organic solar cells and thus well

understood [84]. Standard glass substrates, each having six P3HT:PCBM-based solar cells

with effective areas of 0.1 cm2 each (as discussed in Experimental section Figure 4.1), were

encapsulated using an adhesive and three different barrier foils, namely plain PET,

PHPS/acrylic/PHPS/acrylic films on PET, and a Mitsubishi barrier foil as shown in Figure

5.39.

Figure 5.39: Schematic view of encapsulation of P3HT:PCBM based solar cell on glass,

rim encapsulated with PHPS based sandwich barrier on PET film.

Glass

Organic solar cell

PHPSAcrylicPHPS

Acrylic

Adhesive Adhesive

PET


120

After lamination, the devices showed efficiencies around 3%. They were placed in a damp

heat chamber at 40oC/85% RH. For measuring the current density vs. voltage

characteristics, the samples were taken out of the damp heat chamber and placed under a

sun simulator in ambient atmosphere. As shown in Figure 5.40, the devices encapsulated

with plain PET died within less than 50 hours of exposure to damp heat due to the ingress

of water and oxygen. On the other hand, the devices encapsulated with PHPS-based barrier

foils and Mitsubishi barrier foils did not show any significant degradation and remained

stable for the period of testing.

a) b)

0 50 100 150 200 250 3000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

PC

E (

%)

Time (h)

PHPS

Mitsubishi

PET

0 50 100 150 200 250 3000

10

20

30

40

50

60

70

FF

(%

)

Time (h)

PHPS

Mitsubishi

PET

c) d)

PHPS

Mitsubishi

PET

0 50 100 150 200 250 3000

2

4

6

8

10

Jsc (m

A/c

m2)

Time (h)

PHPS

Mitsubishi

PET

0 50 100 150 200 250 3000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Voc (

V)

Time (h)

Figure 5.40: Efficiency of P3HT:PCBM based organic solar cells encapsulated with plain

PET foils (PET), commercial Mitsubishi barrier foils (Mitsubishi), or PHPS-based barrier

foils (PHPS, for details see text) upon exposure to damp heat (40 °C/85% RH). (Published

in reference [184] and reproduced with permission from John Wiley and Sons).


121

5.2.9 Encapsulation of OSCs by direct deposition of PHPS

After the optimization of PHPS-based barrier layers on PET foils and their successful

testing as encapsulation for OSC devices, PHPS-based barrier layers with a thickness of

170 nm were coated directly on P3HT:PCBM-based solar cells. Two different top electrode

thickness have been used i.e. 50 nm and 100 nm. This is just to avoid the diffusion of PHPS

solvent and possible harm to OSC by deep UV. One, two and three layers on top of each

other were coated by doctor blading on top of the OSCs at room temperature and

subsequently curing each layer by simultaneous exposure to deep UV light (172 nm) and

heat (T = 100 °C) for 3 min. The performance of devices before coating, after coating and

curing is shown in Figure 5.41.

--

50 nm -- --

100 nm --

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

PC

E [

%]

initial

coated

cured

--

50 nm -- --

100 nm --0

10

20

30

40

50

60

70

FF

[%

]

--

50 nm -- --

100 nm --0.0

0.1

0.2

0.3

0.4

0.5

0.6

Vo

c [

V]

--

50 nm -- --

100 nm --0

2

4

6

8

10

12

Jsc [

mA

/cm

²]

--

50 nm -- --

100 nm --0

50

100

150

200

250

Lig

ht

Inje

ctio

n @

1.0

V [

mA

/cm

²]

--

50 nm -- --

100 nm --1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

Le

aka

ge

[A

/cm

²]

Figure 5.41: Device performance of the OSC devices with evaporated silver electrode of

thickness 50 nm and 100 nm, and each device directly coated with one, two and three PHPS

layers each having thickness of 170 nm, showing performance in terms of PCE, FF, Voc,

Jsc,light injection and Leakage of initial (grey), after coating (red) and after curing (green).


122

Both OSC devices with 100 nm and 50 nm thick top silver electrode survived the process,

and no degradation in terms of efficiency is observed hence; neither detrimental

interactions between the PHPS solution and the OSC nor any damage by deep UV light

were observed (Figure 5.41). Therefore, for further experiments, top electrode with 50 nm

thickness is used, because at this thickness silver does not exhibit a significant gas barrier

effect but protects the solar cell from the deep UV light during the curing of PHPS. The

encapsulated devices were subsequently exposed to accelerated lifetime tests. In damp heat

at 40 °C/85% RH, the devices fail rapidly, due to the formation of cracks in the barrier film

and subsequent delamination and ingress of water and oxygen (Figure 5.42).

a)

b)

Figure 5.42: Organic solar cells directly coated with 170 nm PHPS a) before, b) after a

few hours of exposure to damp heat (40oC/ 85%RH) [186].

In order to investigate why the barrier failed catastrophically, OSC structure without top

silver electrode were coated with PHPS coatings (1 layer ~200 nm thickness) with and

without curing and were subsequently positioned in damp heat.

Two different behaviors were observed, when PHPS is cured completely and is place in

damp heat 65oC/ 85 RH, the layer faces catastrophic failures and delamination occurs

(Figure 5.43a,b), as a consequence of the brittleness of the fully cured PHPS films, even

minor stress at the interface to the substrate, due to different thermal expansion coefficients

or due to uptake of water into one of the layers, most probably the PEDOT:PSS layer, and

subsequent volume expansion, leads to delamination of the films. The second behavior is

when PHPS is not cured, spiral cracks are observed, which indicates a possible reaction

between PHPS and PDOT.PSS. This is because PDOT.PSS is of acidic nature and can react

with PHPS, as a result cracks appear which subsequently lead to delamination Figure 5.43

(c,d).


123

Therefore, in order to prevent such catastrophic failures, the use of soft interlayer between

PHPS and OSC is inevitable. The soft interlayer will not only act as a separator but also

compensate the possible volume expansion of PDOT.PSS and will prevent cracking of

PHPS cured layer.

a) b)

c)

d)

Figure 5.43: Cracking in the PHPS layers coated on OSC device without top silver

electrode, after exposure to damp heat 65oC/ 85 RH, a) device coated with cured PHPS

on top, b) micrograph of the device shown highlighted section of (a), shwoing

delaminated areas, c) device coated with un-cured PHPS and d) micrograph of

highlighted section of (c) showing spiral crack..


124

5.2.10 Intermediate layer of ZnO to avoid delamination

In order to decouple the PHPS barrier film from the substrate, intermediate layers have to

be inserted between substrate and barrier layer, which are able to compensate the

mechanical stress. We chose to test two different materials for this purpose, namely ZnO

nanoparticles and a UV-curable acrylic adhesive. Both materials serve the purpose of

decoupling the PHPS barrier and the substrate, ZnO due to its nanoparticulate nature and

the acrylic adhesive due to its elasticity. In addition, both materials provide additional UV

protection during PHPS curing. The layers were deposited on top of the OSC, and cured

subsequently. In the case of ZnO, the 100 nm thick layer was cured by annealing at 120 °C

for 1 min. In the case of the acrylic adhesive, the 5 µm thick layer was cured by exposure

to UV light in inert atmosphere. Subsequently, the PHPS solution was doctor bladed on top

of the intermediate layers and cured by exposure to deep UV light as shown in Figure 5.44.

Figure 5.44: Schematic diagram of an organic solar cell (OSC) on glass, directly coated

with two PHPS layers (170 nm each), alternating with three layers of Rolic (~ 5 µm each)

along with ZnO (100 nm) as a separating layer[186].

OSCs packaged in this way do not show any signs of cracking after exposure to damp heat

as shown in Figure 5.45. Consequently, the barrier remains stable and does not crack, which

makes it ideally suited for encapsulating OSC with direct coating. Hence, directly coated

devices are subjected to lifetime testing.

Glass substrate

Acrylic

Organic solar cell

PHPS

Acryic

PHPS

Acrylic

ZnO


125

Figure 5.45: Organic solar cells directly coated with two PHPS layers (170 nm each),

alternating with three layers of Rolic (~ 5 µm each) along with ZnO (100 nm) as a

separating layer after a few hours of exposure to damp heat (40oC/ 85%RH) [186].

5.2.11 Lifetime tests

Directly coated devices were subjected to lifetime tests under damp heat 40oC/ 85 RH and

sun test and performance was monitored with time intervals. Normalized power conversion

efficiency (PCE) of the test devices are shown in Figure 5.46. During ~700 hours of

exposure to damp heat 65oC/ 85 RH, the initial power conversion efficiency (PCE) of 3%

drops by 20 %. This drop is almost exclusively due to a loss in jsc (see inset of Figure 5.46),

which suggests that the performance loss is mainly caused by ingress of oxygen and

subsequent doping of the active layer [90], rather than by delamination of the PEDOT:PSS

layer after water ingress.

Under constant illumination with 1 sun at 65 °C block body temperature in ambient air the

PHPS-coated devices lose around 29% of the initial performance within 350 hours. The

degradation of the latter is again mainly due to loss of jsc, which indicates that also under

these conditions diffusion of oxygen through the coating and subsequent doping of the

active layer is the culprit. Since the degradation effect caused by oxygen is much more

detrimental in the presence of light [6], more performance loss is observed compared to the

respective devices exposed to damp heat in the dark. The reason for the diffusion of oxygen


126

is the formation of the cracks in the barrier despite of ZnO layer. The oxygen ingresses

through the cracked areas and reacted with the active layer which resulted in loss of Jsc and

subsequently losing PCE.

0 100 200 300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0N

orm

alized J

sc

Time (h)

Damp heat

Sun test

0 100 200 300 400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

No

rma

lized J

sc

Time (h)

Damp heat

Sun test

No

rma

lize

d P

CE

Time (h)

Damp heat

Sun test

Figure 5.46: Time traces of normalized power conversion efficiency (PCE) of

P3HT:PCBM based OSCs which have been directly coated with a barrier film stack,

consisting of 2 PHPS layers (thickness ~200 nm each), alternating with 3 interlayers of

acrylic adhesive (each ~ 5 µm thick), subjected to accelerated lifetime testing under

damp heat (40°C, 85% RH) and sun soaking at 65 °C. Inset shows the normalized Jsc of

the same devices(Published in reference [184] and reproduced with permission from

John Wiley and Sons).

5.2.12 Investigation on device failure in sun test:

For further investigations, the degradation devices were analyzed by optical microscope.

Visible cracks are observed in the device as shown in Figure 5.47. These defects served as

the channels for the diffusion of oxygen and thus, the devices degraded


127

a)

b)

c)

Figure 5.47: Optical micrographs of cracks in the sun tested device, a) bright field image,

b) dark field image and c) bottom illuminated image.

5.2.13 Conclusion

It is demonstrated that the flexible organic electronic devices can be encapsulated without

efficiency losses by direct coating of PHPS on top of the devices with roll-to-roll

compatible methods and subsequent conversion to SiO2 by deep UV light treatment in

ambient air. The devices encapsulated with such barriers exhibited stable performance for

several hundred hours in accelerated lifetime tests. The quality of the barrier films was

hardly affected by several thousands of bending cycles as long as the thickness of individual

PHPS layers was below 200 nm. Maximum flexibility along with optimum barrier

properties was achieved by fabricating multilayer stacks of alternating ultrathin PHPS

layers and thin interlayers of organic polymers. The polymer interlayers compensate for the

shear stress upon bending and at the same time act as planarization layers, which enhances

the quality of the PHPS barrier films. Mechanical decoupling of the directly coated barriers

from the device surface by flexible interlayers is shown to avoid cracking of the cured

PHPS layers in accelerated lifetime tests caused by the thermal expansion of the device. A

fast, robust, and quantitative endpoint control of the PHPS curing process based on FTIR

peak ratios has been established.

The direct coating of PHPS-based barrier films is ideally suited for temporary protection

of organic electronics, e.g., for storage or transport before integration into the final product,

such as insulating glass windows or other façade elements. The bill of materials, based on

present prices for small scale orders, of a directly coated barrier of the structure shown in

Figure 5.44 is around 40 €/m2, and thus in the same range as the cost of encapsulation with

medium quality barrier films. This makes full in-line encapsulation of electronic devices


128

by roll-to-roll printing/coating methods possible, even of 3D objects, and thus

revolutionizes the backend processing of printed PV.

CONCLUSION

129

CONCLUSION

In this thesis it is shown that coated barriers are a viable alternative to the encapsulation of

flexible opto-electronic devices into barrier films prepared by vacuum assisted methods.

Moreover, direct coating of devices instead of lamination has been demonstrated.

Following Fick’s 1st law of diffusion as a guideline, two approaches towards coatable

barriers have been chosen, namely enhancing tortuosity by filling glass flakes into PVB

films and reducing accessible area by silica coatings obtained by UV curing of

perhydropolysilazanes. Both methods provide good barrier quality at high transparency and

good flexibility. Glass flakes based barriers proved to be intrinsically flexible, whereas the

brittleness of PHPS based barriers had to be accounted for by a PHPS/organic polymer

multilayer approach.

Both, PVB/glass flakes and PHPS/organic polymer based barrier systems could be

successfully applied for the life time enhancement of flexible organic solar cells under

accelerated testing conditions.

Both encapsulation techniques have been optimized with respect to throughput in view of

application in R2R manufacturing. This makes full in-line encapsulation of electronic

devices by R2R printing/coating methods possible, even of 3D objects, and thus

revolutionizes the backend processing of printed PV.

BIBLOGRAPHY

130

BIBLOGRAPHY

[1] L. Wang et al., “Enhanced moisture barrier performance for ALD-encapsulated

OLEDs by introducing an organic protective layer,” J. Mater. Chem. C, vol. 5, no.

16, pp. 4017–4024, 2017.

[2] S. Kim et al., “Degradation of blue-phosphorescent organic light-emitting devices

involves exciton-induced generation of polaron pair within emitting layers,” Nat.

Commun., vol. 9, no. 1, pp. 1–11, 2018.

[3] G. Sherafatipour et al., “Degradation pathways in standard and inverted DBP-C 70

based organic solar cells,” Sci. Rep., vol. 9, no. 1, pp. 1–11, 2019.

[4] S. K. Gupta, S. Banerjee, A. Singh, L. S. Pali, and A. Garg, “Modeling of

degradation in normal and inverted OSC devices,” Sol. Energy Mater. Sol. Cells,

vol. 191, pp. 329–338, 2019.

[5] Z. Shan, J. Shi, W. Xu, C. Li, and H. Wang, “Organic field-effect transistors based

on biselenophene derivatives as active layers,” Dye. Pigment., vol. 171, p. 107675,

2019.

[6] A. Seemann, H.-J. Egelhaaf, C. J. Brabec, and J. A. Hauch, “Influence of oxygen on

semi-transparent organic solar cells with gas permeable electrodes,” Org. Electron.,

vol. 10, no. 8, pp. 1424–1428, Dec. 2009.

[7] P. Maisch et al., “Inkjet printing of semitransparent electrodes for photovoltaic

applications,” Org. Photovoltaics XVII, vol. 9942, p. 99420R, 2016.

[8] Y. Wang, X. Ke, Z. Xiao, L. Ding, R. Xia, and H. Yip, “Organic and solution-

processed tandem solar cells with 17.3% efficiency,” Science (80-. )., vol. 1098, no.

September, pp. 1094–1098, 2018.

[9] L. Lucera et al., “Guidelines for Closing the Efficiency Gap between Hero Solar

Cells and Roll-To-Roll Printed Modules,” Energy Technol., vol. 3, no. 4, pp. 373–

384, Apr. 2015.

[10] C. N. Hoth, P. Schilinsky, S. A. Choulis, S. Balasubramanian, and C. J. Brabec,

“Solution-Processed Organic Photovoltaics,” in Applications of Organic and

Printed Electronics, E. Cantatore, Ed. Boston, MA: Springer US, 2013, pp. 27–56.

[11] J. A. Hauch, P. Schilinsky, S. A. Choulis, S. Rajoelson, and C. J. Brabec, “The

impact of water vapor transmission rate on the lifetime of flexible polymer solar

cells,” Appl. Phys. Lett., vol. 93, no. 10, pp. 2006–2009, 2008.

[12] C. S. Wu, J. Y. Liao, S. Y. Fang, and A. S. T. Chiang, “Flexible and transparent

BIBLOGRAPHY

131

moisture getter film containing zeolite,” Adsorption, vol. 16, no. 1–2, pp. 69–74,

2010.

[13] P. E. Burrows et al., “Ultra barrier flexible substrates for flat panel displays,”

Displays, vol. 22, no. 2, pp. 65–69, 2001.

[14] S. Logothetidis et al., “Ultra high barrier materials for encapsulation of flexible

organic electronics,” Eur. Phys. J. Appl. Phys., vol. 51, no. 3, p. 33203, 2010.

[15] J. Fahlteich et al., “29.1: Ultra-High Barriers for Encapsulation of Flexible Displays

and Lighting Devices,” in SID Symposium Digest of Technical Papers, 2013, vol.

44, pp. 354–357.

[16] J. Ahmad, K. Bazaka, L. J. Anderson, R. D. White, and M. V. Jacob, “Materials and

methods for encapsulation of OPV: A review,” Renew. Sustain. Energy Rev., vol.

27, pp. 104–117, Nov. 2013.

[17] W. Huang et al., “Low temperature PECVD SiNx films applied in OLED

packaging,” Mater. Sci. Eng. B, vol. 98, no. 3, pp. 248–254, Apr. 2003.

[18] A. A. Dameron, S. D. Davidson, B. B. Burton, P. F. Carcia, R. S. McLean, and S.

M. George, “Gas Diffusion Barriers on Polymers Using Multilayers Fabricated by

Al2O3 and Rapid SiO2 Atomic Layer Deposition,” J. Phys. Chem. C, vol. 112, no.

12, pp. 4573–4580, 2008.

[19] A. Morlier, S. Cros, J. P. Garandet, and N. Alberola, “Gas barrier properties of

solution processed composite multilayer structures for organic solar cells

encapsulation,” Sol. Energy Mater. Sol. Cells, vol. 115, pp. 93–99, 2013.

[20] E. S. Tsurko et al., “Large Scale Self-Assembly of Smectic Nanocomposite Films

by Doctor Blading versus Spray Coating: Impact of Crystal Quality on Barrier

Properties,” Macromolecules, vol. 50, no. 11, pp. 4344–4350, 2017.

[21] L. Blankenburg and M. Schrödner, “Perhydropolysilazane derived silica for flexible

transparent barrier foils using a reel-to-reel wet coating technique: Single- and

multilayer structures,” Surf. Coatings Technol., vol. 275, pp. 193–206, Aug. 2015.

[22] C. Lungenschmied, G. Dennler, G. Czeremuszkin, M. Eche, H. Neugebauer, and N.

Serdar Sariciftci, Flexible encapsulation for organic solar cells, vol. 6197. 2006.

[23] P. J. Brewer, B. A. Goody, Y. Kumar, and M. J. T. Milton, “Accurate measurements

of water vapor transmission through high-performance barrier layers,” Rev. Sci.

Instrum., vol. 83, no. 7, 2012.

[24] V. Siracusa, “Food packaging permeability behaviour: A report,” Int. J. Polym. Sci.,

vol. 2012, no. i, 2012.

BIBLOGRAPHY

132

[25] K. Cooksey, “Food packaging, principles and practices, 2nd ed., by Gordon

Robertson. CRC, Taylor and Francis: Boca Raton, FL, 2006. ISBN 0-8493-3775-5:

550 pages.,” Packag. Technol. Sci., vol. 21, no. 1, pp. 57–59, Jan. 2008.

[26] D. S. Lee, K. Yam, and L. Piergiovanni, Food packaging science and technology.

2008.

[27] S. Zeman and L. ’ubomír Kubík, “Permeability of Polymeric Packaging Materials,”

Tech. Sci., vol. 10, no. 1, pp. 33–34, Nov. 2007.

[28] J. S. McBride, T. A. Massaro, and S. L. Cooper, “Diffusion of gases through

polyurethane block polymers,” J. Appl. Polym. Sci., vol. 23, no. 1, pp. 201–214, Jan.

1979.

[29] T. Dependence, R. Temperature, and S. Mahajan, “Arrhenius-Type Equation Learn

more about Arrhenius-Type Equation Deformation Twinning Advances in

polymeric materials for modified atmosphere packaging ( MAP ),” 2002.

[30] B. Flaconneche, J. Martin, and M. H. Klopffer, “Permeability, Diffusion and

Solubility of Gases in Polyethylene, Polyamide 11 and Poly (Vinylidene Fluoride),”

Oil Gas Sci. Technol., vol. 56, no. 3, pp. 261–278, 2001.

[31] C. Mo, W. Yuan, W. Lei, and Y. Shijiu, “Effects of Temperature and Humidity on

the Barrier Properties of Biaxially-oriented Polypropylene and Polyvinyl Alcohol

Films,” J. Appl. Packag. Res., vol. 6, no. 1, pp. 40–46, 2014.

[32] Bedane, “Mass transfer of water vapor, carbon dioxide and oxygen on modified

cellulose fiber-based materials,” Nord. Pulp Pap. Res. J., vol. 27, no. 02, pp. 409–

417, 2012.

[33] S. A. Stern, S.-M. Fang, and H. L. Frisch, “Effect of pressure on gas permeability

coefficients. A new application of ‘free volume’ theory,” J. Polym. Sci. Part A-2

Polym. Phys., vol. 10, no. 2, pp. 201–219, 1972.

[34] “No Title 补充材料,” no. c, pp. 2–6.

[35] V. V. Krongauz, “Diffusion in polymers dependence on crosslink density: Eyring

approach to mechanism,” J. Therm. Anal. Calorim., vol. 102, no. 2, pp. 435–445,

2010.

[36] “Permeability, Diffusivity, and Solubility of Gas and Solute Through Polymers

Introduction,” no. 1, pp. 29–99, 1866.

[37] H. Feng, “Modeling of vapor sorption in glassy polymers using a new dual mode

sorption model based on multilayer sorption theory,” Polymer (Guildf)., vol. 48, no.

BIBLOGRAPHY

133

10, pp. 2988–3002, 2007.

[38] S. Logothetidis et al., “Ultra high barrier materials for encapsulation of flexible

organic electronics,” Eur. Phys. Journal-Applied Phys., vol. 51, no. 3, 2010.

[39] O. Miesbauer, M. Pfrogner, C. Steiner, J. Fahlteich, and K. Noller, “Multilayer

barrier structures on ETFE substrate films.”

[40] A. Sorrentino, M. Tortora, and V. Vittoria, “Diffusion behavior in polymer-clay

nanocomposites,” J. Polym. Sci. Part B Polym. Phys., vol. 44, no. 2, pp. 265–274.

[41] C. Lu and Y.-W. Mai, “Permeability modelling of polymer-layered silicate

nanocomposites,” Compos. Sci. Technol., vol. 67, no. 14, pp. 2895–2902, Nov. 2007.

[42] R. K. Bharadwaj, “Modeling the barrier properties of polymer-layered silicate

nanocomposites,” Macromolecules, vol. 34, no. 26, pp. 9189–9192, 2001.

[43] E. L. Cussler, S. E. Hughes, W. J. Ward, and R. Aris, “Barrier membranes,” J. Memb.

Sci., vol. 38, no. 2, pp. 161–174, 1988.

[44] E. Picard, A. Vermogen, J. Gerard, and E. Espuche, “Barrier properties of nylon 6-

montmorillonite nanocomposite membranes prepared by melt blending: Influence of

the clay content and dispersion stateConsequences on modelling,” J. Memb. Sci.,

vol. 292, no. 1–2, pp. 133–144, Apr. 2007.

[45] B. Tan and N. L. Thomas, “A review of the water barrier properties of polymer/clay

and polymer/graphene nanocomposites,” J. Memb. Sci., vol. 514, pp. 595–612, 2016.

[46] J. P. Cerisuelo, R. Gavara, and P. Hernández-Muñoz, “Diffusion modeling in

polymer–clay nanocomposites for food packaging applications through finite

element analysis of TEM images,” J. Memb. Sci., vol. 482, pp. 92–102, May 2015.

[47] C. Lu and Y.-W. Mai, “Influence of Aspect Ratio on Barrier Properties of Polymer-

Clay Nanocomposites,” Phys. Rev. Lett., vol. 95, no. 8, Aug. 2005.

[48] L. E. Nielsen, “Models for the Permeability of Filled Polymer Systems,” J.

Macromol. Sci. Part A - Chem., vol. 1, no. 5, pp. 929–942, Aug. 1967.

[49] W. A. Wakeham and E. A. Mason, “Diffusion through multiperforate laminae,” Ind.

Eng. Chem. Fundam., vol. 18, no. 4, pp. 301–305, 1979.

[50] R. Aris, “On a problem in hindered diffusion,” Arch. Ration. Mech. Anal., vol. 95,

no. 2, pp. 83–91, 1986.

[51] K. Bhunia, S. Dhawan, and S. S. Sablani, “Modeling the Oxygen Diffusion of

Nanocomposite-based Food Packaging Films,” J. Food Sci., vol. 77, no. 7, pp. N29–

N38, Jul. 2012.

[52] W. R. Falla, M. Mulski, and E. L. Cussler, “Estimating diffusion through flake-filled

BIBLOGRAPHY

134

membranes,” J. Memb. Sci., vol. 119, no. 1, pp. 129–138, 1996.

[53] G. D. Moggridge, N. K. Lape, C. Yang, and E. L. Cussler, “Barrier films using flakes

and reactive additives,” Prog. Org. coatings, vol. 46, no. 4, pp. 231–240, 2003.

[54] W. T. Brydges, S. T. Gulati, and G. Baum, “Permeability of glass ribbon-reinforced

composites,” J. Mater. Sci., vol. 10, no. 12, pp. 2044–2049, 1975.

[55] G. Choudalakis and A. D. Gotsis, “Permeability of polymer/clay nanocomposites: A

review,” Eur. Polym. J., vol. 45, no. 4, pp. 967–984, Apr. 2009.

[56] G. H. Fredrickson and J. Bicerano, “Barrier properties of oriented disk composites,”

J. Chem. Phys., vol. 110, no. 4, pp. 2181–2188, Jan. 1999.

[57] A. A. Gusev and H. R. Lusti, “Rational Design of Nanocomposites for Barrier

Applications,” Adv. Mater., vol. 13, no. 21, pp. 1641–1643, 2001.

[58] M. Minelli, M. G. Baschetti, and F. Doghieri, “Analysis of modeling results for

barrier properties in ordered nanocomposite systems,” J. Memb. Sci., vol. 327, no.

1–2, pp. 208–215, Feb. 2009.

[59] R. D. Maksimov, S. Gaidukov, J. Zicans, and J. Jansons, “Moisture permeability of

a polymer nanocomposite containing unmodified clay,” Mech. Compos. Mater., vol.

44, no. 5, pp. 505–514, 2008.

[60] A. Greco and A. Maffezzoli, “Two-dimensional and three-dimensional simulation

of diffusion in nanocomposite with arbitrarily oriented lamellae,” J. Memb. Sci., vol.

442, pp. 238–244, Sep. 2013.

[61] S. Nazarenko, P. Meneghetti, P. Julmon, B. G. Olson, and S. Qutubuddin, “Gas

barrier of polystyrene montmorillonite clay nanocomposites: Effect of mineral layer

aggregation,” J. Polym. Sci. Part B Polym. Phys., vol. 45, no. 13, pp. 1733–1753.

[62] N. K. Lape, E. E. Nuxoll, and E. L. Cussler, “Polydisperse flakes in barrier films,”

J. Memb. Sci., vol. 236, no. 1–2, pp. 29–37, Jun. 2004.

[63] B. Xu, Q. Zheng, Y. Song, and Y. Shangguan, “Calculating barrier properties of

polymer/clay nanocomposites: Effects of clay layers,” Polymer (Guildf)., vol. 47,

no. 8, pp. 2904–2910.

[64] A. Sorrentino, M. Tortora, and V. Vittoria, “Diffusion behavior in polymer-clay

nanocomposites,” J. Polym. Sci. Part B Polym. Phys., vol. 44, no. 2, pp. 265–274,

Jan. 2006.

[65] K. Müller et al., “Review on the Processing and Properties of Polymer

Nanocomposites and Nanocoatings and Their Applications in the Packaging,

Automotive and Solar Energy Fields,” Nanomaterials, vol. 7, no. 4, p. 74, Mar. 2017.

BIBLOGRAPHY

135

[66] V. M. Drakonakis, A. Savva, M. Kokonou, and S. A. Choulis, “Investigating

electrodes degradation in organic photovoltaics through reverse engineering under

accelerated humidity lifetime conditions,” Sol. Energy Mater. Sol. Cells, vol. 130,

pp. 544–550, 2014.

[67] J. Nelson, “Polymer: Fullerene bulk heterojunction solar cells,” Mater. Today, vol.

14, no. 10, pp. 462–470, 2011.

[68] K. Wang, C. Liu, T. Meng, C. Yi, and X. Gong, “Inverted organic photovoltaic

cells,” Chem. Soc. Rev., vol. 45, no. 10, pp. 2937–2975, 2016.

[69] D. T. Fakult, “Process Development for Inkjet Printing of Organic Photovoltaics

Prozessentwicklung für den Tintenstrahldruck organischer Photovoltaik.”

[70] M. Pope and C. E. Swenberg, “Electronic Processes in Organic Crystals,” Oxford

Univ. Press USA 2nd Ed., p. 455, 1982.

[71] P. Wellmann, Materialien der Elektronik und Energietechnik. 2017.

[72] C. Ma, “Printed Organic Thin Film Solar Cells,” Printed Electronics. pp. 201–250,

02-Jun-2016.

[73] S. Torabi et al., “Strategy for enhancing the dielectric constant of organic

semiconductors without sacrificing charge carrier mobility and solubility,” Adv.

Funct. Mater., vol. 25, no. 1, pp. 150–157, 2015.

[74] L. H. Slooff et al., “Determination of the intrinsic diode parameters of polymer solar

cells,” Energy Procedia, vol. 31, pp. 11–20, 2012.

[75] N. K. Elumalai and A. Uddin, “Open circuit voltage of organic solar cells: An in-

depth review,” Energy Environ. Sci., vol. 9, no. 2, pp. 391–410, 2016.

[76] J. Yan and B. R. Saunders, “Third-generation solar cells: A review and comparison

of polymer:fullerene, hybrid polymer and perovskite solar cells,” RSC Adv., vol. 4,

no. 82, pp. 43286–43314, 2014.

[77] K. Vandewal, K. Tvingstedt, A. Gadisa, O. Inganäs, and J. V. Manca, “Relating the

open-circuit voltage to interface molecular properties of donor:acceptor bulk

heterojunction solar cells,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 81,

no. 12, pp. 1–8, 2010.

[78] Y. Zhang, I. D. W. Samuel, T. Wang, and D. G. Lidzey, “Current Status of Outdoor

Lifetime Testing of Organic Photovoltaics,” Adv. Sci., vol. 5, no. 8, 2018.

[79] A. Seemann et al., “Reversible and irreversible degradation of organic solar cell

performance by oxygen,” Sol. Energy, vol. 85, no. 6, pp. 1238–1249, 2011.

[80] M. Jørgensen, K. Norrman, and F. C. Krebs, “Stability/degradation of polymer solar

BIBLOGRAPHY

136

cells,” Sol. Energy Mater. Sol. Cells, vol. 92, no. 7, pp. 686–714, Jul. 2008.

[81] S. A. Gevorgyan et al., “Lifetime of organic photovoltaics: Status and predictions,”

Adv. Energy Mater., vol. 6, no. 2, pp. 1–17, 2016.

[82] G. A. dos Reis Benatto, N. Espinosa, and F. C. Krebs, “Life-Cycle Assessment of

Solar Charger with Integrated Organic Photovoltaics,” Adv. Eng. Mater., vol. 19, no.

8, pp. 1–7, 2017.

[83] S. A. Gevorgyan et al., “Improving, characterizing and predicting the lifetime of

organic photovoltaics,” J. Phys. D. Appl. Phys., vol. 50, no. 10, p. 103001, Mar.

2017.

[84] M. Stephen, S. Karuthedath, T. Sauermann, K. Genevicius, and G. Juška,

Degradation effects on charge carrier transport in P3HT:PCBM solar cells studied

by Photo-CELIV and ToF, vol. 9184. 2014.

[85] S. Karuthedath, T. Sauermann, and H. Egelhaaf, “The e ff ect of oxygen induced

degradation on charge carrier dynamics in P3HT : PCBM and Si- PCPDTBT :

PCBM thin fi lms and solar cells †,” no. iv, pp. 3399–3408, 2015.

[86] N. Grossiord, J. M. Kroon, R. Andriessen, and P. W. M. Blom, “Degradation

mechanisms in organic photovoltaic devices,” Org. Electron. physics, Mater. Appl.,

vol. 13, no. 3, pp. 432–456, 2012.

[87] W. R. Mateker and M. D. McGehee, “Progress in Understanding Degradation

Mechanisms and Improving Stability in Organic Photovoltaics,” Adv. Mater., vol.

29, no. 10, 2017.

[88] J. Adams et al., “Water Ingress in Encapsulated Inverted Organic Solar Cells:

Correlating Infrared Imaging and Photovoltaic Performance,” Adv. Energy Mater.,

vol. 5, no. 20, p. 1501065, Oct. 2015.

[89] M. O. Reese et al., “Pathways for the degradation of organic photovoltaic P3HT :

PCBM based devices,” vol. 92, pp. 746–752, 2008.

[90] A. Früh, “PMMA as an effective protection layer against the oxidation of P3HT and

MDMO-PPV by ozone,” pp. 1891–1901, 2018.

[91] V. I. Madogni, B. Kounouhéwa, A. Akpo, M. Agbomahéna, S. A. Hounkpatin, and

C. N. Awanou, “Comparison of degradation mechanisms in organic photovoltaic

devices upon exposure to a temperate and a subequatorial climate,” Chem. Phys.

Lett., vol. 640, pp. 201–214, Nov. 2015.

[92] D. Chi, S. Qu, Z. Wang, and J. Wang, “High efficiency P3HT:PCBM solar cells with

an inserted PCBM layer,” J. Mater. Chem. C, vol. 2, no. 22, pp. 4383–4387, 2014.

BIBLOGRAPHY

137

[93] A. Guerrero and G. Garcia-Belmonte, “Recent Advances to Understand Morphology

Stability of Organic Photovoltaics,” Nano-Micro Lett., vol. 9, no. 1, p. 10, 2016.

[94] M. Giannouli, V. M. Drakonakis, A. Savva, P. Eleftheriou, G. Florides, and S. A.

Choulis, “Methods for Improving the Lifetime Performance of Organic

Photovoltaics with Low-Costing Encapsulation,” ChemPhysChem, vol. 16, no. 6,

pp. 1134–1154, Apr. 2015.

[95] C. T. Howells et al., “Influence of perfluorinated ionomer in PEDOT:PSS on the

rectification and degradation of organic photovoltaic cells,” J. Mater. Chem. A, vol.

6, no. 33, pp. 16012–16028, 2018.

[96] L. Piergiovanni and S. Limbo, Oil Derived Polymers. 2013.

[97] D. Raheem, “Application of plastics and paper as food packaging materials - An

overview,” Emirates J. Food Agric., vol. 25, no. 3, pp. 177–188, 2013.

[98] P. M. Budd and N. B. McKeown, “Highly permeable polymers for gas separation

membranes,” Polym. Chem., vol. 1, no. 1, pp. 63–68, 2010.

[99] G. Dennler et al., “A new encapsulation solution for flexible organic solar cells,”

Thin Solid Films, vol. 511–512, pp. 349–353, Jul. 2006.

[100] M. Schmid et al., “Properties of Whey-Protein-Coated Films and Laminates as

Novel Recyclable Food Packaging Materials with Excellent Barrier Properties,” Int.

J. Polym. Sci., vol. 2012, pp. 1–7, 2012.

[101] F. Hammann and M. Schmid, “Determination and Quantification of Molecular

Interactions in Protein Films: A Review,” Materials (Basel)., vol. 7, no. 12, pp.

7975–7996, Dec. 2014.

[102] M. Schmid, W. Zillinger, K. Müller, and S. Sängerlaub, “Permeation of water

vapour, nitrogen, oxygen and carbon dioxide through whey protein isolate based

films and coatings—Permselectivity and activation energy,” Food Packag. Shelf

Life, vol. 6, no. Supplement C, pp. 21–29, Dec. 2015.

[103] C. S. Reig, A. D. Lopez, M. H. Ramos, and V. A. C. Ballester, “Nanomaterials: a

Map for Their Selection in Food Packaging Applications: NANOMATERIALS: A

MAP FOR FOOD PACKAGING APPLICATIONS,” Packag. Technol. Sci., vol.

27, no. 11, pp. 839–866, Nov. 2014.

[104] S. Seethamraju, P. C. Ramamurthy, and G. Madras, “Performance of an ionomer

blend-nanocomposite as an effective gas barrier material for organic devices,” RSC

Adv., vol. 4, no. 22, p. 11176, 2014.

[105] J. H. Yeo, C. H. Lee, C.-S. Park, K.-J. Lee, J.-D. Nam, and S. W. Kim, “Rheological,

BIBLOGRAPHY

138

morphological, mechanical, and barrier properties of PP/EVOH blends,” Adv.

Polym. Technol., vol. 20, no. 3, pp. 191–201, 2001.

[106] V. Siracusa, C. Ingrao, A. Lo Giudice, C. Mbohwa, and M. Dalla Rosa,

“Environmental assessment of a multilayer polymer bag for food packaging and

preservation: An LCA approach,” Food Res. Int., vol. 62, pp. 151–161, Aug. 2014.

[107] S. Seethamraju, P. C. Ramamurthy, and G. Madras, “Encapsulation for Improving

the Efficiencies of Solar Cells,” in Materials and Processes for Solar Fuel

Production, vol. 174, B. Viswanathan, V. Subramanian, and J. S. Lee, Eds. New

York, NY: Springer New York, 2014, pp. 23–40.

[108] J. Fahlteich et al., “The Role of Defects in Single- and Multi-Layer Barriers for

Flexible Electronics,” Soc. Vac. Coaters 57th Annu. Tech. Conf. Proc., vol. 57, pp.

403–413, 2015.

[109] D. R. Paul and L. M. Robeson, “Polymer nanotechnology: Nanocomposites,”

Polymer (Guildf)., vol. 49, no. 15, pp. 3187–3204, Jul. 2008.

[110] M. Stöter, S. Rosenfeldt, and J. Breu, “Tunable Exfoliation of Synthetic Clays,”

Annu. Rev. Mater. Res., vol. 45, no. 1, pp. 129–151, Jul. 2015.

[111] A. A. Gokhale and I. Lee, “Recent Advances in the Fabrication of Nanostructured

Barrier Films,” J. Nanosci. Nanotechnol., vol. 14, no. 3, pp. 2157–2177, Mar. 2014.

[112] T. D. Papathanasiou, “Barrier Properties of Flake-Filled Membranes: Review and

Numerical Evaluation,” J. Plast. Film Sheeting, vol. 23, no. 4, pp. 319–346, Oct.

2007.

[113] Y. Cui, S. Kumar, B. Rao Kona, and D. van Houcke, “Gas barrier properties of

polymer/clay nanocomposites,” RSC Adv., vol. 5, no. 78, pp. 63669–63690, 2015.

[114] X. Hou, Y. Wang, G. Sun, R. Huang, and C. Zhang, “Effect of polyaniline-modified

glass flakes on the corrosion protection properties of epoxy coatings,” High Perform.

Polym., vol. 29, no. 3, pp. 305–314, 2017.

[115] F. Higashide, K. Omata, Y. Nozawa, and H. Yoshioka, “Permeability To Oxygen of

Poly(Methyl Methacrylate) Films Containing Silane-Treated Glass Flakes.,” J

Polym Sci Polym Chem Ed, vol. 15, no. 8, pp. 2019–2028, 1977.

[116] L. Peponi, D. Puglia, L. Torre, L. Valentini, and J. M. Kenny, “Processing of

nanostructured polymers and advanced polymeric based nanocomposites,” Mater.

Sci. Eng. R Reports, vol. 85, no. 1, pp. 1–46, 2014.

[117] C. W. Chiu, T. K. Huang, Y. C. Wang, B. G. Alamani, and J. J. Lin, “Intercalation

strategies in clay/polymer hybrids,” Prog. Polym. Sci., vol. 39, no. 3, pp. 443–485,

BIBLOGRAPHY

139

2014.

[118] S. Sinha Ray and M. Okamoto, “Polymer/layered silicate nanocomposites: a review

from preparation to processing,” Prog. Polym. Sci., vol. 28, no. 11, pp. 1539–1641,

Nov. 2003.

[119] J. Gaume, C. Taviot-Gueho, S. Cros, A. Rivaton, S. Thérias, and J. L. Gardette,

“Optimization of PVA clay nanocomposite for ultra-barrier multilayer encapsulation

of organic solar cells,” Sol. Energy Mater. Sol. Cells, vol. 99, pp. 240–249, 2012.

[120] K. Yano, A. Usuki, A. Okada, T. Kurauchi, and O. Kamigaito, “Synthesis and

properties of polyimide–clay hybrid,” J. Polym. Sci. Part A Polym. Chem., vol. 31,

no. 10, pp. 2493–2498, 1993.

[121] P. B. Messersmith and E. P. Giannelis, “Synthesis and barrier properties of poly(ε‐

caprolactone)‐layered silicate nanocomposites,” J. Polym. Sci. Part A Polym. Chem.,

vol. 33, no. 7, pp. 1047–1057, 1995.

[122] S.-Y. Zhou, J.-B. Chen, X.-J. Li, X. Ji, G.-J. Zhong, and Z.-M. Li, “Innovative

enhancement of gas barrier properties of biodegradable poly(butylene succinate)

nanocomposite films by introducing confined crystals,” RSC Adv., vol. 6, no. 4, pp.

2530–2536, 2016.

[123] M. Stöter et al., “Nanoplatelets of sodium hectorite showing aspect ratios of ≈20 000

and superior purity,” Langmuir, vol. 29, no. 4, pp. 1280–1285, 2013.

[124] E. S. Tsurko et al., “Can high oxygen and water vapor barrier nanocomposite

coatings be obtained with a waterborne formulation?,” J. Memb. Sci., vol. 540, no.

June, pp. 212–218, 2017.

[125] F. Li, X. Jiang, J. Zhao, and S. Zhang, “Graphene oxide: A promising nanomaterial

for energy and environmental applications,” Nano Energy, vol. 16, pp. 488–515,

Sep. 2015.

[126] H. M. Kim and H. S. Lee, “Water and oxygen permeation through transparent

ethylene vinyl alcohol/(graphene oxide) membranes,” Carbon Lett., vol. 15, no. 1,

pp. 50–56, 2014.

[127] R. K. Singh, R. Kumar, and D. P. Singh, “Graphene oxide: strategies for synthesis,

reduction and frontier applications,” RSC Adv., vol. 6, no. 69, pp. 64993–65011,

2016.

[128] V. V Neklyudov, N. R. Khafizov, I. A. Sedov, and A. M. Dimiev, “New insights

into the solubility of graphene oxide in water and alcohols,” Phys. Chem. Chem.

Phys., vol. 19, no. 26, pp. 17000–17008, 2017.

BIBLOGRAPHY

140

[129] T. Kuilla, S. Bhadra, D. Yao, N. H. Kim, S. Bose, and J. H. Lee, “Recent advances

in graphene based polymer composites,” Prog. Polym. Sci., vol. 35, no. 11, pp.

1350–1375, Nov. 2010.

[130] I.-H. Tseng, Y.-F. Liao, J.-C. Chiang, and M.-H. Tsai, “Transparent

polyimide/graphene oxide nanocomposite with improved moisture barrier property,”

Mater. Chem. Phys., vol. 136, no. 1, pp. 247–253, Sep. 2012.

[131] C.-H. Chang et al., “Novel anticorrosion coatings prepared from

polyaniline/graphene composites,” Carbon N. Y., vol. 50, no. 14, pp. 5044–5051,

Nov. 2012.

[132] J.-T. Chen et al., “Enhancing polymer/graphene oxide gas barrier film properties by

introducing new crystals,” Carbon N. Y., vol. 75, pp. 443–451, Aug. 2014.

[133] T. Kim, J. H. Kang, S. J. Yang, S. J. Sung, Y. S. Kim, and C. R. Park, “Facile

preparation of reduced graphene oxide-based gas barrier films for organic

photovoltaic devices,” Energy Environ. Sci., vol. 7, no. 10, pp. 3403–3411, 2014.

[134] A. Pakdel, Y. Bando, and D. Golberg, “Nano boron nitride flatland,” Chem. Soc.

Rev., vol. 43, no. 3, pp. 934–959, 2014.

[135] L. H. Li, T. Xing, Y. Chen, and R. Jones, “Boron Nitride Nanosheets for Metal

Protection,” Adv. Mater. Interfaces, vol. 1, no. 8, pp. 1–21, 2014.

[136] J. J. Tweddle, R. Rockley, S. J. Catchpole, and S. Norris, “Improving the Efficiency

of Hermetic Packaging using Novel Gettering Solutions.”

[137] I. Topolniak et al., “Applications of polymer nanocomposites as encapsulants for

solar cells and LEDs: Impact of photodegradation on barrier and optical properties,”

Polym. Degrad. Stab., vol. 145, pp. 52–59, 2017.

[138] L. Hu, M. Li, C. Xu, and Y. Luo, “Perhydropolysilazane derived silica coating

protecting Kapton from atomic oxygen attack,” Thin Solid Films, vol. 520, no. 3, pp.

1063–1068, Nov. 2011.

[139] Y. Iwamoto, K. Sato, T. Kato, T. Inada, and Y. Kubo, “A hydrogen-permselective

amorphous silica membrane derived from polysilazane,” J. Eur. Ceram. Soc., vol.

25, no. 2–3, pp. 257–264, Jan. 2005.

[140] M. Menning, C. Schelle, A. Duran, J. J. Damborena, M. Guglielmi, and G. Brusatin,

“Investigation of glass-like sol-gel coatings for corrosion protection of stainless steel

against liquid and gaseous attack,” J. sol-gel Sci. Technol., vol. 13, no. 1–3, pp. 717–

722, 1998.

[141] R. Bertoncello, M. Monti, and C. Sada, “Silica thin-films from perhydropolysilazane

BIBLOGRAPHY

141

for the protection of ancient glass,” 2016.

[142] J. Wang et al., “Scratch-Resistant Improvement of Sol-Gel Derived Nano-Porous

Silica Films,” J. Sol-Gel Sci. Technol., vol. 18, no. 3, pp. 219–224, Aug. 2000.

[143] A. Yamano and H. Kozuka, “Perhydropolysilazane-derived silica–

polymethylmethacrylate hybrid thin films highly doped with spiropyran: Effects of

polymethylmethacrylate on the hardness, chemical durability and photochromic

properties,” Thin Solid Films, vol. 519, no. 6, pp. 1772–1779, Jan. 2011.

[144] “Effect of Added Ethanol in Atmospheric-Pressure Chemical Vapor Deposition

Reaction Using Tetraethoxysilane and Ozone,” Jpn. J. Appl. Phys., vol. 33, no. 5R,

p. 2703, 1994.

[145] H. Chatham, “Oxygen diffusion barrier properties of transparent oxide coatings on

polymeric substrates,” Surf. Coatings Technol., vol. 78, no. 1, pp. 1–9, Jan. 1996.

[146] Y. Leterrier, “Durability of nanosized oxygen-barrier coatings on polymers,” Prog.

Mater. Sci., vol. 48, no. 1, pp. 1–55, Jan. 2003.

[147] J. Madocks, J. Rewhinkle, and L. Barton, “Packaging barrier films deposited on PET

by PECVD using a new high density plasma source,” SVC - Smart Mater. Symp.,

vol. 119, no. 3, pp. 268–273, Jun. 2005.

[148] M. J. Bennett, A. T. Tuson, C. F. Knights, and C. F. Ayres, “Oxidation protection of

alloy IN 738 LC by plasma assisted vapour deposited silica coating,” Mater. Sci.

Technol., vol. 5, no. 8, pp. 841–852, Aug. 1989.

[149] D. S. Wuu, W. C. Lo, L. S. Chang, and R. H. Horng, “Properties of SiO2-like barrier

layers on polyethersulfone substrates by low-temperature plasma-enhanced

chemical vapor deposition,” Thin Solid Films, vol. 468, no. 1–2, pp. 105–108, Dec.

2004.

[150] G. A. Garzino-Demo and F. L. Lama, “Friction and wear of uncoated or SiO2-coated

329 stainless steel and of uncoated or AlN-coated aluminium surfaces,” Surf.

Coatings Technol., vol. 68–69, no. Supplement C, pp. 507–511, Dec. 1994.

[151] “Flame Hydrolysis Deposition of SiO 2 -TiO 2 , Glass Planar Optical Waveguides

on Silicon,” Jpn. J. Appl. Phys., vol. 22, no. 12R, p. 1932, 1983.

[152] R. Visser, “Barix Multilayers flexible organic electronics,” 2006.

[153] T. Y. Cho et al., “Moisture barrier and bending properties of silicon nitride films

prepared by roll-to-roll plasma enhanced chemical vapor deposition,” Thin Solid

Films, vol. 660, no. April, pp. 101–107, 2018.

[154] M. D. Clark, M. L. Jespersen, R. J. Patel, and B. J. Leever, “Ultra-thin alumina layer

BIBLOGRAPHY

142

encapsulation of bulk heterojunction organic photovoltaics for enhanced device

lifetime,” Org. Electron., vol. 15, no. 1, pp. 1–8, Jan. 2014.

[155] H. J. Seul, H. G. Kim, M. Y. Park, and J. K. Jeong, “A solution-processed silicon

oxide gate dielectric prepared at a low temperature via ultraviolet irradiation for

metal oxide transistors,” J. Mater. Chem. C, vol. 4, no. 44, pp. 10486–10493, 2016.

[156] T. Homma, T. Katoh, Y. Yamada, and Y. Murao, “A Selective SiO2 Film-Formation

Technology Using Liquid-Phase Deposition for Fully Planarized Multilevel

Interconnections,” J. Electrochem. Soc., vol. 140, no. 8, pp. 2410–2414, 1993.

[157] N. A. Gerrero-Gerrero, M. L. Mendoza-Lópes, J. F. Péres-Robles, A. Mansano-

Ramires, Y. V. Vorob’ev, and J. J. Péres-Bueno, “The action of molasses on

properties of coatings obtained with electrodeposition from a SiO2 aqueous

suspension,” Inorg. Mater., vol. 45, no. 4, pp. 450–456, Apr. 2009.

[158] K. Nakajima, H. Uchiyama, T. Kitano, and H. Kozuka, “Conversion of Solution-

Derived Perhydropolysilazane Thin Films into Silica in Basic Humid Atmosphere at

Room Temperature,” J. Am. Ceram. Soc., vol. 96, no. 9, pp. 2806–2816, Sep. 2013.

[159] J. D. Mackenzie and E. P. Bescher, “Physical properties of sol-gel coatings,” J. Sol-

Gel Sci. Technol., vol. 19, no. 1, pp. 23–29, 2000.

[160] M. D’Arienzo, R. Scotti, B. Di Credico, and M. Redaelli, Synthesis and

Characterization of Morphology-Controlled TiO 2 Nanocrystals, 1st ed., vol. 177.

Elsevier B.V., 2017.

[161] Y. Kobayashi, H. Yokota, Y. Fuchita, A. Takahashi, and Y. Sugahara,

“Characterization of gas barrier silica coatings prepared from perhydropolysilazane

films by vacuum ultraviolet irradiation,” J. Ceram. Soc. Japan, vol. 121, no. 1410,

pp. 215–218, 2013.

[162] T. Ohishi, S. Sone, and K. Yanagida, “Preparation and Gas Barrier Characteristics

of Polysilazane-Derived Silica Thin Films Using Ultraviolet Irradiation,” Mater. Sci.

Appl., vol. 05, no. 03, pp. 105–111, 2014.

[163] A. Lukacs, “Polysilazane precursors to advanced ceramics,” Am Ceram Soc Bull,

vol. 86, no. 1, pp. 9301–9306, 2007.

[164] L. Prager et al., “Conversion of Perhydropolysilazane into a SiOx Network

Triggered by Vacuum Ultraviolet Irradiation: Access to Flexible, Transparent

Barrier Coatings,” Chem. - A Eur. J., vol. 13, no. 30, pp. 8522–8529, Oct. 2007.

[165] A. Morlier, S. Cros, J.-P. Garandet, and N. Alberola, “Structural properties of

ultraviolet cured polysilazane gas barrier layers on polymer substrates,” Thin Solid

BIBLOGRAPHY

143

Films, vol. 550, pp. 85–89, Jan. 2014.

[166] Z. Zhang, Z. Shao, Y. Luo, P. An, M. Zhang, and C. Xu, “Hydrophobic, transparent

and hard silicon oxynitride coating from perhydropolysilazane: Silicon oxynitride

coating from perhydropolysilazane,” Polym. Int., vol. 64, no. 8, pp. 971–978, Aug.

2015.

[167] “Sol-Gel Novel approaches for preparation of nanoparticles,” 2017.

[168] C. R. Krüger and E. G. Rochow, “Polyorganosilazanes,” J. Polym. Sci. Part A

Polym. Chem., vol. 2, no. 7, pp. 3179–3189, 1964.

[169] T. Ohishi and Y. Yamazaki, “Formation and Gas Barrier Characteristics of

Polysilazane-Derived Silica Coatings Formed by Excimer Light Irradiation on PET

Films with Vacuum Evaporated Silica Coatings,” Mater. Sci. Appl., vol. 08, no. 01,

pp. 1–14, 2017.

[170] H. Kozuka, M. Fujita, and S. Tamoto, “Polysilazane as the source of silica: the

formation of dense silica coatings at room temperature and the new route to organic–

inorganic hybrids,” J. Sol-Gel Sci. Technol., vol. 48, no. 1–2, pp. 148–155, Nov.

2008.

[171] T. Kubo, E. Tadaoka, and H. Kozuka, “Formation of silica coating films from spin-

on polysilazane at room temperature and their stability in hot water,” J. Mater. Res.,

vol. 19, no. 02, pp. 635–642, Feb. 2004.

[172] J.-S. Lee, J.-H. Oh, S.-W. Moon, W.-S. Sul, and S.-D. Kim, “A Technique for

Converting Perhydropolysilazane to SiO[sub x] at Low Temperature,” Electrochem.

Solid-State Lett., vol. 13, no. 1, p. H23, 2010.

[173] A. Morlier, S. Cros, J.-P. Garandet, and N. Alberola, “Thin gas-barrier silica layers

from perhydropolysilazane obtained through low temperature curings: A

comparative study,” Thin Solid Films, vol. 524, pp. 62–66, Dec. 2012.

[174] Y. Naganuma, T. Horiuchi, C. Kato, and S. Tanaka, “Low-temperature synthesis of

silica coating on a poly(ethylene terephthalate) film from perhydropolysilazane

using vacuum ultraviolet light irradiation,” Surf. Coatings Technol., vol. 225, pp.

40–46, Jun. 2013.

[175] S. D. Kim, P. S. Ko, and K. S. Park, “Perhydropolysilazane spin-on dielectrics for

inter-layer-dielectric applications of sub-30 nm silicon technology,” Semicond. Sci.

Technol., vol. 28, no. 3, 2013.

[176] S. A. C. Abarca, O. Flores, A. L. G. Prette, and G. S. Barroso, “Synthesis and

Thermal Characterization of Silicon-based Hybrid Polymer,” Chem. Eng. Trans.,

BIBLOGRAPHY

144

vol. 32, pp. 1621–1626, 2013.

[177] T. Ohishi and K. Yanagida, “Preparation and gas barrier characteristics of

polysilazane derived multi-layered silica thin films formed on alicyclic polyimide

film using ultraviolet irradiation,” Front. Nanosci. Nanotechnol., vol. 2, no. 5, pp.

173–178, 2016.

[178] T. M. Hashem, M. Zirlewagen, and A. M. Braun, “Simultaneous photochemical

generation of ozone in the gas phase and photolysis of aqueous reaction systems

using one VUV light source,” Water Sci. Technol., vol. 35, no. 4, pp. 41–48, 1997.

[179] Z. Falkenstein, “Surface cleaning mechanisms utilizing VUV radiation in oxygen

containing gaseous environments Surface cleaning mechanisms utilizing VUV

radiation in oxygen containing gaseous environments.”

[180] T. Niizeki, S. Nagayama, Y. Hasegawa, N. Miyata, M. Sahara, and K. Akutsu,

“Structural Study of Silica Coating Thin Layers Prepared from

Perhydropolysilazane: Substrate Dependence and Water Penetration Structure,”

Coatings, vol. 6, no. 4, p. 64, 2016.

[181] T. Ohishi, “Gas barrier characteristics of a polysilazane film formed on an ITO-

coated PET substrate,” J. Non. Cryst. Solids, vol. 330, no. 1–3, pp. 248–251, Nov.

2003.

[182] A. Morlier, S. Cros, J.-P. Garandet, and N. Alberola, “Gas barrier properties of

solution processed composite multilayer structures for organic solar cells

encapsulation,” Sol. Energy Mater. Sol. Cells, vol. 115, pp. 93–99, Aug. 2013.

[183] L. Prager et al., “Photochemical approach to high-barrier films for the encapsulation

of flexible laminary electronic devices,” Thin Solid Films, vol. 570, no. PartA, pp.

87–95, 2014.

[184] I. A. Channa, A. Distler, M. Zaiser, C. J. Brabec, and H. Egelhaaf, “Thin Film

Encapsulation of Organic Solar Cells by Direct Deposition of Polysilazanes from

Solution,” Adv. Energy Mater., vol. 1900598, p. 1900598, 2019.

[185] G. L. Graff, R. E. Williford, and P. E. Burrows, “Mechanisms of vapor permeation

through multilayer barrier films: Lag time versus equilibrium permeation,” J. Appl.

Phys., vol. 96, no. 4, pp. 1840–1849, 2004.

[186] T. A. Rangreez and R. Mobin, “Ormocer Learn more about Ormocer Polymer

composites for dental fillings Recent advances in posterior resin composite

restorations,” 2019.

[187] C. Sanchez, B. Julián, P. Belleville, and M. Popall, “Applications of hybrid organic–

BIBLOGRAPHY

145

inorganic nanocomposites,” J. Mater. Chem., vol. 15, no. 35–36, p. 3559, 2005.

[188] H. K. Schmidt, “Das Sol-Gel-Verfahren: Anorganische Synthesemethoden,” 2001.

[189] T. Thomé, M. C. G. Erhardt, A. A. Leme, I. Al Bakri, A. K. Bedran-Russo, and L.

E. Bertassoni, “Emerging Polymers in Dentistry,” in Advanced Polymers in

Medicine, F. Puoci, Ed. Cham: Springer International Publishing, 2015, pp. 265–

296.

[190] L. M. Cavalcante, L. F. J. Schneider, N. Silikas, and D. C. Watts, “Surface integrity

of solvent-challenged ormocer-matrix composite,” Dent. Mater., vol. 27, no. 2, pp.

173–179, Feb. 2011.

[191] L. F. J. Schneider, L. M. Cavalcante, N. Silikas, and D. C. Watts, “Degradation

resistance of silorane, experimental ormocer and dimethacrylate resin-based dental

composites,” J. Oral Sci., vol. 53, no. 4, pp. 413–419, 2011.

[192] S. Amberg-Schwab, U. Weber, A. Burger, S. Nique, and R. Xalter, “Development

of Passive and Active Barrier Coatings on the Basis of Inorganic–Organic

Polymers,” Monatshefte für Chemie - Chem. Mon., vol. 137, no. 5, pp. 657–666,

May 2006.

[193] S. Amberg-Schwab, M. Hoffmann, H. Bader, and M. Gessler, “Inorganic-organic

polymers with barrier properties for water vapor, oxygen and flavors,” J. sol-gel Sci.

Technol., vol. 13, no. 1, pp. 141–146, 1998.

[194] “Ultra barriers based on ORMOCERS.” [Online]. Available:

https://fbgs.com/technology/ormocer-coating/.

[195] S. Amberg-schwab and D. C. Isc, “Sustainable functional coatings for future

packaging : barrier and antimicrobial coatings,” no. February, 2016.

[196] POLO Fraunhofer, “Transparent High Barrier Film,” Fraunhofer FEP, vol. 2.1,

2013.

[197] S. Amberg-Schwab, “Development and Upscaling of Nanoscale Transparent Barrier

Lacquers based on Hybrid Polymers ",” AIMCAL Eur. Tech. Conf. AIMCAL

Eur. Web Coat. Conf., 2012.

[198] K. VaÅ¡ko, K. Noller, M. Mikula, S. Amberg-Schwab, and U. Weber, “Multilayer

coatings for flexible high-barrier materials,” Open Phys., vol. 7, no. 2, Jan. 2009.

[199] Y. Zhang, M. Ishida, Y. Kazoe, Y. Sato, and N. Miki, “Water-vapor permeability

control of PDMS by the dispersion of collagen powder,” IEEJ Trans. Electr.

Electron. Eng., vol. 4, no. 3, pp. 442–449, 2009.

[200] K. C. Chang et al., “Synergistic effects of hydrophobicity and gas barrier properties

BIBLOGRAPHY

146

on the anticorrosion property of PMMA nanocomposite coatings embedded with

graphene nanosheets,” Polym. Chem., vol. 5, no. 3, pp. 1049–1056, 2014.

[201] H. Bi et al., “Deposition of PEG onto PMMA microchannel surface to minimize

nonspecific adsorption,” Lab Chip, vol. 6, no. 6, pp. 769–775, 2006.

[202] J. Granstrom, J. S. Swensen, J. S. Moon, G. Rowell, J. Yuen, and A. J. Heeger,

“Encapsulation of organic light-emitting devices using a perfluorinated polymer,”

Appl. Phys. Lett., vol. 93, no. 19, pp. 1–4, 2008.

[203] J. B. Chae, J. O. Kwon, J. S. Yang, D. Kim, K. Rhee, and S. K. Chung, “Optimum

thickness of hydrophobic layer for operating voltage reduction in EWOD systems,”

Sensors Actuators, A Phys., vol. 215, pp. 8–16, 2014.

[204] G. Rowell, “Identification and Suppression of Instabilities in CytopTM films for

Encapsulation of Organic Photovoltaic Devices,” University of California Santa

Barbara, 2009.

[205] A. Bulusu et al., “Engineering the mechanical properties of ultrabarrier films grown

by atomic layer deposition for the encapsulation of printed electronics,” J. Appl.

Phys., vol. 118, no. 8, p. 085501, Aug. 2015.

[206] J. Granstrom, A. Roy, G. Rowell, J. S. Moon, E. Jerkunica, and A. J. Heeger,

“Improvements in barrier performance of perfluorinated polymer films through

suppression of instability during film formation,” Thin Solid Films, vol. 518, no. 14,

pp. 3767–3771, 2010.

[207] E. Manias, A. Touny, L. Wu, K. Strawhecker, B. Lu, and T. C. Chung,

“Polypropylene/montmorillonite nanocomposites. Review of the synthetic routes

and materials properties,” Chem. Mater., vol. 13, no. 10, pp. 3516–3523, 2001.

[208] P. N. Dunlap, “Glass flake reinforced composites as optical materials,” Appl. Opt.,

vol. 30, no. 13, p. 1701, 2009.

[209] A. Solodovnyk et al., “Optimization of Solution-Processed Luminescent Down-

Shifting Layers for Photovoltaics by Customizing Organic Dye Based Thick Films,”

Energy Technol., vol. 4, no. 3, pp. 385–392, 2016.

[210] B. Lipovšek et al., “Optical model for simulation and optimization of luminescent

down-shifting layers filled with phosphor particles for photovoltaics,” Opt. Express,

vol. 23, no. 15, p. A882, 2015.

[211] B. Lipovšek, J. Krč, and M. Topič, “Optical model for thin-film photovoltaic devices

with large surface textures at the front side,” Inf. MIDEM, vol. 41, no. 4, pp. 264–

271, 2011.

BIBLOGRAPHY

147

[212] C. A. Faick and A. N. Finn, “THE INDEX OF REFRACTION OF SOME SODA-

LIMESIL- By,” 1923.

[213] M. A. Marcus, F. Ignatovich, and J. Rd, “No Title,” 2016.

[214] L. Forbes, “Texturing, reflectivity, diffuse scattering and light trapping in silicon

solar cells,” Sol. Energy, vol. 86, no. 1, pp. 319–325, 2012.

[215] J. Hepp et al., “Infrared Absorption Imaging of Water Ingress Into the Encapsulation

of ( Opto- ) Electronic Devices,” IEEE J. Photovoltaics, vol. PP, pp. 1–7, 2018.

[216] J. Y. Kim, S. Noh, J. Kwak, and C. Lee, “Analysis of Annealing Process on

P3HT:PCBM-Based Polymer Solar Cells Using Optical and Impedance

Spectroscopy,” J. Nanosci. Nanotechnol., vol. 13, no. 5, pp. 3360–3364, 2013.

[217] H. Kim, W. W. So, and S. J. Moon, “Effect of thermal annealing on the performance

of P3HT/PCBM polymer photovoltaic cells,” J. Korean Phys. Soc., vol. 48, no. 3,

pp. 441–445, 2006.

[218] S. Karuthedath, T. Sauermann, and H. Egelhaaf, “The e ff ect of oxygen induced

degradation on charge carrier dynamics in P3HT : PCBM and Si- PCPDTBT :

PCBM thin fi lms and solar cells †,” no. iv, pp. 3399–3408, 2015.

[219] M. H. El Haddad, N. E. Dowling, T. H. Topper, and K. N. Smith, “J integral

applications for short fatigue cracks at notches,” Int. J. Fract., vol. 16, no. 1, pp. 15–

30, 1980.

[220] V. M. Sglavo and D. J. Green, “Fatigue limit in fused silica,” J. Eur. Ceram. Soc.,

vol. 21, no. 5, pp. 561–567, 2001.

[221] D. Bonamy et al., “Nanoscale damage during fracture in silica glass,” Int. J. Fract.,

vol. 140, no. 1–4, pp. 3–14, 2006.

[222] H. Hintz, H. Egelhaaf, H. Peisert, and T. Chassé, “Photo-oxidation and ozonization

of poly ( 3-hexylthiophene ) thin fi lms as studied by UV / VIS and photoelectron

spectroscopy,” vol. 95, pp. 818–825, 2010.

LIST OF TABLES

a

LIST OF TABLES

TABLE 1: SOME CHARACTERISTICS OF THE MODELS DEVELOPED TO STUDY REGULARLY DISTRIBUTED AND PERPENDICULARLY

ORIENTED FILLERS IN POLYMER NANOCOMPOSITES. 𝛼 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜, 𝛷 =

𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙𝑒𝑟, 𝜎 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑙𝑖𝑡𝑠. ---------------------------------------------- 18

TABLE 2: SOME CHARACTERISTICS OF THE MODELS DEVELOPED TO STUDY RANDOMLY DISTRIBUTED AND PERPENDICULARLY

ORIENTED FILLERS IN POLYMER NANOCOMPOSITES. 𝛼 = 𝑎𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜, 𝛷 =

𝑣𝑜𝑙𝑢𝑚𝑒 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑙𝑒𝑟, 𝛾 = 𝑠𝑡𝑎𝑐𝑘𝑖𝑛𝑔 𝑝𝑎𝑟𝑎𝑚𝑒𝑡𝑒𝑟, 𝜇 = 𝑔𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐𝑎𝑙 𝑓𝑎𝑐𝑡𝑜𝑟 . ------------- 20

TABLE 3: DEGRADATION OF THE OSC PARAMETERS AND THEIR POSSIBLE EFFECT ON THE DEVICE PERFORMANCE AS DESCRIBED BY

GROSSIORD ET AL,. [86] -------------------------------------------------------------------------------------------------------- 30

TABLE 4: IRRADIATION CONDITION AND WVTR OF EACH SAMPLE ------------------------------------------------------------------- 50

TABLE 5: WVTR OF PHPS COATED ON POLYIMIDE SUBSTRATE ON BOTH SIDES VIA SPIN COATING AND CURED VIA VUV

IRRADIATION AT DIFFERENT TEMPERATURES FOR 20 MINUTES (DATA EXTRACTED FROM [162]). ------------------------- 51

TABLE 6: BIFUNCTIONAL SILANES R’ (CH2)NSI(OR)3, FEW FUNCTIONAL ORGANIC GROUPS R’ FOR PRODUCING AN ORGANIC

NETWORK AND FUNCTIONALIZATION OF THE MATRIX. DATA EXTRACTED FROM [192]. ------------------------------------ 54

TABLE 7: MATERIALS USED IN THE EXPERIMENTS ------------------------------------------------------------------------------------- 61

TABLE 8: COATING PARAMETERS FOR COATING PHPS LAYERS FROM AN AMOUNT OF 70 µL ON PET SUBSTRATE, SUBSEQUENTLY

CURED WITH DEEP UV IRRADIATION IN COMBINATION WITH TEMPERATURE. ----------------------------------------------- 63

TABLE 9: MOISTURE PERMEATION OF THE PVA LAYER HAVING DIFFERENT THICKNESS VALUES. ----------------------------------- 75

TABLE 10: CALCULATED MOISTURE PERMEATION VALUES OF PVOH AND ITS COMPOSITES AT CONDITIONS 40OC / 65%RH; FILM

THICKNESS OF 100 µM IN ALL CASES EXCEPT PET (125 µM). ---------------------------------------------------------------- 76

TABLE 11: WVTR (@40OC/85 % RH) OF PVB FILMS FILLED WITH DIFFERENT VOLUME CONCENTRATIONS OF GLASS FLAKES OF Α

~ 200. --------------------------------------------------------------------------------------------------------------------------- 93

TABLE 12: WVTR (@40OC/85 % RH) OF PVB FILMS FILLED WITH DIFFERENT CONCENTRATION OF GLASS FLAKES WITH Α ~400

ASPECT RATIO GLASS FLAKES WITH DIFFERENT CONCENTRATIONS. ----------------------------------------------------------- 93

TABLE 13: WVTR (@40OC/85 % RH) OF PVB FILMS FILLED WITH DIFFERENT CONCENTRATIONS OF GLASS FLAKES WITH Α ~2000.

------------------------------------------------------------------------------------------------------------------------------------ 93

TABLE 14: OTR OF PVB FILMS FILLED WITH DIFFERENT CONCENTRATIONS OF GLASS FLAKES WITH Α = 2000. ------------------ 94

TABLE 15: PARAMETERS FOR COATING 70 µL PHPS ON PET SUBSTRATE---------------------------------------------------------- 109

LIST OF FIGURES

b

LIST OF FIGURES

FIGURE 1.1. WATER VAPOR TRANSMISSION RATES (WVTR) AND OXYGEN TRANSMISSION RATES (OTR) OF BULK POLYMERS, FOOD

PACKAGING, AS WELL AS OF SOLUTION AND VACUUM PROCESSED HIGH QUALITY BARRIERS. (REPRODUCED FROM [22] WITH

THE PERMISSION FROM ELSEVIER, WITH MODIFICATIONS) .................................................................................... 7

FIGURE 1.2. ILLUSTRATION OF THE AMOUNTS OF WATER TRANSMITTED THROUGH BARRIER FILMS OF THE SIZE OF A FOOTBALL FIELD

(5000 M2) OVER A PERIOD OF 1 MONTH AT THE WVTR VALUES GIVEN (IN G.M-2.DAY-1). DATA EXTRACTED FROM [23]. 7

FIGURE 2.1: RELATION OF WATER VAPOR PRESSURE VS TEMPERATURE (DATA TAKEN FROM DORTMUND DATA BANK, LICENSED BY

CC BY 3.0) ............................................................................................................................................. 11

FIGURE 2.2: MOISTURE PERMEATION OF BIAXIALLY ORIENTED POLYPROPYLENE AND BIAXIALLY ORIENTED PVA, A) MOISTURE

PERMEATION DEPENDENCE ON TEMPERATURE AT 50% RH, B) MOISTURE PERMEATION DEPENDENCE ON RELATIVE

HUMIDITY (RH%) AT 23OC, C) OTR VALUES OF BIAXIALLY ORIENTED PVA AT DIFFERENT TEMPERATURES AT 50% RH AND

D) OTR VALUES OF BIAXIALLY ORIENTED PVA AT DIFFERENT RELATIVE HUMIDITY (RH%) AT 23˚C (COPIED FROM [31]

LICENSED BB CC BY 4.0) . .......................................................................................................................... 13

FIGURE 2.3: SCHEMATIC DIAGRAM A FILM (I) WITHOUT FILLERS OFFERING NO HINDRANCE, (II) FILM FILLED WITH REGULARLY

ARRANGED PLATELETS PERPENDICULAR TO THE DIRECTION OF DIFFUSION, CREATING A TORTUOUS PATH. (REPRODUCED

FROM [45] WITH PERMISSION FROM ELSEVIER). .............................................................................................. 17

FIGURE 2.4: THE ORDER PARAMETER S FOR THREE DIFFERENT CASES; WHEN ALL FILLER PARTICLES ARE PARALLEL TO THE DIFFUSION

DIRECTION (S=-1/2), WHEN THEY ARE PERPENDICULARLY ORIENTED (S=1) AND WHEN THEY ARE RANDOMLY ORIENTED

(S=0). (REPRODUCED WITH MODIFICATIONS FROM [42] WITH PERMISSION FROM AMERICAN CHEMICAL SOCIETY (ACS))

............................................................................................................................................................. 21

FIGURE 2.5: A) SCHEMATIC STRUCTURE OF A TYPICAL ORGANIC SOLAR CELL SHOWING A GLASS OR PET: POLYETHYLENE

TEREPHTHALATE (SUBSTRATE), INDIUM TIN OXIDE: ITO (BOTTOM ELECTRODE), ZNO: ZINC OXIDE (ELECTRON EXTRACTION

LAYER), BLEND OF P3HT:PCBM (ACTIVE LAYER), PEDOT:PSS: POLY(3,4-ETHYLENEDIOXYTHIOPHENE) POLYSTYRENE

SULFONATE (HOLE EXTRACTION LAYER) AND SILVER:AG (TOP ELECTRODE) B) ENERGY BAND DIAGRAM OF A NORMAL CELL

STRUCTURE AND C) ENERGY BAND DIAGRAM OF AN INVERTED CELL STRUCTURE. DATA EXTRACTED FROM [69]. ........... 25

FIGURE 2.6: EQUIVALENT CIRCUIT OF AN ORGANIC SOLAR CELL (ONE DIODE MODEL) (REPRODUCED FROM [74] WITH PERMISSION

FROM ELSEVIER) ....................................................................................................................................... 27

FIGURE 2.7: A SCHEMATIC DIAGRAM OF FEW PROCESSES RESPONSIBLE FOR DEGRADATION IN OSC WITH P3HT:PCBM AS

PHOTOACTIVE LAYER, (REPRODUCED FROM [80] WITH PERMISSION FROM ELSEVIER). ............................................ 28

FIGURE 3.1: OTR AND WVTR OF DIFFERENT BULK POLYMERS NORMALIZED TO 100 µM THICKNESS. [56] PE-LD =POLYETHYLENE

LOW DENSITY, PE-HD= POLYETHYLENE HIGH DENSITY, PP=POLYPROPYLENE, PS=POLYSTYRENE, BOPP=BIAXIALLY ORIENTED

POLYPROPYLENE, PLA=POLYLACTIC ACID, PVC=POLYVINYL CHLORIDE, PA6=POLYAMIDE 6, LCP=LIQUID CRYSTALLINE

POLYMER, EVOH=ETHYL VINYL ALCOHOL, PAN= POLYACRYLONITRILE, PEN= POLYETHYLENE NAPTHALENE, PET=

POLYETHYLENE TEREPHTHALATE, PVDC= POLYVINYLIDENE CHLORIDE, PC= POLYCARBONATE, PVC-P=POLYVINYL

CHLORIDE-PLASTICIZED, PVC-U= POLYVINYL CHLORIDE-UNPLASTICIZED, PVB= POLYVINHYLA BUTYRAL. (REPRODUCED

FROM [100] LICENSED BY CC BY 3.0). ......................................................................................................... 33

LIST OF FIGURES

c

FIGURE 3.2 SCHEMATIC REPRESENTATION OF CLAY MORPHOLOGY WHEN MIXED WITH POLYMERS. (REPRODUCED FROM [116]

WITH PERMISSION FROM ELSEVIER) .............................................................................................................. 36

FIGURE 3.3: LIFETIME OF ORGANIC SOLAR CELLS TESTED UNDER IRRADIATION WITH A SOLAR SIMULATOR (AM 1.5G, 30 OC,

AMBIENT RH 30-40%): NORMALIZED POWER CONVERSION EFFICIENCY (PCE) OF OSCS ENCAPSULATED WITH PET FILM,

PVA COATED PET FILM, AND PET COATED WITH PVA-MMT 5 WT% NANOCOMPOSITE (COPIED FROM [119] WITH

PERMISSION FROM ELSEVIER). ..................................................................................................................... 37

FIGURE 3.4: WVTR (G/M2.H) OF COMPOSITES OF POLYANILINE / GRAPHENE AND POLYANILINE / CLAY AS FUNCTION OF GRAPHENE

LOADING. (COPIED FROM [45] WITH PERMISSION FROM ELSEVIER)..................................................................... 39

FIGURE 3.5: RELATIVE OXYGEN PERMEABILITY FOR PVA, MIXTURE OF PVA/GO COATING AND HYBRID PVA/GO FOR 0.07 VOL%

LAYER IN COMPARISON TO PREDICTIVE PERMEATION CURVES PROPOSED BY THREE MODELS ( I.E. NIELSEN, MODIFIED

NIELSEN AND CUSSLER) FOR DIFFERENT ASPECT RATIOS (𝛼) (COPIED FROM [132] WITH PERMISSION FROM ELSEVIER, 40

FIGURE 3.6: TRANSPARENCY VS REDUCTION IN PERMEATION FOR DIFFERENT FILLER TYPES AND LOADINGS IN POLYMER MATRICES.

............................................................................................................................................................. 41

FIGURE 3.7: TRANSMISSION AND APPEARANCE OF CURED PHPS FILMS, A) SHOWING THE TRANSPARENT APPEARANCE, B)

BENDABLE TRANSPARENT CURED PHPS COATING AND C) TRANSMISSION SPECTRA OF PET FILM AND DIFFERENT TYPES OF

SIO2 COATINGS.(COPIED FROM [169] LICENSED BY CC BY 4.0) ........................................................................ 44

FIGURE 3.8: FTIR SPECTRA OF UNCURED PHPS AND PHPS CURED WITH DIFFERENT METHODS. A) IR SPECTRA OF UNCURED PHPS

(SOLID LINE) AND IR SPECTRA OF PHPS CURING AT 180 °C UNDER MOISTURIZED ATMOSPHERE FOR 300 MIN (DASHED

LINE), B) IR SPECTRA OF UNCURED PHPS (SOLID LINE), PHPS CURED AFTER EXPOSURE TO AMMONIA VAPOR FOR 60

MINUTES (DASHED LINE), C) IR SPECTRA OF UNCURED PHPS (SOLID LINE) AND PHPS CURED BY SUBMERGING INTO 20%

AQUEOUS HYDROGEN PEROXIDE SOLUTION FOR 10 MINUTES. (COPIED FROM [165] WITH PERMISSION FROM ELSEVIER)

............................................................................................................................................................. 47

FIGURE 3.9: TEMPERATURE DEPENDENCE OF WATER VAPOR TRANSMISSION RATES OF THE POLYSILAZANE DERIVED SIO2 COATINGS

(2 COATES) (COPIED FROM [169] LICENSED BY CC BY 4.0). ............................................................................ 52

FIGURE 3.10:PERFORMANCE OF ORGANIC SOLAR CELLS IN TERMS OF NORMALIZED POWER CONVERSION EFFICIENCY (PCE) AND

NORMALIZED SHORT CIRCUIT CURRENT (JSC) DURING THE EXPOSURE TO AM 1.5, 10 0 0 W M –2 LIGHT SOAKING,

ENCAPSULATED WITH (A) PET HAVING THICKNESS OF 50 Μ M, (B) A COMMERCIAL BARRIER, (C) PHPS BASED BARRIER

HAVING ONE PHPS (250 NM) COAT ON BOTH SIDES OF PET, (D) PHPS BASED BARRIER, HAVING 5 LAYER STRUCTURE

(PET/PHPS250 NM/PHPS250 NM/PVA1 Μ M /PHPS250 NM/PHPS250 NM) ON ONE SIDE OF PET AND (E) UN-ENCAPSULATED

OSC DEVICE DEGRADED UNDER IRRADIATION IN GLOVEBOX. (COPIED FROM [182] WITH PERMISSION FROM ELSEVIER). 53

FIGURE 3.11: SCHEMATIC DIAGRAM FOR ROLL-TO-ROLL PRODUCTION OF ORMOCER/INORGANIC OXIDE HYBRID BARRIER FILMS.

(RE DRAWN FROM [167]). ......................................................................................................................... 55

FIGURE 3.12:(A) CHEMICAL STRUCTURE FOR CYTOP TM (B) SPIN-COATED CYTOP FILM ON GLASS SUBSTRATE UNDER ATOMIC

FORCE MICROSCOPE (COPIED FROM [202] WITH PERMISSION FROM AIP PUBLISHING). ......................................... 57

FIGURE 3.13: CHARACTERIZATION OF HYDROPHOBICITY IN TERMS OF WATER DROPLET CONTACT ANGLES AND THICKNESS VALUES

OF FILMS WITH RESPECT TO WEIGHT PERCENTAGES (WT%) OF CYTOP IN SOLUTION. IMAGES BELOW DROPLETS ARE

MEASURED BY ATOMIC FORCE MICROSCOPY (AFM) (COPIED FROM [203] WITH PERMISSION FROM ELSEVIER). .......... 58

LIST OF FIGURES

d

FIGURE 3.14: CALCIUM DEGRADATION MECHANISM, TYPE A CA FILMS AS A FUNCTION OF TIME FOR VARYING CYTOP FILM

THICKNESSES AND TTYPE-B CA FILMS AS A FUNCTION OF TIME FOR VARYING CYTOP FILM THICKNESSES HAVING SINX

INTERLAYER (COPIED FROM [205] WITH PERMISSION FROM AIP PUBLISHING). ..................................................... 59

FIGURE 4.1: DETAILS OF THE OSC DEVICE, A) LAYOUT OF THE COMPLETE OSC DEVICE, B) SCHEMATIC DIAGRAM OF THE WORKING

CELL........................................................................................................................................................ 64

FIGURE 4.2: SCHEMATIC DIAGRAMS OF THE ENCAPSULATED SOLAR CELLS, A) SOLAR CELLS ENCAPSULATED WITH TRADITIONAL

LAMINATION OF THE BARRIER FILMS USING EPOXY AS AN ADHESIVE, B) DIRECTLY COATED SOLAR CELL. ........................ 65

FIGURE 4.3: A) SCHEMATIC DIAGRAM SHOWING CUP TEST USING WATER B) CUP TEST USING DESICCANTS, C) ALUMINUM CUP

ACCORDING TO ASTM STANDARD E96, B) SYSTECH 7002 METHOD................................................................. 66

FIGURE 4.4: A) PHOTOGRAPH OF THE WVTR DEVICE (SYSTECH 7002), B) SCHEMATIC VIEW OF THE PERMEATION CELL OF

SYSTECH 7002 DEVICE SHOWING THE FLOW OF THE DRY AND WET NITROGEN THROUGH THE CELL CHAMBER. ........... 67

FIGURE 4.5: SCHEMATIC VIEW OF THE OXYGEN PERMEATION CELL ................................................................................ 67

FIGURE 5.1: FT-IR TRANSMISSION SPECTRA OF PVA FILMS AND ITS COMPOSITES WITH CLAY CONCENTRATION OF 6 VOL. % IN THE

RANGE OF 1500–500 CM-1. ....................................................................................................................... 72

FIGURE 5.2: OPTICAL MICROGRAPHS OF THE PVA-CLAY NANOCOMPOSITE, WHERE PVA CONTAINS A) 2 VOL%, B) 4 VOL%, AND

C) 6 VOL% MMT-NA+ NANOCLAY, RESPECTIVELY. .......................................................................................... 73

FIGURE 5.3: UV-VIS SPECTRA OF PVA AND ITS COMPOSITES WITH DIFFERENT CLAY CONCENTRATIONS COATED ON GLASS

SUBSTRATES, A) TOTAL TRANSMITTANCE SPECTRA OF PVA AND ITS COMPOSITES WITH CLAY, B) TOTAL TRANSMITTANCE AT

600 NM AS A FUNCTION OF CLAY CONTENT, C) DIFFUSE TRANSMITTANCE SPECTRA OF PVA AND ITS COMPOSITES WITH CLAY,

D) DIFFUSE TRANSMITTANCE AT 600 NM AS A FUNCTION OF CLAY CONTENT. ......................................................... 74

FIGURE 5.5: BLOCKING EFFECT OF PVOH LAYERS HAVING DIFFERENT THICKNESSES. ........................................................ 76

FIGURE 5.6: WATER WEIGHT LOSS VS TIME FROM CUPS SEALED WITH FILMS OF PVA AND ITS COMPOSITES WITH MMT CLAY AT

40OC / 65%RH........................................................................................................................................ 77

FIGURE 5.7: EVOLUTION OF RELATIVE PERMEABILITY OF PVA FILMS WITH INCREASING CONTENT OF NANOCLAY PARTICLES.

(TRIANGLES – EXPERIMENTAL DATA. DDOTTED LINE - CALCULATED DATA) ACCORDING TO THE BHARADWAJ MODEL FOR AN

ASPECT RATIO OF Α = 500 AND ORDER PARAMETERS OF S = 0. ........................................................................... 78

FIGURE 5.8: RECIPROCAL WVTR OF PVA AND ITS MMT-NA+ CLAY COMPOSITE WITH LOADING CONCENTRATIONS OF 0-10

VOLUME %.VS NUMBER OF BENDING CYCLES WITH BENDING RADIUS OF 5 CM. EACH LAYER HAS A THICKNESS OF 100 µM.

............................................................................................................................................................. 79

FIGURE 5.9: OPTICAL MICROGRAPHS OF GLASS FLAKES. (A) GLASS FLAKES WITH THICKNESS ~ 1 µM, (B) GLASS FLAKES WITH

THICKNESS ~0.5 µM, (C) GLASS FLAKES WITH THICKNESS OF ~0.1 µM .................................................................. 81

FIGURE 5.10: MICROGRAPHS OF PVB FILMS CONTAINING 15 VOL% GLASS FLAKES OF AR = 400. A) OPTICAL MICROGRAPH (TOP

VIEW) (B) SEM CROSS SECTION OF A SEMI-POLISHED PVB/GLASS FLAKES COMPOSITE FILMC) EDX IMAGE OF THE

HIGHLIGHTED SECTION (FULLY POLISHED) OF THE PVB/GLASS FLAKES LAYER WITH SI MAPPING. ................................ 82

FIGURE 5.11: CONFOCAL MICROGRAPH SHOWING THE SURFACE OF THE PVB FILLED WITH 25 VOL% OF THE GLASS FLAKES, A)

GLASS FLAKES WITH A.R~200, B) GLASS FLAKES WITH A.R~400, AND C) GLASS FLAKES WITH A.R~2000. ................. 83

LIST OF FIGURES

e

FIGURE 5.12: SPECTRA OF A PRISTINE PVB FILM HAVING A THICKNESS OF ~ 70 µM, A) TOTAL TRANSMISSION (OPEN TRIANGLES IN

BLACK), DIFFUSE TRANSMISSION (FULL TRIANGLES IN RED), B) TOTAL REFLECTANCE (FULL SQUARE IN BLACK) AND DIFFUSE

REFLECTANCE (BLACK SQUARES). .................................................................................................................. 84

FIGURE 5.13: TRANSMITTANCE AND REFLECTANCE SPECTRA OF PVB CONTAINING GLASS FLAKES. A) TOTAL TRANSMITTANCE AND

DIFFUSE TRANSMITTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS FLAKES (A.R = 200), B) TOTAL

TRANSMITTANCE AND DIFFUSE TRANSMITTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS FLAKES (A.R = 400),

C) TOTAL TRANSMITTANCE AND DIFFUSE TRANSMITTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS FLAKES (A.R

= 2000), D) TOTAL REFLECTANCE AND DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS FLAKES

(A.R = 200), E) TOTAL REFLECTANCE AND DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF GLASS

FLAKES OF A.R = 400, AND F) TOTAL REFLECTANCE AND DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL%

OF GLASS FLAKES OF A.R = 2000. ................................................................................................................ 85

FIGURE 5.14: SIMULATED DIRECT/TOTAL TRANSMISSION OF A SINGLE GLASS FLAKE FILLED IN PVB MATRIX AT A WAVELENGTH OF

550 NM FOR DIFFERENT POLARIZATIONS OF THE INCIDENT LIGHT. ....................................................................... 87

FIGURE 5.15: SIMULATED TILT (Θ) AND ROTATION ANGLES OF FLAKES (Φ), FOR CONDITIONS A) VARIATION OF THE TILT ANGLE OF

THE FLAKES (0≤ 𝜃 ≤ 180𝑜) AND FIX ROTATION, B) FIXED TILT, ROTATION AROUND THE VERTICAL AXIS (0≤ 𝜙 ≤

360𝑜) AND C) RANDOM TILT AND RANDOM ROTATION AROUND THE VERTICAL AXIS, WHERE 𝑅 IS A VECTOR NORMAL TO

FLAKE SURFACE. ........................................................................................................................................ 87

FIGURE 5.16: SIMULATED TRANSMISSION (TOTAL AND DIFFUSE) (@550 NM) OF PVB FILMS FILLED WITH GLASS FLAKES AT

DIFFERENT PARTICLE VOLUME CONCENTRATIONS HAVINGRANDOM TILT, RANDOM ROTATION AROUND THE VERTICAL AXIS.

............................................................................................................................................................. 88

FIGURE 5.17: TRANSMISSION AND REFLECTANCE SPECTRA OF PVB CONTAINING GLASS FLAKES AFTER COATING EPOXY ON BOTH

SIDES OF PVB FILLED WITH FLAKES. A) TOTAL TRANSMISSION AND DIFFUSE TRANSMISSION SPECTRA OF PVB FILLED WITH

5-15 VOL% OF 200 ASPECT RATIO GLASS FLAKES, B) TOTAL TRANSMISSION AND DIFFUSE TRANSMISSION SPECTRA OF PVB

FILLED WITH 5-15 VOL% OF 400 ASPECT RATIO GLASS FLAKES, C) TOTAL TRANSMISSION AND DIFFUSE TRANSMISSION

SPECTRA OF PVB FILLED WITH 5-15 VOL% OF 2000 ASPECT RATIO GLASS FLAKES, D) TOTAL REFLECTANCE AND DIFFUSE

REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF 200 ASPECT RATIO GLASS FLAKES, E) TOTAL REFLECTANCE AND

DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF 400 ASPECT RATIO GLASS AND F) TOTAL REFLECTANCE

AND DIFFUSE REFLECTANCE SPECTRA OF PVB FILLED WITH 5-15 VOL% OF 2000 ASPECT RATIO GLASS FLAKES. ............ 90

FIGURE 5.18: DEPENDENCE OF DIFFUSE TRANSMISSION OF PVB FILMS @ 550 NM ON GLASS FLAKE LOADING BEFORE (BLACK

SQUARES) AND AFTER (RED DOTS) COATING EPOXY ON BOTH SIDES OF THE FILM, A) LAYERS OF FLAKES WITH Α ~200 B)

LAYERS OF FLAKES WITH Α ~400, AND C) LAYERS OF FLAKES WITH Α ~2000. ........................................................ 91

FIGURE 5.19: EXPERIMENTAL BARRIER IMPROVEMENT FACTOR OF THE BARRIER (PVB/GLASS FLAKES COMPOSITES) COMPARED

WITH BIF OF COMPOSITED ACCORDING TO BHARDWAJ’S MODEL. EXPERIMENTAL BIF OF FLAKES WITH Α ~200 (CLOSED

BLACK CIRCLE), BHARDWAJ’S SIMULATED (DOTTED BLACK LINE), EXPERIMENTAL BIF OF FLAKES WITH Α ~400 (BLUE CLOSED

TRIANGLE) AND BHARDWAJ SIMULATED (DOTTED BLUE LINE) AND EXPERIMENTAL BIF OF FLAKES WITH Α ~2000 (DARK

YELLOW SQUARE) AND BHARDWAJ SIMULATED (DOTTED DARK YELLOW LINE). VS THE GLASS FLAKES VOLUME

CONCENTRATION IN THE PVB LAYERS. ........................................................................................................... 92

LIST OF FIGURES

f

FIGURE 5.20: SEM CROSS SECTION IMAGE OF PVB FILLED WITH 25 VOL% OF GLASS FLAKES OF Α ~2000, HIGHLIGHTED AREA

SHOWS THE DEFECTS WITHIN THE LAYER. ........................................................................................................ 92

FIGURE 5.21: BARRIER IMPROVEMENT FACTOR OF THE PVB FILM FILLED WITH GLASS FLAKES (Α ~2000) AGAINST OXYGEN (RED

DOTS) AND MOISTURE (BLACK SQUARE) VS THE VOLUME FRACTION OF GLASS FLAKES. ............................................ 94

FIGURE 5.22: WVTR OF PVB FILM WITH 15 VOL% GLASS FLAKES (Α ~ 2000) VS. NUMBER OF BENDING CYCLES. ................. 95

FIGURE 5.23: UV–VIS SPECTRA OF P3HT FILMS ON GLASS ENCAPSULATED WITH A) A PVB LAYER AND B) A PVB FILLED WITH 25

VOL% OF Α ~2000 GLASS FLAKES DURING EXPOSURE TO THE LIGHT OF A SUN SIMULATOR IN AMBIENT AIR AT 65 C. C)

NORMALIZED ABSORBANCE LOSS AT 525 NM OF P3HT FILMS ENCAPSULATED WITH PLAIN PVB, PVB/GLASS FLAKES FILLED

BARRIER. ................................................................................................................................................. 96

FIGURE 5.24: DAMP HEAT DEGRADATION TEST (40 °C/85% RH) OF P3HT:PCBM BASED DEVICES ENCAPSULATED WITH PRISTINE

PVB FILMS AND PVB FILMS FILLED WITH 15% V/V GLASS FLAKES (Α = 2000). JSC: SHORT CIRCUIT CURRENT. VOC: OPEN

CIRCUIT CURRENT. FF: FILL FACTOR, PCE:. POWER CONVERSION EFFICIENCY. ........................................................ 97

FIGURE 5.25: SUN DEGRADATION TEST OF P3HT:PCBM BASED SOLAR CELLS ENCAPSULATED IN THREE DIFFERENT BARRIERS:

MITSUBISHI BARRIER FILM, PRISTINE PVB FILM, AND PVB FILM FILLED WITH 15% V/V GLASS FLAKES (Α = 2000). JSC: SHORT

CIRCUIT CURRENT. VOC: OPEN CIRCUIT CURRENT. FF: FILL FACTOR, PCE: POWER CONVERSION EFFICIENCY. .................. 99

FIGURE 5.26:FTIR SPECTRA OF A 500 NM THICK PHPS FILM COATED ON PE SUBSTRATE CURED WITH DEEP UV LIGHT FOR ~25

MINUTES WITH IRRADIATION DISTANCE OF 100 MM (RED CURVE), 30 MM (BLUE CURVE) AND 5 MM (PURPLE CURVE).

........................................................................................................................................................... 103

FIGURE 5.27: : FTIR SPECTRA OF PHPS FILM (500 NM THICK) CURED WITH THE COMBINATION OF HEAT (100OC) AND

IRRADIATION WITH 172 NM WAVELENGTH LIGHT AT A DISTANCE OF 5 MM FOR 15 MINUTES. ................................. 104

FIGURE 5.28. A) FTIR SPECTRA OF A 500 NM THICK PHPS FILM, CURED BY DEEP UV LIGHT FOR THE TIMES SPECIFIED IN THE

FIGURE. THE CORRESPONDING WVTR VALUES ARE GIVEN NEXT TO THE SPECTRA. B) FTIR PEAK RATIOS (1050 CM-1/(830

CM-1, OPEN CIRCLES) OF FIGURE 5.28 (A), CORRELATED WITH THE CORRESPONDING WVTR VALUES (FULL SQUARES) AT

DIFFERENT CURING TIMES (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION FROM JOHN WILEY AND

SONS). .................................................................................................................................................. 106

FIGURE 5.29: A) FTIR SPECTRA OF A 800 NM THICK PHPS FILM, CURED BY AN EXPOSURE TO DAMP HEAT @ 65OC/85% RH FOR

THE TIMES SPECIFIED IN THE FIGURE. THE CORRESPONDING WVTR VALUES ARE GIVEN NEXT TO THE SPECTRA. B) FTIR PEAK

RATIOS (1050 CM-1/(830 CM-1, OPEN CIRCLES) OF FIGURE 5.29A, CORRELATED WITH THE CORRESPONDING WVTR

VALUES (FULL SQUARES) AT DIFFERENT CURING TIMES. ................................................................................... 108

FIGURE 5.30. FTIR SPECTRA OF A 400 NM THICK PHPS FILM (RED CURVE) AND OF A STACK OF FOUR 100 NM THICK PHPS LAYERS

(BLACK CURVE), BOTH CURED WITH DEEP UV LIGHT FOR A TOTAL OF 12 MINUTES. (PUBLISHED IN REFERENCE [184] AND

REPRODUCED WITH PERMISSION FROM JOHN WILEY AND SONS). ..................................................................... 110

FIGURE 5.31: HYDROPHOBIC NATURE OF THE PHPS, A) DROPLETS OF WATER ON UN-CURED PHPS SURFACE, B) CONTACT ANGLE

(C.A) OF 92.8O ON UN-CURED PHPS, C) C.A OF 88.3O OF PHPS AFTER CURING AND D) CONTACT ANGLE OF PET

SUBSTRATE 80O. ..................................................................................................................................... 111

FIGURE 5.32: OPTICAL CALCIUM TEST OF PHPS BARRIER (270 NM) ON PET FILM (D = 125 µM) AFTER 50 BENDING CYCLES

(BENDING RADIUS OF 3 CM). THE PET/PHPS BARRIER FILM HAS BEEN LAMINATED (USING UV CURABLE ADHESIVE) ON A

CALCIUM FILM OF 200 NM IN THICKNESS, EVAPORATED ON A GLASS SLIDE. TESTING CONDITIONS: 65OC / 85 %RH. IMAGES

LIST OF FIGURES

g

TAKEN AFTER A) 1 H, B) 2 H, C) 5 H, D) 8 H AND E) 10 H (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH

PERMISSION FROM JOHN WILEY AND SONS). ................................................................................................ 112

FIGURE 5.33. RECIPROCAL WVTR VS. NUMBER OF BENDING CYCLES FOR STACKS OF DIFFERENT NUMBERS OF PHPS LAYERS ON

PET FILM ( D = 125 µM) AT A BENDING RADIUS OF 3 CM (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH

PERMISSION FROM JOHN WILEY AND SONS).. ............................................................................................... 112

FIGURE 5.34: TTRANSMISSION SPECTRA OF PET FILM AND OF PHPS/ACRYLIC/PHPS BARRIER COATED ON PET FILM ......... 114

FIGURE 5.35: NORMALIZED RECIPROCAL WVTR OF SANDWICH COATINGS WITH ALTERNATING PHPS AND ORGANIC LAYERS (SEE

INSET) VS. THE NUMBER OF BENDING CYCLES WITH A BENDING RADIUS OF 3 CM. EACH PHPS LAYER HAS A THICKNESS OF

~170 NM, WHILE THE ORGANIC LAYERS ARE ~ 5-10 UM THICK. FOR THE ORGANIC INTERLAYERS, TWO NON-CROSS LINKED

POLYMERS, NAMELY PVDF (BLACK SQUARES) AND PVB (RED CIRCLES) WERE USED. (PUBLISHED IN REFERENCE [184] AND

REPRODUCED WITH PERMISSION FROM JOHN WILEY AND SONS). ..................................................................... 115

FIGURE 5.36: RECIPROCAL WVTR OF SANDWICH COATINGS WITH ALTERNATING PHPS AND ORGANIC LAYERS (SEE INSET) VS. THE

NUMBER OF BENDING CYCLES WITH A BENDING RADIUS OF 3 CM. EACH PHPS LAYER HAS A THICKNESS OF ~170 NM, WHILE

THE ORGANIC LAYERS ARE ~ 5-10 µM THICK. FOR THE ORGANIC INTERLAYERS, TWO UV-CURABLE ADHESIVES ON EPOXY

(BLACK SQUARES) AND ACRYLIC BASIS (RED CIRCLES) WERE USED (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH

PERMISSION FROM JOHN WILEY AND SONS). ................................................................................................ 116

FIGURE 5.37: SCHEMATIC VIEW OF ENCAPSULATION OF P3HT FILMS ON GLASS, RIM ENCAPSULATED WITH PHPS BASED

SANDWICH BARRIER ON PET FILM. (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION FROM JOHN

WILEY AND SONS). .................................................................................................................................. 117

FIGURE 5.38: UV/VIS SPECTRA OF P3HT FILMS ON GLASS ENCAPSULATED WITH A) A PLAIN PET, B) A PHPS BASED BARRIER, C)

MITSUBISHI ( A COMMERCIAL BARRIER) AND D) GLASS DURING EXPOSURE TO THE LIGHT OF A SUN SIMULATOR IN AMBIENT

AIR AT 65OC. THE INCREASE OF ABSORPTION AT 400 NM IN (B) AND (C) IS DUE TO THE YELLOWING OF THE PET SUBSTRATES.

E) NORMALIZED ABSORBANCE LOSS AT 525 NM OF P3HT FILMS ENCAPSULATED WITH PLAIN PET, PHPS BASED BARRIER,

MITISUBISHI AND GLASS ON GLASS. (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION FROM JOHN

WILEY AND SONS). .................................................................................................................................. 118

FIGURE 5.39: SCHEMATIC VIEW OF ENCAPSULATION OF P3HT:PCBM BASED SOLAR CELL ON GLASS, RIM ENCAPSULATED WITH

PHPS BASED SANDWICH BARRIER ON PET FILM. ........................................................................................... 119

FIGURE 5.40: EFFICIENCY OF P3HT:PCBM BASED ORGANIC SOLAR CELLS ENCAPSULATED WITH PLAIN PET FOILS (PET),

COMMERCIAL MITSUBISHI BARRIER FOILS (MITSUBISHI), OR PHPS-BASED BARRIER FOILS (PHPS, FOR DETAILS SEE TEXT)

UPON EXPOSURE TO DAMP HEAT (40 °C/85% RH). (PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION

FROM JOHN WILEY AND SONS). ................................................................................................................. 120

FIGURE 5.41: DEVICE PERFORMANCE OF THE OSC DEVICES WITH EVAPORATED SILVER ELECTRODE OF THICKNESS 50 NM AND 100

NM, AND EACH DEVICE DIRECTLY COATED WITH ONE, TWO AND THREE PHPS LAYERS EACH HAVING THICKNESS OF 170 NM,

SHOWING PERFORMANCE IN TERMS OF PCE, FF, VOC, JSC,LIGHT INJECTION AND LEAKAGE OF INITIAL (GREY), AFTER COATING

(RED) AND AFTER CURING (GREEN). ............................................................................................................ 121

FIGURE 5.42: ORGANIC SOLAR CELLS DIRECTLY COATED WITH 170 NM PHPS A) BEFORE, B) AFTER A FEW HOURS OF EXPOSURE TO

DAMP HEAT (40OC/ 85%RH) [186]. ......................................................................................................... 122

LIST OF FIGURES

h

FIGURE 5.43: CRACKING IN THE PHPS LAYERS COATED ON OSC DEVICE WITHOUT TOP SILVER ELECTRODE, AFTER EXPOSURE TO

DAMP HEAT 65OC/ 85 RH, A) DEVICE COATED WITH CURED PHPS ON TOP, B) MICROGRAPH OF THE DEVICE SHOWN

HIGHLIGHTED SECTION OF (A), SHWOING DELAMINATED AREAS, C) DEVICE COATED WITH UN-CURED PHPS AND D)

MICROGRAPH OF HIGHLIGHTED SECTION OF (C) SHOWING SPIRAL CRACK............................................................. 123

FIGURE 5.44: SCHEMATIC DIAGRAM OF AN ORGANIC SOLAR CELL (OSC) ON GLASS, DIRECTLY COATED WITH TWO PHPS LAYERS

(170 NM EACH), ALTERNATING WITH THREE LAYERS OF ROLIC (~ 5 µM EACH) ALONG WITH ZNO (100 NM) AS A

SEPARATING LAYER[186]. ......................................................................................................................... 124

FIGURE 5.45: ORGANIC SOLAR CELLS DIRECTLY COATED WITH TWO PHPS LAYERS (170 NM EACH), ALTERNATING WITH THREE

LAYERS OF ROLIC (~ 5 µM EACH) ALONG WITH ZNO (100 NM) AS A SEPARATING LAYER AFTER A FEW HOURS OF EXPOSURE

TO DAMP HEAT (40OC/ 85%RH) [186]. ..................................................................................................... 125

FIGURE 5.46: TIME TRACES OF NORMALIZED POWER CONVERSION EFFICIENCY (PCE) OF P3HT:PCBM BASED OSCS WHICH HAVE

BEEN DIRECTLY COATED WITH A BARRIER FILM STACK, CONSISTING OF 2 PHPS LAYERS (THICKNESS ~200 NM EACH),

ALTERNATING WITH 3 INTERLAYERS OF ACRYLIC ADHESIVE (EACH ~ 5 µM THICK), SUBJECTED TO ACCELERATED LIFETIME

TESTING UNDER DAMP HEAT (40°C, 85% RH) AND SUN SOAKING AT 65 °C. INSET SHOWS THE NORMALIZED JSC OF THE

SAME DEVICES(PUBLISHED IN REFERENCE [184] AND REPRODUCED WITH PERMISSION FROM JOHN WILEY AND SONS).

........................................................................................................................................................... 126

FIGURE 5.47: OPTICAL MICROGRAPHS OF CRACKS IN THE SUN TESTED DEVICE, A) BRIGHT FIELD IMAGE, B) DARK FIELD IMAGE AND

C) BOTTOM ILLUMINATED IMAGE. ............................................................................................................... 127

LIST OF ABBREVIATIONS, SYMBOLS AND CONSTANTS

i


∆𝑚 Change of flux in gram

∆𝑡 Change of time

∆𝑝 Change of pressure

𝜏 Tortuous path

𝜋 Pi ~ 3.152

Φ Volume fraction

Å Angstrom

ASTM American standard for testing of materials

ALD Atomic layer deposition

A.R Aspect ratio

AFM Atomic force microscope

ATR Attenuated total reflection

AIP Aluminum iso-propoxide

atm Atmospheric

𝛼 Aspect ratio

BA Benzyl alcohol

BIF Barrier improvement factor

BOPP Biaxially oriented polypropylene

C.A Contact angle

cm3 Cubic centimeter


j

𝐷 Co-efficient of diffusion

DUV Deep ultra violet

EVOH Ethylene vinyl alcohol

EVA Ethylene vinyl acetate

FF Fill factor

FTIR Fourier-transform infrared spectroscopy

GO Graphene oxide

gALD Gas atomic layer deposition

g gram

HgLP Low pressure mercury lamps

HDPE High density polyethylene

HEC Hectorite

HTL Hole transport layer

ITO Indium tin oxide

IMI ITO-metal(ag)-ITO

I Intensity

Io Initial intensity

Jsc Short circuit current

kJ/mol Kilo joul per mole

L Length

LDPE Low density poly ethylene


k

LCP Liquid crystal polymer

MMT Montmorillonite

mm/s Millimeter per second

m2 Square meter

N2 Nitrogen

nm nanometer

OFETs Organic field-effect transistors

OLED Organic light emitting diodes

O2 Oxygen

OPV Organic photovoltaic

OSC Organic solar cells

OEDs Organic electronic devices

OTR Oxygen transmission rate

O3 Oxone

ORMOCERS Organically modified ceramics

PV Photovoltaic

PET Polyethylene terephthalate

PEN Polyethylene naphthalene

PAN Polyacrylonitrile

PECVD Plasma enhanced chemical vapor deposition

PEPVD Plasma enhanced physical vapor deposition

https://en.wikipedia.org/wiki/Montmorillonite


l

PS Polystyrene

PTFE Polytetrafluorethylene

PCdC Polycarbonate di chloride

PC Poly carbonate

PA Poly amide

PLA Poly lactic acid

PA 6 Polyimide 6

PVA Polyvinyl alcohol

PCE Power conversion efficiency

PBS Polybutylene succinate

PEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate

PC60BM Phenyl-C61-butyric acid methyl ester

P3HT Poly(3-hexylthiophene)

PVOH Poly vinyl alcohol

PVB Polyvinyl butylene

PMMA Poly(methyl methacrylate)

PHPS Perhydropolysilazane

RH Relative humidity

R Resistance

𝑟 Radius

sALD Solution atomic layer deposition

https://en.wikipedia.org/wiki/Polybutylene_succinate


m

SEM Scanning electron microscope

S Coefficient of solubility

TFT Thin-film-transistor

TEOS Tetraethyl orthosilicate

TEAOH Tetraethylammonium hydroxide

TGA Thermo gravimetric analysis

UVA Ultraviolet (category A)

UK United kingdom

UV Ultra violet

µm Micro meter

µl Micro liters

Voc Open circuit current

Vol% Volume percent

VUV Vacuum ultraviolet

Wt% Weight percent

W Width

WVTR Water vapor transmission rate

Wm-2 Watt per sq. meter

x Thickness

ZnO Zinc oxide

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