Energetic inversion of singlet/triplet interfacial charge-transfer states for reduced energy loss in organic solar cells

Compared to inorganic and perovskite counterparts, organic solar cells (OSCs) suffer from much more severe nonradiative energy loss due to the nongeminate recombination via triplet states at the donor:acceptor interfaces. To suppress the triplet recombination in OSCs, how to optimize the interfacial energetic landscape is very crucial. Here, we find that the lowest spin-triplet interfacial charge transfer state (TCT1) can be simultaneously hybridized with multiple local-excitation (LE) triplet states below the photovoltaic gap in the state-of-the-art OSCs based on A-D-A acceptors. More importantly, when the TCT1 state is close to the upper side of the LE triplet manifold, the hybridization will result in considerable energetic inversion between the singlet and triplet charge-transfer states. This is beneficial to destabilize the TCT1 state at close donor:acceptor separation and suppress the back charge transfer to the LE triplet states. Our work sheds light on molecular design towards reducing the triplet recombination under low driving force for higher-efficiency OSCs. Significant CT inversion can be achieved via hybridization of the triplet CT state with high-lying local triplet states, which benefits to concurrently reduce the triplet recombination and driving force for higher-efficiency organic photovoltaics.