Toward Long Exciton Diffusion Length in Nonfullerene Small Molecule Acceptors for Organic Solar Cells: Effect of Molecular Packing

Exciton diffusion length (LD) is a key parameter that determines the power conversion performance of organic solar cells (OSCs). A long diffusion length is needed so that excitons can diffuse to the donor–acceptor interface, where charges are separated to generate photocurrent. However, state-of-the-art high-performance nonfullerene acceptor OSCs still suffer from a limited LD (∼10–25 nm). Moreover, a systematic understanding of the relationship between molecular packing and LD is still lacking. In this study, we combined Density Functional Theory (DFT) with Kinetic Monte Carlo (KMC) simulations to investigate the effect of molecular packing on LD of Y6 derivatives. Our results reveal that LD is highly sensitive to the molecular packing, with calculated values ranging from 34 to 64 nm. The variation in LD can be as much as 88% across different molecular packings, influenced not only by exciton coupling but also by intermolecular hopping distances. Furthermore, we find that exciton diffusion is more efficient along the molecular plane rather than perpendicular to it. These findings provide important guidance for understanding the structure–property relationship governing exciton diffusion and designing high-performance OSCs.