Bernd Schmid-Solar Cells

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Thin Film Solar Cells


Bernd Schmidt

01.02.2008


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Physical Basics

Semiconductors

Metal: VB / CB overlap SC: energy gap eV

Isolator: high energy gap (> 4.5eV) chargetransport

withelectrons

and withelectron-

hole-pairs


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Physical Basics

pn-junction, photovoltaic effect

Fermi-niveau adjusts in crystal. Pairs may be seperated at thepn-junction.


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Physical Basics

Shockley-Queisser Limit

Shockley-Queisser Limit:

The efficiency of a photovoltaic cell islimited by energy losses (particularly 1 and

2). Thereby, the classical cells have anefficiency maximum of 33%.

1. low energy photons2. relaxation from higher energies

3. energy difference between layers

4. passage into contacting electrodes

5. recombination


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Forms of Solar Cells

1st Generation

mono or poly crystallinesemiconductor-plates, doped and

contactedadvantage: simple fabrication,allready existing for electronicsdisadvantage: nonelastic,expensive, waste of material


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2nd Generation

thin film cells with crystalline,

amorph or organic semiconductorsadvantage: cheap, elastic, lesswastedisadvantage: lower efficiency,lower durability


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3rd Generation

tandem cells with multiple thinfilmsadvantage: higher efficienciesdisadvantage: difficultimplementation (more energylosses)

Shockley-Queisser-Limit:max. efficiency 33%energy losses with higher or lowerenergies


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Fabrication of Solar Cells

mono crystalline wavers

High-purity silicon is melted and and grownas a single crystal. Then the crystal issawed to wavers and polished, doped andcontacted.advantage: wavers are allready produced

disadvantage: 50% of the material are lost


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poly crystalline silicon

Silicon is melted and cast on a polycrystalline surface. It crystallizes alsopoly crystalline.

advantage: fabrication even cheaper than wavers, user-definedform of crystalsdisadvantage: still much waste, lower efficiency

h F l S l C ll


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Edge-defined Film-fed Growth (EFG)- Silicon

Octagonal thin crystalsare grown. The crystalscan be several m thick.

advantage: nearly no waste, directly thin filmsdisadvantage: lower efficiency (more imperfections)

Thi Fil S l C ll


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Generation of thin Films

1. sputtering of a transparentconductive oxyd layer (TCO) onglass

2. structuring of the TCO layer with alaser

3. deposition of thin doped siliconlayers

4. silicon structuring with laser

5. sputtering of back contact (Al)

6. back contact structuring with laseradvantage: very precise fabrication, nearly no wastedisadvantage: (in case of silicon) special atmosphere / vacuum

needed

Thi Fil S l Cells


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Doping

Diffusion:

donators / acceptors gaseous inspecial atmosphere

diffusion into crystal at about

800 1200

C concentration defined by

endurance and temperature

Ion Implantation: ion beam shot on crystal

concentration defined by energyand angle of beam



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Thin Film Cells

CdTe-Cells

large-scaled semiconductor-films possible

deposition and doping also in oxygenic atmosphere

no surface sealing

band gap 1.5 2.4eV



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Thin Film Cells

CIS-Cells

(nearly)user-defined bandgap (various similarmaterials)

better tandem cells(less crystallinestructureimperfections)

very elastic

even textile solar cells

cheap



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Thin Film Cells

Organic Semiconductors

Spaghetti and Peas

no more doping

no sputtering

InkJet printing

Plastic Solar Cells

extremely elastic

sensitization by pigments



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Thin Film Cells

Spaghetti and Peas

The absorber (spaghetti) take the energy of the photons passing it tothe acceptors (peas). The acceptors transport the electrons to the back

contact (Al).Caused by this process splitting there are much less recombination andmore re-adsorbtion processes.



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Thin Film Cells

Sensitization

1. photon absorption

2. electron transfer to SC

3. electron transport in SC

4. ohmic back contact

5. small voltage at front contact

6. small resistance in electrolyte

7. fast regeneration

fast injection of electrons intoTiO2 (fs ps)

slow back transfer (nsms)

slow recombination in thepigment (60ns)



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Applications

Applications

cheap power supply (e.g.Eldorado)

small integrated solar cells

architecticalimplementation of solarcells with shading effects



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Sources

Sources

Prof. Dr. Derck Schlettwein, Dunnschicht-Solarzellen - Alternativen zur klassischen Si-Technologie

Photovoltaik Neue Horizonte, ForschungsVerbund Sonnenenergie, Berlin 2004

H. Stroppe Physik fur Studenten der Natur- und Ingenieurwissenschaften, Fachbuchverlag Leipzig, Munchen 2005

D.C.Giancoli Physik 3. Auflage, Pearson, Munchen 2006

M. Lux-Steiner und G. Willeke, Physikalische Blatter 57, 47 (2001)

http://www.iea-pvps.org/ar04/che.htm

W.Jaegermann, D.Wohrle, M.Kunst, VW-Foundation

TCO fur Dunnschicht- Solarzellen, ForschungsVerbund Sonnenenergie, Berlin 2001

C.Brabec, A.Cravino, D.Meissner, N.S.Sariciftci, T.Fromherz, M.Rispens, L.Sanchez, J.C.Hummelen, Adv. Funct.Mat.11, 374 (2001)

http://www.fz-juelich.de/ste/datapool/Energieplattform/Vortrag%20Energieplattform%202.pdf

http://www.uni-saarland.de/fak7/fze/AKE Archiv/AKE2004F/AKE2004F Vortraege/AKE 2004F 05Glunz Photovoltaik uf.pdf

http://epub.ub.uni-muenchen.de/1368/1/senior stud 2007 01 02.pdf

http://www.halbleiter.org/waferherstellung/index?thema=dotieren

http://www.imtek.de/anwendungen/content/upload/vorlesung/2005/mst t&p 04 duennschichttechnik (teil 3 vom 05.12.2005).pdf

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Bernd Schmid-Solar Cells