Asymmetric alkoxy side chain engineering on A–DA′D–A non-fullerene acceptors: an effective strategy to enhance crystallinity and electron mobility

Introducing an alkoxy side chain (ASC) to high performance non-fullerene acceptors (NFAs) is a simple but effective strategy to enhance device efficiency. However, the intrinsic mechanism is still an open question, and in particular, its influence on electronic structure, morphology, and charge carrier mobility is still not well understood. In this work, we have selected A-DA'D-A type BZ4F to study the effects of symmetric (experimentally reported) and asymmetric (newly designed in this work) ASC engineering on these properties. The results show that the hybridization effects are evident in the energy of frontier molecular orbitals and averaged electrostatic potential of asymmetric molecules. The introduction of asymmetric ASCs generally enhances electron mobility by promoting molecular planarity, strengthening AA (A: terminal acceptor) face-on stacking, and reducing reorganization energy. The positional variation of the oxygen atom within ASC units can finely modulate ASC orientation and molecular planarity. BZ4F-O-2-asy exhibits the highest electron mobility. Our results demonstrate that introducing an asymmetric ASC with <i>n</i> = 1 (<i>i.e.</i>, featuring one saturated carbon between the oxygen atom and the molecular backbone) onto the DA'D core represents a promising modification strategy.