Reducing the energetic disorder is crucial to improve the efficiencies of organic photovoltaics. Given the high performance of both fused-ring and nonfused-ring A-D-A type acceptors, a fundamental question arises: is a fused-ring D-unit necessary to obtain low energetic disorder? Here, we have systematically investigated the energetic disorder for electrons (described by the standard deviation of the lowest unoccupied molecular orbital (LUMO) energy, σLUMO) in representative fused-ring and nonfused-ring A-D-A type acceptors by combining molecular dynamics simulations with density functional theory calculations. The results point out that the σLUMO is dominated by the dynamic disorder for both fused-ring and nonfused-ring systems. Moreover, for all these acceptors, the LUMO is delocalized over the entire molecular backbone, which benefits to reduce the electron-vibration coupling. Consequently, both fused-ring and nonfused-ring systems exhibit low σLUMO values of 48-56 and 50-71 meV, respectively. Compared to the fused-ring systems with similar conjugation lengths, the σLUMO is slightly increased for the nonfused-ring systems due to the extra rotation-induced static disorder. Notably, the σLUMO of the nonfused-ring systems can be effectively reduced by extending the D-units and restricting the conformational rotation. This work provides helpful insights for developing cost-effective nonfused-ring acceptors with low energetic disorder.
Han et al. (Tue,) studied this question.