May 27, 2004

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European Materials Research Society Spring 2004
Chalcogenide Solar Cells Choosing the Window
Jim Sites
Colorado State University
Collaborators
Markus Gloeckler, Alex Pudov, and Ana Kanevce
(CSU) Falah Hasoon and Miguel Contreras (NREL)
Hans Schock (IPE) and Tokio Nakada
(AGU)
Funding US National Renewable Energy Laboratory
(NREL) Japanese New Energy Development
Organization (NEDO) Special thanks to Markus
Gloeckler for assistance with figures
May 27, 2004
Photovoltaics Laboratory
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Approach
(1) Device-physics approach to the selection of
window layers for fabricating high-performance
solar cells with CdTe and CIGS absorbers.
Device physics no means the whole story, but may
give useful direction even when material
structure or other factors play a major role (2)
Large range of possible band gaps will be
considered. (3) Attempt to be quantitative. (4)
Focus on two areas (a)
Window absorption how much of an effect?
(b) Conduction-band offset what
happens when it changes?
May 27, 2004
Photovoltaics Laboratory
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Choosing the Window Outline

• Photon considerations Window absorption.
• Conduction-band offset problem I Big spikes (and
  their red-kink precursor) that limit current.
• Conduction-band offset problem II The cliff
  problem that limits voltage.
• How much slack does one get in choosing the
  window?
• Conclusions.

May 27, 2004
Photovoltaics Laboratory
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Short-Wavelength Current CdS Windows on CdTe
Short-Wavelength Collection
Same current loss should apply for CI(G)S cells.
Granata, Sites, Contreras-Puente and Compaan,
IEEE PVSC-25, 853 (1996)
May 27, 2004
Photovoltaics Laboratory
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Current Loss with Alternative Windows
Calculated Values
Absorption spectra based on that of CdS, but
shifted in energy.
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Photovoltaics Laboratory
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Fractional Current Loss
Larger fraction with smaller current from
larger-gap absorber.
For 100-nm window layer
May 27, 2004
Photovoltaics Laboratory
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Efficiency Contours
Parameters for record CIGS cells ?EC effects
neglected
100 nm window VOC Eg 550 meV Fill-factor
80
Record CIGS Cell
May 27, 2004
Photovoltaics Laboratory
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Choosing the Window Outline

• Photon considerations Window absorption.
• Conduction-band offset problem I Big spikes (and
  their red-kink precursor) that limit current.
• Conduction-band offset problem II The cliff
  problem that limits voltage.
• How much slack does one get in choosing the
  window?
• Conclusions.

May 27, 2004
Photovoltaics Laboratory
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Sign Convention for ?EC
Smaller Gap Absorber
Larger Gap Absorber
Spike can impede photoelectrons (may be bad)

Cliff slows forward electrons in
interfacial-recombination region (also may be bad)
Some consensus on ?EC magnitudes between theory,
experiment, and numerical simulations of J-V
curves
May 27, 2004
Photovoltaics Laboratory
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Earlier Red-Kink (Solarex Cells)
Dark and Red-light J-V Curves
Also seen In cells from NREL, Boeing, and
Siemens/Shell
May 27, 2004
Photovoltaics Laboratory
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Producing a Red Spectrum
Use a high-pass filter
Series of high-pass filters with
different-wavelength cut-offs
600-nm high-pass filter
Red kink with CdS occurs when no photons are
above 2.4 eV
May 27, 2004
Photovoltaics Laboratory
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The Red Kink in CdS/CIS
Conduction Band at V 0 (light/dark difference
exaggerated)
NREL CdS/CIS J-V
Compensated CdS
CdS barrier impedes electron transport blue
photons may generate sufficient electron-hole
pairs in CdS to alter trap occupation and
mitigate the effect. Can be a serious problem if
no blue photons present. Usually not a problem
with white light, but small kink sometimes seen.
May 27, 2004
Photovoltaics Laboratory
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Kink Depends on CdS Thickness (Simulation)
Weaker kink with thinner CdS. (Also seen
experimentally)
Conduction Band. Impact of barrier increases
with CdS thickness.
More generally strength of kink varies with the
carrier densities of CdS and TCO, and with the
CdS defect density.
May 27, 2004
Photovoltaics Laboratory
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Kink Disappears at Higher Eg (NREL Cells)
Conduction-band offset decreases changes from
spike to cliff


May 27, 2004
Photovoltaics Laboratory
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Choosing the Window Outline

• Photon considerations Window absorption.
• Conduction-band offset problem I Big spikes (and
  their red-kink precursor) that limit current.
• Conduction-band offset problem II The cliff
  problem that limits voltage.
• How much slack does one get in choosing the
  window?
• Conclusions.

May 27, 2004
Photovoltaics Laboratory
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Effect of Interfacial Recombination on VOC
Vary ?EC by expanding Eg (simulated) See Poster
P3.9 (Gloeckler) Lack of spike allows
significant interfacial recombination Effect of
?EC at constant Eg discussed by several groups
CdS Window
May 27, 2004
Photovoltaics Laboratory
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CdS or Alternative Windows?
Vary the window and hence the offset
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Photovoltaics Laboratory
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But, kink can return!
CIGS Absorber (Eg 1.15 eV)
CdS Window (IPE) InS(O,OH) Window
(IPE)
Note ZnS(O,OH) from AGU yields similar curves
Good Superposition
Red Cut-off 2.4 eV
Red Cut-off 2.8 eV
See Poster P3.8 (Pudov)
May 27, 2004
Photovoltaics Laboratory
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Choosing the Window Outline

• Photon considerations Window absorption.
• Conduction-band offset problem I Big spikes (and
  their red-kink precursor) that limit current.
• Conduction-band offset problem II The cliff
  problem that limits voltage.
• How much slack does one get in choosing the
  window?
• Conclusions.

May 27, 2004
Photovoltaics Laboratory
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Efficiency Picture
Vary the offset independently of Eg
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Photovoltaics Laboratory
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Choosing the Window Material
Match absorber and window materials so ?EC is in
optimal range
Big Spike
Small Spike or Cliff
Offset Values from Zhang,Wei, and Zunger, JAP 83,
3192 (1998)
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Photovoltaics Laboratory
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Conclusions

• From a device-physics perspective, the optimal
  choice of window material for chalcogenide solar
  cells varies with the band gap of the absorber.
• A general problem for CdS windows is low blue
  response.
• A problem for CdS on low-gap absorbers (CIS) is a
  big spike that impedes current. Mitigated by
  thin, high-carrier-density, or photoconductive
  CdS.
• A problem for CdS on high-gap absorbers (CdTe or
  CGS) is the lack of a barrier to inhibit
  interfacial recombination.
• At room temperature, a single window material is
  optimal over an approximate 300-meV range of
  absorber band gap.

May 27, 2004
Photovoltaics Laboratory