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.