Understanding the origin of the S-curve in conjugated polymer/fullerene photovoltaics from drift-diffusion simulations

Abstract

We utilize drift-diffusion modeling to investigate the cause of S-shaped current-voltage curves in organic solar cells. We find that even a many order-of-magnitude mismatch of the carrier mobilities is insufficient to generate S-shaped J-V curves. Instead, S-shaped J-V curves result when a sigmoid-shaped electron mobility profile is entered into the calculation. This suggests that S-curves in bulk heterojunction photovoltaics are caused by factors that affect the extraction of electrons near the device cathode. Such factors could include surface recombination, partially blocking contacts caused by interfacial layers, or vertical phase separation of the fullerenes away from the cathode interface.