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Interface and composition engineering towards stable
and efficient organic-inorganic perovskite solar cells
Optimierung von Grenzflächen und Zusammensetzung für stabile und
effiziente organisch-anorganische Perowskit-Solarzellen
Der Technischen Fakultät
der Friedrich-Alexander-Universität
Erlangen-Nürnberg
zur
Erlangung des Doktorgrades Dr.-Ing.
vorgelegt von
Haiwei Chen
aus Hubei, China
Als Dissertation genehmigt
von der Technischen Fakultät
der Friedrich-Alexander-Universität Erlangen-Nürnberg
Tag der mündlichen Prüfung: 29.01.2018
Vorsitzende des Promotionsorgans: Prof. Dr.-Ing. Reinhard Lerch
1.Gutachter: Prof. Dr. Christoph J. Brabec
2.Gutachter: Prof. Dr. Dirk W. Schubert
i
Acknowledgments
First of all, I am deeply grateful to my supervisor Prof. Prof. Christoph J. Brabec for his
professional guidance, constant support and patience. Without his fruit full discussions,
enthusiasm and encouragement, this thesis would hardly have been completed.
I also owe a great debt of gratitude to all my colleagues at iMEET for their valuable help about
my research and life. I am indebted to many of my colleagues Shi Chen, Dr. Yi Hou, Dr. Ning Li,
Dr. Andres Osvet who have contributed a lot to this thesis. I sincerely thank Dr. Siegfried Eigler
and Dr. Christian E. Halbig for their great help about oxo-graphene. In addition, I would like to
thank Xiaofeng Tang for his great help with SEM characterization. I am deeply grateful Dr. Ole
Lytken for his valuable help with XPS characterization. Besides, I would like to thank Chen Xie,
Dr. Hong Zhang and for their AFM help. I acknowledge Dr. Jens Adams, Felix Hoga, Dr.
Thomas Hellmuller, Simon Kahmann, Stefan Langner, and Andrej Classen for their technical
support and discussion about lifetime test. I would like to thank Ening Gu for her help with XRD
measurement.
I want to express my gratitude to Corina Winkler, Leonid Kuper and other colleagues for
providing necessary facilities, clean lab environment and warm help for my research. I am
grateful to Claudia Koch, Ulrike Knerr, Manuela Baumer and Mr. Batentschuk for the kind help
about conferences and my daily lives during my PhD study.
I would like to show my gratitude to the CSC, which supports my research and daily lives in
German.
Last but not the least, I sincerely acknowledge my beloved family and my friends for their
understanding and constant support during this time.
Haiwei Chen
ii
Erlangen
December, 2017
iii
Abstract
In the past few years, organic-inorganic perovskite solar cells have drawn considerable attention
because of their excellent optoelectronic properties, easy-processability and low cost. A high
power-conversion efficiency of over 22% has been achieved for organic-inorganic perovskite
solar cells, which is a promising candidate as a low-cost photovoltaic technology. However, the
intrinsic instability of perovskite solar cells owing to moisture or water hampers their large-scale
practical application in ambient environment. This thesis targets on the development and
illustrating related mechanisms the high-efficiency and stable perovskite solar cells.
In the first part of this thesis, a novel hole transporting material (solution-processable sulfated
graphene oxide, SGO) is exploited. It is employed as the hole transporting materials instead of
hydrophilic PEDOT:PSS for organic-inorganic perovskite solar cells. An impressive power
conversion efficiency (PCE) of 15.2% was achieved for the resulting perovskite devices with the
planar inverted architecture. In addition, the resulting device shows a higher open-circuit voltage
of close to 1.1 V than its counterparts based on PEDOT:PSS. Moreover, approximately 92% of
its original PCE of the unpackaged perovskite solar cell is kept under ambient atmosphere and in
the dark after around 1900 h. Moreover, without encapsulation, approximately 80% of the initial
PCE for perovskite device in combination with SGO hole transporting layer is retained under 0.5
sun illumination and nitrogen atmosphere after around 500 h. Besides, ~60% of initial PCE for
organic-inorganic perovskite solar cells without encapsulation is maintained under 0.5 sun light
soaking and ambient atmosphere with the temperature lower than 30 °C after ~1000 h. It suggests
SGO layer play a role of effectively blocking the diffusion of moisture into the perovskite film,
which leads to dramatically improved photo-stability and environmental stability of unsealed
perovskite devices. This research illustrates the importance of exploiting hydrophobic interfacial
iv
materials and inhibiting the diffusion of moisture into the perovskite solar cells, which is vital for
efficient perovskite devices with impressive lifetime.
The second part of this thesis develops aqueous processed [6,6]-phenylC61butyric acid methyl
ester (PCBM) nanoparticles (PCBM NP). Organic-inorganic perovskite solar cells were
fabricated via employing an ultrathin aqueous-processed PCBM NP layer as the electron
transporting layer. PCBM in chlorobenzene (PCBM CB) have been used as electron transport
layer for high-efficiency perovskite solar cells. However, its solvent is toxic and PCBM CB layer
can be washed off the solvent of perovskite precursor such as DMF and DMSO. To increase its
resistance to solvents of perovskite precursor, aqueous processed PCBM NP were developed.
With the green and environmentally safe processing, the perovskite solar cells are fabricated and
optimized as a function of the processing conditions. Then, the lifetime of devices based on
PCBM CB and PCBM NP is compared (under 1 sun light soaking in nitrogen). Aggressive
long-term stability measurements of perovskite devices based on PCBM NP and PCBM CB
layers are carried out. The long-term stability of the unencapsulated devices based on PCBM NP
and PCBM CB is compared under constant light illumination (1 sun). The PCE of the PCBM
NP-based perovskite device maintains 86.2% of its original value within the 920 h. By contrast,
the PCBM CB-based device exhibits worse stability, retaining only 62.7% of its original PCE
after 920 h. Interface engineering plays a key role in improving the PCE and lifetime of
organic-inorganic perovskite solar cells.
In the third part of this thesis, composition engineering is employed to exploit an efficient and
stable perovskite solar cell. The optimized perovskite solar cell based on FA0.85Cs0.15PbI2.4Br0.6
exhibits the best power-conversion efficiency and superior photo-stability. There is only slight
decrease for the FA0.85Cs0.15PbI2.4Br0.6-based perovskite solar cells after ~500 h under constant 1
sun illumination in nitrogen, indicating composition engineering is vital for enhancing the
photostability of perovskite solar cells.
v
Zusammenfassung
Organisch-anorganische Perowskit-Solarzellen haben, aufgrund ihrer exzellenten
optoelektronischen Eigenschaften und ihrer einfachen Herstellung via Druckverfahren, in den
letzten Jahren stark an Aufmerksamkeit gewonnen. Bisher wurden einige der höchsten
Wirkungsgrade von über 22% erreicht, was sie zu einem vielversprechenden Kandidaten
einer kostengünstigen Photovoltaiktechnologie macht. Jedoch erschwert die Instabilität von
Perowskit-Solarzellen gegenüber Feuchtigkeit und Wasser die großflächige Umsetzung der
Technologie in natürlichen Umgebungen. Um stabile und hocheffiziente
Perowskit-Solarzellen entwickeln zu können, ist das Ziel der Arbeit die Untersuchung und
Beschreibung der verschiedenen Degradationsmechanismen.
Im ersten Teil der Arbeit wird auf die Entwicklung einer SGO, welche das hydrophile
PEDOT:PSS als Lochtransportmaterial in Organometalltrihalogenid Perowskit-Solarzellen
ersetzen soll, eingegangen. Die mit SGO hergestellten Solarzellen basierend auf einer
planar-invertierten Zellarchitektur weisen deutlich höhere Wirkungsgrade (PCE: engl. power
conversion efficiency) von bis zu 15,2% und, viel wichtiger, volle Leerlaufspannungen (Voc:
engl. open circuit voltage) von bis zu 1.1 V auf. Des Weiteren blockiert SGO erfolgreich das
Eindringen von Feuchtigkeit in das Bauteil, was zu einer signifikant besseren
Umweltstabilität von unverpackten Perowskit-Solarzellen führt. Bei einer Lagerung an Luft,
einer kontinuierlicher Beleuchtung von 0,5 Sonnen und Temperaturen von unter 30 °C ist die
PCE nach 500 Stunden auf 80% und nach etwa 1000 Stunden auf ca. 60% der ursprünglichen
Effizient gefallen. Darüber hinaus behalten die unverpackten Solarzellen nach einer 1900
stündigen Aufbewahrung im Dunkeln und unter Umgebungsbedingungen 92% der
Ausgangseffizient. Unsere Resultate untermauern, dass das Kontrollieren der Wasserdiffusion
in Perowskit-Solarzellen durch das Entwickeln von Zwischenschichten ein entscheidender
Schritt in Richtung langandauernder Umweltstabilität ist.
Der zweite Teil der Arbeit behandelt die Entwicklung und Untersuchung von PCBM
Nanopartikeln (PCBM-NP), welche durch einen wasserbasierten Prozess synthetisiert werden.
vi
Dabei werden organisch-anorganische Perowskit-Solarzellen mit einem dünnen Film aus
PCBM-NP, welche als elektronenleitende Schicht dient, hergestellt. PCBM, verarbeitet aus
einer Chlorbenzollösung, wurde schon als Elektronenleiter in hocheffizienten
Perowskit-Solarzellen angewendet. Jedoch, aufgrund der Giftigkeit von Chlorbenzol und der
Tatsache, dass DMF und DMSO, welche als Lösungsmittel der Perowskitvorstufe dienen,
PCBM abwaschen, wurde ein wasserbasierter Beschichtungsprozess entwickelt, um PCBM
widerstandsfähiger gegenüber diesen Lösungsmittel zu machen. Im Folgenden, basierend auf
einer umweltfreundlichen Herstellung, wurden Perowskit-Solarzellen als eine Funktion der
Prozessparameter hergestellt und optimiert. Danach wurden aggressive
Langzeitstabilitätsmessungen der Solarzellen basierend auf PCBM-CB und PCBM-NP
durchgeführt und die Ergebnisse miteinander verglichen (Testbedingungen:
Beleuchtungsstärke von einer Sonne unter einer Stockstoffatmosphäre). Solarzellen basierend
auf PCBM-NP behielten nach 920 Stunden 86,2% ihrer Ausgangseffizienz, wobei die
PCBM-CB basierten Solarzellen mit 62,7% Ausgangseffizienz eine deutlich schlechtere
Stabilität aufweisen. Das Entwickeln von Zwischenschichten spielt eine Schlüsselrolle bei
der Verbesserung der Effizienz und Lebenszeit von organisch-anorganischen
Perowskit-Solarzellen.
Im dritten Teil der Arbeit wird die Zusammensetzung der Perowskitschicht verändert, um
eine effiziente und stabile Perowskit-Solarzelle zu erzeugen. Mit FA0.85Cs0.15PbI2.4Br0.6 wurde
die optimalste Zusammensetzung gefunden, welche den besten Wirkungsgrad und eine
außergewöhnliche Photostabilität aufweist. Bei einer konstanten Beleuchtung von einer
Sonne und unter Stickstoff, sind nach etwa 500 Stunden nur geringe Leistungsverluste der
FA0.85Cs0.15PbI2.4Br0.6 basierten Solarzellen zu erkennen.
vii
Ag Silver
Al Aluminum
Au Gold
ZnO Zinc oxide
Ta-WOx Tantalum doped tungsten oxide
PDCBT Poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,
5′-2,2′-bithiophene]
PC60BM [6,6]-Phenyl-C61-Butyric-acid-Methyl ester
NP Nanoparticle
PEDOT:PSS Poly(ethylenedioxythiophene):poly(styrene sulfonic acid)
P3HT Poly(3-hexylthiophene-2,5-diyl)
spiro-MeOTAD 2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluoren
e
PTAA poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]
ETL Electron transporting layer
HTL Hole transporting layer
HTM Hole transporting materials
ITO Indium tin oxide
J-V Current density-voltage
EQE External quantum efficiency
VB Valence band
Abbreviations
viii
CB Conduction band
HOMO Highest Occupied Molecular Orbital
LUMO Lowest Unoccupied Molecular Orbital
WF Work function
AM 1.5G Air Mass 1.5 Global
PV Photovoltaic
Eg Bandgap
PL Photoluminescence
KP Kelvin probe
AFM Atomic Force Microscopy
SEM Scanning electron microscope
XPS X-ray photoelectron spectroscopy
XRD
X-ray diffraction
ix
Symbols
PCE Power-conversion efficiency %
FF Fill Factor
Voc Open-circuit voltage V
Jsc Short Current density mA/cm2
A Device active area cm2
I Light intensity mW/cm2
EF Fermi level eV
Pin Input power mW/cm2
R Resistance Ωcm2
T Temperature
Φ Work function eV
θ Contact angle o
1
Contents
ACKNOWLEDGMENTS ................................................................................................................ I
ABSTRACT ................................................................................................................................. III
ZUSAMMENFASSUNG ............................................................................................................... V
ABBREVIATIONS ................................................................................................................ VII
SYMBOLS ............................................................................................................................... IX
CHAPTER 1 ............................................................................................................................. 1
INTRODUCTION.................................................................................................................... 1
1.1 Photovoltaic devices .................................................................................................................................. 2
1.2 Evolution of perovskite solar cells ............................................................................................................ 4
1.2.1 Evolution of perovskite semiconductors ................................................................................................... 4
1.2.2 Intrinsic properties of organic-inorganic perovskite .................................................................................. 5
1.2.2.1 High absorption coefficient .................................................................................................................... 5
1.2.3 Progress of perovskite solar cells .............................................................................................................. 7
1.2.4 Architecture of perovskite solar cells ........................................................................................................ 8
1.2.5 Depositing techniques ............................................................................................................................... 9
1. 3 Stability challenges of perovskite devices .............................................................................................. 11
1.4 Aim and content of this thesis ................................................................................................................. 13
CHAPTER 2 ........................................................................................................................... 16
THEORY ................................................................................................................................ 16
2.1 Work principles of perovskite solar cells ................................................................................................ 17
2.1.1 Current density-voltage (J-V) characteristic ............................................................................................ 20
2
2.2 Intrinsic stability of perovskite materials ............................................................................................... 22
2.3 Degradation process of perovskite .......................................................................................................... 24
2.3.1 Moisture-induced degradation ................................................................................................................. 24
2.3.2 Thermal-induced degradation .................................................................................................................. 26
2.3.3 photo-induced degradation ...................................................................................................................... 26
CHAPTER 3 ........................................................................................................................... 28
STATE OF THE ART ............................................................................................................ 28
3.1 Interface engineering .............................................................................................................................. 28
3.2 Composition engineering ........................................................................................................................ 34
3.3 Electrode engineering ............................................................................................................................. 37
CHAPTER 4 ........................................................................................................................... 41
EXPERIMENTAL SECTION .............................................................................................. 41
4.1 Materials ................................................................................................................................................. 42
4.1.1 Perovskite layer ....................................................................................................................................... 42
4.1.2 ETMs and HTMs ..................................................................................................................................... 42
4.2 Device Fabrication .................................................................................................................................. 45
4.2.1 Spin-coating technique ............................................................................................................................ 45
4.2.2 P-i-n architecture ..................................................................................................................................... 46
4.2.3 N-i-p architecture..................................................................................................................................... 46
4.3 Device Characterization ......................................................................................................................... 47
4.3.1 J-V and EQE measurement ...................................................................................................................... 47
4.3.2 Lifetime test ............................................................................................................................................. 47
4.3.3 ATR-FTIR ................................................................................................................................................ 48
4.3.4 AFM ........................................................................................................................................................ 49
4.3.5 Scanning electron microscope (SEM) ..................................................................................................... 49
3
4.3.6 Photoluminescence spectroscopy (PL) .................................................................................................... 50
4.3.7 Surface energy ......................................................................................................................................... 50
4.3.8 X-ray photoelectron spectroscopy ........................................................................................................... 50
4.3.9 X-ray powder diffraction (XRD) ............................................................................................................. 50
4.3.10 Raman spectroscopy .............................................................................................................................. 51
CHAPTER 5 ........................................................................................................................... 52
5.1 Solution processed SGO ......................................................................................................................... 53
5.2 Morphologies of SGO ............................................................................................................................. 57
5. 3 AFM characterization of perovskites .................................................................................................... 58
5.4 J-V and EQE characteristics .................................................................................................................. 59
5.5 Work function ......................................................................................................................................... 61
5.6 Hysteresis ................................................................................................................................................ 62
5.7 Lifetime characterization ....................................................................................................................... 64
5.8 Contact angle .......................................................................................................................................... 67
5.9 WVTR measurement .............................................................................................................................. 68
5.10 ATR-FTIR measurement ...................................................................................................................... 70
5.11 Conclusions ........................................................................................................................................... 71
CHAPTER 6 ........................................................................................................................... 72
AQUEOUS PCBM NANOPARTICLES FOR EFFICIENT AND STABLE
PEROVSKITE SOLAR CELLS .......................................................................................... 72
6.1 Aqueous PCBM nanoparticles ................................................................................................................ 73
6.2 SEM characterization ............................................................................................................................. 76
6.3 XPS characterization .............................................................................................................................. 77
6.4 Contact angle measurement ................................................................................................................... 78
6.5 SEM images and XRD of perovskites ..................................................................................................... 79
4
6.6 UV-Vis absorption ................................................................................................................................... 80
6.7 Hysteresis ................................................................................................................................................ 81
6.8 Box plots of performance ........................................................................................................................ 82
6.9 J-V and EQE characteristics .................................................................................................................. 83
6.10 Lifetime test .......................................................................................................................................... 84
6.11 Conclusion ............................................................................................................................................. 85
CHAPTER 7 ........................................................................................................................... 86
COMPOSITION ENGINEERING TOWARDS EFFICIENT AND STABLE
PEROVSKITE SOLAR CELLS .......................................................................................... 86
7.1 Composition engineering ........................................................................................................................ 87
7.2 UV-Vis Characterization ........................................................................................................................ 90
7.3 SEM characterization ............................................................................................................................. 91
7.4 Boxplots of photovoltaic performance .................................................................................................... 92
7.5 J-V and EQE characteristics .................................................................................................................. 93
7.6 Lifetime test ............................................................................................................................................ 93
CHAPTER 8 ........................................................................................................................... 96
SUMMARY AND OUTLOOK ............................................................................................. 96
8.1 Summary ................................................................................................................................................. 96
8.2 Outlook ................................................................................................................................................... 98
REFERENCES ..................................................................................................................... 103
1
Chapter 1
Introduction
In this chapter, the evolution of photovoltaic technology is reviewed, followed by introducing
the origin of organic-inorganic hybrid perovskite materials. Their optical and electronic
properties are briefly discussed. Structures and techniques for fabricating perovskite devices
are also noted. Besides, challenges for the stability of perovskite solar cells are discussed.
2
1.1 Photovoltaic devices
With the enormous development of society, consumption fossil fuel resources increased
dramatically in the last several decades. It caused huge environmental problems such as
pollution, global warming and climate change. In addition, the global energy crisis caused by
the limited natural energy resources and increasing price is an urgent issue the world is facing
nowadays. Several kinds of potential renewable energy such as wind energy, nuclear energy
and solar energy have been exploited to solve these problems. Solar energy has attracted
extensive attention since it is eco-friendly and sustainable [1]. A total of around 170,000
TW-hours of solar energy strikes the Earth every hour continuously. It’s more than 10,000
times the humanity’s energy needs over an entire year. Solar panels with ~2% power
conversion efficiency (PCE) covering 10% the world’s deserts (around 1.5% of the total land
area on Earth) are sufficient to cover all of the world’s energy needs. Some commercial solar
panels based on silicon and CdTe have been installed in several countries and areas. However,
their further large-scale application is still limited by its high cost and related pollution
problems.
The history of photoelectric effect dated back to as early as 1839, which was first observed by
a French physicist, Edmund Bequerel. He found small amount of electric current was
generated when certain materials were exposed to light. The first photovoltaic cell was built
in 1883. However, it only exhibited ~1%，which is far from practical application. Then the
PCE was significantly increased to 6% by Bell Laboratories in 1954. After that, photovoltaic
devices attracted extensive attention. But its widespread use was still limited because it was
too expensive to produce. In the 1960s, benefiting from the space programs, the photovoltaic
devices and related fundamental mechanism were extensively investigated. Its reliability was
drastically improved and the fabrication cost started to decrease. In the 1970s, energy crisis
made photovoltaic technology recognized as an available alternative source of energy for
non-space applications. Nowadays, the highest PCE of 26.7 % was obtained in a practical
crystalline Si solar cell, which pinpoints a path to approaching to the theoretical
thermodynamic limit of 29.4%[2]. However, its large-scale application is still limited by its
complicated fabrication process, high cost and pollution problems. Therefore, to solve these
3
problems, third-generation photovoltaic devices such as quantum dot solar cells,
dye-sensitised solar cells and perovskite solar cells have been extensively studied[3, 4]. In
principle, the upper limit of tandem solar cells is higher than that of single-junction solar
cells.
Figure 1.1 Golden triangle for organic-inorganic perovskite solar cells showing the
relationship about stability, efficiency and cost.
Desired perovskite solar cells should exhibit high power-conversion efficiency and stability,
whereas have low cost. Since perovskite solar cells are solution-processable at low
temperature, it is energy-saving. Some raw materials for fabricating perovskite solar cells
such as methylammonium and lead iodide are earth-abundant and relatively cheap. The
perovskite solar cells exhibit low cost and high power conversion efficiency close to Si solar
cells. However, its stability under illumination and ambient condition is far behind its
counterparts such as the silicon solar cells, which impedes its large-scale practical
application.
Table 1. Summarization of representative research-cell efficiencies, including relevant
parameters such as power conversion efficiency and the open circuit voltage (Adapted after
[5] with permission from Wiley-VCH).
4
1.2 Evolution of perovskite solar cells
1.2.1 Evolution of perovskite semiconductors
Hybrid organic-inorganic perovskite materials have been investigated for more than a century.
However, their initial semiconductor applications such as light-emitting diodes and thin-film
transistors, began in the last twenty years. A number of exciting physical properties such as
low exciton binding energy, long carrier diffusion length and high absorption coefficient have
been extensively investigated[6-9]. The hybrid organic-inorganic perovskite semiconductor
material takes the common ABX3 structure and is usually composed of an inorganic or
organic monovalent cation, A=Cs+, ethylammonium (EA) CH3CH2NH3+, n-butylammonium
(BA) CH3(CH2)3NH3+, methyl-ammonium (MA) CH3NH3
+ and formamidinium (FA)
Classification Voc
(V)
Jsc
(mA/cm2)
FF
(%)
PCE
(%)
GaInAsP/GaInAs 2.024 19.51 82.5 32.6 ± 1.4
Si (crystalline) 0.740 42.5 84.7 26.6 ± 0.5
Si (multicrystalline)
Perovskite
CIGS
0.6717 40.55 80.9 22.0 ± 0.4
1.144 24.92 79.6 22.7 ± 0.8
0.7411 37.76 80.6 22.6 ± 0.5
CdTe 0.8872 31.69 78.5 22.1 ± 0.5
GaInP 1.4932 16.31 87.7 21.4 ± 0.3
Organic 0.8150 20.27 73.5 12.1 ± 0.3
Dye 0.744 22.47 71.2 11.9 ± 0.4
Perovskite/Si 1.651 18.09 79.0 23.6 ± 0.6
5
CH3(NH2)2+), a divalent cation, B = (Pb2+; Ge2+ and Sn2+), and an anion X = (Cl-; Br-; I-;
SCN-; BF4- and PF6
-;)[10-14].
There are mainly four kind of crystal structures for organic-inorganic perovskite solar cells:
(a) zero-dimensional like CH3NH3PbI3, (b) one-dimensional like (CH3NH3)2PbI4, (c)
two-dimensional like (C10H21NH3)2PbI4, and (d) three-dimensional like
(CH3NH3)4PbI6•2H2O[15]. Zero-dimensional perovskites are the most widely used
perovskites now. The bandgap of perovskites can be tuned by substituting cations or anions,
which renders them as absorbers for semitransparent perovskite solar cells with various
colour. The resulting solar cells can be integrated into the building as windows. Currently,
solar cells based on two-dimensional perovskite exhibit significantly enhanced photo-stability
and chemical stability during operation in comparison with other kinds of perovskite.
However, its power conversion efficiency is still much lower than solar cells based on other
perovskites like CH3NH3PbI3. Researchers around the world make tremendous effort to
developing novel perovskites with various composition. Combining the perovskites with
different crystal structures may reach a balance between the power conversion efficiency and
lifetime under various conditions.
1.2.2 Intrinsic properties of organic-inorganic perovskite
1.2.2.1 High absorption coefficient
The absorption coefficient of CH3NH3PbI3 was calculated to be 1.5× 104 cm-1 at 550 nm via a
CH3NH3PbI3/TiO2 composite film. It implied that the penetration depth for light with
wavelength of 550 nm is 660 nm. In contrast, the absorption coefficient at 700 nm was 0.5×
104 cm-1, indicating a penetration depth of 2 μm for 700 nm light. Overwhelming majority of
incident light can be absorbed by the perovskite film with a thickness of around 2 μm, which
rendered it as a strong candidate for the absorbers of thin-film solar cells[16-18]. It is much
thinner than its counterparts such as crystalline silicon solar cells which usually have a
thickness of hundreds micrometer. Therefore, it significantly reduces the consumption of
perovskites.
6
Figure 1.2 A representative UV-Vis absorption spectrum for the CH3NH3PbI3/TiO2 composite
film. Absorption coefficient α was calculated from the equation T=I/I0=exp(-αl) (1-1, Adapted
from [16] with permission from Nature Publishing Group). T, I0, I and l are transmittance,
incident light intensity, transmitted light intensity and TiO2 layer thickness, respectively.
1.2.2.2 Long charge diffusion length
When the incident light is absorbed in the organic-inorganic perovskite materials,
electron-hole pairs were generated in perovskite. Bound electron-hole pairs, the primary
photoexcited species generated in the absorption process, play an important role in
understanding the way that the photovoltaic devices function. These carriers could still exist
as free carriers or became excitons depending on the exciton binding energy [17, 18]. It has
been demonstrated that a direct and accurate spectroscopic measurement of the exciton
binding energy can be carried out by using high magnetic fields. It can both transport holes
and electrons. It is demonstrated to be only 16 meV for CH3NH3PbI3 films at low
temperatures, which is much smaller than those of previous reports and comparable to the
exciton binding energies of ~25 meV. It leads to the quite long diffusion length of the
three-dimensional organic-inorganic perovskite materials[19]. It is relatively longer than that
of polymer solar cells. Therefore, most charge can be transferred to the electrodes and
extracted before being quenched.
7
1.2.2.3 Easy processing and low cost
The raw materials for organic-inorganic perovskite precursor are soluble in solvents such as
DMF and DMSO, which means perovskite film are solution-processable. Perovskite can be
fabricated either via vapour techniques or in solution like spin-coating, which requires
relatively low temperatures below 150 , suggesting its relatively simpler process and low
cost compared with conventional solar cells[20]. In addition, it has been demonstrated that
the charge transporting layers and some electrodes like carbon electrode can be processed via
printing techniques at low temperatures (＜150 )[21]. It means the consumption of energy
for fabricating perovskite solar cells is much lower than that of its counterparts such as
silicon solar cells. It allows for the fabrication of flexible perovskite solar cells on transparent
plastic substrates such as PEDOT:PSS/polyethylene terephthalate (PET) or indium tin
oxide-coated PET substrates. Flexible conducting plastic substrates can be potentially
cheaper than the rigid conducting glass counterparts[22]. Besides, exploiting flexible
perovskite modules allows for its large-scale production via roll-to-roll manufacturing, thus
reducing industrial cost and permitting practical application.
1.2.3 Progress of perovskite solar cells
Their first application in solar cells was reported by Miyasaka and coworkers in 2009[23]. In
2011, the power conversion efficiency (PCE) of liquid organic-inorganic CH3NH3PbI3
sensitized solar cells was dramatically improved to 6.54%[24] . However, their stability was
limited due to dissolution of organic-inorganic CH3NH3PbI3 in the solvent of electrolyte such
as γ-butyrolactone. To tackle down this critical problem, a small molecule
2,2(7,7-(tetrakis-(N,N-di-p-methoxyphenylamine)-9,9-(spirobifluorene))) (spiro-MeOTAD),
a solid organic electrolyte, has been employed to fabricate all-solid-state perovskite solar
cells. Organic-inorganic CH3NH3PbI3 solar cells based on spiro-MeOTAD have obtained a
PCE of around 9.7%[25]. Perovskite solar cells combined with CH3NH3PbI2Cl and
mesoporous TiO2 obtain a PCE of 7.6%. Perovskite solar cells with insulating Al2O3 further
improve the PCE to 10.9%[26]. Al2O3 acts as framework for the organic-inorganic perovskite
film. In recent years, perovskite solar cells have shown an impressive breakthrough and fast
evolution with rapid increases in power conversion efficiency, from initial reports of ~9% in
8
2012 to ~22% in 2017[26]. It has been reported that the levelized cost of electricity of
perovskite solar cells is calculated to be $ 0.035-0.049 per kilowatt-hour with a PCE of above
12 % and lifetime of ~15 years[27]. Its cost is lower than those of traditional power sources
and other photovoltaic technologies. The final target of perovskite solar cells is to achieve
high-efficiency (above 25%) perovskite solar cells with long lifetime (e.g. 20 years). It
probably leads to the large-scale commercial application of organic-inorganic perovskite solar
cells.
1.2.4 Architecture of perovskite solar cells
Figure 1.3 Two kinds of common architectures of perovskite solar cells: p-i-n architecture
(Glass/ITO/hole-transporting layer/perovskite/electron-transporting layer/top contact) and
n-i-p architecture (Glass/ITO/electron-transporting layer/perovskite/hole-transporting
layer/top contact).
In the p-i-n type architecture, hole-transporting materials such as CuSCN, NiO or
PEDOT:PSS are first deposited on the transparent conductive oxide (TCO) coated substrates
such as indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO) and annealed with
hotplate to obtain a planar or mesoporous films[28, 29]. Then, the perovskite layer is
fabricated on the hole-transporting layer by spin-coating and annealing under ~100.
Subsequently, the electron-transporting layer is deposited on the perovskite layer. Finally, the
photovoltaic device is completed by evaporating low work-function metal such as silver on
top.
9
For the n-i-p architecture, electron transporting materials such as TiO2,
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM), ZnO or SnO2 are deposited on a
TCO-coated glass substrate. Then, a perovskite layer is deposited on the electron transporting
layer and annealed under ~100, which is followed by depositing a hole transporting
material such as spiro-MeOTAD, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA),
or poly[5,5′-bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bithiophene]
(PDCBT) on top of the perovskite layer. Finally, the photovoltaic device is completed by
evaporating a high work function metal such as gold or printing carbon electrodes on
top[30-32].
1.2.5 Depositing techniques
Figure 1.4 Schematic illustration of the experimental setup for perovskite deposition via (a)
one-step spin coating solution-processed organic-inorganic perovskite precursor with toluene
dripping, (b) dual source vapor deposition techniques, (c) two-step spin coating and (d)
doctor-blading. (Reproduced from [33-37] with permission from Nature Publishing Group
and Royal Society of Chemistry)
Several kinds of techniques such as one-step spin-coating, two-step spin-coating, dual source
vapor deposition and doctor-blading are used to deposit the organic-inorganic perovskite
10
films (shown in Figure 1.4). Dual source vapor deposition is carried out under high vacuum.
The other three kinds of techniques are vacuum-free, which is helpful to the fast large-scale
production. The ideal perovskite films should has no pinholes, an optimum thickness and
large crystals, thus preventing the direct contact between the ETL and HTL and promoting
efficient charge separation. It can effectively reduce the internal recombination in the
perovskite solar cells. Depositing techniques have a great effect on the morphology, thickness
and quality of the perovskite films. The performance of perovskite devices is highly
dependent on the quality of the perovskite films and the contact between the perovskite films
and the charge transport layers. Therefore, choosing the appropriate depositing techniques is
important for achieving high-performance perovskite solar cells.
1.2.5.1 One-step spin coating
The process for the one-step spin coating method is shown in Figure 1.4a. The perovskite
precursor solution containing the mixing of AX (e.g. methylammonium iodide and
formamidinium iodide) and BX2 (e.g. PbI2 and SnI2) in solvents such as dimethylformamide
(DMF) and dimethyl sulfoxide (DMSO) dropped on the TCO/glass substrates and
spin-coated at sufficient revolutions per minute (RPM) to obtain the desired film thickness.
The thickness of perovskite layer is mainly affected by the concentration of the precursor
solution and the spin speed. Typically, toluene or chlorobenzene is added onto the spinning
substrate at the last 3 seconds of the spinning step. After that, the substrates are blowed with
N2 and annealed to achieve perovskite layer.
1.2.5.2 Dual source vapor deposition
The dual source vapor deposition method is also a strong candidate for producing high
performance perovskite solar cells, which was reported by Snaith et al. in 2013. AX and BX2
are placed in separate crucibles and evaporated simultaneously in a certain evaporation ratio
and annealed to crystallize the perovskite on substrates (shown in Figure 1.4b). It can
precisely control the thickness and composition of the perovskite film. Therefore, it can be
employed to fabricate perovskite layer apart from ABX3, for example, A0.9BX3,A0.95BX3,
A1.05BX3. Since it is completed under high vacuum, it takes longer time to prepare the
perovskite films than the vacuum-free techniques.
11
1.2.5.3 Two-step spin coating
The two-step spin coating method is an alternative technique that has the ability to fabricate
high-quality perovskite films. It is developed by Michael Gratzel and widely used by many
research groups. First, BX2 solution γ-butyrolactone is spin-coated on the substrate and
annealed at around 70 for appropriate time. After the BX2 layer is cooled down to room
temperature, it is dipped into AX in isopropanol solution and annealed to form crystallized
perovskite layer (shown in Figure 1.4c). The concentration of AX and BX2 solution has an
effect of the perovskite morphology.
1.2.5.4 Doctor-blading technique
Although the above-mentioned methods have been demonstrated to be successful for
fabricating high-efficiency perovskite solar cells, they are not compatible with large-scale
production like the roll-to-roll process. Some low-cost and scalable deposition techniques
such as ultrasonic spray-coating and inkjet-printing were also used for fabricating perovskite
films. However, the PCEs of the resulting perovskite devices were relatively lower than those
prepared by spin-coating. The doctor-blading method could be strong candidates for
fabricating high-efficiency perovskite solar cell with large grain size (shown in Figure 1.4d).
This method has been demonstrated to be efficient for fabricating large-area perovskite solar
cells with high efficiency. It is a competitive method for manufacture perovskite modules on
a large scale in future.
1. 3 Stability challenges of perovskite devices
Although these kinds of solar cells have shown promising efficiencies with potential for
higher performance, however their practical application is still limited by their poor stability
compared to leading PV technologies[38]. In the past few years, intensive endeavours have
been directed at improving the lifetime of perovskite solar cells. Preliminary research has
illustrated the relevant degradation mechanisms in the perovskite materials, the interface
layers and electrodes. The degradation of perovskite solar cells is mainly due to the
degradation of perovskite material, charge transporting materials and the failure of top
12
electrodes under ambient atmosphere or light soaking[39-42]. Perovskites are sensitive to
moisture and tend to react with water and form hydrate and other byproducts such as HI and
methylamine.
Some perovskites are unstable under ultraviolet light illumination and degrade. Electrodes
like gold can diffuse through the hole-transporting material and into the perovskite. The
acidic PEDOT:PSS inclines to absorbing water in environment and is corrosive to the
electrodes. PCBM molecules tend to form dimers in the operating process. It is detrimental to
the lifetime of the perovskite solar cells.
Composition engineering such as ternary and binary cation (anion) has been exploited to
improve the long-term stability. The commonly used perovskite, MAPbI3, is sensitive to
moisture. Managing the photons and decreasing the bandgap of light harvesters contribute to
enhancement of light harvesting capability. FAPbI3 has an ideal band gap close to the single
junction optimum, thus allowing for higher power conversion efficiency. However, pure
FAPbI3 exist either as a photoinactive hexagonal δ-phase or a photoactive perovskite α-phase
since it lacks structural stability at low temperature [43, 44]. The α-phase is sensitive to
moisture or solvents.
Although pure inorganic cesium-containing perovskite exhibit superior thermal stability,
CsPbBr3 does not have an good band gap for solar cells[44]. Therefore, it can absorb and
convert only part of the visible light. Although the perovskite phase of CsPbI3 has a more
suitable band gap of 1.73 eV, it exists as a photoinactive δ-phase at low temperature[45]. It
can form the stable photoactive perovskite phase at temperatures beyond 300 [44]. It is
mainly because of thermal or structural instabilities. Developing organic-inorganic perovskite
semiconductors with ternary and binary cation (anion) is a potential strategy to obtain
perovskite materials with enhanced thermal and structural stability. Perovskite
semiconductors with binary cations have been demonstrated to be more structurally and
thermally stable than the pure MA or FA perovskite materials[45]. Doping FAPbI3 with only a
small amount of MA can lead to a more suitable crystallization and the black phase FA
perovskite. It indicates that smaller cations like MA play a vital role in the formation of the
structurally and thermally stable photoactive black phase of FA perovskite [46]. Cs+ can also
be employed to dope the FAPbI3 and contribute to its stabilization.
13
Another effective strategy is to expoit two-dimensional layered perovskite semiconductors
such as (BA)2(MA)2Pb3I10 (n=3), (BA)2(MA)3Pb4I13 (n=4)
(CH3NH3)2(C6H5(CH2)2NH3)2Pb3I10 (n = 3) or 2D-3D heterostructured perovskite
materials[47]. Although two-dimensional layered perovskite materials exhibit improved
moisture resistance, a relatively low power conversion efficiency of 12.5% is achieved due to
their enlarged bandgap and exciton binding energy. Developing two-dimensional perovskites
with superior bandgap can potentially improve the power-conversion efficiency while
maintaining relatively higher stability.
Covering the surface of perovskite layer with a water-resisting layer has been demonstrated to
be another promising way to improve the moisture resistance of perovskite devices [48].
Benzene-amine molecules, hydrophobic tertiary and quaternary alkyl ammonium cations
have been used to functionalize the surface of perovskite film and enhance its
moisture-resistance [49]. It reduces the possibility of absorption of water molecule on the
perovskite surface. Simply covering the perovskite solar cells with hydrophobic
encapsulation film is also an alternative and effective solution to blocking the diffusion of
water molecule into the organic-inorganic perovskite layer, which greatly enhance its
environmental stability.
1.4 Aim and content of this thesis
This thesis aims at improving the lifetime and understanding related fundamental mechanism
of degradation process. The organic-inorganic perovskite layers are usually fabricated by
thermal-evaporation under high vacuum or by spin-coating in a glovebox under low
temperature. Therefore, its stability is not very satisfying due to their low formation energy,
which lags far behind the commercial photovoltaic devices such as CdTe solar cells. In the
last few years, tremendous efforts have been made to enhance their lifetime. This thesis was
focused on developing solution-processable charge transport layer and perovskite layers with
ideal composition for efficient and stable perovskite devices. The SGO has been shown to be
stable hole-transport materials for efficient perovskite solar cells. At the same time, it exhibits
reasonable power-conversion efficiency. In addition, aqueous-processed PCBM NP open a
novel avenue towards partially tackling the stability issue of perovskite devices. Besides, the
14
effect of perovskite composition on the photovoltaic performance and photostability were
investigated.
In the first chapter of this thesis, the evolution of organic-inorganic perovskite materials and
the device architectures for perovskite solar cells are surveyed. The progress of corresponding
perovskite devices in the last several years is reviewed. The superior intrinsic properties of
organic-inorganic perovskite semiconductors are also summarized. In addition, the current
challenges for the stability of organic-inorganic perovskite devices and potential solutions are
discussed in detail.
In Chapter 2, a brief overview addresses the working principles of organic-inorganic
perovskite solar cells and related possible degradation mechanism.
Chapter 3 introduces the recent progress of efficient and stable perovskite solar cells.
State-of-the-art device performance and lifetime are summarized. Various strategies for
enhancing the lifetime of perovskite solar cells while maintaining high power-conversion
efficiency are also introduced.
Chapter 4 introduces the chemicals used in this thesis including solvents, raw materials for
preparing perovskite precursor, charge transporting materials, etc. The deposition methods
used in this thesis and the process for fabricating perovskite solar cells are discussed in detail.
In addition, related characterization methods employed for this thesis are also illustrated in
detail.
In Chapter 5, the influence of two kinds of hole-transporting materials on the lifetime of
unencapsulated perovskite solar cells has been investigated under various conditions such as
ambient condition, nitrogen atmosphere and illumination. Firstly, a high efficiency of close to
15.2% has been obtained employing the SGO hole transporting layer and ~ 60% of their
original PCE for unpackaged SGO solar cells is maintained under 0.5 suns light soaking after
1000 h. Replacing PEDOT:PSS layer with SGO layer significantly improves the
environmental lifetime of unsealed perovskite solar cells with inverted architecture under 0.5
suns light soak. It implies that on one hand SGO layer acts as an effective hole-transporting
material. On the other hand, it facilitates stabilizing perovskite devices by slowing down the
moisture ingress into perovskite layer on account of its superior hydrophobic property in
15
comparison with PEDOT:PSS. Besides, solar cells based on SGO show a superior lifetime
compare to that of PEDOT:PSS based devices under ambient atmosphere in the dark. It
illustrates the significance of exploiting novel hydrophobic hole transporting materials with
low water vapour transmission constant toward organic-inorganic perovskite solar cells with
promising long lifetime.
Chapter 6 In this chapter, high-efficiency organic-inorganic perovskite solar cells have been
fabricated via employing an ultrathin aqueous-processed PCBM nanoparticle layer. In
addition, its effect on device performance and lifetime has been investigated.
PCBM NP has been used as electron transport layer for high-efficiency perovskite solar cells.
However, its solvent is toxic and PCBM layer can be washed off the solvent of perovskite
precursor such as DMF and DMSO. To increase its resistance to solvents of perovskite
precursor, aqueous processed PCBM NP is developed. Subsequently, the resistance of PCBM
CB and aqueous PCBM NP is studied by SEM, XPS and contact angle measurement. With
the green and environmentally safe processing, the perovskite solar cells are fabricated and
optimized as a function of the processing conditions. Then, the lifetime of devices based on
PCBM CB and PCBM NP is compared (under 1 sun light soaking in nitrogen). PCBM NP
based devices exhibit higher Voc, FF and PCE than those of PCBM CB devices. In addition,
PCBM NP-based devices also show superior photo-stability under 1 sun light soaking in
nitrogen.
Chapter 7 In this chapter, composition engineering is employed to develop perovskite solar
cells with high efficiency and long-term stability. Perovskite solar cells based on four kinds of
organic-inorganic perovskite materials (FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2,
FA0.7Cs0.3PbI2.4Br0.6 and FA0.7Cs0.3PbI1.8Br1.2) are prepared and subjected to light stress under
1 sun light soaking in nitrogen. At last, the device based on FA0.85Cs0.15PbI2.4Br0.6 gives
superior photovoltaic performance and photo-stability.
The main achievements in this thesis are summarized in chapter 8. Besides, the main
limitations and potential strategies for tackling these problems are depicted. Improved device
architectures, deposition processes and interface and composition engineering will play a
vital role in improving the PCE and long-term stability, which might facilitate the large-scale
practical application of perovskite solar cells.
16
Chapter 2
Theory
This chapter introduces the optical and electronic properties of perovskite semiconductors. In
addition, working-principles including charge generation, charge recombination, charge
transport and charge extraction of perovskite solar cells are also depicted. Besides, the
degradation mechanisms of perovskite solar cells are discussed.
17
2.1 Work principles of perovskite solar cells
Figure 2.1 Working Principle of the perovskite solar cells with (a) n-i-p
(FTO/ETL/perovskite/HTL/Metal electrodes) and (b) p-i-n (FTO/HTL/perovskite/ETL/Metal
electrodes) structure. The HOMO, LUMO, valence band and conduction band alignment with
respect to the vacuum.
The incident light is transmitted by a transparent conductive oxide and absorbed by the
organic-inorganic perovskite semiconductors between the hole-tranporting layer and electron
transporting layer. In contrast to organic photovoltaic devices, the absorption of photons in
perovskite semiconductors does not result in the formation of a long lifetime exciton.
Absorbed photons can generate electron-hole pairs that further lead to free charge carriers [50,
51]. Charge carriers in the perovskite exhibit much longer diffusion length than those in
organic solar cells. Subsequently, the charge carriers were transported by corresponding
charge transporting layers and extracted at the corresponding electrode, thus creating a
current. The shell electrons in the perovskites interact with each other and form the valance
band and conduction. The bandgap is the energy gap between the valance band maximum and
conduction band minimum. There is a positive correlation between the band gap and the
electrons/electrons interaction and atomic nucleus/shell electrons interaction. Replacing the
perovskite with atoms with various electronegativity can adjust its bandgap. Perovskites with
different bandgap show different color, which could be used for fabricating colorful windows.
The shell electrons escape easier from the lager atoms since they have weaker attraction to
valance electrons, which leads to a smaller bandgap.
18
A typical perovskite solar cell comprises an electron transporting layer, a perovskite layer, a
hole transporting layer and an electrode. Perovskite solar cells are divided into two kinds of
structures: p-i-n and n-i-p structures. A typical photovoltaic process in perovskite solar cells
includes the following four steps:
1) Light absorption. Hybrid organic-inorganic perovskite materials have been shown to be
promising light absorbers with high absorption coefficient (α)[52]. For example, the MAPbI3
film has an absorption onset of about 800 nm, a direct bandgap of ~ 1.55 eV and high
absorption coefficient in the visible range (105 cm-1). Therefore, a relatively thin perovskite
film (~300-500 nm) can efficiently harvest sunlight and convert it into electricity. The
organic-inorganic perovskites can transport both holes and electrons with long diffusion
length.
2) Charge separation. Free charges are generated upon the light incident into the perovskite
layer. Then, charge separation can occur either by injecting photo-generated holes into the
hole transporting layer or injecting electrons into the electron transporting layer ETL, which
has been demonstrated to occur on similar timescale[53]. For example, the excited electrons
generated by the organic-inorganic perovskite molecules upon illumination can be injected
into the conduction band of TiO2, whereas holes generated by the organic-inorganic
perovskite molecules can be injected into the HOMO of HTL like P3HT. There is
recombination between the perovskites and the hole transporting materials or the electron
transporting materials. Reducing the above-mentioned recombination contributes to
improving the short-current density and PCE.
3) Charge transport. Free holes near the ETL/perovskite interface have to diffuse through the
perovskite absorber layer before they are extracted at the HTL/perovskite interface, which
resulted in possibility of recombination.
Table 2.1 Summarization of representative organic-inorganic hybrid perovskites and
charge-transport layers and work function of representative metal electrodes used in
perovskite solar cells with respect to the vacuum [54].
19
Valence band(eV) Conduction band (eV) Work function (eV)
MAPbI3 -5.4 -3.9
FAPbI3 -5.4 -4.0
MAPbBr3 -5.5 -3.4
MAPb I2Br -5.4 -3.6
PC61/71BM -6.0 -4.3 - -3.8
TiO2 -8.0 -4.3
ZnO -7.8 -4.4
pp-Spiro-OMeTAD -5.2 -2.3
pm-Spiro-OMeTAD -5.3 -2.3
po-Spiro-OMeTAD -5.2 -2.2
SGT-407 -5.3 -2.3
P3HT -5.0 -3.0
PTAA -5.1 -1.8
NiO -5.4- -5.3 -1.8
CuSCN -5.3 -1.5
CuI -5.3 -2.2
ITO/FTO -4.7- -4.4
Au -5.1
Al -4.3
Ag -4.7
Ni -5.0
20
Then, holes are transported to the HTL/electrodes interface by HTL. Likewise, similar
considerations apply to the electrons near the ETL/perovskite interface. Developing charge
transporting materials those are in good match with the conduction band and valence band
facilitate the charge transfer in the perovskite solar cells.
4) Charge extraction. Free electrons and holes are extracted at the HTL/electrodes and
ETL/electrodes interfaces, respectively. The evaporated or printed electrodes should have an
ideal work function for hole or electron transfer. In addition, the electrodes should be
intrinsically stable towards illumination and moisture, which could improve the lifetime of
perovskite solar cells.
2.1.1 Current density-voltage (J-V) characteristic
The current density-voltage (J-V) curve with and without illumination is one of the most
important characterization methods in describing the performance of a perovskite solar cell.
When perovskite devices are exposed to illumination, holes and electrons are generated in
perovskite layers and extracted to the corresponding electrodes because of the built-in electric
field, thus creating a current in the external circuit. Perovskite solar cells exhibit a typical p-n
junction diode behavior in the dark, allowing the electric current to pass through the
perovskite solar cells when a certain forward bias at the voltage for which the diode opens is
applied. When the applied forward or reverse bias is close to the threshold voltage, the
electric current that passes through the organic-inorganic perovskite solar cell should be as
low as possible.
The typical J-V characteristics of a perovskite solar cell with and without illumination are
shown in Figure 2.2. Several parameters describing the performance of perovskite solar cells
such as power conversion efficiency (PCE), short-circuit current density (Jsc), open-circuit
voltage (Voc) and fill factor (FF) can be extracted from the J-V curve. The point at which
maximal power can be obtained from a solar cell is denoted as maximal power point (MPP).
Tuning the band gap of the perovskite films has a great effect on the Voc. Radiative
recombination is unavoidable because of the reciprocity between emission and absorption,
thus limiting the Voc of MAPbI3 to be 1.33V[55]. Approaching to this thermodynamic limit is
21
essential to reach the Shockley-Queisser efficiency limit. Another voltage loss results from
non-radiative recombination, which can be aquired by measuring the emission yield (EQEEL)
of the electroluminescence (EL) spectra.
Figure 2.2 The typical illuminated and dark J-V curves of perovskite solar cells with a
sweeping speed of 100mV/s. Illumination with various light intensity was supplied with a
Newport Sol 1A solar simulator.
MAPbI3 was thought as a direct bandgap semiconductor, thus exhibiting relatively low
non-radiative recombination. In addition, optimum selectivity between perovskite layers and
charge transport layers can reduce the surface recombination, thus resulting in an increase of
Voc and enhancement of power conversion efficiency.
Light management is essential for minimizing the parasitic losses of the perovskite devices.
In addition, the thickness of organic-inorganic perovskite layer should be appropriate to
capture incident light. Narrowing the bandgap of perovskite semiconductors by doping
MAPbI3 with FA or Sn can result in higher photocurrent. However, it partially reduces the Voc.
The main photocurrent losses for the perovskite device result from reflection losses,
transmission losses and parasitic absorption by the TCO, charge transporting layers and top
electrodes, which leads to the decrease of internal quantum efficiency. Applying
anti-reflection layer on top of transparent conductive electrodes of the perovskite solar cells
22
can reduce the reflection loss and increase short-current density. Use transparent or colorless
hole transporting layers or electron transporting layers can partially reduce the loss of
incident light.
Morphology engineering of perovskite film plays an important role in minimizing the internal
quantum efficiency losses. Perovskite devices with larger crystal size exhibited a higher
internal quantum efficiency than those based on organic-inorganic perovskite with small
crystal size, thus leading to high photocurrent approaching to theoretical model
limitations[56].
Currently, inverted organic-inorganic perovskite devices have shown the highest FF
(87%)[57], which is close to its theoretical limit of 91% (for MAPbI3). Solution-processed
perovskite films with thickness of hundreds of nanometers is sufficient to obtain a PCE of
beyond 20%. There is as anti-correlation between the thickness of perovskite film and the FF,
which is because increasing film thickness enlarges the diffusion lengths before the charge
carriers are extracted by the electrodes, thus increasing the recombination. In order to achieve
an impressive power conversion efficiency, it is necessary to reach a balance between the
thickness and FF. The series resistor (Rs) and parallel resistor (Rp) have a great effect on the
FF. In an ideal perovskite solar cell, the Rs and Rp should be close to zero and infinite
respectively.
2.2 Intrinsic stability of perovskite materials
Despite the high PCE, the poor stability remains a major challenge for organic-inorganic
perovskite solar cells. Long-term stability is essential for their large-scale application.
Chemical degradation caused by humidity is the most observed in a wide range of hybrid
organic-inorganic perovskites. When organic-inorganic perovskite materials are exposed to
ambient environment, the hydroscopic nature of the organic ammonium cation and potential
solubility of PbX2 in water could lead to the degradation of perovskite semiconductors.
Organic-inorganic perovskites can react with water and form hydrate, which further
decompose and generate HI and methylamine. Illumination such as ultraviolet light or visible
light is also detrimental to some organic-inorganic perovskites like MAPbI3.
23
The hybrid organic-inorganic perovskite semiconductor material takes the common ABX3
structure and is usually composed of an inorganic or organic monovalent cation, A=Cs+,
n-butylammonium (BA) CH3(CH2)3NH3+, methyl-ammonium (MA) CH3NH3
+ and
formamidinium (FA) CH3(NH2)2+), a divalent cation, B = (Pb2+; Ge2+ and Sn2+) and an anion
X = (Cl-; Br-; I-; SCN-; BF4- and PF6
-;).
Organic-inorganic perovskite materials can form various preferential crystal structures
depending on the size and interaction between the A cation and the corner-linked BX6
octahedra. The Goldschmidt tolerance factor (t) has been used for empirically predicting
which structure perovskite materials tend to form. In order to be incorporated into the lattice
of perovskite materials, all of the cations employed in perovskite devices should fulfil the
requirement of the tolerance factor. The tolerance factor (t) can be calculated from the ionic
radius of the atoms [42, 58]. In order to improve the stability of perovskite materials,
increasing its complexity by adding more inorganic elements like Cs and enhancing the
entropy of mixing. It successfully stabilize usually unstable perovskite materials (like the
non-photoactive “yellow” phase of FAPbI3)[42]. All combinations of Cs, MA, and FA cations
such as Cs/MA, Cs/FA and Cs/MA/FA were selected because they all form a photoactive
perovskite “black” phase and show unexpected properties. For example, the Cs/MA/FA triple
cation perovskite solar cells are more thermally stable and reproducible than MA/FA-based
solar cells. In order to achieve perovskite materials with superior stability, various cations and
anions have been explored for fabricating perovskites. In general, if the tolerance factor of
materials is in the range of 0.71-1.0, perovskite structure can be formed. The
inorganic-organic hybrid perovskite materials with a tolerance factor of 0.9-1.0 tend to form a
cubic structure. A distorted perovskite structure with tilted octahedra is formed when the
tolerance factor of inorganic-organic hybrid perovskite materials is in the range of 0.71-0.9.
When the t>1 or t<0.71, non-perovskite structures are formed. Although the rule was initially
developed for inorganic perovskite oxides, the trend still partially applies to inorganic-organic
hybrid perovskite materials. However, most APbI3 or APbBr3 perovskite materials with
monovalent cations do not have an appropriate Goldschmidt tolerance factor between 0.71
and 1.0 for a photoactive perovskite. It implies that most elemental cations are too small for
constructing perovskites. Massive efforts have been devoted to searching for perovskites with
appropriate tolerance factor. The relevance between t and the perovskite structure is shown in
24
Figure 2.3.
Figure 2.3 The relationship between the crystal structure and tolerance factor of inorganic
and organic-inorganic perovskite semiconductors including δo-CsPbI3, δH-FAPbI3 and α-
FAxCs1-xPbI3. (Reproduced after [58] with permission from American Chemical Society)
2.3 Degradation process of perovskite
2.3.1 Moisture-induced degradation
Compared to traditional robust inorganic photovoltaic materials, organic-inorganic
perovskite materials have not shown long enough lifetime for practical application. Stability
to moisture and light is extremely critical for the commercialization of this novel photovoltaic
technology.
The precise degradation process of organic-inorganic perovskites in the presence of moisture
is under discussion. H2O is considered as a Lewis base. Hybrid organic-inorganic perovskites
react with water via coordination between the H2O and the proton of ammonium in
CH3NH3PbI3, thus leading to the apparently irreversible degradation in the ambient
25
atmosphere[59]. The loss of methyl-amine and the formation of yellowish PbI2 have been
reported in previous literatures, while more recent reports rather imply the partially reversible
formation of (CH3NH3)4PbI6:H2O hydrate complexes as an intermediate step, which will
further decompose into CH3NH2, HI and PbI2. In the presence of H2O, the possible
decomposition process of hybrid perovskites is as follows.
Figure 2.4 Possible degradation mechanism of organic-inorganic perovskite in the presence
of moisture. It displays the reaction between the water and the perovskite and the resulting
products such as hydrate complexes, HI and CH3NH2. (Redrawn after [60] with permission
from Wiley-VCH)
A H2O molecule is necessary to initiate the degradation process and an excess H2O molecule
is required to dissolve the byproducts such as methylammonia and hydrogen iodide. The
degradation process might be driven by the phase changes of both HI and CH3NH2. It leads to
the formation of a yellowish film, which is shown to be PbI2. Since the HI and CH3NH2 are
volatile, the loss of these components accelerate the decomposition of the organic-inorganic
perovskite.
However, apart from the moisture-induced degradation process, other factors that may lead to
degradation should also be taken into consideration, such as phase transition, heat stress and
light-induced trap-state formation. The degradation process of organic-inorganic perovskites
is substrate-dependent.
26
2.3.2 Thermal-induced degradation
When CH3NH3PbI3 is heated to temperatures higher than 150 , a reversible degradation
process occurs. CH3NH3PbI3 decomposes into CH3NH2, HI and PbI2 via an endothermic
reaction, while PbI2 can react with CH3NH3I and form CH3NH3PbI3.
The ion diffusion in the perovskite film is induced by light and thermally activated[61]. The
ionic diffusion coefficient is associated with the activation energy barrier and the degradation
of perovskite devices. CH3NH3PbI3 exhibits low activation energy barrier and a high
diffusion coefficient, which results in poor stability under illumination and dark. What’s
worse, its activation energy decreases with increasing temperature, thus further accelerating
ion transport[61]. In contrary, perovskite with mixed MA/FA cation has a higher activation
energy barrier and the lower diffusion coefficient of ions, thus resulting in a better lifetime
under illumination and dark. Formamidinium-based perovskites have shown higher thermal
stability than those of methylammonium-based perovskites[62]. However, the former is more
sensitive to moisture than the latter because of its high hygroscopicity. In addition, Cesium
was demonstrated to stabilize the organic-inorganic perovskite FAPbI3[42]. Small amount of
Cs doping in FAPbI3 enhanced its thermal stability. Cs doping not only facilitate the film
formation but also reduce the trap states, thereby the short circuit current density and
power-conversion efficiency were improved.
2.3.3 photo-induced degradation
Photo-induced degradation is one of the main reasons for the unsatisfactory lifetime of
CH3NH3PbI3 perovskite solar cells when exposed to environmental atmosphere. When
subjected to dry air and light, a fast degradation of perovskite solar cells is observed in only
several minutes to hours[63]. In the presence of light and oxygen, photo-generated electrons
in the organic-inorganic perovskites react with oxygen and form superoxide, which
deprotonates the CH3NH4+ and leads to the degradation of perovskites. Therefore,
encapsulating the perovskite solar cells in inert atmosphere like nitrogen potentially supress
the degradation under illumination.
After the CH3NH3PbI3 is exposed to blue laser in vacuum for 120 min, metallic lead was
27
found. It is demonstrated by the peak for the metallic lead in XPS spectrum. After 480 min of
illumination, the decomposition of organic-inorganic perovskite is saturated[64]. Ultraviolet
light can also accelerate the degradation of perovskites and interfacial charge transporting
layer. Applying a protection layer that blocks or absorbs ultraviolet light on top of perovskite
devices is an effective way to slowing down the degradation. Crosslinking the interfacial
layer can also inhibit the degradation of perovskite solar cells.
Other possible reasons for the degradation: (1) the adsorption of oxygen/moisture by hole
transporting materials/electron transporting materials. (2) incomplete coverage of the
organic-inorganic perovskite film by the electron transport materials. Therefore the
perovskite layer is not fully protected by the ETL layer and resulted in a fast reaction between
the electrodes and the perovskite layer. It may render the perovskite exposed to the ambient
atmosphere and accelerate the degradation process. It has been demonstrated that metal
electrodes can diffuse into the beneath contact layer by depth profile measurement or
TEM[65, 66]. In addition, the fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM)
layer itself also degrades in ambient environment by reaction with moisture or oxygen and
exhibits typical peak in X-ray photoelectron spectrum (XPS) because of the water-PCBM
interaction. Besides, when exposed to light, photodimerization of PC60BM occurs, which is
partially responsible for the device degradation [67, 68]. By simulating the parameters of the
photovoltaic devices, it is found that the dimerization results in the decrease of the effective
charge carrier mobility, thus affecting short circuit current and FF of the photovoltaic devices.
However, when perovskite solar cells are subjected to annealing, the performance loss of the
corresponding device and the dimerization are demonstrated to be reversible.
28
Chapter 3
State of the art
The recent development of high-efficiency and stable perovskite solar cells is reviewed in this
chapter. In addition, the challenges that hamper the development perovskite solar cells with
long-term stability are discussed.
Organic-inorganic perovskite solar cells generally consist of electron transporting layer,
perovskite semiconductors, hole transporting layers and top metal electrodes. Each
component plays an important role in improving the long-term stability and photovoltaic
performance of the perovskite solar cells. Optimizing the interface materials, perovskite
materials and design of device architectures is vital not only for obtaining high efficiency but
also for long-term stability. Ideal charge transporting layers in perovskite solar cells are films
which should be thin enough to reduce resistive losses and uniformly and continuously cover
the whole current collector at the same time. It can also supresses the internal recombination
in the perovskite solar cells. However, it is hard to reach the balance when the size of the
perovskite solar cells increases. Several organic-inorganic perovskites tend to be sensitive to
moisture, exploiting hydrophobic interfacial layers with reasonable conductivity is helpful for
improving its stability. In addition, some perovskites are inclined to degrading under
ultraviolet light. Exploiting interfacial materials with capability of blocking ultraviolet light is
an effective way to enhancing its stability towards ultraviolet light. Therefore, tremendous
research efforts have been devoting to developing novel materials and architectures.
3.1 Interface engineering
A number of organic polymers such as poly(3,4-ethylenedioxythiophene) polystyrene
29
sulphonate (PEDOT:PSS), PTAA, PDCBT, poly(3-hexylthiophene-2,5-diyl) (P3HT), and
2,2′,7,7-tetrakis(N,N-p-dimethoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) have
been used as p-type hole transport layers, while PCBM, C60 and their derivatives have been
used as n-type electron transport layers[69-71]. Although organic ETL and HTL allow for
high PCEs and reduced hysteresis, they are still limited by the resulting device stability and
its cost. Several inorganic charge transport layers (CuSCN, CuI, p-doped (p+) NixMg1-xO and
NiO as the hole transport layers; SnO2, ZnO2 and TiO2 as the electron transport layers) have
been employed for the fabrication of efficient and stable perovskite solar cells[72, 73]. The
above-mentioned metal oxides have shown superior stability and much higher carrier
mobility than those aforementioned organic charge transport materials. It is partially
attributed to their superior intrinsic stability
A robust and stable inorganic hole-transporting layer has been developed for high-efficiency
and large-area perovskite solar cells. N-doped (n+) TiOx and p-doped (p+) NixMg1-xO have
been used as electron transporting material and hole transporting material instead of organic
charge transporting materials for a planar MAPbI3-PCBM film architecture, respectively[74].
N-doped (n+) TiOx and p-doped (p+) NixMg1-xO were achieved by substituting Ti4+ ions and
Ni(Mg)2+ ions on the TiOx matrix and NixMg1-xO lattice by Nb5+ and Li+ ions, respectively.
This strategy resulted in significant increase in the electrical conductivity, which allowed
thicker oxide films to be used for rapid selective extraction of one type of charge carriers
while efficiently blocking the other type by reducing pinholes and eliminating cracks over
large areas. Therefore, the hole transporting layer effectively reduce their recombination at
the interface. The resulting large-area perovskite solar cells exhibited a certified high power
conversion efficiency (>15%), which is partially due to the impressive reproducibility and
homogeneity. In addition, no hysteresis was found in the corresponding current-voltage
curves for the resulting photovoltaic devices. The devices exhibit long lifetime
maintaining >90% of the initial PCE after light soaking (under 1 sun) for 1000 hours (at the
short-circuit condition). After the devices were encapsulated by a covering glass and a UV
curable adhesive, 97% of their initial PCE was remained after storing them in the dark and
dry air for 1000 hours. Although the device stability was dramatically enhanced, the judicious
control of the doped inorganic charge transport layer for the perovskite solar cells might
result in much complexity for large-scale production. Further development of stable inorganic
30
charge tranporting materials prepared by simpler method is required.
Figure 3.1 The architecture of organic-inorganic perovskite solar cells based on Ti(Nb)Ox
and NiMgLiO layers. Ti(Nb)Ox and NiMgLiO act as electron and hole extraction layer,
repectively.
Perovskite solar cells with inorganic hole transport materials (p-type NiOx nanoparticles) and
electron transporting material (n-type ZnO nanoparticles) has been reported by You et al.
Compared to perovskite solar cells based on organic charge transporting materials, they
exhibit good long-term stability and an uncertified PCE of 16.1%. The average PCE of 15.0%
is still far inferior to that of devices based on fullerenes. Because of their low energy of
formation, organic-inorganic hybrid perovskites are prone to degradation in ambient
environment. Therefore, both the electron transporting layer and hole transporting layer play
a vital role in protecting the perovskite semiconductors from moisture and air. At the same
time, the charge transporting layers should exhibit low light absorption capability so that in
order to reduce the loss of incident light, which is beneficial to increase the short circuit
current and power conversion efficiency. Then, it is crucial to select low-cost charge transport
layer with good energy match, efficient charge transport and long-term stability. Besides,
nanostructured metal-oxide semiconductors can be prepared via solution process either from
corresponding precursor or nanoparticles at low temperature.
In addition, the fullerene PCBM layer itself also degrades in ambient environment by reaction
with moisture or oxygen and exhibits typical peak in X-ray photoelectron spectrum (XPS)
because of the water-PCBM interaction. Besides, when exposed to light, photodimerization
31
of PC60BM occurs, which is partially responsible for the device degradation [68]. By
simulating the parameters of the photovoltaic devices, it is demonstrated that the dimerization
results in the decrease of the effective charge carrier mobility, thus affecting short circuit
current and FF of the photovoltaic devices. However, when subjected to annealing, the
performance loss of the corresponding device and the dimerization were shown to be
reversible.
Figure 3.2 Schematic illustration of energy level the NiOx-based perovskite solar cell versus
vacuum level. NiOx and ZnO are employed as solution-processed metal oxide hole and
electron transport layers, respectively.
The intrinsic instability of hybrid organic-inorganic perovskite materials owing to moisture
hampers their practical application in ambient environment. To tackle this issue, fullerene
was crosslinked with silane molecules with hydrophobic functional groups, which results in
highly water-resistant fullerene layer[75]. The resulting fullerene layer successfully blocks
the diffusion moisture into the organic-inorganic perovskite layers and thus protects them
from moisture-caused degradation. In addition, it has been demonstrated that the crosslinking
fullerene exhibit higher conductivity than that of the normal fullerene without crosslinking.
Methylammonium iodide have been shown to be an effective n-dopant for the fullerene layer
via anion-induced electron transfer, which further enhances its conductivity over 100-fold.
Compared to the devices based on non-crosslinked C60-SAM, those devices based crosslinked
and doped fullerene show longer recombination lifetime and a high PCE of 19.5% with no
32
hysteresis. Besides, the corresponding lifetime was dramatically improved in comparison to
non-crosslinked C60-SAM. Around 90% of their initial efficiencies were maintained after the
unencapsulated perovskite devices were exposed to an ambient atmosphere for 30 days.
An alternative strategy to stabilize the perovskite layer is to functionalize it with hydrophobic
tertiary and quaternary alkyl ammonium cations via a simple surface functionalization
method, which act as an efficient water-resisting layer on the perovskite surface.[76] They
successfully block the diffusion of moisture and air into the perovskite layer and hinder its
fast degradation. The resulting functionalized perovskite materials show improved surface
hydrophobicity, which leads to an enhanced moisture stability. The PCE for the
corresponding devices decreased to 90.4% of its initial value for ~500 h under a high relative
humidity of 90 ± 5%. By contrast, the PCE decreased to 23.8% and 59.6% of its original
value for MA and TMA devices, respectively. It indicates TEA plays a vital role in enhancing
the moisture stability of perovskite devices. Interestingly, photovoltaic performance of TEA
based devices is still on par with that of MA devices. It offers efficient method for enhancing
the humidity tolerance of organic-inorganic perovskite materials.
The original tilt angle between the (100) surface and the two surface Pb5c-I1c (Pb5c and I1c
represent the five-coordinated surface Pb atoms and the surface I atom coordinated with one
Pb atom, respectively) bonds is approximately 33.4, which is close to the value of 34.7 in
the bulk perovskite. When the surface MA was replaced with the tertiary and quaternary alkyl
ammonium cations, these tilt angles were increased to 59.5 and 98.7.
The adsorption energy of water molecules on the surface Pb5c sites of the tetragonal
CH3NH3PbI3 (100) surface is -0.52 eV. Since the surface unoccupied conduction band
minimum around Pb5c atoms points directly to the vacuum, water should be easily adsorbed
at the Pb5c sites of the MA surface. After replacing the surface MA by TMA, the surface I1c is
shifted upwards above the Pb5c. The surface hydrophobic ammonium cations increased the
water-resistance of Pb5c atoms. The adsorption energy of water molecules on the TMA
surface was calculated to be -0.68 eV by density functional theory calculations, which is
close to the value on the MA surface. By contrast, the adsorption energy on the TEA surface
is smaller (-0.43 eV) due to the dramatical structural change. It indicated that TEA
functionalized perovskite materials superior water-resistance than those functionalized by
33
MA and TMA.
Zhao et al. also functionalize the perovskite with amine functional molecules through a facile
post-deposition process [49]. The amine solution was spin-coated on the perovskite layer. The
passivation effect of three kinds of structurally similar benzene-amine molecules (aniline,
benzylamine, and phenethylamine) were experimentally and theoretically investigated
(denoted as unmodified FAPbI3, A-FAPbI3, BA-FAPbI3 and PA-FAPbI3, respectively). The
presence of the benzene-amine molecule and its chemical bonding to the FAPbI3 were further
demonstrated by the Fourier transform infrared spectroscopy (FTIR) method.
Then the abovementioned functionalised perovskite films were exposed to 50±5% relative
humidity in air for 4 months. It is intriguing that the BA-FAPbI3 showed superior stability
without color change, which is further confirmed by the corresponding XRD result,
indicating no impurity in the XRD patterns. In contrast, the unmodified FAPbI3 film became
yellowish within 72 h. Despite that aniline and phenethylamine have similar chemical
structures with the benzylamine, the A-FAPbI3 and PA-FAPbI3 exhibited much worse
water-resistance. Only the benzylamine-modified perovskite films exhibited the best
water-resistivity, which is due to its optimal steric molecule arrangement.
After being exposed to 50±5% relative humidity in air for 72 h, the A-FAPbI3 and PA-FAPbI3
films showed a noticeable extra XDR peak for δ-FAPbI3, implying that the configuration
could have a significant effect on the passivation of perovskite. In order to clarify the
phenomenon, the water adsorption energy (Ead) of on various perovskite films was achieved
via DFT calculation. The Ead of unmodified FAPbI3, A-FAPbI3, BA-FAPbI3 and PA-FAPbI3
were calculated to be -0.58, -0.54, -0.40, and -0.40 eV, respectively. It indicated BA-FAPbI3
and PA-FAPbI3 gave better water resistance. The BA-FAPbI3 devices gave a high efficiency
of 19.2% and a Voc of 1.12 V, implying a relatively low non-radiative recombination loss.
Then, unmodified FAPbI3 devices, BA-FAPbI3 devices (full cells) and BA-FAPbI3/TiO2/FTO
samples (half cells) were exposed to ambient atmosphere with 50±5% relative humidity. The
unmodified FAPbI3 devices almost died in 72 h, while 50% of the original performance was
maintained for the BA-FAPbI3 devices after being exposed to ambient air for 3000 h. The
hole transporting layer (spiro-MeOTAD) and gold electrode were deposited on half cells right
before the J-V test. It is intriguing that there is almost no degradation during the lifetime test
34
for 3000 h.
Figure. 3.3 Molecular structure of three kinds of structurally similar benzene-amine
molecules: aniline, benzylamine, and phenethylamine; photographs of unmodified FAPbI3,
A-FAPbI3, BA-FAPbI3 and PA-FAPbI3 films exposed to 50±5% relative humidity in air under
different period (fresh, 3 days, 4 months). (Reproduced after [49] with permission from
Wiley-VCH)
3.2 Composition engineering
Saliba et al. calculated the tolerance factor for the MA, FA and alkali metals (Li, Na, K, Rb,
Cs) (shown in Figure 3.4)[77]. The alkali metals were investigated because they exhibit
desirable oxidation stability. The result shows that only MAPbI3, FAPbI3 and CsPbI3 fall into
the range of “established perovskites” with a photoactive black perovskite phase. Li, Na and
K are apparently outside of the aforementioned range rendering them too small for
consideration. Although the ionic radius of Rb+ (152 pm) is smaller than that of Cs+ (167 pm),
CsPbI3 can form photoactive black perovskite phases while RbPbI3 form photoinactive
35
yellow non-perovskite phases.
Figure 3.4 Tolerance factor of APbI3 perovskite semiconductors, A = Li, Na, K, Rb, Cs, MA
and FA. Only Cs, MA and FA are suitable for forming perovskite. Rs is really close to the
established perovskite area. (Reproduced after [77] with permission from American Chemical
Society)
Despite not being in the range of “established perovskites” with a photoactive black
perovskite, the tolerance factor of RbPbI3 is really close to the range. It implies that the small
and oxidation-stable Pb+ can be potentially incorporated into the lattice of a photoactive
perovskite with multiple A-cation. Four kinds of perovskite materials with FA as the majority
cation were investigated: RbFA, RbMAFA, RbCsMAFA and RbCsFA.
The ratio of Rb in the perovskite with the optimized composition is around 5-10 %.
Interestingly, the resulting perovskite materials showed excellent photovoltaic properties. A
champion PCE of 21.8% and stabilized efficiencies of 21.6% as well as an
electroluminescence of 3.8% were obtained. A high open-circuit voltage of 1.24 V was
achieved for solar cells based on a band gap of 1.63 eV, which means a non-radiative
recombination loss of 0.39 V. It is comparable to those of mature conventional photovoltaic
technologies.
The resulting devices were subjected to constant AM1.5G illumination and maximum power
point tracking at 85°C for 500 h in a nitrogen atmosphere. 95% of its original PCE was
36
retained for the polymer-coated solar cells during the 85°C step. It implies that Rb can
stabilize the black FA perovskite phase.
Despite that three-dimensional hybrid organic-inorganic perovskites exhibit excellent
optoelectronic properties such as its high absorption coefficient, high charge carrier mobility
and long carrier diffusion length, the practical application of corresponding photovoltaic
devices is still hindered by its environmental instability and photostability under light soaking.
Some two-dimensional hybrid organic-inorganic perovskite materials have been exploited
and the corresponding devices gave promising long-term stability. However, its PCE is still
far inferior to those of devices based on three-dimensional counterparts. It is mainly because
that the organic cations acts like isolated spacing layers between the conducting inorganic
slabs, which hampers the out-of-plane charge transport. To tackle this problem,
near-single-crystalline layered perovskite (BA)2(MA)n-1PbnI3n+1 (n=3 or 4) films were
developed [47]. The crystallographic planes of the inorganic component in the
aforementioned perovskite films show a highly preferential out-of-plane alignment relative to
the surface of top electrodes, thus leading to efficient charge transport. A champion PCE of
12.52% without hysteresis for two-dimensional perovskite solar cells was obtained. In
addition, superior long-term lifetime was achieved when corresponding photovoltaic devices
were subjected to light soaking, humidity and heat stress. 60% of original PCE of unsealed
two-dimensional perovskite solar cells after being kept under constant 1 sun illumination for
over 2,250 hours. Besides, they also exhibit superior stability in comparison to their
three-dimensional counterparts under air exposure (65% relative humidity). There is no
degradation after the encapsulated devices are subjected to the constant 1 sun illumination or
humidity for over 2250 hours.
MAPbI3 is sensitive to heat, humidity and light. The effect of substitution of its cation or
anion on is as follows. Partial substitution MA+ with FA+ could improve its resistance to heat,
humidity, light and oxygen, while replacing MA+ with Cs+ could increase its light, heat and
humidity resistance[78]. Aliphatic and aromatic alkylammonium are helpful to enhance
humidity resistance and oxygen resistance of perovskite materials. The resistance of MAPbI3
to heat, light and humidity could be enhanced by substituting MA+ with larger ammonium
cations. Partial substituting I- with Br- or Cl- can increase the humidity resistance of
perovskite, while doping perovskite with SCN- could improve its resistance to light and
37
humidity.
In the past several years, cesium lead halides perovskite such as solar cells have attracted lots
of attention because of enhanced stability[79]. The ionic radius of Cs+ is appropriate for
constructing three-dimensional perovskites. CsPbI3 exhibit a cubic phase and a band gap of
1.73 eV for photovoltaic application[79], which is larger than that of MAPbI3. Thus, higher
energy is needed to excite electrons from its valance band to conduction band. One
photoactive structure of CsPbI3 is the cubic phaseα-CsPbI3. It is usually prepared under a high
temperature beyond 310 and stable at room temperature in inert atmosphere[80]. Partially
replacing of MA+ in MAPbI3 perovskite with Cs+ can improve its stability. Nanoscale CsPbI3
quantum dots exhibit superior stability than that of the bulk α-CsPbI3 bulk materials at room
temperature. The perovskite solar cells based on this nanoscale CsPbI3 quantum dots exhibit a
high PCE of more than 10% and superior thermal stability than their organic-inorganic hybrid
counterparts like MAPbI3. Pure CsPbI3 inclines to form the nonphotoactive δ-phase below
320°C, which is thermodynamically preferred.
3.3 Electrode engineering
When solar cells are exposed to high temperature (above 70 ), metal electrodes tend to
diffuse into through the beneath charge transport layer and into the organic-inorganic
perovskite, thus resulting in irreversible degradation. Then, effective strategies are either
replacing metal electrodes with stable electrodes like carbon electrodes of replace small
molecule charge transport materials with conductive polymer. Domanski et al. introduce a
chromium (Cr) interlayer between the gold electrode and hole-transporting layer
(spiro-MeOTAD), which has successfully slowed down the mitigation of gold through the
hole-transporting layer and into the organic-inorganic perovskite layer[81]. The resulting
devices with Cr interlayer exhibited improved stability. In addition, insertion a
chromium-chromium oxide interlayer between the perovskite and top electrodes, which
effectively inhibits the reaction between the metal electrodes and the perovskite. The
resulting flexible perovskite solar cells give a high power-per-weight of 23 W g-1 and more
importantly show a stabilized power conversion efficiency of 12% [82].
38
Han et al. reported a hole-conductor-free (5-AVA)x(MA)(1-x)PbI3 based perovskite solar cells
using porous carbon as a back contact material[83]. The 5-AVA cations not only template the
template of organic-inorganic perovskite crystals in the TiO2, but also induce its preferential
growth. The resulting (5-AVA)x(MA)1-xPbI3 perovskite with mixed cations exhibits less
defect and superior surface contact with the TiO2 compared to MAPbI3. Carbon materials are
intrinsically and chemically stable, which avoids the diffusion of electrodes through the
charge transport layer and into the perovskite layer. Thick porous carbon electrodes can
efficiently block the diffusion of moisture throughout the electrode into the perovskite layer.
Besides, porous carbon is a powerful adsorbent for oxygen, which effectively blocks the
diffusion of oxygen into the perovskite material. The unsealed device was stable under
constant 1 sun illumination in ambient environment for over 1000 h.
Figure 3.5 Energy band alignment of fully printable (5-AVA)x(MA)(1-x)PbI3 based perovskite
devices with thick porous carbon electrodes with respect to the vacuum. Approximately 1 μm
mesoporous TiO2 layer and 2μm mesoporous ZrO2 layer were screen-printed on the
FTO/glass. Perovskite precursor was drop-casted and infiltrated on the TiO2 layer. Carbon
layer with a thickness of around 10 μm was coated on top.
Table 3.1 Summarization of representative perovskite solar cells with long-term stability,
including relevant parameters such as storage condition and the rate of decreasing efficiency
39
Device configuration Encapsulation Test
duration
Storage condition PCE
degradation
percentage
Reference
FTO/c-TiO2/TEA-perovskite/
Spiro-OMeTAD/Ag
No 500 h 55 ± 5% relative
humidity, dark
~9.6% [76]
FTO/Li-TiO2/FAPbBr3/
Spiro-OMeTAD /Au
No 180 d ambient
condition
10% [84]
FTO/NiMgLiO/MAPbI3/
PCBM/Ti(Nb)Ox/Ag
Yes 1000 h 45° to 50°C, 1sun, less than
10%
[74] No 7 days RT, dark, air ~5 %
FTO/TiO2/polymer-scaffold
perovskite /Spiro-OMeTAD/Au
No 300 h 70% relative
humidity, dark
35% [85]
FTO/TiO2/RbxCsMAFA/
PTAA/Au
No 500h 85°C, 1sun, N2 5 % [77]
FTO/c-TiO2/m-TiO2/
(Perovskite:X PbI2)/
Spiro-OMeTAD/Au
No 2300h 1 sun, air 2.3% [86]
FTO/c-TiO2/m-TiO2/(Perovskite:
X PbI2)/Spiro-OMeTAD/Au
No 2500 h Dry air stable [87]
FTO/c-TiO2/Li-doped m-TiO2/
Csx(MA0.17FA0.83)(1-x)Pb(I0.83Br0.17
)3/spiro-OMeTAD/gold
No 250 h dry atmosphere in
the dark
10% [44]
FTO/TiO2/HCl-assisted
perovskite/
Spiro-OMeTAD/Au
No two and a
half month
Black,
30%-60%
humidity,
T=18-25
5% [88]
FTO/TiO2/mCVT perovskite/
HTM/Ag
No 100 d in air, 40%
relative
humidity
stable [89]
Table 3.1 (continued.)
40
Device configuration Encapsulation Test
duration
Storage condition PCE
degradation
percentage
Reference
FTO/PEDOT:PSS/2D
perovskites/PCBM/Al
Yes 2250 h 1 sun stable [47]
TiO2/ZrO2/(5-AVA)x(MA)1-xPbI3/
Carbon
No 1008 h RT, 1 sun, air stable [83]
ITO/PTAA/MAPbI3/Cu No 816 h 25 °C, ~55%
relative
humidity, air
2% [90]
41
Chapter 4
Experimental Section
This chapter introduces the materials, fabrication process for perovskite solar cells and
characterization methods in the thesis.
42
4.1 Materials
4.1.1 Perovskite layer
PbI2 and Methylammonium iodide (CH3NH3I) were purchased from Sigma-Aldrich and
Dyenamo, respectively. FAI and PbBr2 were purchased from Dyesol company. They are used
as received.
Table 4.1 Perovskite materials used in this thesis. Monovalent cations include MA+, FA+ and
Cs+. Monovalent anions include Br- and I-.
Material
abbreviation
Solvents
Perovskites
MAPbI3 DMF:DMSO (1:1)
MAPbI3 (with 5% PbI2
excess)
DMF:DMSO (4:1)
FA0.85Cs0.15PbI0.8Br0.2 DMF:DMSO (4:1)
FA0.85Cs0.15PbI0.6Br0.4 DMF:DMSO (4:1)
FA0.7Cs0.3PbI0.8Br0.2 DMF:DMSO (4:1)
FA0.7Cs0.3PbI0.6Br0.4 DMF:DMSO (4:1)
4.1.2 ETMs and HTMs
Table 4.2 Inorganic electron transporting materials used for fabrication of perovskite solar
cells in this thesis.
43
Material
abbreviation
Provider Solvents
ETLs
PCBM Solenne chlorobenzene
ZnO nanograde isopropanol
PCBM nanoparticles - water
Table 4.3 Organic and inorganic HTLs used in this thesis.
Material
abbreviation
Provider Solvents
HTLs
PEDOT:PSS (4083) Heraeus water
P3HT Merck water
Ta-WOx Nanograde isopropanol
SGO - water/isopropanol
PDCBT One material chlorobenzene
Synthesis of Ta-WOx: Firstly, tungsten and tantalum salts were dissolved in organic solvents
and stirred to prepare a precursor. The Ta-WOx nanoparticles were prepared by flame spray
synthesis with the above-mentioned precursor. Subsequently, the precursor was added into a
spray nozzle, dispersed by oxygen and lighted by a mixed oxygen-methane flame. After that,
the resulting off-gas was then filtered by a steel mesh filter by a vacuum pump. The Ta-WOx
nanoparticles achieved was gathered from the filter mesh. Then the nanoparticles were
dispersed in isopropanol or ethanol with an undisclosed dispersant to get a stable suspension.
Solution-processed sulfated graphene oxide (SGO) was synthesized with graphite crystals
(grade 3061) purchased from Asbury Carbons Inc. Sulfuric acid, Potassium permanganate
44
and hydrogen peroxide were ordered from Sigma-Aldrich® and used as received. Double
distilled water was from Carl Roth®. A Sigma 4K15 centrifuge equipped with 200 ml plastic
centrifuge tube was used for centrifugation. A Bandelin UW3200 sonotrode combined with a
plane titan tip and a maximum power of 200 W was used for ultrasonication. SGO was
synthesized according to previous literature, but with minor modification[91]. Firstly,
graphite crystals (1 g) were added into cold conc. H2SO4 (96%, 24 mL, below 10 °C) with
stirring. After that, KMnO4 (2 g) was added into the dispersion and stirred for 16 h.
Subsequently, 20 mL H2SO4 (20%) into the mixture in 4 h with continuously adding water
(100 mL). Then, 50 mL hydrogen peroxide (5%) was added into it to reduce insoluble
manganese components. The dispersion was always kept beneath 10 °C provided by a
refrigerating coil. The SGO was collected with repeated centrifugation at 1500 RCF for 6
times. Then, it is subjected to pulsed ultrasonication for 4 minutes (20 W), which resulted in
its delamination. The resulting dispersion was centrifuged and washed at a speed of 1500
RCF for several times to obtain monolayer particles. Then, it was centrifuged at 13000 RCF
for 45 min to remove the partial smallest particles. At the end, isopropanol was added into the
SGO suspension. A 0.25 mg/ml stable yellowish SGO suspension 1:1 (vol:vol)
water/isopropanol was obtained.
45
Figure 4.1 Chemical structures of charge transporting materials used for fabricating
perovskite solar cells in the thesis.
To prepare a 40 wt.% MAPbI3 precursor solution，PbI2 and CH3NH3I were added in a mixture
of DMF and DMSO (2:1 v/v) with molar ratio of 1:1 and stirred at 60 overnight.
Subsequently, it is filtered with a 1.3 cm diameter and 0.45 mm pore syringe filter. A 1.4 M
FA0.83MA0.17Pb1.1Br0.50I2.80 precursor solution was similarly prepared using a mixture of DMF
and DMSO (4:1 v/v). A PCBM solution in chlorobenzene of 5mg/ml and 20mg/ml were
prepared by adding PCBM in the chlorobenzene and stirred at 60 °C for 2 hours. The PDCBT
was dissolved in chlorobenzene at concentrations of 5 mg/mL and stirred for 2 hours at
60 °C.
4.2 Device Fabrication
4.2.1 Spin-coating technique
Spin-coating technique is currently the predominant procedure used to apply homogeneous
thin films with thickness of the order of micrometers and nanometers to flat substrates. An
important equipment used for spin-coating is called a spin processor, spin coater or spinner. It
one of the most low-cost for fabricating solution-processed organic-inorganic perovskite solar
cells. However, it is difficult for simple spin-casting to obtain a uniform large-area perovskite
film with homogeneous thickness and impressive crystallinity compared to evaporation
methods. A typical spin-coating process is as follows. First, to obtain precursor for
spin-coating, some organic materials or inorganic salts are dissolve in certain solvents such as
chlorobenzene and dimethyl formamide and stirred for certain time at certain temperature.
The solvent for the precursor is normally volatile and simultaneously evaporates during the
spin-coating process. Second, the precursor is manually (using a syringe) or automatically
(with a dispense unit and dispense nozzles) applied onto the center of a substrate fixed on the
sample holder of the spin-coater and spinned at high speed such as 4000 rpm for tens of
seconds. The precursor is spread by centrifugal force to and finally off the edge of the
substrates. Finally, the substrate is annealed at certain temperature and a thin film with
46
desired thickness on substrates is achieved.
4.2.2 P-i-n architecture
At first, FTO substrates with a size of 25 mm*25 mm is patterned by laser. Then, the
patterned FTO substrates were cleaned with toluene, acetone and isopropanol by
ultrasonication. After that, PEDOT:PSS were spin-coated on the cleaned FTO substrates and
annealed at 140 °C for 10 min under ambient atmosphere. In addition, SGO suspension was
deposited on the FTO substrates by spin-coating and followed by annealing at 120 °C for 10
min under ambient condition. The perovskite precursor solution was spin-coated onto the
FTO/PEDOT:PSS substrates or FTO/SGO substrates at 4000 rpm for 35 sec. For solvent
dripping, 300 μL of toluene was dropped onto the precrystallized perovskite film during the
last 5 s of the spinning process. Then, the perovskite film is blown by an N2 stream to
partially remove the residual solvents followed by annealing at 100 ºC for 10 minutes.
Subsequently, PCBM(20 mg/ml) in chlorobenzene solution was spin-coated onto the
perovskite film at a speed of 2000 rpm for 30 sec. After that, ZnO nanoparticle suspensions
(provided by NanoGrade) was spin-coated on the PCBM layer at 3000 rpm. At the end, 150
nm Al was evaporated on top of ZnO layer with mask to form a 10.4 mm2 active area under
10-6 mbar.
4.2.3 N-i-p architecture
PCBM in chlorobenzene solution or aqueous PCBM nanoparticles were spin-coated on the
ITO glass substrates at 2000rpm for 30 sec. After that, the films were annealed at 100 ºC for
10 minutes. Subsequently, MAPbI3 or FA0.83MA0.17Pb1.1Br0.50I2.80 precursor solution was
deposited on the fullerene layer at 4000 rpm for 35 sec. For solvent dripping, 300 μL of
toluene was dropped onto the precrystallized perovskite film during the last 5 s of the
spinning process. Subsequently, the perovskite film is blown by an N2 stream to partially
remove the residual solvents. Then, the substrates was annealed at 100 ºC for 10 minutes and
a shining perovskite film like a mirror is achieved. Subsequently, PDCBT in chlorobenzene
solution was spin-coated onto the perovskite layer at 1000 rpm for 30 sec and annealed at 80
ºC for 5 min. After that, Ta-WOx suspension (nanoparticle suspensions, provided by
47
NanoGrade) was spin-coated on top at 1000 rpm for 30 sec and annealed at 80 ºC for 5 min.
At last, 100 nm Au was evaporated on Ta-WOx layer through a shadow mask under 10-6 mbar
to form an active area of 10.4 mm2.
4.3 Device Characterization
4.3.1 J-V and EQE measurement
J-V measurement was carried out by a source measurement unit purchased from BoTest in
combination with a Newport Sol 1A solar simulator. The light source was calibrated with
AM1.5G spectra at 100 mW/cm2 by using a certified silicon reference cell. For the light
intensity dependent measurement, a series of neutral colour density filters were used to tune
the light intensity, thus allowing the intensity to range from 0.4 to 100 mW/cm2. All
perovskite solar cells were tested under ambient atmosphere.
The EQE of a solar cell is a vital parameter because it gives information on the current that a
given solar cell will produce when illuminated by incident photons of a particular wavelength.
The EQE measurement was carried out by an Enli Technology EQE measurement system
(Taiwan). The light intensity of the light source at each wavelength was calibrated by using a
certified single-crystal Si photovoltaic cell. It is defined as the ratio of number of carriers
collected by the perovskite solar cell to the number of photons incident on the solar cell. All
the EQE curves for perovskite solar cells in this thesis are achieved without bias voltage.
4.3.2 Lifetime test
To avoid the effect of moisture and oxygen in the ambient environment, pressure-tight
aluminum chambers with nitrogen was used flow to hold perovskite solar cells. Transparent
glass windows on the chambers permit investigation of the light stress on the stability of
perovskite solar cells. In addition, cooling system beneath the chambers is used to cool the
photovoltaic devices to low temperature (below 30 ), thus eliminating the effect of the heat
on lifetime of the perovskite cells. Light-induced degradation of perovskite solar cells was
48
carried out under one sun equivalent illumination provided by white light LEDs (X
BRIDGELUX-BXRA-30E0800-B-00), which also minimizes the thermal degradation.
Current-voltage curves were recorded using a Keithley 2400 source meter. All solar cells in
the chamber were held at maximum power point. The current-voltage curves for all
degradation duration were automatically recorded via a LabVIEW interface. Both moisture
and oxygen concentrations in the chamber were kept below 0.5 p.p.m. for the lifetime
duration in N2 atmosphere.
Figure 4.2 (a) Home-built setups for investigating the lifetime of organic-inorganic
perovskite solar cells. The illumination is provided by white light LEDs (X
BRIDGELUX-BXRA-30E0800-B-00). The inert atmosphere with constant temperature is
provided by an airtight metal chamber. (b) Current-voltage curves were recorded using a
Keisight B2901A source meter. (c) A multiplexer for controlling measurement of each solar
cell in the chamber.
4.3.3 ATR-FTIR
Nowadays, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is
the most widely used FTIR tool, which has been shown to be very useful in various
applications. When the IR beam is incident into a crystal of relatively higher refractive index,
it reflects from the internal surface of the crystal and creates an evanescent wave which casts
49
orthogonally into the sample in close contact with the ATR crystal. Some of the evanescent
wave is absorbed by the specimen, while the reflected radiation is partially returned to the
detector. ATR-FTIR generally allows qualitative or quantitative analysis of samples with little
specimen preparation.
4.3.4 AFM
The AFM consists of a cantilever made of silicon or silicon nitride with a sharp tip (probe)
which is used to scan the sample surface. The tip radius of curvature is on the order of
nanometers. When the tip approaches close to a specimen surface, forces between the
specimen and the tip result in a deflection of the cantilever. When a laser beam is reflected off
the cantilever, the signal is achieved. Then the signal in cantilever deflection or oscillation is
rectified to a setpoint value by minimizing the error signal through a feedback-controlled
piezo. At last, the corrected signal is transformed into a high-resolution image of the surface.
The AFM can be operated in a several modes such as tapping modes and contact modes.
Generally, common imaging modes include contact modes and non-contact or "tapping"
modes where the cantilever is oscillated or vibrated at certain frequency. In this thesis, AFM
was employed to investigate the surface roughness and morphology of thin films. All
topographical measurements in this thesis were carried out with AFM (Veeco Model D3100)
with tapping mode.
4.3.5 Scanning electron microscope (SEM)
SEM is an effective method to characterize the morphology, which produces images of
samples by scanning the surface of sample with a focused beam of electrons. The electrons
interact with atoms in the samples, generating information about the sample's surface
morphology and composition. The top-view SEM surface images were achieved with the
electron beam of the FEI JEOL7601F SEM using an acceleration voltage of 2 kV with a
current of 100 pA. In addition, energy dispersive spectroscopy in the SEM can be used to
analyze the elemental component in the sample
50
4.3.6 Photoluminescence spectroscopy (PL)
The room temperature PL spectra were recorded with a home-made PL setup. Choosing an
appropriate laser wavelength is essential to avoid unwanted fluorescence interference. The PL
excitation wavelength for this thesis was set to 450 nm. PL measurements were carried out on
perovskite films grown on different charge transporting layer. When a sample is illuminated
by the blue laser, PL is generated from the sample after the absorption of photons. The
intensity and peak position of PL depends on the material and laser wavelength.
4.3.7 Surface energy
The contact angle for interface materials used in this thesis such as SGO was measured with a
contact angle instrument from Dataphysics (model OCA20). The values of contact angles for
various interface materials were calculated with the SCA20-U software and the
Owens-Wendt and Kaelble method. In this thesis, the liquid for the surface energy
measurement is water. Contact angle measurements is carried out under ambient atmosphere.
4.3.8 X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultrahigh
vacuum (UHV) chamber with a pressure below 5×10−10 mbar employing a Scienta ESCA 200
spectrometer in combination with a hemispherical electron energy analyzer and a
monochromatic Al Kα X-ray source (1486.6 eV). It is a powerful tool that provides important
quantitative and chemical state information from the surface of the sample. The typical
analysis depth of XPS is only several nanometers.
4.3.9 X-ray powder diffraction (XRD)
XRD is a powerful method for identifying the atomic and molecular structure of minerals and
other crystalline materials. X-ray measurement was carried out by exposing powder samples
to filtered Cu-Kα radiation. The XRD data is collected by a Panalytical X'pert powder
diffractometer with a solid-state X'Celerator stripe detector. Typically a few milligrams of
51
powder or a single crystal are fixed on the sample holder and then exposed to an X-ray beam.
Diffraction data can be obtained from powdered samples, thin films or single crystals. The
resulting diffraction data are compared against those in a XRD database to search for a match.
Additionally, it can also be employed to estimate crystallite size.
4.3.10 Raman spectroscopy
Raman spectroscopy is an effective tool to quantitatively determine the defects in the
semiconductors. It can also be used for determining light-trapping capability of
semiconductors. Since it is non-destructive to products, it can be used in production line for
quality control.
52
Chapter 5
Efficient and Environmentally Stable Perovskite Solar
Cells Employing Sulfated Graphene Oxide as the
Anode Interfacial Layer
In this chapter, solution-processable single layers of sulfated graphene oxide (SGO) was
developed and employed as anode interfacial layer for organometal tri-halide perovskite solar
cells. The resulting solar cells with planar inverted architecture give a high PCE of 15.2%.
Moreover, they exhibit the full Voc of close to 1.1 V. Besides, sulfated graphene oxide can
effectively inhibit the ingress of water vapor into the device, thus significantly improving its
environmental stability. The unpackaged cells keep 80% of the initial PCE under illumination
after at 500 h. Approximately 60% of the initial PCE of the unpackaged perovskite solar cells
is kept after ~1000 h under 0.5 sun illumination and environmental atmosphere with the
temperature lower than 30 °C. In addition, 92% of its initial PCE of the unencapsulated
perovskite devices is retained after approximately 1900 h in ambient atmosphere and without
illumination. Our findings may provide a promising and facile approach to efficient and
environmentally stable photovoltaic devices.
Part of this chapter has been published in [52](Reproduced with permission from Royal Society of
Chemistry) in collaboration with Yi Hou, Christian E. Halbiq, Siegfried Eigler, Shi Chen, Ning Li, Cesar
Omar Ramirez Quiroz, Fei Guo, Xiaofeng Tang, Nicola Gasparini, Simon Kahmann, Ievgen Levchuk,
Andres Osvet, Hong Zhang, Christoph J. Brabec. Haiwei Chen fabricated the perovskite solar cells, carried
out the measurements including J-V characterization, lifetime test etc., analyzed the data and wrote part of
the resulting manuscript. Haiwei Chen, Yi Hou, Shi Chen Christoph J. Brabec，Xiaofeng Tang et al.
designed the experiments. Simon Kahmann performed the FTIR-ATR measurement and related analysis.
Xiaofeng Tang carried out the SEM measurement. Cesar Omar Ramirez Quiroz prepared the ZnO
dispersion. Christian E. Halbiq and Sieqfried Eigler performed the synthesis and characterization of SGO
such as the Raman characterization. AFM characterization of ZnO and perovskites on different hole
53
transporting layer layers was carried out by Hong Zhang. Hong Zhang also performed the contact angle
measurement and related analysis. Ning Li, Fei Guo, Christoph J. Brabec, Nicola Gasparini, Ievgen
Levchuk, Andres Osvet revised this manuscript.
5.1 Solution processed SGO
Device operational lifetime is one of the most important factors determining the success and
performance potential of photovoltaic technologies. Lifetime issues are of immense concern
in developing high-efficiency and commercial organometal tri-halide perovskite solar cells
because of their water soluble Pb-containing component[92]. In the early stages of perovskite
device research, unsatisfying thermal and structural stability has been reported and seems to
be one of the main roadblocks towards the large-scale practical application of perovskite solar
cells[93]. Tremendous efforts have been devoted to understand and illuminate the exact
degradation mechanism of organic-inorganic perovskites in the presence of moisture.
Previous studies have focused on the decomposition of perovskite, which leads to the
formation of methyl-amine and PbI2. More recent works rather imply moisture in ambient
environment has a detrimental effect on the stability of the perovskite solar cells and leads to
predominant degradation of the perovskite solar cells. When exposed to ambient environment,
moisture not only causes perovskite to degrade and turn into PbI2 but also complexes with it
and forms a hydrate (CH3NH3)4PbI6:H2O product[94]. Gradual changes in perovskites lead to
the loss of power conversion efficiency of a perovskite devices, thus limiting its lifetime.
Interestingly, oxygen has been demonstrated to have little influence on the lifetime of
perovskite solar cells[59]. However, in the presence of light, the perovskite will be oxidized
by oxygen and moisture，thus leading to photo-oxidative bleaching of the perovskite layer.
To solve the stability problem of the perovskite devices, lots of efforts have recently been
devoted to developing novel materials. For example, perovskite materials such as
FA1-xCsxPbI3, (C6H5(CH2)2NH3)2(CH3NH3)2[Pb3I10] and (CH3NH3Pb(SCN)2) with superior
photo- or moisture-stability have been developed[95, 96]. In addition, phosphonic acid
ammonium derivatives have been reported to cross-link perovskite grains, thereby enhancing
the environmental stability of the perovskite solar cells[38].
54
A simple and potential method to guarantee perovskite devices with long lifetime is to
package the perovskite solar cells with barriers or adhesives with low water vapor
transmission rate (WVTR)[48, 97]. It has been demonstrated that poly(methyl methacrylate)/
hydrophobic carbon nanotube composites and Teflon, as effective barriers, inhibit the
diffusion of moisture into perovskite materials and improve environmental stability of
perovskite cells[97]. In 2014, Han’s group used a carbon layer as a top electrode and a
mositure-retaining film for hole transporting material-free perovskite devices. The resulting
unencapsulated perovskite solar cells obtained a high PCE of 12.8% and promising lifetime
under 1 sun illumination[83]. In another work by Wei et al., a free-standing carbon layer was
deposited and used as the cathode of HTM-free perovskite devices, exhibiting a PCE of
13.53% and good lifetime[98]. It has been also proved that HTM-free perovskite devices
show promising lifetime when subjected to high temperature and continuous outdoor
illumination[99].
Organic-inorganic perovskite devices can be fabricated in different architectures like
mesoporous structures and planar structures[100, 101]. An inverted planar architecture
（p-i-n）is chosen for this study owing to its promising compatibility with large-area solution
production at fairly low temperatures as well as hysteresis free. Generally, perovskite solar
cells with inverted planar structure contain a conducting transparent electrode such as indium
and fluorine doped tin oxide （ITO and FTO）), a hole-transporting film, an organic-inorganic
perovskite film, an electron-transporting film like phenyl-C61-butyric acid methyl ester
(PCBM) with a buffer layer like ZnO and a top cathode (e.g. Ag, Au and Al)[101, 102].
Various conjugated polymers (e.g. poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine), Poly
(3,4-ethylenedioxythiophene) polystyrene sulphonate (PEDOT:PSS), polythiophene) have
been employed as HTMs for perovskite solar cells[103, 104]. PEDOT:PSS is the most
popular HTM due to its excellent properties, such as superior film forming properties, low
cost, green solvent and large-scale roll-to-roll manufacturing probability[105]. Nonetheless, it
also has shortcomings like being acidic and hygroscopic. The high saturation concentration of
moisture results in a fast accumulation of moisture in the interface layers. All these
shortcomings of PEDOT:PSS seems to be not evitable. Only recently, it has been
demonstrated by us water-free PEDOT:PSS solutions which could be used for fabricating
electron blocking layer on top of perovskite devices in the regular cell architecture
(n-i-p)[106]. Several inorganic hole transporting layers like graphene oxide, CuSCN and
55
NiOx have been recently employed as alternative HTMs for hysteresis-free organic-inorganic
perovskite devices with the inverted architecture[28, 107, 108]. Li+ doped NixMg1-xO and
NiOx are employed to replace PEDOT:PSS, which leads to almost non-hysteresis perovskite
solar cells with superior environmental stability. Reduced graphene oxide has been employed
as electron blocking layer for perovskite solar cells. The PCE of resulting devices from
around 10% to 6% in 120 h in ambient environment[108]. Reduced graphene oxide and
graphene oxide are fabricated from natural graphite under rigorous oxidation conditions.
During the liquid exfoliation process of the carbon framework, nanoscale huge holes are
prone to be introduced[109]. In contrast to the synthesis for graphene oxide, a milder way
was exploited to largely avoid the generation of in-plane defects[110]. Besides, functional
groups were generated and organic sulfate esters functional groups were formed in the
graphene derivative with approximately one organosulfate on 20-30 C atoms[91]. What is
more, the negative charge state of the functionalized graphene derivatives is determined by
the organosulfate groups[111].
In this chapter, solution-processed sulfated graphene oxide (SGO) was used to replace
hydrophilic PEDOT:PSS as hole transporting layer for organic-inorganic perovskite devices.
The resulting solar cells in the inverted planar architecture give a superior photovoltaic
performance with a PCE of close to 15.2% with SGO as an anode interfacial material. In
addition, a high Voc of close to 1.1 V is achieved. Moreover, SGO effectively inhibits the
ingress of moisture into the device stack leading to dramatically improved environmental
stability of unsealed devices. Approximately 80% of the initial PCE is retained after at 500 h
under 0.5 sun illumination and N2 atmosphere. Unsealed perovskite solar cells keep ~60% of
the initial PCE under 0.5 sun illumination and environmental atmosphere with the
temperature lower than 30 °C for ~1000 h. In addition, without encapsulation, 92% of its
initial PCE of the perovskite device is retained under environmental atmosphere and in the
dark after approximately 1900 h. Our results has demonstrated that controlling the moisture
ingress into perovskite devices with various interface engineering is an essential method
towards enhanced lifetime under ambient condition. The schematic architecture of the
photovoltaic solar cells and structure of the SGO are shown in Figure 5.1. The SGO is the
hole transporting layers, which blocks the electron transport. PCBM is the electron
transporting layer, which blocks the hole transport. ZnO is a buffer layer between the top Al
electrode and the PCBM layer, which effectively improves the ohmic contact. SGO material
56
was synthesized through a milder chemical synthesis method according to the literature [107].
Fewer defects are generated in the synthesis process compared to the harsh synthesis
condition for graphene oxide. As can be seen in Figure 5.1, the SGO material is
functionalised with OSO3- and -OH group.
Figure 5.1 The schematic architecture and energy level alignment vesus vacuum of the
photovoltaic solar cells. SGO and PCBM were employed as hole and electron transporting
materials, respectively. A ZnO layer is used as buffer layer between the PCBM and Al. The
chemical structure of the SGO is also shown in the figure.
Figure 5.2. Raman spectra of a) SGO and b) graphene (G1). ID/IG ratio of c) SGO and d)
graphene spectra as a function of FWHM 2D peak(Adapted from [52] with permission from
57
Royal Society of Chemistry). Christian E. Halbiq and Sieqfried Eigler carried out the
synthesis and characterization of SGO such as the Raman characterization. They provided
three batches.
Most of the carbon lattice in the SGO is retained after the mild oxidation and exfoliation in
contrary to material achieved by the Hummer’s method. Besides, sulphate ester groups
attached on both sides of the single-layer SGO sheets are not hydrolysed since reaction
temperature is always maintained beneath 10°C. 2D and ID/IG for SGO are 196±66 and
1.02±0.04, respectively. 2D and ID/IG for SGO for graphene are 79±11 and 2.12±0.34,
respectively. The defect density inside the SGO lattice is analysed by statistical Raman
measurement and determined to be only ~0.8 %.
5.2 Morphologies of SGO
Figure 5.3 a) An AFM image of SGO on a Si/SiO2 substrate. The SGO layer is coated on the
substrate from the SGO dispersion b) a scanning electron microscope (SEM) image of SGO
on the FTO/glass. The SGO is spin-coated on the single crystalline silicon substrate.
(Reproduced after [52] with permission from Royal Society of Chemistry). Xiaofeng Tang
carried out the SEM measurement. AFM characterization of SGO was carried out by Hong
Zhang.
The AFM image was achieved with a tapping mode by depositing SGO sample on a Si/SiO2
substrates. As can be seen in Figure 5.3, the size of SGO ranges from 1 μm to 10 μm, which
is larger than the crystalline size of perovskite crystal. It is consistent with the previous
58
literature that the SGO sheets cover part of the FTO surface[112]. The discontinuous SGO
layer between the FTO glass and perovskite layer leads to a higher coverage of perovskite
with larger crystalline size.
5. 3 AFM characterization of perovskites
Figure 5.4 AFM morphology images of a) perovskite film grown on the PEDOT:PSS/FTO
glass, b) perovskite film grown on the SGO/FTO glass; c) ZnO layer on the
SGO/perovskite/PCBM composite layer. All the layers are fabricated with spin-coating
method (Reproduced after [52] with permission from Royal Society of Chemistry). The AFM
images are obtained by tapping mode. AFM characterization of ZnO and perovskites on
different hole transporting layer layers was carried out by Dr. Hong Zhang.
As can be seen in Figure 5.4, perovskite precursor solution was spin-coated on the SGO film
and PEDOT:PSS film, followed by annealing at 100 to form the organic-inorganic
perovskite film. The mean crystalline size of perovskite crystals formed on SGO layer is
much bigger than those formed on PEDOT:PSS layer. The biggest crystalline size of
59
perovskite crystal on SGO layer is approximately 600 nm, while the largest crystalline size
for the perovskite crystal on PEDOT:PSS layer is only approximately 350 nm. Besides, the
organic-inorganic perovskite layer grown on the SGO layer has fewer pinholes and is more
compact. It decreases the probability of the direct contact between the perovskite and
interface, thereby lowering the related recombination. The mean size of ZnO nanoparticles is
only approximately 30 nm. The resulting ZnO layer effectively isolate the Al electrode and
the perovskite layer, thus slowing down the degradation.
5.4 J-V and EQE characteristics
Figure 5.5 (a) J-V curves and (b) EQE curves of SGO/CH3NH3PbI3/PCBM/ZnO/Al devices
(blue squares) and PEDOT:PSS/CH3NH3PbI3/PCBM/ZnO/Al devices (green circles)
(Redrawn after [52] with permission from Royal Society of Chemistry). 38 SGO-based
devices and 26 PEDOT:PSS-based devices were fabricated and characterized. J-V curves
were obtained with a sweeping speed of 100mV/s. Illumination with various light intensity
was provided with a Newport Sol 1A solar simulator. The EQE curves were obtained by an
Enli Technology EQE measurement system (Taiwan). The light intensity at each wavelength
of the light source was calibrated with a certified single-crystal Si photovoltaic cell.
Figure 5.5 shows the J-V curves of a champion solar cell in combination with SGO
compared to a reference solar cell based on PEDOT:PSS hole transporting layer. In order to
obtain an excellent PCE, it is essential to achieve a reasonable high Voc and a high Jsc
simultaneously. Whereas, it is one of the most crucial issues for various photovoltaic devices
to reach a balance between the two parameters. The hero SGO-based perovskite device
exhibits a high PCE of 15.2%, a Voc of 1.08 V, a Jsc of 18.06 mA/cm2, and a FF of 77.7%,
60
while the reference solar cell with PEDOT:PSS hole transporting layer gives a PCE of around
10.8%, with a Voc of 0.928 V, a Jsc of 17.1 mA/cm2, and a FF of 68.6%. Jsc for the SGO-based
device and PEDOT:PSS-based device is similar, which is further confirmed with EQE
measurements under AM 1.5G illumination with deviations beneath around 5% (see Figure
5.5b). However, substituting the PEDOT:PSS interfacial layer with SGO hole transporting
layer significantly enhances Voc and FF, thus leading to a superior PCE.
Group 1 2 3 4 5 6
PCE(%) 10-11 11-12 12-13 13-14 14-15 15-16
Counts 2 7 10 13 5 1
Table 5.1 Photovoltaic efficiency distribution of 38 perovskite solar cells based on SGO hole
transporting layer. J-V curves of these solar cells were obtained with a sweeping speed of
100mV/s.
The distribution of power conversion efficiency for perovskite solar cells based on SGO hole
transporting layer are summarized and exhibited in Table 5.1. As can be seen in the table,
most of the devices gave a PCE between 12% and 14%, showing a reasonable reproducibility.
The highest PCE for this kind of perovskite device was beyond 15%, while there were solar
cells showing PCE beneath 11%. More efforts such as improving the quality of SGO and
perovskite film is essential to enhance the reproducibility and power conversion efficiency.
61
Figure 5.6 Comparison between the dark J-V characteristics of perovskite solar cells based
on SGO and PEDOT:PSS (Reproduced after [52] with permission from Royal Society of
Chemistry). The dark J-V curves of these solar cells were obtained with a sweeping speed of
100mV/s. They corresponded to the champion devices based on SGO and PEDOT:PSS.
The series resistance can be calculated from the dark J-V curves of perovskite solar cells. The
series resistance for the solar cells based on SGO is calculated to be 9.9 Ω cm2, whereas the
reference device based on PEDOT:PSS exhibits a higher series resistance of 15.5 Ω
cm2(Figure 5.6). The relatively lower series resistance of SGO-based devices partially
contributes to a higher FF of 77.7% than that of PEDOT:PSS-based devices. In addition, as
can be seen from the AFM images for these two kinds of perovskite films (Figure 5.4), the
perovskite layer on SGO layer shows larger crystal size than that on PEDOT:PSS, which
may result in the strong field dependence of the solar cells based on SGO from the 4th into the
3rd quadrant.
5.5 Work function
Table 5.2 Work function of SGO and PEDOT:PSS characterized by Kelvin probe in the
ambient condition (Redrawn after [52] with permission from Royal Society of Chemistry).
62
Work function (eV)
SGO -5.2
PEDOT:PSS -5.0
The work function of PEDOT:PSS film and SGO film measured by Kelvin probe is
demonstrated to be -5.0 and -5.2 eV, respectively (Table 5.2). The higher work function of
SGO film than PEDOT:PSS film dominantly leads to the higher Voc. Besides, superior match
between the valence band of perovskite and the work function of SGO film might facilitate
the charge transport, thus enhancing the Jsc and PCE of perovskite solar cells.
5.6 Hysteresis
Figure 5.7 Hysteresis performance of perovskite solar cells based on SGO and PEDOT:PSS
films. The J-V curves of these perovskite solar cells were obtained with a sweeping speed of
100mV/s.
Hysteresis performance of perovskite solar cells based on SGO and PEDOT:PSS films was
also investigated and compared (shown in Figure 5.7). Although the SGO-based device
exhibited higher Voc and Jsc, slight hysteresis was observed in its J-V characteristics. There is
almost no difference for the Jsc extracted from the forward-scan and reverse-scan J-V curves.
Interestingly, there is almost no hysteresis in the PEDOT:PSS-based perovskite solar cells.
63
Figure 5.8 (a) Short circuit current density as a function of the incident light intensity for
various solar cells: SGO/CH3NH3PbI3/PCBM/ZnO/Al devices (blue squares) and
PEDOT:PSS/CH3NH3PbI3/PCBM/ZnO/Al devices (gree circles) (Reproduced after [52] with
permission from Royal Society of Chemistry), (b) Voc as a function of the light intensity for
SGO based device and PEDOT:PSS based device.
In addition, in order to illuminate the recombination mechanism of the solar cells,
light-intensity-dependent photocurrent characterization was carried out. The relationship
between the incident light intensity (I) and Jsc generally obeys the power law (Jsc∝I α). In
Figure 5.8a, the light intensity dependence of short circuit current density for these two kinds
of solar cells was plotted on a log-log scale and fitted to the power law. There is almost no 2nd
order recombination in solar cells based on SGO at Jsc conditions when the exponent α ≈
1[113]. Interestingly, PEDOT:PSS based devices give a lower α = 0.93, suggesting a small
but obvious contribution from 2nd order recombination.
The light intensity dependence of Voc in Figure 5.8b for both types of devices was also
plotted. A slope of 1.42 kT/q for SGO device and 1.27 kT/q for PEDOT:PSS device were
extracted, indicating a stronger dependency of Voc on the light intensity and additional
trap-assisted recombination losses in the case of SGO device.
64
5.7 Lifetime characterization
The lifetime performance of the organic-inorganic perovskite devices with PEDOT:PSS and
SGO hole transporting layers was studied with a home-built equipment. LED lamps provide
illumination without ultraviolet light for the lifetime measurement so as to avoid its negative
effect on the light soaking lifetime. Unpackaged devices based on SGO and PEDOT:PSS are
subjected to three different conditions: (1) storage under environmental condition and in the
dark, (2) storage under 0.5 suns illumination in nitrogen atmosphere and (3) storage under
0.5 suns illumination in ambient condition. The related photovoltaic performance evolution
with time is systematically evaluated so as to compare the long-term stability of the resulting
solar cells. Under three different conditions, solar cells based on SGO exhibit superior
lifetime compared to those devices based on PEDOT:PSS. Solar cells based on SGO give
unusually long lifetime under environmental atmosphere in the dark. Approximately 92% of
initial PCE for solar cells was maintained after being stored under environmental atmosphere
(a relative humidity between 30-50% and temperature of around 15-25 ) for over 1900 h.
In contrary to SGO devices, only approximately 50% of initial PCE for solar cells based on
PEDOT:PSS layer is kept after about 400 h.
Figure 5.9 PCE evolution as a function of time for unencapsulated perovskite solar cells
based on a) PEDOT:PSS compared to unencapsulated solar cells based on b) SGO subjected
to various conditions: 1) storage under an ambient atmosphere and in the dark, 2)
illumination under 0.5 suns and in nitrogen atmosphere, 3) illumination under 0.5 suns in
ambient atmosphere (Light-induced degradation of perovskite solar cells was under one sun
equivalent illumination provided by white light LEDs (X
65
BRIDGELUX-BXRA-30E0800-B-00) without ultraviolet light, which minimizes the
degradation caused by ultraviolet light and the thermal degradation . Current-voltage curves
were achieved using a Keithley 2400 source meter. All solar cells were kept under open
voltage. All J-V curves were achieved under AM 1.5 G illumination with a sweeping speed of
100mV/s. (Redrawn after [52] with permission from Royal Society of Chemistry).
When subjected to 0.5 suns illumination in nitrogen atmosphere, solar cells based on SGO
give superior photo-stability than those based on the PEDOT:PSS. In addition, both two kinds
of unpackaged solar cells give a similar trend in spite of the much smaller difference. Solar
cells based on PEDOT:PSS only maintain 54% of their initial PCE after around 670 h. In
contrast, solar cells based on the SGO still keep 74% of their initial PCE. When subjected to
ambient atmosphere (a RH of between 30-50 % and temperature of around 20-30 ) and
illumination of around 0.5 suns, lifetime for these two kinds of solar cells show the most
dramatical discrepancy. Unpackaged solar cells based on PEDOT:PSS almost lose over 95%
of their initial PCE after only 50 h. In contrast, unpackaged SGO devices keep ~60% of their
initial PCE under the same condition after 1000 h. It indicates the solar cells based on
PEDOT:PSS almost fail completely after 50 h. By contrary, the solar cells based on SGO
only show little degradation. These measurements with the observation were summarized.
The unsealed perovskite solar cells based on SGO exhibit superior lifetime compared to
those based on PEDOT:PSS under the three different conditions. It suggests that SGO layer
may act as a barrier slowing down the diffusion of moisture into the organic-inorganic
perovskite film.
Figure 5.10 The J-V characteristics of fresh and aged perovskite solar cells based on
66
PEDOT:PSS (a) and SGO (b) layer (storage in the dark and an ambient environment). The
J-V curves of these solar cells were obtained with a sweeping speed of 100mV/s.
As can be seen in Figure 5.10, after storage in the dark and ambient atmosphere for 264h,
both Voc and Jsc decreased. Jsc significantly decreased after 504h, while the Voc recovered.
The decrease of Jsc might result from the degradation of perovskite layer and the PEDOT:PSS
interface. By contrast, although Jsc and Voc of the SGO-based device decreased, its FF
increased a bit. Even after storage in the dark and an ambient environment for 1960 h, a
promising PCE and FF were still maintained.
Figure 5.11 The J-V characteristics of fresh and aged perovskite solar cells based on
PEDOT:PSS (a) and SGO (b) layer (The J-V curves of these solar cells were obtained with a
sweeping speed of 100mV/s); (c) the UV-Vis absorption spectra of fresh and aged
organic-inorganic perovskite devices in combination with PEDOT:PSS and SGO hole
transporting layer (under continuous 0.5 sun light soaking and an ambient atmosphere)
(Redrawn after [52] with permission from Royal Society of Chemistry).
67
Jsc of perovskite solar cells based on PEDOT:PSS decreased dramatically after storage under
constant 1 sun illumination and an ambient environment for 24h, while the Voc increased
slightly. It further decreased closed to zero in only 48 h. The decrease of Jsc might mainly
result from the degradation of perovskite layer and the PEDOT:PSS interface under
illumination. In contrast, Jsc of the SGO-based device decreased steadily in the first 48 h, its
Voc increased a bit, while maintaining a stable FF. After 1036 h, the Jsc decreased to ~60% of
original value, while a high Voc was still maintained. It is further demonstrated by the UV-Vis
absorption result. As can be seen in Figure 5.11c, UV-Vis absorption spectra for the solar
cells based on PEDOT:PSS exhibit a dramatical change at a RH of 30-50 % and under 0.5
suns illumination after only 96 h. In contrast, only negligible or almost no changes for the
UV-Vis absorption spectra of SGO based devices are found after 240 h. It is in good
correlation with previous report by Kelly et al[59]. The three main features in these spectra
are attributed to the generation of a mixture of PbIx hydrate complexes, perovskite and PbI2.
Visible light is reported by Li and co-workers to induce and accelerate the ion migration. It
well explains why perovskite solar cells exhibit a faster degradation under illumination in
ambient atmosphere than those counterparts stored under ambient condition and in the
dark[61].
5.8 Contact angle
Figure 5.12 The contact angle for a drop of water: a) on PEDOT:PSS film (18) and b) on
SGO film (48) (measured under room temperature and ambient atmosphere). Dr. Hong
Zhang carries out the contact angle measurement. Contact angle is measured an instrument
from Dataphysics (model OCA20) under ambient atmosphere. The liquid for the surface
energy measurement is water.
68
As can be seen in Figure 5.12a and Figure 5.12b, SGO film shows a larger contact angle of
48° in comparison with that of PEDOT:PSS film (18°), indicating higher hydrophobicity for
SGO film than that of PEDOT:PSS film. Thus, the SGO layer can effectively block the
ingress of moisture into the organic-inorganic perovskite film. The superior lifetime of solar
cells based on SGO is mainly attributed to its improved hydrophobicity and probable
superior moisture barrier properties in comparison with PEDOT:PSS.
5.9 WVTR measurement
Figure 5.13 Scheme of water transmission measurement at a RH of 85% and temperature of
85 . (Redrawn after [52] with permission from Royal Society of Chemistry) (An ultraviolet
curable epoxy adhesive (Katiobond LP 655, Delo) is used to bond the hole transporting layer
coated FTO/glass on Al cap. The weight of cups is measured by Ohaus scale with readability
of up to 0.01mg and capacity of up to 220g.)
69
Figure 5.14 The weight increase of PEDOT:PSS/FTO/glass and SGO/FTO/glass as a
function of time at 85% RH/85 . (Redrawn after [52] with permission from Royal Society
of Chemistry). After five trials, a ultraviolet curable epoxy glue (Katiobond LP 655, Delo) is
demonstrated to be effective to bond the SGO or PEDOT:PSS coated FTO glass on the Al
cup The weight of cups is measured by Ohaus scale with readability of up to 0.01mg and
capacity of up to 220g.
In order to further compare the role of PEDOT:PSS and SGO in inhibiting the diffusion of
moisture into the perovskite film, PEDOT:PSS/FTO/glass and SGO/FTO/glass are sealed on
the open mouth of an Al cup filled with Calcium Chloride (Figure 5.14). A ultraviolet curable
epoxy glue (Katiobond LP 655, Delo) is employed to bond the SGO or PEDOT:PSS coated
FTO glass on the Al cup. After that, these Al cups are stored at a RH of 85% and temperature
of 85 provided by a climate chamber. Ideally, the water vapor diffusion rate for the glue
should be dramatically lower than the SGO or PEDOT:PSS in order to avoid the effect of
glue. The water vapor transmission constant D for the glue (Delo Katiobond LP 655) is
previously determined to be around 1.1× 10-12 cm2/s under a RH of 90% and temperature of
at 60 °C, which is in excellent consistent with the WVTR value provided by the supplier
(WVTR is 6.1 gr/m2/d, [www.delo.de])[114]. Recently, the diffusion coefficient of moisture
in PEDOT:PSS film is demonstrated to be a value of D = ~5.0 ×10−10 m2 s−1, which is around
two orders of magnitude higher than that of our glue[115]. Therefore, the PEDOT:PSS
hole-transporting layer mainly contribute to the water vapour diffusion. As can be seen in
70
Figure 5.14, the WVTR in PEDOT:PSS based cups is at least 10 fold higher than those of
SGO based cups.
5.10 ATR-FTIR measurement
Having illustrated that water vapor indeed can enter perovskite solar cells through the
PEDOT:PSS layer, the degradation behavior of the corresponding cells under ambient
environment and light soaking is next analyzed by attenuated total reflectance-Fourier
transform infrared spectroscopy. ATR-FTIR spectroscopy is used to further understand and
illustrate the degradation process on a molecular level. The 3200 cm-1 is corresponded to the
N-H stretch vibration in the CH3NH3- group (shown in Figure 5.15)[36]. After being
subjected to ambient environment and light soaking for 240 h, the peak intensity for the N-H
stretch in the CH3NH3- group in the PEDOT:PSS solar cells dramatically declines, indicating
a severe loss of methyl amine in PEDOT:PSS solar cells. It is reported that any excess iodide
in perovskite can react with water and generate hydroiodic acid and react with the
methylammonium cation to form methylamine[116]. Both hydroiodic acid and methylamine
are extremely volatile. The loss of hydroiodic acid and methylamine will speed up the rate of
generation of the hydrated phase to attain the equilibrium. By contrast, the peak for the N-H
stretch vibration of perovskite in SGO based solar cells decreases only slightly (Figure 5.15),
thereby indicating significantly reduced degradation for SGO based devices.
Figure 5.15 Attenuated total reflection Fourier transform infrared spectra of fresh and aged
perovskite devices based on (a) PEDOT:PSS layer and (b) SGO layer (under room
71
temperature, ambient atmosphere and 0.5 suns illumination) (Redrawn after [52] with
permission from Royal Society of Chemistry). Simon Kahmann carried out the FTIR-ATR
measurement and related analysis.
5.11 Conclusions
In summary, the influence of two kinds of hole-transporting materials on the lifetime of
unencapsulated perovskite solar cells was investigated under various conditions. Firstly, a
high efficiency of close to 15.2% has been obtained employing the SGO hole transporting
layer and ~ 60% of their original PCE for unpackaged SGO solar cells is maintained under
0.5 suns light soaking after 1000 h. Replacing PEDOT:PSS layer with SGO layer
significantly improved the environmental lifetime of unsealed perovskite solar cells with
inverted architecture under 0.5 suns light soak. It implies that on one hand SGO layer acts as
an effective hole-transporting material. On the other hand, it facilitates stabilizing perovskite
devices by slowing down the moisture ingress into perovskite layer on account of its superior
hydrophobic property in comparison with PEDOT:PSS. Besides, solar cells based on SGO
show a superior lifetime compare to that of PEDOT:PSS based devices under ambient
atmosphere in the dark. It illustrate the significance of exploiting novel hydrophobic hole
transporting materials with low water vapour transmission constant toward perovskite solar
cells with promising long lifetime.
72
Chapter 6
Aqueous PCBM Nanoparticles for Efficient and
Stable Perovskite Solar Cells
Interface engineering plays a key role in improving the power conversion efficiency and
lifetime of organic-inorganic perovskite solar cells. In this chapter, high-efficiency
organic-inorganic perovskite solar cells are fabricated via employing an ultrathin
aqueous-processed PCBM nanoparticle layer. In addition, its effect on device performance
and lifetime was investigated.
PCBM in chlorobenzene has been used as electron-transport layer for high-efficiency
perovskite solar cells. However, its solvent is toxic and PCBM CB layer can be washed off
the solvent of perovskite precursor such as DMF and DMSO. To increase its resistance to
solvents of perovskite precursor, aqueous processed PCBM NP is developed. Subsequently,
the resistance of PCBM CB and aqueous PCBM nanoparticles is studied by SEM, XPS and
contact angle measurement. With the green and environmentally safe processing, the
perovskite solar cells are fabricated and optimized as a function of the processing conditions.
Then, the lifetime of devices based on PCBM CB and PCBM nanoparticles is compared
(under 1 sun light soaking in nitrogen).
Contribution to this chapter:
Chen Xie prepared the PCBM nanoparticle dispersion. Dr. Ole Lytken carried out the XPS
characterization. Haiwei Chen fabricated the fullerene-based perovskite solar cells, carried
out the measurements including J-V characterization, lifetime test etc., analyzed the data and
wrote part of the resulting manuscript. Haiwei Chen, Chen Xie, Christoph J. Brabec designed
73
the experiments. The SEM images of fullerenes and perovskite film on different fullerenes
was obtained by Mr. Xiaofeng Tang. Ening Gu carried out the XRD measurement and
prepared related samples.
6.1 Aqueous PCBM nanoparticles
Perovskite solar cells have attracted considerable attention in the past several years as a
strong candidate for sustainable future energy source. However, its photovoltaic performance
is still inferior to that of inorganic solar cells such silicon solar cells. Several main challenges
urgrntly need to be solved: poor environmental stability and moisture sensitivity. Currently,
almost all the parts in the perovskite devices can be processed from solutions or inks, which
allows for large-scale production like ink-jet printing or roll-to-roll coating. Consequently, it
enables the low fabrication costs and the opportunity to use flexible substrates like TCO/PET.
Moreover, advantages such as color tunability, semitransparency, mechanical flexibility and
thinness enable its application in building-integrated photovoltaics (e.g. photovoltaic walls or
windows). Considerable research efforts have been dedicated to developing novel perovskite
semiconductors and charge transporting layers with high charge carrier mobility and
optimized energy alignment, thus leading to the increasing performance of the perovskite
solar cells.
Despite that the highest efficiency achieved in mesoscopic perovskite devices is already
higher than the commercial CdTe and CIGS solar cells, the charge transporting layers such as
PCBM and PDCBT are still processed by toxic aromatic or chlorinated solvents (e.g.
1,2-dichlorobenzene (DCB), chlorobenzene (CB) and chloroform (CF)), which are not
environmentally friendly. The toxicity of these solvents varies with their chemical structure.
Eco-friendly solvents are liquids with very low toxicity and should not be harmful for
people’s health and the environment.
In addition, using these highly toxic solvents increases the cost and complexity of the process
for fabricating perovskite devices, thus hampering their large-scale practical application.
Therefore, replacing commonly used toxic solvents with relatively environmentally friendly
solvents for the manufacture of perovskite devices is helpful to the practical application of the
perovskite technology.
74
Several approaches have been exploited to disperse or dissolve the conjugated polymer or
PCBM in water: (1) functionalizing the conjugated polymer or fullerenes with nonionic
alcohol and glycol side chains. (2) nanoparticle dispersions of hydrophobic fullerenes or
polymers in water or (3) functionalizing the conjugated polymer or PCBM with ionic side
chains such as sulfonic acid or carboxylic acid.
Landfester et al. developed a useful miniemulsion technique disperse conjugated materials in
water. The nanoscale morphology conjugated materials is fixed in a single nanoparticle,
which is stabilized. In addition, nanoparticles can be dispersed in some eco-friendly solvents
like water. When the nanoparticle layer is subjected to the thermal annealing, the aqueous
NPs merge and form larger donor and acceptor domains. Besides, after the deposition and
thermal annealing, the stabilizers in the active layer may be reduced, but not eliminated. It
hampers the bulk-heterojunction formation, which leads to a relatively lower PCE and shorter
lifetime than that of conventional bulk heterojunction polymer solar cells.
An alternative and successful strategy to deposit charge transporting semiconductors like
PCBM or P3HT from environmentally friendly solvents is dispersing these materials in
alcohol or water. Fabricating the uniform charge transporting layers from the eco-friendly
solvents requires efforts to control their morphology. Colsmann et al. developed a
nanoparticle dispersion approach without stabilizers, which permits control of the
nanoparticle size and for processing employing water as solvent for hydrophobic conducting
conjugated polymers. A high PCE of 4.1% was obtained, which is comparable to the
state-of-the-art OPV device fabricated from chlorinated solvents[117].
The PCBM film processed from chlorobenzene is partially soluble in some solvents such as
DMF and DMSO. To solve this problem, some efforts have been dedicated to increasing its
resistance to solvents. For example, crosslinking materials were used to crosslink the PCBM
molecules. The resulting PCBM layer is more resistant to solvents. Here, PCBM nanoparticle
dispersion in water with stabilizers is used to fabricate the PCBM layer. It can be processed
under low temperature. The processing versatility of PCBM renders its combination with
temperature-sensitive and low-cost substrate materials such as polyethylene terephthalate
film.
In this section, the use of aqueous processed PCBM nanoparticles as electron transport layer
75
for constructing efficient perovskite solar cells is present. The resulting PCBM NP devices
show long-term stability.
Figure 6.1 Scheme of perovskite solar cells using PCBM CB and PCBM NP as electron
transport layer. The perovskite has 5% PbI2 excess. PDCBT was used as hole-tranporting
layer, while the WOx was used as buffer layer.
The scheme of the perovskite solar cells based on PCBM NP or PCBM CB is shown in the
Figure 6.1. Firstly, PCBM CB or PCBM NP were deposited on the glass/ITO substrates from
PCBM CB in chlorobenzene solution or PCBM NP dispersion in water. After that,
CH3NH3PbI3 with 5% PbI2 excess was fabricated on the PCBM layer. Then, a
hole-transporting layer PDCBT was deposited on top from PDCBT chlorobenzene solution
and annealed at 100 for 5 min. Subsequently, WOx was spin-coated on the PDCBT layer
and annealed at 100 for 5 min. Finally, a 100 nm-thick gold was evaporated on top to
complete the solar cell.
Figure 6.2 Scheme of the preparation of dispersion of PCBM nanoparticles in water. The
76
miniemulsion is formed by adding fullerene in chloroform into the aqueous solution
containing stabilizer and ultrasonication with a Hielscher UPS200S ultrasonic finger in an ice
bath.
The process for the PCBM nanoparticle synthesis is shown in the Figure 6.2: Firstly, PCBM
was added into CHCl3 and stirred overnight at 50 °C. Sodium dodecyl sulfate (SDS) was
dissolved in Milli-Q-water and kept at 40 °C. Subsequently, the PCBM chloroform solution
was added into SDS water solution at a ratio of 1:6 and stirred for 1 h. Then, the mixture was
ultrasonicated with a Hielscher UPS200S ultrasonic finger in an ice bath. After sonication,
the miniemulsion system was heated and kept at 70 °C for 3h with constant stirring. The
chloroform was gradually removed from the miniemusion via evaporation. The excess
surfactant in the particle dispersion was removed using Amicon® ultra-15 centrifuge filter
(cutoff 10K). The dispersion was added into the filter and centrifuged at 5000 rpm for 20 min.
The supernatant liquid was discarded and around 15 mL water was added into the retentate.
Then, it was centrifuged again. This centrifuge process was repeated for 5 times to remove
the surfactant. Finally, moderate amount of water was added into the retentate to get the
aqueous PCBM NP dispersion.
6.2 SEM characterization
The surface coverage and morphology of perovskite layer are important to the performance
of organic-inorganic perovskite solar cells. The surface morphology of ITO/glass is shown in
the Figure 6.3a. The size of ITO crystals ranges from tens of nanometers to beyond 100
nanometer. The PCBM NP dispersion was spin-coated on the ITO/glass substrate and
annealed at 100 . The surface of the PCBM NP film was rough and some PCBM NP could
be seen on the surface (shown in Figure 6.3b). After the PCBM NP film was washed by
DMF/DMSO mixture and annealed it at 100 , the surface became smoother and PCMB
nanosheets were formed (shown in Figure 6.3c). The coverage of the PCBM layer on the
ITO/glass was significantly improved, thus decreasing the possibility of contact between the
organic-inorganic perovskites and ITO. It resulted in less recombination between the
perovskite and the ITO electrodes. The PCBM CB on the ITO/glass was also rough (shown in
Figure 6.3d). However, there was much less PCBM left on the ITO/glass after being washed
77
by the DMF/DMSO, implying the PCBM CB layer is almost washed away (shown in Figure
6.3e). It meant the PCBM NP layer exhibits higher solvent resistance than the PCBM CB
layer.
Figure 6.3 SEM images of a) ITO, b) PCBM NP on ITO, c) Washed PCBM NP on ITO, d)
PCBM CB on ITO and e) Washed PCBM CB on ITO. The accelerating voltage for obtaining
the SEM images is 2 kV and high vacuum. The SEM images of fullerenes was obtained by
Xiaofeng Tang.
6.3 XPS characterization
As can be seen in Figure 6.4, the XPS spectra exhibited a main C1s peak at 285 eV which
was corresponded to C–C/C–H bonds. The peak at 289.2 eV with lower intensity was typical
COO- or -C(=O)O bonds. Before washing PCBM NP or PCBM CB film with DMF/DMSO
mixture, the peaks at 285 eV for both of them showed almost similar intensity, implying
similar PCBM thickness for PCBM NP and PCBM CB film. However, after being washed by
the DMF/DMSO mixture, the peak for PCBM NP decreased to half of its original intensity,
which was double of the peak intensity for washed PCBM CB film. More PCBM NP was left
than that of PCBM CB after the solvent washing. It meant the PCBM NP film has superior
solvent resistant than that of PCBM CB film.
78
300 298 296 294 292 290 288 286 284 282
Inte
nsi
ty (
a.u
.)
Binding energy [eV]
PCBM NP
washed PCBM NP
PCBM CB
washed PCBM CB
Figure 6.4 XPS spectra of PCBM NP (black), washed PCBM NP (red), PCBM CB (blue) and
washed PCBM CB (pink). A main C1s peak at 285 eV was corresponded to C–C/C–H bonds.
The peak at 289.2 eV with lower intensity was typical COO- or -C(=O)O bonds. XPS
characterization of various fullerene layers was carried out by Dr. Ole Lytken.
6.4 Contact angle measurement
To further investigate the influence of the DMF/DMSO washing on the PCBM CB film and
PCBM NP film, contact angle measurement was carried out. The contact angle of pure
ITO/glass was 53.2 º(shown in Figure 6.5a). After the PCBM CB layer was deposited on the
ITO/glass, the corresponding contact angle was significantly increased to 91.6 º (shown in
Figure 6.5b). It indicated the PCBM CB layer is more hydrophobic than that of ITO surface.
However, the contact angle was decreased to 76.º after being washed by DMF/DMSO
mixture, indicating most of PCBM was partially washed away, which was consistent with the
SEM result (shown in Figure 6.5c). By contrast, the PCBM NP film exhibited a contact angle
of 89.1 º, while the contact angle was slightly increased to 91.2 º (shown in Figure 6.5d and
e). It might be because the coverage of the PCBM NP on the ITO surface was improved
despite partial PCBM NP was washed off by the DMF/DMSO mixture. It was consistent with
the top-view SEM images of the PCBM NP film and PCBM NP film washed by the
DMF/DMSO.
79
Figure 6.5 Contact angle of water on (a) ITO (53.2 º) (b) PCBM CB/ITO (91.6 º) (c) Washed
PCBM CB/ITO (76.0 º) (d) PCBM NP/ITO (89.1 º) (e) Washed PCBM NP/ITO (91.2 º).
Contact angle measurement was carried out with an instrument from Dataphysics (model
OCA20) under ambient atmosphere. The liquid for the surface energy measurement of
fullerenes is water.
6.5 SEM images and XRD of perovskites
In addition, the perovskite films fabricated on the PCBM NP layer and PCBM CB layer were
compared by the top-view SEM images. As can be seen in the Figure 6.6a and Figure 6.6b,
the crystal size and morphology of the perovskite were similar. The average crystal size of
organic-inorganic perovskites was approximately 250 nm. This was further confirmed the
XRD measurement. Similar XRD spectra with similar intensity were achieved (shown in
Figure 6.7).
Figure 6.6 Top-view SEM images of perovskite on a) PCBM NP, b) PCBM CB. Some
crystals were marked with dashed circles. The SEM images of perovskites was obtained by
80
Xiaofeng Tang.
Figure 6.7 XRD spectra of perovskite films on PCBM CB and PCBM NP layer on ITO
substrates. X-ray results were obtained by exposing powder samples to filtered Cu-Kα
radiation. The XRD data in the 2θ range of 10 to 70º is achieved by a Panalytical X'pert
powder diffractometer with a solid-state X'Celerator stripe detector. Ening Gu contributes to
the XRD measurement and related sample preparation.
The four strongest peaks in the XRD spectra were attributed to the MAPbI3 perovskite on
PCBM NP film and PCBM CB film, which was consistent with former literatures[118]. The
four peaks with the highest intensity were attributed to (110), (220), (310) and (312) crystal
planes of the MAPbI3 perovskite. The small peak at 12.6° was attributed to the PbI2
impurities in the samples. It was because that there is excess of PbI2 when the perovskite
precursor is prepared. Distribution of PbI2 on the organic-inorganic perovskite surface acted
as a passivation layer, In comparison with less PbI2 region, PbI2-rich region showed longer
lifetime because of effective suppressed defect trapping[119]. Whereas, the effect of
incorporating excess PbI2 into perovskite solar cells on their lifetime was still a debatable
issue which needs further clarification.
6.6 UV-Vis absorption
81
Figure 6.8 UV-Vis absorption spectrum in the range of 450-800 nm of perovskite films on
PCBM CB and PCBM NP layer on ITO substrates.
Then, UV-Vis absorption of perovskite films on PCBM CB and PCBM NP layer on ITO
substrates was carried out. The two absorption spectra almost overlaped with each other,
indicating thickness of the two kinds of perovskite films was almost same. The onset of the
UV-Vis spectrum was close to 800 nm, suggesting a band gap of around 1.5 eV, which was
consistent with previous reports[120, 121]. The absorption intensity at 500 nm was higher
than 2, which meant the perovskite film could absorb almost all the incident light with a
wavelength of 500 nm. However, only partial near-infrared light could be absorbed by the
organic-inorganic perovskite and converted to electricity.
6.7 Hysteresis
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Figure 6.9 Comparison of hysteresis of perovskite solar cells based on PCBM NP and PCBM
CB layer. (One sun equivalent illumination was provided by Newport Sol 1A solar simulator.
Current-voltage curves were achieved using a Keithley 2400 source meter. All J-V curves
with forward and reverse scans for the perovskite solar cells were achieved with a sweeping
speed of 100mV/s.)
Hysteresis performance of perovskite solar cells based on PCBM NP and PCBM CB films
was also investigated and compared. The PCBM CB based device exhibited worse hysteresis
than that of PCBM NP device. There was almost no difference for the Jsc extracted from the
forward-scan and reverse-scan J-V curves, which could be partially attributed to the similar
perovskite film thickness. However, PCBM NP based devices showed higher Voc (1.04V) and
higher fill factor than those of PCBM CB devices. From the above-mentioned SEM images,
XRD spectra and UV-Vis absorption spectra, the morphology for the perovskite films on
PCBM NP and PCBM CB layer had been demonstrated to be similar. Therefore, the
difference of the hysteresis performance for these two kinds of photovoltaic devices was
mainly caused by the electron transporting layers (PCBM NP and PCBM CB layer). The
coverage of PCBM NP layer washed by DMF/DMSO is much better than that of the washed
PCBM CB layer.
6.8 Box plots of performance
Photovoltaic performance of perovskite solar cells based on PCBM NP and PCBM CB layer
was measured and compared. Parameters describing the photovoltaic performance were
extracted from J-V characteristics and summarized in the box plots (shown in Figure 6.10).
The average PCE of PCBM NP device was higher than that of the PCBM CB devices, while
the Jsc of these two kinds of devices were close. It might result from the perovskite layer with
similar thickness and similar absorption capability. In addition, the PCBM NP devices
exhibited higher average Voc and FF than those of PCBM CB devices. It might be attributed
to the superior coverage of the PCBM NP layer on the FTO/glass, which effectively reduce
the internal recombination in the perovskite solar cells.
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Figure 6.10 Box plots for comparison of photovoltaic performance of perovskite solar cells
based on PCBM NP and PCBM CB layer. The plots are summarized from 8 devices from
each group. All J-V curves with forward and reverse scans for the perovskite solar cells were
achieved with a sweeping speed of 100mV/s.
6.9 J-V and EQE characteristics
Figure 6.11 J-V curves and external quantum efficiency curves of champion perovskite solar
cells based on PCBM NP and PCBM CB layer. (All J-V curves for the perovskite solar cells
were achieved with a sweeping speed of 100mV/s. EQE curves was achieved with no bias
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voltage.)
The J-V characteristics and EQE curves of champion devices based on PCBM NP and PCBM
CB were shown in Figure 6.11. Due to the higher Voc and FF, the PCBM NP devices gave a
PCE of 16.5%, which is much higher than that of PCBM CB devices (13.5%). The PCBM
NP layer exhibited better coverage than that of PCBM NP layer after being washed by the
DMF/DMSO mixture, which might reduce the recombination between the perovskite and
ITO electrodes, thus enhancing the FF and Voc. The similar Jsc for these two kinds of devices
is consistent with the integrated short-circuit current from EQE curves.
6.10 Lifetime test
Aggressive long-term stability measurements of perovskite devices based on PCBM NP and
PCBM CB layers were carried out (Figure. 6.12). The long-term stability of the
unencapsulated devices based on PCBM NP and PCBM CB was compared under constant 1
sun light illumination. The normalized PCE as a function of testing time under constant 1 sun
illumination was shown in Figure 6.12. The PCE of the PCBM NP perovskite device
maintained 86.2% of its original value within the 920 h. By contrast, the PCBM CB device
exhibited worse stability, retaining only 62.7% of its original PCE after 920 h.
Figure 6.12 Evolution of photovoltaic performance of perovskite solar cells based on PCBM
NP and PCBM CB layer as a function of time (under open-voltage, under constant 1sun light
soaking in N2).
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6.11 Conclusion
In conclusion, PCBM NP-strategy was demonstrated to be helpful to increase the
solvent-resistance of PCBM layer. In addition, PCBM NP-based perovskite devices exhibited
superior photo-stability than that of PCBM CB-based devices. A PCE of 16.5% was obtained
for the PCBM NP-based perovskite solar cells, which was much higher than that of
perovskite solar cells based on PCBM CB layer. In addition, the PCBM NP-based perovskite
solar cells exhibited superior photo-stability than the PCBM CB-based devices under
constant 1 sun illumination in N2 atmosphere.
It demonstrated the importance of exploiting novel electron transporting layers with higher
solvent-resistance, which partially contributes to the perovskite solar cells with longer
lifetime and higher power conversion efficiency. Further improvement could be expected
from electron transporting layer with superior solvent-resistance and conductivity, which
needs intensive research efforts.
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Chapter 7
Composition engineering towards efficient and
stable perovskite solar cells
Organic-inorganic perovskite solar cells with various components exhibit different
photovoltaic performance. Organic cations like ammonium tend to react with moisture and
form volatile molecules and accelerate the degradation of perovskite solar cells. Therefore,
inorganic cations are introduced into the perovskite component.
To investigate the effect of composition on the power conversion efficiency and the
photo-stability, perovskite solar cells were fabricated based on four kinds of perovskite
materials (FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2, FA0.7Cs0.3PbI2.4Br0.6 and
FA0.7Cs0.3PbI1.8Br1.2). The relationship between light utilization and bandgap adjustment was
studied. In addition, the effect of band gap on the lifetime under illumination was also
investigated. The morphology of the various perovskite film is studies by SEM. In addition,
UV-Vis absorption of these perovskites was investigated. The power conversion efficiency
for these devices was measured and compared. Besides, these photovoltaic devices were
subjected to light stress under nitrogen atmosphere to test their photo-stability.
Contribution:
Haiwei Chen, Shi Chen, Christoph J. Brabec，et al. the designed the experiments of this
chapter. Haiwei fabricated the perovskite solar cells with four kinds of organic-inorganic
perovskites and carried out the morphology characterization and lifetime under ~0.5 suns
light soaking.
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Dr. Thomas Hellmuller and Andrej Classen partially carried out the degradation and analysis
part of this chapter. The SEM images of organic-inorganic perovskite film was obtained by
Xiaofeng Tang.
7.1 Composition engineering
In perovskite materials, A cations such as Cs+, CH3NH3+ and CH(NH2)2
+ occupy the central
position of the three-dimensional perovskite lattice, which plays a vital role in determining its
structure and dimensionality[121-123]. It directly affects the physical properties and stability
of the material. In addition, it contribute to the charge compensation in the perovskite lattice.
However, it does not mainly contribute to determining the band alignment. The organic cation
in hybrid inorganic-organic hybrid perovskite usually has a nonspherical geometry shape and
rotates constantly. Therefore, its absolute size is difficult to be determined. Zero-dimensional
organic-inorganic perovskites are the most widely used perovskites in solar cells now. The
bandgap of perovskites can be tuned by substituting cations or anions, which renders them as
absorbers for semitransparent perovskite solar cells with different colour. The resulting
perovskite solar cells can also be integrated into the building as windows. Although the exact
tolerance factor for a particular semiconductor is hard to be achieved due to the geometrical
shape of the organic cation, a larger cation like FA+ usually leads to a higher tolerance
factor[124]. But qualitative analysis of the cation size is helpful to illustrate the structure
transition in these semiconductors. The most investigated alternative candidate for the
substitution of MA+ has been the FA+, which was firstly reported by Baikie and Mathews et
al[125]. FAPbI3 has a superior band gap close to the optimized band gap for single-junction
solar cells than that of MAPbI3. It significantly enhances the short circuit current density and
related power conversion efficiency. Two kinds of phases with different crystal structures
could exist in solution-processed FAPbI3 films. The photoactive phase (black phase) has a
perovskite structure (denoted as α-phase), while the nonphotoactive phase (yellow phase) has
hexagonal structure, which was denoted as the δ-phase[126, 127]. The FA-based perovskite
semiconductor exhibits a t value of 0.88, which is higher than that for MA-based perovskite
(0.83). A cation in the formamidinium lead iodide is relatively larger than that in
methylammonium lead iodide, which leads to a higher transition temperature for the
formamidinium lead iodide[127]. It is considered to not directly determine the band structure.
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However, its size plays an important role in determining the entire network by enlarging or
compressing it. By replacing the MA+ with FA+, the stability and performance for the FAPbI3
is expected to be improved simultaneously.
Moreover, hydrogen bonding to the inorganic matrix is enhanced when replacing MA+ with
FA+, resulting in the tetragonal-to-quasi cubic structural evolution and the superior thermal
stability of FAPbI3 in nitrogen and oxygen[128]. The enhancement of thermal stability maybe
due to the more stable cubic phase of FAPbI3 compared to MAPbI3.Since the FA+ has more
protons than those of MA+, thus resulting in a higher probability of forming hydrogen bonds
and reducing the extent of Sn oxidation in FASnI3 semiconductor.
In addition, compared to the MAPbI3, the light stability in ambient atmosphere for FAPbI3 is
also improved. MA+ in the MAPbI3 perovskite will release protons under light illumination.
Then, the proton combines with the I- ion and leads to the formation of HI, which results in
the poor photo-stability of MAPbI3[129]. By contrast, it is more difficult for FA+ in FAPbI3 to
release protons since the FA+ is stabilized by the resonance properties of the C-N bonds, thus
leading to a relatively better photo-stability under light illumination[130].
All-inorganic perovskite materials without volatile organic cations may exhibit long-term
stability. In the past few years, Cesium Lead halides perovskite such as CsPbI3 solar cells
have drawn intensive attention because of improved stability[79]. The ionic radius of Cs+ is
appropriate for the three-dimensional structure. The all-inorganic CsPbX3 compound with the
most ideal band gap for photovoltaic applications is CsPbI3 with a cubic phase and a band
gap of 1.73 eV[79]. Its bandgap is wider than that of MAPbI3, Therefore, a higher energy is
required to excite electrons from the valance band to the conduction band. Cs+ is the most
appropriate cation since geometrical constraints of the organic-inorganic perovskite structure
require a large A-site cation. The photoactive α-CsPbI3 with a cubic phase is usually obtained
at a temperature over 310 and stable at room temperature when kept in inert
atmosphere[80]. Although it has been used for fabrication of photovoltaic devices, α-CsPbI3
immediately transforms into the nonphotoactive yellow phase (orthorhombic phase, denoted
as δ-phase (Eg=2.82 eV), when exposed to ambient environment[91]. The relatively high
transition temperature and phase instability in ambient atmosphere hamper its practical
application as photovoltaic material. Replacing of MA+ in MAPbI3 with Cs+ could further
improve its stability. When MAPbI2Br and CsPbI2Br were subjected to heat and moisture
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stress for 300 mins (at 85 in RH of 20-25%), there is almost no change in UV-Vis
absorption and XRD pattern for the CsPbI2Br, indicating significantly improved thermal
stability. In contrast, the UV-Vis absorption for MAPbI2Br semiconductor dramatically
decreased in the duration, which suggested worse stability under the same condition.
However, this approach results in an undesired increase of the bandgap. Nanoscale phase
stabilization of CsPbI3 quantum dots is an alternative strategy to stabilize the α-CsPbI3 at
room temperature, which is much lower than the phase transition temperature for α-CsPbI3
bulk materials. The resulting perovskite solar cells based on this all-inorganic material exhibit
a high PCE of over 10% and higher thermal stability than their organic hybrid counterparts.
Pure CsPbI3 tend to form the nonphotoactive yellow δ-phase below 320°C, which is
thermodynamically preferred.
Figure 7.1 The structure of the perovskite solar cells based on FA0.85Cs0.15PbI2.4Br0.6,
FA0.85Cs0.15PbI1.8Br1.2, FA0.7Cs0.3PbI2.4Br0.6 and FA0.7Cs0.3PbI1.8Br1.2. P3HT nanoparticles and
PCBM were used as hole transporting layer and electron transporting layer for the solar cells,
respectively.
Here, the FA+ and I- are partially replaced with Cs+ and Br-, thus combining both of their
advantages. Solar cells based on four kinds of perovskite semiconductors
(FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2, FA0.7Cs0.3PbI2.4Br0.6 and FA0.7Cs0.3PbI1.8Br1.2)
are fabricated. The P3HT NP and PCBM act as hole-transport layer and electron transport
layer because of the fact that their energy levels match well with the perovskite
semiconductors.
The aim of partially substituting the A cation and X anion is to achieve a more stable cubic
phase and the ideal position of the conduction band of the organic-inorganic perovskite
semiconductors, which plays an important role in enhancing the stability.
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FAI, CsI and PbBr2 are ordered from Dyesol Ltd. PbI2 are purchased from Sigma-Aldrich. All
materials are used as purchased. FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2,
FA0.7Cs0.3PbI2.4Br0.6 and FA0.7Cs0.3PbI1.8Br1.2 precursor are prepared by dissolving appropriate
salts in DMF/DMSO mixture (VDMF:VDMSO=4:1).
ITO glass are successively washed with sonication in toluene, aceton and isopropanol. Then,
it is dried with nitrogen flow and treated with O3 plasma to remove the organic residual. After
that, P3HT nanoparticles in water is spin-coated on the ITO glass at 2000 rpm for 30 s and
annealed at 100 for 10 min. Subsequently, 60 μL of perovskite precursor is dropped on the
P3HT film and spin-coated at 4000 rpm for 35 s. At the last 3 s of this process, 300μL
chlorobenzene is dropped on the film. Then, the film is blown with nitrogen flow and
annealed at 150 for 15 min. 60 μL PCBM in chlorobenzene is deposited on the perovskite
film via spin-coating at 2000 rpm for 30 s and annealed at 100 for 2 min. ZnO
nanoparticles in isopropanol was deposited on the PCBM layer via spin-coating at 3000 rpm
and annealed at 100 for 5 min. At last, 100 nm-thick Ag layer is evaporated on top to
finish a photovoltaic device with an active area of 10.4 mm2.
7.2 UV-Vis Characterization
Light harvesting is one of the most important steps towards making full use of almost all the
incident light. Ideally, the absorption spectrum of organic-inorganic perovskite layer should
cover the whole visible light range. Therefore, photons with different wavelength in the
visible light range can contribute to the photo-current generation, which leads to higher short
circuit current density. The UV-Vis absorption of various perovskite films
(FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2, FA0.7Cs0.3PbI2.4Br0.6 and FA0.7Cs0.3PbI1.8Br1.2)
is carried out by a UV-Vis-NIR spectrometer (Lambda 950, from Perkin Elmer). As shown in
the Figure 7.2, the FA0.85Cs0.15PbI2.4Br0.6 and FA0.7Cs0.3PbI2.4Br0.6 films exhibited wider
absorption range than those of FA0.7Cs0.3PbI1.8Br1.2 and FA0.85Cs0.15PbI1.8Br1.2 film, suggesting
narrower band gap for FA0.85Cs0.15PbI2.4Br0.6 and FA0.7Cs0.3PbI2.4Br0.6 films. The bandgap of
FA0.7Cs0.3PbI1.8Br1.2 and FA0.85Cs0.15PbI1.8Br1.2 is calculated to be approximately 1.66 eV,
while the band gap for the FA0.85Cs0.15PbI2.4Br0.6 and FA0.7Cs0.3PbI2.4Br0.6 is around 1.6 eV.
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Figure 7.2 The UV-vis absorption spectra of FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2,
FA0.7Cs0.3PbI2.4Br0.6 and FA0.7Cs0.3PbI1.8Br1.2 film on the P3HT/ITO/glass substrate ranging
from the wavelength of 400 nm to 800 nm.
7.3 SEM characterization
Figure 7.3 The top-view SEM images of FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2,
FA0.7Cs0.3PbI2.4Br0.6 and FA0.7Cs0.3PbI1.8Br1.2 film on the P3HT/ITO/glass substrate. The SEM
images of perovskites was obtained by Xiaofeng Tang.
The morphology of organic-inorganic perovskite layer has a vital effect of the resulting
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photovoltaic performance. Benign morphology with fewer pinholes can result in less
recombination and higher short circuit current density. Then, top-viewed morphology of these
films is investigated by SEM. As shown in Figure 7.3a, crystallinity of the
FA0.85Cs0.15PbI2.4Br0.6 film is good and few pinholes are observed. The crystal size is ~ 350
nm. By contrast, the FA0.85Cs0.15PbI1.8Br1.2 film has more pinholes and smaller crystal size
(shown in Figure 7.3b). Although the average crystal size for both FA0.7Cs0.3PbI2.4Br0.6 and
FA0.7Cs0.3PbI1.8Br1.2 films is similar, there are some excess materials at the grain boundaries
(shown in Figure 7.3c and Figure 7.3d). It might hamper the effective charge transfer in the
perovskite materials.
7.4 Boxplots of photovoltaic performance
Figure 7.4 Boxplots of photovoltaic performance (PCE, Jsc, Voc, FF) of perovskite solar cell
based on FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2, FA0.7Cs0.3PbI2.4Br0.6 and
FA0.7Cs0.3PbI1.8Br1.2. The plots for 6 devices from each group are summarized. All J-V curves
with forward and reverse scans are achieved for the perovskite solar cells with a sweeping
speed of 100mV/s.
As shown in Figure 7.4, the FA0.85Cs0.15PbI2.4Br0.6-based devices exhibit the best
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performance, while the FA0.85Cs0.15PbI1.8Br1.2-based devices showed worse performance,
which maybe because of the worse quality of the corresponding perovskite films. There are
pinholes in the FA0.85Cs0.15PbI1.8Br1.2 films. In addition, FA0.7Cs0.3PbI2.4Br0.6-based devices
gave better average PCE and FF than those of FA0.7Cs0.3PbI1.8Br1.2-based devices. However,
the deviation for the Voc of FA0.85Cs0.15PbI2.4Br0.6 is worse than other devices.
7.5 J-V and EQE characteristics
Figure 7.5 the J-V and EQE characteristics of champion FA0.85Cs0.15PbI2.4Br0.6-based
perovskite solar cells. (The J-V curve for the perovskite solar cell was achieved with a
sweeping speed of 100mV/s. The EQE curve was achieved with no bias voltage.)
As can be seen in Figure 7.5a, our champion FA0.85Cs0.15PbI2.4Br0.6-based perovskite cell
shows a PCE of 11.8%, a Jsc = 18.4 mA/cm2, a Voc = 0.893 V and a fill factor (FF) = 72.1%. It
indicates the FA0.85Cs0.15PbI2.4Br0.6 is the optimum composition for perovskite solar cells with
superior Voc and FF. In addition, the superior photovoltaic performance partially results from
its benign perovskite crystallinity. Cross calibration by EQE measurements confirmed the Jsc
values recorded under 1 illumination with deviations less than about 5% (see Figure 7.5b).
7.6 Lifetime test
Then, the stability of perovskite solar cell based on FA0.85Cs0.15PbI2.4Br0.6,
FA0.85Cs0.15PbI1.8Br1.2, FA0.7Cs0.3PbI2.4Br0.6 and FA0.7Cs0.3PbI1.8Br1.2 under light stress is
investigated. There is almost no decrease for the FA0.85Cs0.15PbI2.4Br0.6-based perovskite
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device after ~500 h under 1 sun illumination in N2, indicating its superior stability (shown in
Figure 7.6a). In contrast, the PCE of FA0.85Cs0.15PbI1.8Br1.2-based perovskite device
decreases to ~54% of its original PCE value (shown in Figure 7.6b). Under the same
condition, only approximately 86% of initial PCE for the FA0.7Cs0.3PbI2.4Br0.6-based
perovskite device is maintained after the light stress (shown in Figure 7.6c), while the PCE
of FA0.7Cs0.3PbI1.8Br1.2-based perovskite device significantly decreases to 50% in ~380 h
(shown in Figure 7.6d). The PCE decrease of FA0.7Cs0.3PbI1.8Br1.2-based perovskite device is
mainly attributed to the decrease of its current density.
Figure 7.6 Evolution of photovoltaic performance parameters of unsealed perovskite solar
cells based on FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2, FA0.7Cs0.3PbI2.4Br0.6 and
FA0.7Cs0.3PbI1.8Br1.2 with time.
7.7 Conclusion
In this chapter, a general strategy to stabilize the perovskite device via composition
engineering is proposed. Organic-inorganic perovskites constitute various promising class of
materials, whereas suffer from the unsatisfying lifetime under light soaking. Solid-state
alloying a perovskite material with a large tolerance factor and a perovskite material with
small tolerance factor is a potential way to obtaining a perovskite material with superior
tolerance factor. As an example of this strategy, four kinds of perovskite materials are
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investigate: FA0.85Cs0.15PbI2.4Br0.6, FA0.85Cs0.15PbI1.8Br1.2, FA0.7Cs0.3PbI2.4Br0.6 and
FA0.7Cs0.3PbI1.8Br1.2. It might be because that partially substituting FA+ with the smaller-size
Cs+ cation could reduce its effective tolerance factor, thus stabilizing the α-phase. On the
other hand, partially substituting Cs+ with the larger-size FA+ cation is helpful to obtain a
stabilized α-phase of CsPbI3. The FA0.85Cs0.15PbI2.4Br0.6-based devices exhibited the best PCE
and long-term stability under light stress.
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Chapter 8
Summary and outlook
All the main achievements and results in this thesis are summarized in this chapter. Interface
and composition engineering have been demonstrated to be effective approaches for
developing efficient and stable perovskite solar cells.
8.1 Summary
The past few years have witnessed a rapid progress of organic-inorganic perovskite cells with
high PCEs of over 22%, which is approaching the record of single-crystalline silicon solar
cells. Further dramatical enhancement of the Jsc and Voc of the perovskite solar cell is a
prerequisite for achieving highly efficient perovskite solar cell in the future. However, their
large-scale commercial application is still challenged by one major drawbacks, which is its
relatively poor stability compared to commercial inorganic solar cells such as silicon
photovoltaic devices. In order to compete with these stable commercial inorganic solar cells,
tremendous efforts have been devoted to improving its long-term stability without sacrificing
photovoltaic performance. The photoelectron generation and power-conversion processes
such as light harvesting and charge separations, charge transportations and charge collections
are intensively investigated. Various strategies has been developed to improve the
photo-stability and chemical stability etc.. For example, hydrophobic materials like PTFE
was employed to encapsulate the perovskite solar cells. The lifetime of perovskite solar cells
was significantly improved because of blocking the diffusion of moisture in the air into the
cells.
The first part of this thesis focuses on the development of solution-processable single layers
of sulfated graphene oxide. Firstly, a high efficiency of close to 15.2% has been obtained
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employing the SGO hole transporting layer and ~ 60% of their original PCE for unpackaged
SGO solar cells is maintained under 0.5 suns light soaking under a white light LED without
UV light after 1000 h. The power conversion efficiency of perovskites based on SGO is
higher than those of PEDOT:PSS. Substituting PEDOT:PSS layer with SGO layer
dramatically improved the environmental lifetime unpackaged perovskite solar cells with
inverted architecture. It indicates that on one hand SGO layer acts as an effective
hole-transporting layer. On the other hand, it contributes to stabilizing perovskite devices by
slowing down the moisture diffusion into perovskite layer because of its superior
hydrophobic property compared to PEDOT:PSS. In addition, solar cells based on SGO layer
show a better lifetime compare to that of PEDOT:PSS based devices under ambient
atmosphere in the dark. It illustrate the significance of exploiting novel hydrophobic hole
transporting materials with low water vapour transmission constant toward perovskite solar
cells with promising long lifetime.
Furthermore, aqueous processed PCBM nanoparticles have been developed and used as
electron transporting materials for perovskite solar cells. The PCBM NP can be deposited on
FTO glasses at low temperature. The processing versatility of PCBM NP leads to its
impressive compatibility with low-cost and temperature-sensitive substrates such as
polyethylene terephthalate films. The PCBM NP processed from water shows superior
solvent resistance than that of PCBM CB layer, which is demonstrated by the SEM and
contact angle measurement. The PCBM CB is almost washed away, while there is more
PCBM NP left on the ITO glass. It indicates that PCBM NP has a superior solvent resistance
than that of PCBM CB. The PCBM NP-based devices give a higher Voc, FF and PCE of
16.5% than those of PCBM CB devices. The long-term stability of the unencapsulated
devices based on PCBM NP and PCBM CB is compared under constant light illumination (1
sun) in nitrogen atmosphere. The PCE of the PCBM NP perovskite device maintains 86.2%
of its original power-conversion efficiency value within approximately 920 h. By contrast,
the PCBM CB device exhibits worse photo-stability, retaining only 62.7% of its original PCE
after 920 h.
Eventually, a composition engineering have been developed and demonstrated to be helpful
to enhance the photo-stability of organic-inorganic perovskite solar cells in the last part of
this thesis. Four kinds of organic-inorganic perovskite materials (FA0.85Cs0.15PbI2.4Br0.6,
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FA0.85Cs0.15PbI1.8Br1.2, FA0.7Cs0.3PbI2.4Br0.6 and FA0.7Cs0.3PbI1.8Br1.2) are employed as
absorbers for perovskite solar cells. The FA0.85Cs0.15PbI2.4Br0.6-based devices exhibit the best
power-conversion efficiency. In addition, promising long-term photo-stability is achieved for
this kind of perovskite solar cell. There is only slight decrease of PCE after being exposed to
1 sun in nitrogen for around 500 h. These results reinforce the importance of composition
engineering and motivate the use of this strategy to improve PCE and long-term stability of
perovskite solar cells.
In summary, various strategies of harvesting incident light and handling charge transport are
exploited towards stable and high-efficiency perovskite solar cells. It provides potential paths
for further improvement. Interfacial engineering is an effective way to improving the lifetime
of perovskite solar cells, while achieving a high PCE at the same time. Doping
organic-inorganic perovskites containing organic cations with inorganic cations such as Cs+
improve the lifetime for perovskite solar cells.
8.2 Outlook
It seems that perovskite photovoltaics will approach to the very highest efficiencies while
retaining the low cost and easy processability in the next few years. Perovskite photovoltaics
may offer a promising avenue for large-scale manufacturing of low-energy-production
photovoltaic modules with earth-abundant raw materials instead of rare metals such as
Indium. The process for manufacturing silicon solar cells and copper indium gallium selenide
solar cells is energy-consuming and time-consuming. However, the perovskite solar cells can
be fabricated within 4 hours with raw materials like PbI2 and CH3NH3I at low temperature
beneath 150 , which is more energy-saving than that of current commercial photovotaic
devices.
In the past few years, the organic-inorganic perovskite-based technology has been dominated
by methylammonium lead halide perovskites. Combining various cations or anions can avoid
their disadvantages while retaining the advantages of the constituents. Perovskites with FA+
and Cs+ cations will be strong candidates for efficient perovskite solar cells with long-term
stability. Composition engineering including developing two-dimensional perovskites or
hybrid two-dimensional/three-dimensional perovskites also plays an important role in
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obtaining high-efficiency and stable perovskite solar cells. These kinds of perovskites have
superior intrinsic stability to moisture and light. Stoichiometric proportion optimization of
ternary organic-inorganic perovskite could lead to perovskite solar cells with reasonable
performance. In addition, developing novel cations is also an alternative way to significantly
improving the lifetime and power-conversion efficiency. It partially solve the problem of
environmental degradation and current-voltage hysteresis.
Developing robust interface materials such as HTLs and ETLs for perovskite photovoltaic
devices is really important for enhancing their efficiency and lifetime. Ideally, the interfaces
should be low-cost, intrinsically stable and hydrophobic, thus constantly shielding the
perovskite from the intrusion and damage of moisture. Cross-linking conducting hole
transporting or electron transporting molecules could also enhance their capability of
blocking moisture while almost maintaining its conductivity. An alternative way is to
improving the intrinsic stability of hole transporting or electron transporting molecules.
Therefore, it leads to an enhanced environmental stability. Nevertheless, few package
methods can fulfill the low moisture vapor diffusion rate requirement for perovskite solar
cells with long lifetime. Therefore, enhanced environmental stability of organic-inorganic
perovskite solar cells is thus desirable to decrease the serious requirements for the hermetic
package with low water vapor transmission rate.
Passivation the grain boundaries with passivation layer such as PbI2 could suppress defect
trapping and enhance the electron and hole lifetime. Thereby, the internal recombination in
the perovskite solar cells is suppressed. It is vital for further improving the performance of
the solar cells. Developing perovskite single crystal with benign grain boundary may be a
potential effective way to reduce the internal recombination, thus resulting in enhanced
photovoltaic performance. Adding Additives in the perovskite precursor solution is also
potentially helpful to facilitate the growth of perovskites and reduce the defects in the
perovskite layer.
Furthermore, wide bandgap perovskite materials maybe strong candidates for sub-cells of
low-cost and high-efficiency tandem cells. In principle, tandem solar cells have a higher
theoretical efficiency limit than that of single-junction devices. Perovskite solar cells and
inorganic solar cells such as silicon and copper indium gallium diselenide can be combined to
fabricate tandem solar cell with four-terminal and two-terminal architechture. Various
100
promising results have been achieved now. A high power conversion efficiency of 23.6% has
been achieved. Further improvement could be expected from reduced internal recombination
and excellent band match, which may realize their practical application. Due to the excellent
stability of inorganic photovoltaic materials of silicon and potential stability of wide band gap
perovskite materials, the combination of wide band gap perovskite and inorganic photovoltaic
materials may result in stable and high-performance (PCE over 25%) tandem cells.
Figure 8.1 Tandem solar cells based on organic-inorganic perovskite in combination with
silicon and copper indium gallium diselenide materials.
101
Publications
[1] Shi Chen, Yi Hou, Haiwei Chen, Xiaofeng Tang, Stefan Langner, Ning Li, Tobias Stubhan, Ening
Gu, Andres Osvet and Christoph J. Brabec. Adv. Energy Mater. 2017, Doi:
10.1002/aenm.201701543.
[2] Shreetu Shrestha, Thilo Michel, Andres Osvet, Ievgen Levchuk, Haiwei Chen, WolfgangHeiss,
Gisela Anton and Christoph J. Brabec. Nature Photonics, 2017, DOI:
10.1038/NPHOTON.2017.94.
[3] Ning Li, Jose Dario Perea, Thaer Kassar, Yi Hou, Nusret S. Guldal, Haiwei Chen, Shi Chen,
Stefan Langner, Christoph J. Brabec. Nature Communications, 2017, 8, 14541.
[4] Jie Min, Yuriy N. Luponosov, Chaohua Cui, Bin Kan, Haiwei Chen, Xiangjian Wan, Yongsheng
Chen, Yongfang Li, and Christoph J. Brabec. Adv. Energy Mater. 2017, 1700465.
[5] Yi Hou, Simon Scheiner, Xiaofeng Tang, Nicola Gasparini, Moses Richter, Ning Li, Peter
Schweizer, Shi Chen, Haiwei Chen, Marcus Halik, and Christoph J. Brabec. Adv. Mater.
Interfaces, 2017, 170000.
[6] Chaohong Zhang, Alexander Mumyatov, Stefan Langner, José Darío Perea, Thaer Kassar, Jie Min,
Lili Ke, Haiwei Chen, Ning Li, Pavel Troshin, and Christoph J. Brabec. Adv. Energy Mater.
2016, 1601204.
[7] Shi Chen, Yi Hou, Haiwei Chen, Moses Richter, Fei Guo, Simon Kahmann, Xiaofeng Tang,
Tobias Andres Osvet, Christoph J. Brabec. Advanced Energy Materials, 2016, 6, 1600132.
[8] Haiwei Chen*, Yi Hou, Christian E. Halbiq, Shi Chen, Hong Zhang, Ning Li, Fei Guo, Xiaofeng
Tang*, Sieqfried Eigler, Christoph J. Brabec. Journal of Materials Chemistry A, 2016, 4, 11604.
[9] Xiaofeng Tang, Marco Brandl, Benjamin May, Yi Hou, Moses Richter, Haiwei Chen, Shi Chen,
Simon Kahmann, Gebhard J. Matt, Christoph J. Brabec. Journal of Materials Chemistry A,
2016, 4, 15896.
[10] Sule Erten-Ela, Haiwei Chen, Andreas Kratzer, Andreas Hirsch and Christoph J. Brabec. New J.
Chem. , 2016,40, 2829-2834.
[11] ZhiPeng Shao, Xu Pan, Haiwei Chen, Li Tao, WenJun Wang, Yong Ding, Bin Pan, Shangfeng
Yang and Songyuan Dai，Energy Environ. Sci., 2014, 7, 2647-2651.
[12] Wenjun Wang,, Xu Pan, Weiqing Liu, Bing Zhang, Haiwei Chen, Xiaqin Fang, Jianxi Yao and
102
Songyuan Dai. Chem. Commun., 2014, 50, 2618-2620.
[13] Haiwei Chen, Xu Pan, Weiqing Liu, Molang Cai, Dongxing Kou, Zhipeng Huo, Xiaqin Fang and
Songyuan Dai, Chem. Commun., 2013, 49, 7277-7279.
[14] Haiwei Chen, Ruo Yuan, Yaqin Chai, Jinfen Wang, Wenjuan Li, Biotechnol. Lett. 2010, 32,
1401-1404
Poster:
Extending the environmental lifetime of unpackaged perovskite solar cells through interfacial design,
2nd International Conference on Perovskite Solar Cells and Optoelectronics (PSCO-2016) - Genova,
Italy, 26-28 September 2016.
103
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