Dissociation of singlet excitons dominates photocurrent improvement in high-efficiency non-fullerene organic solar cells

In organic solar cells, the singlet and triplet excitons dissociate into free charge carriers with different mechanisms due to their opposite spin state. Therefore, the ratio of the singlet and triplet excitons directly affects the photocurrent. Many methods were used to optimize the performance of the low-efficiency solar cell by improving the ratio of triplet excitons, which shows a long diffusion length. Here we observed that in high-efficiency systems, the proportion of singlet excitons under linearly polarized light excitation is higher than that of circularly polarized light. Since the singlet charge transfer state has lower binding energy than the triplet state, it makes a significant contribution to the charge carrier generation and enhancement of the photocurrent. Further, the positive magnetic field effect reflects that singlet excitons dissociation plays a major role in the photocurrent, which is opposite to the case of low-efficiency devices where triplet excitons dominate the photocurrent.