Ultrafast charge photogeneration in conjugated polymer thin films

We report femtosecond transient absorption spectroscopy on thin films of bl ue-emitting derivatives of polyindenofluorene (PIF). Probe wavelength and p ump intensity dependence measurements allow the separation of the broadly o verlapped LB, exciton and charge pair absorption spectra. We find that char ge pairs are produced within the instrument resolution (similar to 100 fs) followed by exciton-exciton bimolecular annihilation on a picosecond timesc ale. Two possible mechanisms for ultrafast charge generation are considered : direct charge separation of nascent 1B(u) excitons by static quenching at intrinsic defects and sequential transitions to produce highly energetic e xcitons that dissociate efficiently. Photophysical modelling of intensity-d ependent data reveals that sequential absorption followed by charge separat ion is the most likely mechanism for ultrafast charge pair generation, with the ratio of excited-state to ground-state absorption cross section limes the effective yield of charge pairs approximate to 0.1. These observations are consistent with previous photocurrent measurements and quantum chemical calculations assigning highly excited states to greatly delocalised excito ns [Kohler et al., Nature 392 (1998) 903]. (C) 2001 Elsevier Science B.V. A ll rights reserved.