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Rate limiting interfacial hole transfer in Sb2S3 solid state solar cells
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Rate limiting interfacial hole transfer in Sb2S3 solid state solar cells

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view article: http://dx.doi.org/10.1039/C3EE43844A ...

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.

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  • 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 ®
  • 2. Sb2S3 Photovoltaics • 1.7 eV band gap • Solution processing • ® 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. 2
  • 3. Motivation • Charge separation by electron and hole transfer • ® 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. 3
  • 4. Transient Absorption Spectroscopy • Trapped holes (sulfide radical species) in Sb2S3 show 560 nm induced abs. • ® Follow with/without CuSCN e S-· h CuSCN Sb2S3 TiO2 4
  • 5. TiO2/Sb2S3 Fitting ® TiO2 Sb2S3 5
  • 6. TiO2/Sb2S3/CuSCN Fitting and kht ® 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 6
  • 7. Diffusion-Transfer Model • ® Model Assumptions • Random walk • No transfer to TiO2 • Pseudo first order transfer to CuSCN • Diffusion-transfer independent of recombination 7
  • 8. Model Results • ® 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 8
  • 9. Diffusion or Diffusion-Transfer? ® • 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) 9
  • 10. Applicability to Photovoltaics • Investigate planar TiO2/Sb2S3/CuSCN photovoltaics • Highest EQE seen in 45nm Sb2S3 film – matches productive absorber thickness estimate • ® Internal Quantum Efficiency (IQE) follows Hole Transfer Efficiency (HTE) 10
  • 11. Conclusions ® • 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 11
  • 12. Thank You ® 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 Visit KamatLab.com for more research from our group or find us on Facebook at facebook.com/kamatlab! 12