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Thin Film Solar Cells
Thin Film Solar Cells
Bernd Schmidt
01.02.2008
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Thin Film Solar Cells
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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Thin Film Solar Cells
Physical Basics
pn-junction, photovoltaic effect
Fermi-niveau adjusts in crystal. Pairs may be seperated at thepn-junction.
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Thin Film Solar Cells
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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Thin Film Solar Cells
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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Thin Film Solar Cells
Forms of Solar Cells
2nd Generation
thin film cells with crystalline,
amorph or organic semiconductorsadvantage: cheap, elastic, lesswastedisadvantage: lower efficiency,lower durability
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Thin Film Solar Cells
Forms of Solar Cells
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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Thin Film Solar Cells
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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Thin Film Solar Cells
Fabrication of Solar Cells
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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Thin Film Solar Cells
Fabrication of Solar Cells
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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Thin Film Solar Cells
Fabrication of Solar Cells
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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Thin Film Solar Cells
Fabrication of Solar Cells
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
Thin Film Solar Cells
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Thin Film Solar Cells
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
Thin Film Solar Cells
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Thin Film Solar Cells
Thin Film Cells
CIS-Cells
(nearly)user-defined bandgap (various similarmaterials)
better tandem cells(less crystallinestructureimperfections)
very elastic
even textile solar cells
cheap
Thin Film Solar Cells
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Thin Film Solar Cells
Thin Film Cells
Organic Semiconductors
Spaghetti and Peas
no more doping
no sputtering
InkJet printing
Plastic Solar Cells
extremely elastic
sensitization by pigments
Thin Film Solar Cells
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Thin Film Solar Cells
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.
Thin Film Solar Cells
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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)
Thin Film Solar Cells
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Applications
Applications
cheap power supply (e.g.Eldorado)
small integrated solar cells
architecticalimplementation of solarcells with shading effects
Thin Film Solar Cells
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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