Optimizing electron and proton transfer in water splitting dye-sensitized solar cells

Visible light water splitting in dye sensitized solar cells has now been reported by several groups who have used transition metal oxides or dyes coupled to catalyst clusters as photosensitizers. Iridium oxide nanoparticles, recently characterized electrochemically by Murray and coworkers, are especially good catalysts for the oxygen evolution reaction in these photoelectrochemical cells. We have found that ruthenium polypyridyl dye molecules coupled to IrO ₂ clusters via malonate linkages can sensitize porous TiO ₂ electrodes, and that the low overall quantum yield for water splitting (∼1%) can be understood in terms of competing back electron transfer pathways. This talk describes several strategies for improving the quantum yield. When a thin layer of a wide-bandgap oxide (Nb ₂O ₅ or ZrO ₂) is added between the sensitizer and TiO ₂, back electron transfer becomes slower and the quantum yield roughly doubles. The timescale of photocurrent decay (tens of seconds) suggests that the local decrease in pH that accompanies oxygen evolution in the porous TiO ₂ film slows down electron transfer from Ir(IV) to Ru(III). We will describe the results of experiments that test this hypothesis, as well as strategies for increasing the fraction of sensitizer molecules that bridge between TiO ₂ and IrO ₂.