Tracing photoinduced electron transfer process in conjugated polymer/fullerene bulk heterojunctions in real time

Ultrafast spectroscopic studies using an optical excitation of a conjugated polymer by sub-10-fs pulses are reported. Phonon modes which are strongly coupled to the electronic excitation of the conjugated polymer are directly observed as coherent oscillations during the pump-probe experiment, mirroring the resonant/nonresonant Raman spectrum of the conjugated polymer. In composites of a conjugated polymer with a fullerene the primary photoexcitation is found to be an ultrafast photoinduced electron transfer. We are able to time resolve for the first time the kinetics of this charge transfer process with a forward transfer time of around tau (et) similar to 45 fs.