Abstract

The efficiency of polymer-based organic photovoltaic (OPV) devices currently approaches 10%. Despite this progress, the fundamental mechanism of charge photogeneration at the microscopic level is still highly debated and poorly understood [1, 2]. Here we present a comprehensive study of the initial time dynamics of the blend poly-3-hexylthiophene: [6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) upon impulsive excitation of the donor moiety with high-time-resolution pump-probe, supported by time-dependent density functional theory (TDDFT) calculations. Experimentally, we observe coherent vibrational motion of the fullerene moiety after impulsive optical excitation of the polymer donor. Comparison with TDDFT simulations evidences coherent electron transfer between donor and acceptor and oscillations of the transferred charge with a ~25-fs period, matching that of the observed vibrational modes. The TDDFT simulations show that strong vibronic coupling promotes delocalization of the photoexcited electronic wave packet across the interface. Both the electronic density and the nuclei display correlated oscillations on the same time scales, which are essential in driving the charge transfer. Additionally, preliminary investigations with two-dimensional electronic spectroscopy (2DES) on the same materials confirm that strong coherent coupling between electronic and vibrational degrees of freedom is the driving force for polaron pair formation. These results provide evidence for the dominant role of vibronic coupling in triggering charge delocalization and charge transfer in the early stages of the dynamics in this class of non-covalently bound donor:acceptor systems [3].

© 2015 IEEE