Hole injection from P1 dye hot-excited states in p-type dye-sensitized films: a fluorescence study

We present a study of the excited state relaxation dynamics of the photosensitizer P1 used in p-type dye-sensitized solar cells. Comparative femtosecond fluorescence upconversion measurements in solution and in films show that the dye undergoes a picosecond electronic relaxation from the bright Franck-Condon (FC) state to a low-emitting charge-transfer (CT) state in polar environment. The fluorescence is moderately quenched in solution and on the mesoporous Al₂O₃ isolator but dramatically more on NiO semiconductor. We assign this sub-picosecond process to the hole injection thus confirming that the electron transfer is from the FC state directly into the NiO valence band.