Precursor States for Charge Carrier Generation in Conjugated Polymers Probed by Ultrafast Spectroscopy

Photocurrent experiments using two femtosecond laser pulses are performed on a photodiode using a ladder-type conjugated polymer as the active layer. With a photon energy of 3.1 eV the first pulse excites singlet excitons. A time-delayed second pulse with a photon energy of 2.49 eV leads to a decrease of the photocurrent by exciton depletion due to stimulated emission. ${\mathrm{S}}_1$ excitons being dissociated during their entire lifetime are identified as the only relevant channel for charge carrier generation. Intrachain polaron pairs are also formed on an ultrafast time scale with a yield of $\approx 10\%$. They can be efficiently dissociated by reexcitation with photons of an energy of 1.9 eV.