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 concns. of electrolyte (I3-/I- in propylene carbonate). Electron transfer from the valence band of NiO to the excited C343 is very fast, occurring from hundreds of femtoseconds to a few picoseconds, but also the subsequent recombination is in tens of picoseconds. Formation of an intermediate, attributed to I2-|NiO(+), was obsd. on the picosecond time scale. Simultaneously the reduced dye was converted back to the C343 ground state, indicating that recombination can be intercepted by I3- redn. Consistent with that interpretation, oxidized NiO and depletion of I3- persisted on the millisecond time scale. Complete dye-sensitized solar cells (DSSCs) with these films as photocathodes gave up to 10-11% incident photon to current conversion efficiency at the C343 visible absorption max., which is the highest value reported for a p-type DSSC. The results elucidate the main mechanism for photocurrent generation in this p-type DSSC, which is important for the understanding and development of this counterpart of the conventional n-type Gratzel cell.