Thin films of mesoscopic metal-oxide semiconductors containing surface-bound dyes could serve as low-cost and robust alternatives to silicon for photovoltaic applications. However, their energy-conversion efficiencies are only half as large as those of silicon. To increase efficiency, my research group is incorporating electrocatalysts to drive multiple-electron-transfer halide redox chemistry at the dye-sensitized photoanode. This design scheme allows for 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 new 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. 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. My research group also recently observed that the rate of self-exchange electron-transfer between surface-anchored dye molecules is highly dependent on the nature of the supporting electrolyte cations. The most rapid self-exchange processes were observed when mixed electrolytes of Li+ and n-tetrabutylammonium+ were utilized, suggesting that a synergism exists between cations. I will also report on my research group’s recent efforts to couple the three processes described above and demonstrate a new world-record iodine-based dye-sensitized solar cell.

The proof-of-concept demonstrations 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.