Investigating the origin of S-shaped photocurrent-voltage characteristics of polymer:fullerene bulk-heterojunction organic solar cells

We herein investigated the origin of the S-shape behaviour exhibited near the compensation voltage in the photocurrent density-voltage (Jph-V) characteristics of organic solar cells. P3HT:PCBM bulk-heterojunction solar cells annealed at a too high temperature show the S-shaped Jph-V characteristics. Optical microscopy images revealed the interface degradation. Utilizing a drift-diffusion model, we found that the S-shape behavior cannot be seen by a sole effect of charge accumulation or imbalance of charge carrier mobilities. Rather, the effect of both combined resulting in strong bimolecular recombination causes a reduction in photocurrent near the compensation voltage, producing the S-shape feature.