Energetic Disorder in A-D-A Type Acceptors for Organic Photovoltaics: Fused-Ring vs. Nonfused-Ring Systems.

Abstract

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, σ_LUMO) 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 σ_LUMO 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 σ_LUMO values of 48-56 and 50-71 meV, respectively. Compared to the fused-ring systems with similar conjugation lengths, the σ_LUMO is slightly increased for the nonfused-ring systems due to the extra rotation-induced static disorder. Notably, the σ_LUMO 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.