view article: http://dx.doi.org/10.1039/C3EE43844A Solid-state sensitized solar cells (SSCs) utilizing semiconductor absorbers overcome the issues of leakage and evaporation encountered in liquid-junction SSCs, and offer the potential for efficient, low cost photovoltaics. For widespread commercialization these solar cells require higher power conversion efficiency than is currently obtained with state-of-the-art devices. One critical component to this is the efficient extraction of photogenerated charges from the semiconductor absorber material. In this study, we decouple the two steps of hole transfer in the Sb2S3/CuSCN system: diffusion of holes in the Sb2S3 absorber layer, and transfer of these holes across Sb2S3–CuSCN interface. We find that interfacial transfer is the major limiting step in the thin (< 20 nm) Sb2S3 films used for high efficiency Sb2S3 photovoltaics. Decoupling of diffusion and interfacial transfer leads to a deeper understanding of the mechanism of hole transfer. This information has implications for the future design of semiconductor-based SSCs as it points to an important, often neglected interface, the absorber-hole conductor interface, which can play an important role in charge extraction.

1. Rate Limiting Interfacial Hole Transfer in
Sb2S3 Solid-State Solar Cells.
Jeffrey A. Christians1, David T. Leighton, and Prashant V. Kamat*1,2
Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556
Chemical and Biomolecular Engineering
2Department of Chemistry and Biochemistry
1Radiation Laboratory, Department of
2014, 7, 1148-1158.
DOI: 10.1039/C3EE43844A
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2. Sb2S3 Photovoltaics
•
1.7 eV band gap
•
Solution processing
•
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Promising efficiency
JSC
VOC
FF
η
12.4 mA cm-2
455 mV
0.59
3.3 %
J. A. Christians and P. V Kamat, ACS Nano, 2013, 7, 7967–7974.
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3. Motivation
•
Charge separation by electron and hole transfer
•
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Slow extraction of holes leads to increased recombination in
CdSe QDSCs
1.
2.
K. Tvrdy, P. A. Frantsuzov, and P. V. Kamat, Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 29–34.
V. Chakrapani, D. R. Baker, and P. V. Kamat, J. Am. Chem. Soc., 2011, 133, 9607–9615.
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4. Transient Absorption Spectroscopy
•
Trapped holes (sulfide radical
species) in Sb2S3 show 560 nm
induced abs.
•
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Follow with/without CuSCN
e
S-·
h
CuSCN
Sb2S3
TiO2
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5. TiO2/Sb2S3 Fitting
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TiO2 Sb2S3
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6. TiO2/Sb2S3/CuSCN Fitting and kht
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CuSCN
TiO2 Sb2S3
•
Fit TiO2/Sb2S3/CuSCN films to
biexponential decay (t > 6ps)
•
Compare average lifetimes
with/without CuSCN to calculate kht
•
Decrease in kht attributed to
diffusion of holes through Sb2S3 to
Sb2S3-CuSCN interface
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7. Diffusion-Transfer Model
•
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Model Assumptions
• Random walk
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No transfer to TiO2
•
Pseudo first order transfer
to CuSCN
•
Diffusion-transfer
independent of
recombination
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8. Model Results
•
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Model solution provides a calculated transient absorption
response
Calculated parameters
• D = 6.8 4.7 10-2 cm2 s-1
• ki = 2.8 0.2 103 cm s-1
• µ = 2.6 1.9 cm2 V-1 s-1
• LD = 180 60 nm
Hole Transfer Biot Number, λ
• λ << 1; interfacial transfer limited
• λ >> 1; diffusion limited
• 20 nm Sb2S3  λ = 0.10 0.01
• 130 nm Sb2S3  λ = 0.64 0.05
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9. Diffusion or Diffusion-Transfer?
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•
Ascribe all limitations to diffusion (i.e. infinitely fast interfacial
transfer)
•
Diffusion only model doesn’t capture dynamics at extreme Sb2S3
film thicknesses
•
Increase in DA with Sb2S3 thickness confirms D & ki
•
Estimation of productive absorber thickness (~50 nm)
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10. Applicability to Photovoltaics
•
Investigate planar
TiO2/Sb2S3/CuSCN
photovoltaics
•
Highest EQE seen in 45nm
Sb2S3 film – matches
productive absorber
thickness estimate
•
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Internal Quantum Efficiency (IQE)
follows Hole Transfer Efficiency (HTE)
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11. Conclusions
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Hole transfer rate decreases nearly an order of magnitude from 20
to 130 nm Sb2S3 films
•
Describe all hole dynamics with diffusion coefficient (D) and
interfacial hole transfer coefficient (ki)
•
Observe contribution of D and ki with hole transfer Biot number, λ
•
Hole transfer from Sb2S3 to CuSCN is limited by transfer across
the Sb2S3-CuSCN interface, not hole mobility
•
Measure hole mobility (μ = 2.6 +/- 1.9 cm2 V-1 s-1) in conditions
resembling Sb2S3 photovoltaics
•
Estimate productive absorber thickness at 50 nm by combining
interfacial transfer, diffusion, and recombination into the apparent
diffusion length
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12. Thank You
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Rate Limiting Interfacial Hole Transfer in Sb2S3
Solid-State Solar Cells.
Jeffrey A. Christians1, David T. Leighton, and Prashant V. Kamat*1,2
Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556
Chemical and Biomolecular Engineering
2Department of Chemistry and Biochemistry
1Radiation Laboratory, Department of
2014, 7, 1148-1158. DOI: 10.1039/C3EE43844A
This research was supported
by the U.S. Department of
Energy
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