Leveraging Iodide Oxidation Electrocatalysts to Overcome Efficiency Limitations in Dye-Sensitized Solar Cells
Joseph Cardon a, Kevin Tkaczibson b, Hsiang-Yun Chen a, Shane Ardo a b
a Department of Chemistry, University of California, Irvine, CA 92617 USA
b Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92617 USA
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Invited Speaker Session, Shane Ardo, presentation 051
DOI: https://doi.org/10.29363/nanoge.hopv.2019.051
Publication date: 11th February 2019

Thin films of inexpensive metal-oxide semiconductors containing surface-bound molecular dyes could serve as low-cost and robust alternatives to silicon for indoor photovoltaic applications. However, the 1 Sun power-conversion efficiencies of dye-sensitized solar cells are only half as large as those of silicon. To increase efficiency, my research group is incorporating electrocatalysts to drive multiple-electron-transfer oxidation of redox shuttles at the dye-sensitized photoanode. This design scheme allows for incorporation of dyes that are weaker oxidants when oxidized, therefore extending dye absorption into the near-infrared spectral region and increasing projected solar-cell efficiencies to beyond 20%.

Effective implementation of this innovative design requires three major developments: (1) near-infrared-absorbing dyes, (2) efficient electrocatalysis of two-electron-transfer iodide oxidation, and (3) generation of the active state of electrocatalysts via single-electron-transfer events with dyes. Toward (1), we have designed and synthesized four Os(II)–polypyridyl dyes that absorb light out to ~1 µm. Toward (2), we have identified two molecular motifs that drive the two-electron-transfer oxidation of iodide at low overpotential, albeit with low electrocatalytic performance. Toward (3), using nanosecond transient absorption spectroscopy we have showed that a dye-sensitized mesoporous thin film of anatase TiO2 nanocrystallites and functionalized with molecular charge acceptors can accumulate multiple oxidizing equivalents by single-electron-transfer events with oxidized dyes and through requisite self-exchange electron-transfer between surface-anchored dye molecules. This is the first report that has unequivocally shown such behavior under conditions of low-fluence (solar) excitation. Monte Carlo simulations support the observed behavior and the results are consistent with a mechanism where ~100 self-exchange electron-transfer events occur between dye molecules prior to oxidation of the molecular charge acceptors. Slow charge recombination between electrons in TiO2 and the oxidized molecules anchored to the surface of TiO2 enabled this demonstration. In follow-on work we have developed models for Monte Carlo simulations of these processes in mesoporous thin films. Simulating both pulsed-light and continuous-wave illumination conditions, we have identified several scenarios where rate-limiting mechanistic behavior changes from first-order to second-order in oxidized dye and combinations thereof. Together, these data may help explain the non-ideal and complex recombination kinetics observed for molecules bound to TiO2 nanocrystallites that cannot be wholly explained by trap-state energy distributions in TiO2.

Collectively, the results described herein validate the new proposed mechanistic processes and suggest that there may in fact be clear pathways to enable dye-sensitized devices with > 20% efficiency.

This material is based upon work supported by the U.S. National Science Foundation under CHE - 1566160.

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