Ultrafast charge photogeneration in conjugated polymer thin films

We report femtosecond transient absorption spectroscopy on thin films of blue-emitting derivatives of polyindenofluorene (PIF). Probe wavelength and pump intensity dependence measurements allow the separation of the broadly overlapped 1Bu exciton and charge pair absorption spectra. We find that charge pairs are produced within the instrument resolution (∼100 fs) followed by exciton–exciton bimolecular annihilation on a picosecond timescale. Two possible mechanisms for ultrafast charge generation are considered: direct charge separation of nascent 1Bu excitons by static quenching at intrinsic defects and sequential transitions to produce highly energetic excitons that dissociate efficiently. Photophysical modelling of intensity-dependent data reveals that sequential absorption followed by charge separation is the most likely mechanism for ultrafast charge pair generation, with the ratio of excited-state to ground-state absorption cross section times the effective yield of charge pairs≈0.1. These observations are consistent with previous photocurrent measurements and quantum chemical calculations assigning highly excited states to greatly delocalised excitons [Köhler et al., Nature 392 (1998) 903].