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Motivation
problem: global warming and climate change
Contents
• Introduction
• Material Properties
• Growth Methods for Thin Films
• Development of CIGS Thin Film Solar Cells
• Fabrication Technology
• Conclusion & Prospect
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
Contents
• Introduction
• Material Properties• Phase diagram• Impurities & Defects
• Growth Methods for Thin Films
• Development of CIGS Thin Film Solar Cells
• Fabrication Technology
• Conclusion & Prospect
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: >1µm
• minority-carrier lifetime: several ns• electron diffusion length: few µm• electron mobility: 1000 cm2 V -1 s-1 (single crystal)
CuFeS2
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: -, -, -phase and Cu2Se
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
Material Properties III
• -phase (CuInSe2):– range @RT: 24-24.5 at% – optimal range for efficient thin film solar cells: 22-24 at % possible at growth temp.: 500-550°C, @RT: phase separation
into +
• -phase (CuIn3Se5)– built by ordered arrays of defect pairs
( VCu, InCuanti sites)
• -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.
Impurities & Defects I
problem: a-phase highly narrowed @RT
– solution: widening -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 precursors
better film morphologypassivation of grain-boundaries higher p-type conductivity reduced defect concentration
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
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
Impurities & Defects III
• electrically neutral nature of structural defects– Ef
defect 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
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
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 300-550°C
– inverted three stage process:•evaporation of In, Ga, Se•deposition of (In,Ga)2Se3
on substrate @ 300°C•evaporation of Cu and Se deposition at elevated T•evaporation of In, Ga, Se
smoother film morphology
highest efficiencyHamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
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
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
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
Development of CIGS Solar Cells I
•
soda lime glasssubstrate 2mm
CIGS absorber 1.6 µm
Mo back contact 1µm
Zn0 front contact 0.5µm
CdS buffer 50nm
www.kolloquium-erneuerbare-energien.uni-stuttgart.de/downloads/Kolloq_2006/Dimmler_EEKolloq-290606.pdf
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/CIGS interface
function: • misfit reduction between CIGS and ZnO layer• protection of CIGS layer
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
Development of CIGS Solar Cells III
Window layer: ZnO• band gap: 3.3 eV• bilayer high- / low-resistivity ZnO deposited by RF-sputtering /
atomic layer deposition (ALD)• resistivity depending on deposition rate (RF-sputtering)/flow
rate (ALD)
• high-resistivity layer:- layer thickness 0.5µm- intrinsic conductivity
• low-resistivity layer:- highly doped with Al (1020 cm-3)- n-type conductivity
function:• transparent front contact R.Menner, M.Powalla: Transparente ZnO:Al2O3 Kontaktschichten für CIGS Dünnschichtsolarzellen
Development of CIGS Solar Cells IV
band gap structure:
• i-ZnO inside space-charge region• discontinuities in conduction band structure
– i-ZnO/CdS: 0.4eV– CdS/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: Relaxationsphänomene im elektrischen Transport von Cu(In,Ga)Se2, 1999.
Contents
• Introduction
• Material Properties
• Growth Methods for Thin Films
• Development of CIGS Thin Film Solar Cells
• Fabrication Technology• Cell processing• Module processing
• Conclusion & Prospect
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
Fabrication Technology II
module processing:– packaging technology nearly identical to crystalline-Si
solar cells
tempered glass as cover glass
Al frame
CIGS-based circuitjunction box with leads
soda-lime glass as substrate
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
ethylene vinyl acetate (EVA) as pottant
Contents
• Introduction
• Material Properties
• Growth Methods for Thin Films
• Development of CIGS Thin Film Solar Cells
• Fabrication Technology
• Conclusion & Prospect
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.jpgwww.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
Thank you for your attention!
References:Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.Meyer, Thorsten: Relaxationsphänomene im elektrischen Transport
von Cu(In,Ga)Se2, 1999.Dimmler, Bernhard: CIS-Dünnschicht-Solarzellen Vortrag, 2006.