Motivation

1
Motivation
problem global warming and climate change
2
Contents

• Introduction
• Material Properties
• Growth Methods for Thin Films
• Development of CIGS Thin Film Solar Cells
• Fabrication Technology
• Conclusion Prospect

3
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

4
Contents

• Introduction
• Material Properties
• Phase diagram
• Impurities Defects
• Growth Methods for Thin Films
• Development of CIGS Thin Film Solar Cells
• Fabrication Technology
• Conclusion Prospect

5
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 gt1µm
• minority-carrier lifetime several ns
• electron diffusion length few µm
• electron mobility 1000 cm2 V -1 s-1 (single
  crystal)

6
CuFeS2
7
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 a-, b-, d-phase and Cu2Se

Hamakawa, Yoshihiro Thin Film Solar Cells,
Springer, 2004.
8
Material Properties III

• a-phase (CuInSe2)
• range _at_RT 24-24.5 at
• optimal range for efficient thin film solar
  cells 22-24 at
• possible at growth temp. 500-550C, _at_RT phase
  separation into ab
• b-phase (CuIn3Se5)
• built by ordered arrays of defect pairs
• ( áVCu, InCuñ anti sites)
• d-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.
9
Impurities Defects I

• problem a-phase highly narrowed _at_RT
• solution widening a-phase region by impurities
• partial replacement of In with Ga
• 20-30 of In replaced
• Ga/(GaIn) 0.3
• Þ band gap adjustment
• incorporation of Na
• 0.1 at Na by precursors
• Þ better film morphology
• Þ passivation of grain-boundaries
• Þ higher p-type conductivity
• Þ reduced defect concentration

Hamakawa, Yoshihiro Thin Film Solar Cells,
Springer, 2004.
10
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

11
Impurities Defects III

• electrically neutral nature of structural defects
• Efdefect complexes lt Efsingle defect
• Þ formation of defect complexes out of certain
  defects
• á2VCu, InCuñ, áCuIn, InCuñ and á2Cui, InCuñ
• no energy levels within the band gap
• grain-boundaries electronically nearly inactive

12
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

13
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 _at_ 300C
• evaporation of Cu and Se
• deposition at elevated T
• evaporation of In, Ga, Se
• Þ smoother film morphology
• Þ highest efficiency

Hamakawa, Yoshihiro Thin Film Solar Cells,
Springer, 2004.
14
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)

• 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

Hamakawa, Yoshihiro Thin Film Solar Cells,
Springer, 2004.
15
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_technolo
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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

17
Development of CIGS Solar Cells I

Zn0 front contact 0.5µm
CdS buffer 50nm
CIGS absorber 1.6 µm
Mo back contact 1µm
soda lime glass substrate 2mm
www.kolloquium-erneuerbare-energien.uni-stuttgart.
de/downloads/Kolloq_2006/Dimmler_EEKolloq-290606.p
df
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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/CIGS interface
• function
• misfit reduction between CIGS and ZnO layer
• protection of CIGS layer

Hamakawa, Yoshihiro Thin Film Solar Cells,
Springer, 2004.
19
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 ZnOAl2O3
Kontaktschichten für CIGS Dünnschichtsolarzellen
20
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 nip

Meyer, Thorsten Relaxationsphänomene 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

22
Fabrication Technology I

• cell processing
• substrate wash 1
• deposition of metal base electrode
• patterning 1
• formation of p-type CIGS absorber

• monolithical integration
• during cell processing
• fabrication of complete modules

• deposition of buffer layer
• patterning 2
• deposition of n-type window layer
• patterning3

• deposition Ni/Al collector grid
• deposition of antireflection coating

Hamakawa, Yoshihiro Thin Film Solar Cells,
Springer, 2004.
23
Fabrication Technology II

• module processing
• packaging technology nearly identical to
  crystalline-Si solar cells

Hamakawa, Yoshihiro Thin Film Solar Cells,
Springer, 2004.
24
Contents

• Introduction
• Material Properties
• Growth Methods for Thin Films
• Development of CIGS Thin Film Solar Cells
• Fabrication Technology
• Conclusion Prospect

25
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

http//img.stern.de/_content/56/28/562815/solar1_5
00.jpg www.kolloquium-erneuerbare-energien.uni-stu
ttgart.de/downloads/Kolloq_2006/Dimmler_EEKolloq-2
90606.pdf
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• 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.