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

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^-I 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 visible absorption maximum, which is the highest value reported for a p-type DSSC. Our results elucidate the main mechanism for photocurrent generation in this p-type DSSC, which is important for the understanding and development of these rarely studied counterpart of conventional n-type "Grätzel cells".