Tailoring short-range mobility at donor–acceptor heterointerfaces through small molecules promotes efficient organic solar cells

Abstract

Understanding how to optimize the electronic processes and material selections are fundamental to fostering the development of organic solar cells (OSCs). In this report, we identified that small molecule donors (SMDs) could act as nanofillers for the voids in the bulk of photoactive films, which are most prominent at donor–acceptor heterointerfaces due to a higher degree of disorder than molecular domains. Meanwhile, although such more compact heterointerfaces are beneficial in singlet exciton dissociation due to more overlapping electronic wavefunctions, they cannot guarantee device improvements as they also influence the recombination probability. Based on our photophysical analysis, incorporating SMDs with weaker molecular polarizability to occupy these heterointerfaces while preserving the intrinsic delocalization of donor and acceptor singlet excitons is necessary to suppress the sub-nanosecond bimolecular recombination losses, owing to the enhanced short-range mobilities that drive free charges from heterointerfaces towards molecular domains. Herein, this is made possible through the simple fluorination of the molecule B1, generating new B1-derivative SMDs. Uncovering this new design strategy for heterointerfaces is expected to serve as one of the frameworks for the era of >20% efficiency OSCs.