1. NEXT GENERATION THIN-FILM SOLAR CELLS
Alison J. Breeze
Solexant Corp.
San Jose, CA USA

2. 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

3. 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 3

4. 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:
Power conversion efficiency:

5. Solar cell characterization: external quantum efficiency (EQE)20 40 60 80
100
200
300
400
500
700
800
900
600
Wavelength (nm)
Unscaled Quantum Efficiency (%)
short-circuit
voltage bias
F(l) = flux density/unit l
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.

6. Theoretical max h and optimal bandgap
Maximize Jsc: maximize absorption  smaller bandgap Eg
Maximize Voc: larger Eg
Theoretical maximum h=31% (Shockley-Queisser) under AM1.5 spectra occurs for Eg=1.4eV
0.50
1.00
1.50
2.00
2.50
0.00
0.10
0.20
0.30
0.40
Band Gap / eV
Efficiency
CIGS
CdTe
J. Nelson, The Physics of Solar Cells. London: Imperial College Press,
2003, ch. 2, p. 33.

7. 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 h
[%]
Jsc
[mA/cm2]
Voc
[V] ff
area
[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
1.07
* Martin A. Green, et al, “Solar Cell Efficiency Tables (Version 31),” Prog.
Photovolt: Res. Appl., vol. 16, 2008, pp

8. 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

9. CIGS and CdTe device structures
substrate configuration
CdTe
superstrate configuration

10. CIGS solar cells Key advantages for CIGS solar cells
tune bandgap eV 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 h via three-stage evaporation recipe
Deposition techniques for CdS: Chemical bath deposition (CBD) common

11. 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

12. 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
1.0
1.1
1.2
1.3
1.4
1.5
1.6
Bandgap (eV) 10 12 14 16 18 20
Efficiency (%)
solar cell
theory optimum
h decrease at higher Eg
from decreased Jsc
due to recombination
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

13. CIGS: Graded Bandgaps
Improve performance with graded bandgap across CIGS thickness
Increase Ga and Eg towards Mo interface
Reduce recombination losses
Improve Voc and Jsc
h increase from 13 to 16%*
Ga ratio
depth
CdS
CIGS Mo
*T. Dullweber et al, “Back surface band gap gradings in Cu(In,Ga)Se2
solar cells,” Thin Solid Films, vol. 387, 2001, pp

14. Thinner CIGS layers reduce material usage and cost
400
600
800
1000
1200
Wavelength (nm) 20 40 60 80
100
External QE(%)
1mm cell
Std cell
reduce material usage
and cost
EQE l decrease due to
absorption limitation
Jsc decrease 2-3mA/cm2
h = 16.9%
Kannan Ramanathan et al, “Properties of High Efficiency CIGS Thin-Film Solar
Cells,” Proc. 31st IEEE Photovoltaics Specialists Conference, 2005

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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)