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 = CuInSe 2 (copper indium diselenide) CIGS = CuIn x Ga 1-x Se 2 (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 cm 2 V -1 s -1 (single crystal)

CuFeS 2

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 Cu 2 Se Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.

Material Properties III  -phase (CuInSe 2 ): at% –optimal range for efficient thin film solar cells: at %  possible at growth temp.: phase separation into  +   -phase (CuIn 3 Se 5 ) –built by ordered arrays of defect pairs (  V Cu, In Cu  anti sites)  -phase (high-temperature phase) –built by disordering Cu & In sub-lattice Cu 2 Se –built from chalcopyrite structure by Cu interstitials Cu i & Cu In anti sites Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.

Impurities & Defects I problem: a-phase highly –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 morphology   passivation 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: V Cu problem: V Se compensating donor –n-type: Cu-rich material, Se deficiency dominant donor: V Se 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 –E f defect complexes < E f single defect  formation of defect complexes out of certain defects  V Cu, In Cu ,  Cu In, In Cu  and  2Cu i, In Cu   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 °C –inverted three stage process: evaporation of In, Ga, Se deposition of (In,Ga) 2 Se 3 on 300°C evaporation of Cu and Se deposition at elevated T evaporation of In, Ga, Se  smoother film morphology  highest efficiency Hamakawa, 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 H 2 Se atmosphere thermal process for conversion into CIGS advantage: large-area deposition disadvantage: use of toxic gases (H 2 Se) Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, –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 H 2 Se

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 MoCu,Ga,In,SeCdSZnO

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 glass substrate 2mm CIGS absorber 1.6 µm Mo back contact 1µm Zn0 front contact 0. 5 µm CdS buffer 50nm

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 Cd Cu - 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 (10 20 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, –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 circuit junction box with leads soda-lime glass as substrate Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 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 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, Meyer, Thorsten: Relaxationsphänomene im elektrischen Transport von Cu(In,Ga)Se2, Dimmler, Bernhard: CIS-Dünnschicht-Solarzellen Vortrag, 2006.