Transcript IRPS 2006 STANDARDS FOR ELECTRONIC PRESENTATION …

NEXT GENERATION THIN-FILM
SOLAR CELLS
Alison J. Breeze
Solexant Corp.
San Jose, CA USA
Purpose
• Provide overview of basic solar cell
characterization measurements
• Introduce prevalent thin-film solar cell
materials and structures
• Discussion of key characteristics and
challenges for CIGS and CdTe devices
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Outline
• Basic solar cell characterization
• Theoretical efficiency limit and parameters
• Overview of leading thin-film solar cell
performances
• Intrinsic degradation: a-Si vs CIGS and CdTe
• Characteristics and challenges
– copper indium gallium diselenide (CIGS)
– cadmium telluride (CdTe)
• Summary
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Solar cell characterization:
current density – voltage (J-V) curves
• Air Mass 1.5 solar spectrum, I = 1000 W/m2
• Open-circuit voltage Voc
• Short-circuit current density Jsc
• Fill factor:
(JV)max
ff 
Jsc Voc
• Power conversion efficiency:

Jsc Voc ff
I
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EQE 
# electrons collected ( )
# photons incident ( )
• short-circuit
• voltage bias
Jsc  q  F( ) EQE( ) d

F() = flux density/unit 
Unscaled Quantum Efficiency (%)
Solar cell characterization:
external quantum efficiency (EQE)
100
80
60
40
20
0
200 300 400 500 600 700 800 900
Wavelength (nm)
EQE for record CdTe device
X. Wu, et al, “16.5%-efficient CdS/CdTe polycrystalline thin film solar cell,”
Proc. 17th EPSEC, 2001, pp. 995–1000.
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Theoretical max  and optimal bandgap
• Maximize Jsc: maximize absorption  smaller bandgap Eg
• Maximize Voc: larger Eg
Theoretical maximum 31% (Shockley-Queisser) under AM1.5
spectra occurs for Eg=1.4eV
Efficiency
0.40
CIGS
0.30
0.20
CdTe
0.10
0.00
0.50
1.00 1.50 2.00
Band Gap / eV
2.50
J. Nelson, The Physics of Solar Cells. London: Imperial College Press,
2003, ch. 2, p. 33.
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Leading thin-film solar cell
technologies
• Copper Indium Gallium Diselenide (CIGS, Cu(In1-xGax)Se2)
• Cadmium Telluride (CdTe)
• Amorphous Silicon (a-Si)
Maximum recorded efficiencies*
device
type

[%]
Jsc
[mA/cm2]
CIGS
19.9±0.3
35.5
0.692 81.0 0.419
CdTe
16.5±0.5
25.9
0.845 75.5 1.032
a-Si
9.5±0.3
17.5
0.859 63.0
Voc
[V]
ff
area
[%] [cm2]
1.07
* Martin A. Green, et al, “Solar Cell Efficiency Tables (Version 31),” Prog.
Photovolt: Res. Appl., vol. 16, 2008, pp. 61-67.
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Intrinsic Initial Degradation: a-Si vs
CIGS and CdTe
• a-Si suffers from intrinsic degradation, “Staebler- Wronski effect”
• connected to amorphous, hydrogenated nature of the material
• associated with light exposure
• stabilizes after initial drop, ~25% efficiency loss
• CIGS and CdTe: small-grained crystalline materials,
not impacted by this type of degradation
All solar cells must be encapsulated to protect against
environmental effects
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CIGS and CdTe device structures
CIGS
CdTe
substrate configuration
superstrate configuration
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CIGS solar cells
Key advantages for CIGS solar cells
• tune bandgap 1.04-1.4eV by variation of Ga fraction
(increase Ga  increase Eg)
• direct bandgap absorber with a~105 cm-1
• substrate structure allows for flexible substrates
Deposition techniques for CIGS
• selenization of precursers with H2Se
• sputtered metals reacted with Se gas
• evaporation of constituent elements
• highest  via three-stage evaporation recipe
Deposition techniques for CdS: Chemical bath deposition
(CBD) common
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Critical Aspects of CIGS
Na doping
• Critical for high performance devices
• promotes CIGS grain growth
• passivates grain boundaries to reduce recomination
• optimium ~0.1% atomic
• diffused from soda lime glass or incorporated separately (ex:
NaF)
Evaporation: from Cu-rich to Cu-poor
• begin with Cu-rich for liquid growth phase
• end with Cu-poor for favorable electronic properties
Control over constituent ratios: difficult using traditional
deposition methods such as co-evaporation
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Uniform variation of Ga/(Ga+In) ratio
increase Eg  increase Voc (max Eg~1.3eV, Voc~0.8eV)
• change primarily in conduction band
• optimum efficiency at Eg=1.14eV
Efficiency (%)
20
solar cell
theory optimum
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 decrease at higher Eg
from decreased Jsc
due to recombination
16
14
12
10
1.0
1.1
1.2 1.3 1.4
Bandgap (eV)
1.5
1.6
Miguel A. Contreras et al, “Diode Characteristics in State-of-the-Art ZnO/CdS/
Cu(In1-xGax)Se2 Solar Cells,” Prog.Photovolt: Res. Appl., vol.13, 2005, p.209-216
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CIGS: Graded Bandgaps
Improve performance with graded bandgap across CIGS
thickness
• Increase Ga and Eg towards Mo interface
• Reduce recombination losses
CdS
CIGS
Mo
  increase from 13 to 16%*
Ga ratio
• Improve Voc and Jsc
depth
*T. Dullweber et al, “Back surface band gap gradings in Cu(In,Ga)Se2
solar cells,” Thin Solid Films, vol. 387, 2001, pp. 11-13.
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Thinner CIGS layers
100
• reduce material usage
and cost
• EQE  decrease due to
absorption limitation
• Jsc decrease 2-3mA/cm2
•  = 16.9%
External QE(%)
Std cell
80
1mm cell
60
40
20
0
400
600 800 1000
Wavelength (nm)
1200
Kannan Ramanathan et al, “Properties of High Efficiency CIGS Thin-Film Solar
Cells,” Proc. 31st IEEE Photovoltaics Specialists Conference, 2005
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CdTe solar cells
Key advantages for CdTe solar cells
• Eg=1.5eV near theoretical optimal value
• direct bandgap absorber with a~105 cm-1
• easier to control than quaternary CIGS system
Wide range of deposition techniques
• Sputtering, close-spaced sublimation, physical vapor
deposition, electrodeposition, screen printing
• Later processing steps result in similar result for all
deposition approaches
• CdS: best results with chemical bath deposition
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CdS optimization and buffer layer
CdS thickness optimization
• No photocurrent generated in CdS  minimize
thickness to maximize transmission to CdTe in blue region
(CdS Eg=2.4eV)
• Too-thin CdS  shunting issue
Thin resistivity buffer layer
• Transparent Cond. Oxide / buffer / CdS / CdTe / electrode
• ex: high resistivity SnO2, In2O3, ZnO, Zn2SnO4
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CdCl2 heat treatment for CdTe
Heat treatment with CdCl2 required for high Jsc
• Methods:
• Soak CdTe film in CdCl2:MeOH, heat treat 400°C
• Heat treat 400°C in presence of CdCl2 vapor
• Effects:
• Recrystalization and grain growth in CdTe
• Establish or increase CdTe p-type doping
• Passivation of grain boundary traps
• CdS/CdTe interfacial mixing: CdTeyS1-y/CdSxTe1-x
• alleviates structural, electrical defects at interface
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Electrode contact to CdTe
Key challenge: ohmic contact to CdTe
• CdTe valence band 5.7eV
• current-limiting Scottky back barrier
Simulated dark and light
J-V curves:
ideal cell (squares)
significant series resistance
(triangles)
significant back contact (circles)
M. Gloeckler and J.R. Sites, “Quantum Efficiency of CdTe Solar Cells in
Forward Bias,” Proc. 19th EPSE, 2004, pp. 1863-1866
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CdTe contact strategies
Approaches for non-blocking contact
• high workfunction metal or degenerate semiconductor (ex: Au,
Sb2Te3, graphite paste)
• Formation of p+ doped layer on CdTe surface to promote
tunnelling thru barrier
• etching to produce p+ Te-rich surface layer
• Cu doping
• acts as p-type dopant in CdTe
• forms Cu2Te (in conjunction with etching)
• other dopants (ex: HgTe), included in graphite paste
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Conclusions
• Basic initial characterizations include J-V and EQE
measurements for determining efficiency and utilization of
solar spectral range
• designing optimal solar cells begins by selecting materials
with the right fundamental properties such as band-gap
value
• State-of-art performance for 3 leading thin-film devices:
efficiencies from 9.5% (a-Si) to 19.9% (CIGS)
• CIGS and CdTe have been pursued due to their bandgaps,
high absorption strengths and other favorable properties,
but key challenges remain including bandgap optimization
and controlling material profiles (CIGS) and back-contact
formation (CdTe)
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