NEXT%20GENERATION%20THIN-FILM%20SOLAR%20CELLS

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NEXT GENERATION THIN-FILM SOLAR CELLS

• Alison J. Breeze
• Solexant Corp.
• San Jose, CA USA

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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 characterizationcurrent 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

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Solar cell characterization external quantum
efficiency (EQE)

• 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. 9951000.
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Theoretical max h and optimal bandgap

• Maximize Jsc maximize absorption ? smaller
  bandgap Eg
• Maximize Voc larger Eg
• Theoretical maximum h31 (Shockley-Queisser)
  under AM1.5 spectra occurs for Eg1.4eV

CIGS
CdTe
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 h Jsc mA/cm2 Voc V ff area cm2
CIGS 19.90.3 35.5 0.692 81.0 0.419
CdTe 16.50.5 25.9 0.845 75.5 1.032
a-Si 9.50.3 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. 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 substrate configuration
CdTe 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 a105 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

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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/(GaIn) ratio

• increase Eg ? increase Voc (max Eg1.3eV,
  Voc0.8eV)
• change primarily in conduction band
• optimum efficiency at Eg1.14eV

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.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
• Improve Voc and Jsc
• h increase from 13 to 16

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

• 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
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CdTe solar cells

• Key advantages for CdTe solar cells
• Eg1.5eV near theoretical optimal value
• direct bandgap absorber with a105 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 Eg2.4eV)
• 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 CdCl2MeOH, heat treat 400C
• Heat treat 400C 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 (triangle
s) 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)