Transcript Folie 1

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: a-, b-, d-phase and Cu2Se
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
Material Properties III
• a-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 a+b
• b-phase (CuIn3Se5)
– built by ordered arrays of defect pairs
( VCu, InCu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.
Impurities & Defects I
problem: a-phase highly narrowed @RT
– solution: widening a-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: 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
– 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
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 efficiency
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
Growth Methods for Thin Films II
sequential process:
– annealing
of from
stacked
elemental layers
selenization
vapor:
• substrate: soda lime glass coated with Mo
• deposition of Cu and In, Ga layers
bysputtering
sputtering
films by
• deposition
Se layer
by atmosphere
evaporation
selenizationofunder
H2Se
• rapid
thermal
process
thermal
process
for conversion into CIGS
advantage: large-area deposition
avoidance
of toxic
H2(H
Se 2Se)
disadvantage:
use of toxic
gases
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
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
Mo
Cu,Ga,In,Se
CdS
ZnO
advantages: low cost production method
flexible modules and high power per weight ratio
disadvantages: lower efficiency
http://www.solarion.net/images/uebersicht_technologie.jpg
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
•
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.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:
• monolithical
integration:
– deposition
Ni/Al
of
buffer
layer grid
substrate wash
#1collector
–
–
–
–
–
during cell #2
processing
deposition
patterning
of
antireflection
coating
metal base electrode
fabrication
ofn-type
complete
modules
deposition
window
layer
patterning of
#1
patterning#3
formation of p-type CIGS absorber
substrate
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
Fabrication Technology II
module processing:
– packaging technology nearly identical to crystalline-Si solar cells
tempered glass as cover glass
ethylene vinyl acetate (EVA) as pottant
soda-lime glass as substrate
Al frame
junction box with leads
CIGS-based circuit
Hamakawa, Yoshihiro: Thin Film Solar Cells, Springer, 2004.
Contents
• Introduction
• Material Properties
• Growth Methods for Thin Films
• Development of CIGS Thin Film Solar Cells
• Fabrication Technology
• Conclusion & Prospect
Conclusion & Prospects
conclusion:
prospects:
•
•
•
•
•
high
increasing
reliability
utilization (solar parks, aerospace etc.)
high
optimization
efficiencyof(≈19%
fabrication
in small
processes
area, ≈13% in large area modules)
less
gainconsumption
in efficiency for
of materials
large areaand
solar
energy
cells
monolithical
possible shortintegration
run of indium and gallium resources
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
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.