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CIS/CIGS Solar Cells
Mawi Seminar WS 07/08 Prof. Dr. H. Fll
Mark-Daniel Gerngro, Julia Reverey
02/04/2008 12:00 - 12:45
A. 241
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Motivation
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http://www.photon-magazine.com/news/news_2004-03%20ap%20sn%20Honda_big.jpg
problem: global warming and climate changeproblem: short running oil resources and raising power demandsolution: solar cells, especially CIS/ CIGS solar cells
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Contents
Introduction
Material Properties Growth Methods for Thin Films
Development of CIGS Thin Film Solar Cells
Fabrication Technology
Conclusion & Prospect
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Introduction
CIS = CuInSe2 (copper indium diselenide)
CIGS = CuInxGa1-xSe2 (copper indium gallium diselenide)
compound semiconductor ( I-III-VI)
heterojunction solar cells high efficiency (19% in small area, 13% in large area modules)
very good stability in outdoor tests
applications:
solar power plants
power supply in aerospace
decentralized power supply
power supply for portable purposes
http://www.baulinks.de/webplugin/2007/i/0732-wuerthsolar1.jpghttp://www.copper.org/innovations/2007/05/images/civilian_flex_panel.jpghttp://www.esa.int/images/ISS_2004_web400.jpghttp://www.rgp.ufl.edu/publications/explore/v12n2/images/thin-film.jpg
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Contents
Introduction
Material Properties
Phase diagram
Impurities & Defects
Growth Methods for Thin Films
Development of CIGS Thin Film Solar Cells
Fabrication Technology
Conclusion & Prospect
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Material Properties I
crystal structure:
tetragonal chalcopyrite structure
derived from cubic zinc blende structure
tetrahedrally coordinated
direct gap semiconductor
band gap: 1.04eV 1.68eV
exceedingly high adsorptivity
adsorption length: >1m minority-carrier lifetime: several ns
electron diffusion length: few m
electron mobility: 1000 cm2V-1 s-1 (single crystal)
Shiyou Chen and X. G. Gong: Physical Review B 75, 205209 2007Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
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Material Properties II
simplified version of the ternary phase diagram
reduced to pseudo-binary phase diagram along the red dashed line
bold black line: photovoltaic-quality material
4 relevant phases: E-, F-, H-phase and Cu2Se
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
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Material Properties III
E-phase(CuInSe2):
range @RT: 24-24.5 at%
optimal range for efficient thin film solar cells: 22-24 at %
possible at growth temp.: 500-550C, @RT: phase separation into E+F
F-phase(CuIn3Se5)
built by ordered arrays of defect pairs
( VCu, InCuanti sites)
H-phase(high-temperature phase)
built by disordering Cu&In sub-lattice
Cu2Se
built from chalcopyrite structure by
Cu interstitials Cui & CuIn anti sites
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
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Impurities & Defects I
problem: a-phase highly narrowed @RT
solution: widening E-phase region by impurities
partial replacement of In with Ga
20-30% of In replaced Ga/(Ga+In) }0.3
band gap adjustment
incorporation of Na
0.1 at % Na by precursorsbetter film morphology
passivation of grain-boundaries
higher p-type conductivity
reduced defect concentration
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
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Impurities & Defects II
doping of CIGS with native defects:
p-type:
Cu-poor material, annealed under high Se vapor pressure
dominant acceptor: VCu
problem: VSe compensating donor
n-type:
Cu-rich material, Se deficiency
dominant donor: VSe
electrical tolerance to large-off stoichiometries
nonstoichiometry accommodated in secondary phase
off-stoichiometry related defects electronically inactive
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Impurities & Defects III
electrically neutral nature of structural defects
Efdefect complexes < Efsingle defect
formation of defect complexes out of certain defects
VCu, InCu, CuIn, InCu and 2Cui, InCu no energy levels within the band gap
grain-boundaries electronically nearly inactive
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Contents
Introduction
Material Properties
Growth Methods for Thin Films
Coevaporation process
Sequential process
Roll to roll deposition
Development of CIGS Thin Film Solar Cells
Fabrication Technology
Conclusion & Prospect
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Growth Methods for Thin Films I
coevaporation process: evaporation of Cu, In, Ga and Se from elemental sources
precise control of evaporation rate by EIES & AAS or mass spectrometer
required substrate temperature between 300-550C
inverted three stage process:
evaporation of In, Ga, Se
deposition of (In,Ga)2Se3
on substrate @ 300C
evaporation of Cu and Sedeposition at elevated T
evaporation of In, Ga, Se
smoother film morphology
highest efficiency
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
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Growth Methods for Thin Films II
sequential process: selenization from vapor:
substrate: soda lime glass coated with Mo
deposition of Cu and In, Ga films by sputtering
selenization under H2Se atmosphere
thermal process for conversion into CIGS
advantage: large-area deposition
disadvantage: use of toxic gases (H2Se)
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
annealing of stacked elemental layers
substrate: soda lime glass coated with Mo
deposition of Cu and In, Ga layers by sputtering
deposition of Se layer by evaporation
rapid thermal process
advantage: large-area deposition
avoidance of toxic H2Se
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Growth Methods for Thin Films III
roll to roll deposition: substrate: polyimide/ stainless steel foil coated with Mo
ion beam supported low temperature deposition of Cu, In, Ga & Se
advantages: low cost production method
flexible modules and high power per weight ratio
disadvantages: lower efficiency
http://www.solarion.net/images/uebersicht_technologie.jpg
Mo Cu,Ga,In,Se CdS ZnO
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Contents
Introduction
Material Properties
Growth Methods for Thin Films
Development of CIGS Thin Film Solar Cells
Cross section of a CIGS thin film
Buffer layer
Window layer
Band-gap structure
Fabrication Technology
Conclusion & Prospect
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Development of CIGS Solar Cells I
soda lime glasssubstrate 2mm
CIGS absorber 1.6 m
Mo back contact 1m
Zn0 front contact 0.5m
CdS buffer 50nm
www.kolloquium-erneuerbare-energien.uni-stuttgart.de/downloads/Kolloq_2006/Dimmler_EEKolloq-290606.pdf
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Development of CIGS Solar Cells II
Buffer layer: CdS deposited by chemical bath deposition (CBD)
layer thickness: 50 nm
properties:
band gap: 2.5 eV
high specific resistance
n-type conductivity
diffusion of Cd 2+ into the CIGS-absorber (20nm)
formation of CdCu- donors, decrease of recombination at CdS/CIGSinterface
function: misfit reduction between CIGS and ZnO layer
protection of CIGS layer
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
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Development of CIGS Solar Cells III
Window layer: ZnO band gap: 3.3 eV
bilayer high- / low-resistivity ZnO deposited by RF-sputtering / atomiclayer deposition (ALD)
resistivity depending on deposition rate (RF-sputtering)/flow rate (ALD)
high-resistivity layer:
- layer thickness 0.5m
- intrinsic conductivity
low-resistivity layer:- highly doped with Al (1020 cm-3)
- n-type conductivity
function:
transparent front contactR.Menner, M.Powalla: Transparente ZnO:Al2O3 Kontaktschichten fr CIGS Dnnschichtsolarzellen
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Development of CIGS Solar Cells IV
band gap structure:
i-ZnO inside space-charge region
discontinuities in conduction band structure
i-ZnO/CdS: 0.4eVCdS/CIGS: - 0.4eV 0.3eV
depends on concentration of Ga
positive space-charge at CdS/CIGS
huge band discontinuities of
valance-band edge
electrons overcome heterojunction
exclusively
heterojunction: n+ip
Meyer, Thorsten: Relaxationsphnomene im elektrischen Transport von Cu(In,Ga)Se2, 1999.
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Contents
Introduction
Material Properties
Growth Methods for Thin Films
Development of CIGS Thin Film Solar Cells
Fabrication Technology
Cell processing Module processing
Conclusion & Prospect
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Fabrication Technology I
cell processing:
substrate wash #1
deposition of metal base electrode
patterning #1
formation of p-type CIGS absorber
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
deposition of buffer layer
patterning #2
deposition of n-type window layer
patterning#3
substrate
deposition Ni/Al collector grid
deposition of antireflection coating
monolithical integration:
during cell processing
fabrication of complete modules
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Fabrication Technology II
module processing:
packaging technology nearly identical to crystalline-Si solar cells
tempered glass as cover glass
Al frame
CIGS-based circuit
junction box with leads
soda-lime glass as substrate
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
ethylene vinyl acetate (EVA) as pottant
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Contents
Introduction
Material Properties
Growth Methods for Thin Films
Development of CIGS Thin Film Solar Cells
Fabrication Technology
Conclusion & Prospect
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Conclusion & Prospects
conclusion:
high reliability
high efficiency (19% in small area, 13% in large area modules)
less consumption of materials and energy monolithical integration
high level of automation
http://img.stern.de/_content/56/28/562815/solar1_500.jpg
www.kolloquium-erneuerbare-energien.uni-stuttgart.de/downloads/Kolloq_2006/Dimmler_EEKolloq-290606.pdf
prospects:
increasing utilization (solar parks, aerospace etc.)
optimization of fabrication processes
gain in efficiency for large area solar cells possible short run of indium and gallium resources
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Thank you for your attention!
sources:
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
Meyer, Thorsten: Relaxationsphnomene im elektrischen Transport von
Cu(In,Ga)Se2, 1999.
Dimmler, Bernhard: CIS-Dnnschicht-Solarzellen Vortrag, 2006.