Charge Dynamics in Organic Photovoltaic Materials: Interplay between Quantum Diffusion and Quantum Relaxation

This paper discusses the mechanism of generation of free charges in organic photovoltaic cells (OPV) from electrostatically bound electron-hole pairs. The efficiency of this process is explained when interfacial charge-transfer (CT) states are generated by direct optical excitation. We used semiclassical quantum dynamics at a short time scale (similar to 100 fs) and Redfield theory at a relatively long time scale (similar to 10-100 ps) to cover both the process of dissociation and the relaxation to the lowest energy state. Our calculations suggest that a CT state with an intermediate electron-hole separation can evolve into a charge-separated (CS) state on ultrafast time scales (similar to 100 fs) as a result of quantum diffusion. On long time scales, however, the CS states ultimately relax to the low-energy CT states due to the interaction with the thermal bath, indicating that the yield of free charge carrier generation is determined by the interplay between ultrafast charge separation, due to quantum diffusion, and the much slower quantum relaxation process.