Photogenerated Carrier Mobility Significantly Exceeds Injected Carrier Mobility in Organic Solar Cells

Charge transport in organic photovoltaic (OPV) devices is often characterized by space-charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi-equilibrium energies in the disorder-broadened density of states (DOS). In contrast, in an operating OPV device the photo-generated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time-resolved experiments and simulations we show that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time-resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that the method of SCLC fails to capture is that of photogenerated carrier thermalization which boosts carrier mobility. We predict that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For