A Theoretical Insight into the Effect of Alkoxy-Induced Noncovalent Conformational Locks in Non-Fullerene Acceptors for Organic Solar Cells

The effect of noncovalent conformational locks (NCLs) on the active-layer materials of organic solar cells (OSC) is important. However, the mechanisms of NCLs formation are not clear. Here, we systematically explore the role of NCLs in A–D–A′–D–A-type non-fullerene acceptors (NFAs) ( BTP-in-4F, BTP-out-4F, DO-4F, and QO-4F ). The results show that the strategy of alkoxy side chains into the central fused ring ( DO-4F ) facilitates the formation of S···O NCLs. These NCL features have a superior molecular planarity and decrease the reorganization energy for exciton dissociation and electron transfer. Conversely, more alkoxy side chains substitution for the central fused ring ( QO-4F ) yields obvious steric hindrance, accompanying marked terminal deformation, which impedes charge transfer processes. Due to the influence of the S···O NCLs in DO-4F, the PBDB-T/DO-4F system exhibits not only better HOMO energy level offsets and electrostatic potential differences but also multiple efficient charge transfer pathways (hot exciton, direct excitation, and intrinsic electric field mechanisms) at the D/A interface. Lastly, this work provides a molecular understanding of how alkoxy chain engineering regulates the acceptor conformations and interface kinetics, providing a robust theoretical basis for the designing of high-efficiency NFAs.