Multi-THz spectroscopy of mobile charge carriers in P3HT:PCBM on a sub-100 fs time scale

The dynamics of mobile charge carrier generation in polymer bulk heterojunction films is of vital importance to the development of more efficient organic photovoltaics. As with conventional semiconductors, the optical signatures of mobile carriers lie in the far-infrared (1-30 THz) although the electrodynamics deviate strongly from the Drude model. The key time scales for the process are sub-100 fs to picoseconds, and is a challenge to perform low energy spectroscopy on these time scales as it is less than the period of oscillation for the probing light. In this work, we demonstrate sub-100 fs spectroscopy of a polymer bulk heterojunction film P3HT:PCBM using a single-cycle, phase-locked and coherently detected multi-THz transient as a probe pulse following femtosecond excitation at 400 nm. By observing changes to the reflected THz transients from the film surface following photoexcitation, we can extract the complex optical conductivity spectrum for the film in snapshots of 40 fs following photoexcitation. We find that for our excitation conditions mobile charges are created in less than 120 fs and are characterized by a spectrum which is characteristic of a two dimensional delocalized polaron. A large fraction of mobile carriers relax to a localized state on a 1 ps time scale. Pump energy dependent photon-to-mobile carrier conversion efficiency supports hot exciton dissociation as a mechanism for such fast mobile carrier generation.