Coumarin 343−NiO Films as Nanostructured Photocathodes in Dye-Sensitized Solar Cells: Ultrafast Electron Transfer, Effect of the I3−/I− Redox Couple and Mechanism of Photocurrent Generation

Abstract

Nanoporous, p-type NiO films were sensitized with coumarin 343 (C343), and the photoinduced electron transfer dynamics was studied in the presence of different concentrations of electrolyte (I3−/I− in propylene carbonate). Electron transfer from the valence band of NiO to the excited C343 is very fast, occurring on time scales from hundreds of femtoseconds to a few picoseconds, but also the subsequent recombination is quite rapid, on the time scale of tens of picoseconds. Nevertheless, formation of an intermediate, attributed to I2−| NiO(+), was observed on the picosecond time scale. Simultaneously the reduced dye was converted back to the C343 ground state, indicating that recombination could be intercepted by I3− reduction. Consistent with that interpretation, we observed oxidized NiO and depletion of I3− persisting on the millisecond time scale. Complete dye-sensitized solar cells (DSSCs) with these films as photocathode gave up to 10−11% incident photon to current conversion efficiency at the C343 vis...