Elucidating the effects of bromine substitution in asymmetric quinoxaline central core-based non-fullerene acceptors on molecular stacking and photovoltaic performances

Abstract

Substitution optimization is an efficient method to construct non-fullerene acceptors (NFAs), and electronegative fluorine and chlorine are generally used for this. However, the research on bromine, which is a larger atom, is scarce. Herein, we synthesized five NFAs with different substitutions to elucidate the effect of bromine substitutions (alkoxy, hydrogen, chlorine, and fluorine were also tested for comparison) based on an asymmetric quinoxaline central core. A novel method involving the local surface electrostatic potential (local ESP) was developed to compare the various electron properties. The results showed that the intrinsic characteristics of bromine, including its large atomic radius, strong polarizability, and weak electronegativity, led to a slightly enhanced local-ESP value of the central core in the brominated NFA (Qx-PhBr), which directly guided the molecular stacking, as proved by the single-crystal analysis. Simultaneously, the donor–acceptor interaction was also weakened due to the bromine substitution. As a result, Qx-PhBr demonstrated tight molecular stacking and an optimized blend film morphology and achieved an excellent PCE of 19.65% in binary organic solar cells. Meanwhile, the optimal molecular stacking and interactions also endowed this efficient system with excellent stability and tolerance to film-thickness. This work emphasizes the potential of brominated NFAs and proposes an innovative method of utilizing the local ESP.