Dye-sensitized materials could serve as low-cost and robust alternatives to silicon for applications in solar-energy-conversion technologies. However, the energy-conversion efficiencies of dye-sensitized constructs must first double, a tall task that is possible if electrocatalysts are incorporated onto the dye-sensitized photoanode. My research group is interrogating fundamental mechanisms of this design strategy. Using nanosecond transient absorption spectroscopy we showed that a dye-sensitized mesoporous thin film consisting of anatase TiO2 nanocrystallites and functionalized with molecular charge acceptors can accumulate multiple oxidizing equivalents 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 excitation. Monte Carlo simulations support the observed behavior and the results are consistent with a mechanism where ~100 self-exchange electron-transfer events between dye molecules anchored to TiO2 were required prior to oxidation of model electrocatalysts. Slow charge recombination between electrons in TiO2 and the oxidized molecules anchored to the surface of TiO2 enabled this discovery. I will also share specific information regarding design criteria required to observe this behavior as well as results from our recent efforts to couple this chemistry to electrocatalysis of iodide oxidation. The general concept of decoupling light absorption and electrocatalysis enables use of dyes with higher-energy ground states, therefore extending dye absorption into the near-infrared spectral region and to projected solar-cell efficiencies beyond 20%. For this work, we employed Os–based coordination compound dyes whose Os(III/II) redox potential is more negative than the potential required to perform iodide oxidation by any single-electron-transfer mechanism. These two proof-of-concept demonstrations validate the new proposed architecture and suggest that there may in fact be clear pathways to enable dye-sensitized devices with &gt; 20% efficiency.

Direct evidence of molecular charge accumulation during solar excitation of molecular dyes, and guidelines for obtaining large quantum yields

253rd National Meeting (2017)

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