Regulation of Molecular Orientations of A-D-A Nonfullerene Acceptors for Organic Photovoltaics: The Role of End-Group pi-pi Stacking

Increasing horizontal and face-on orientations of the active molecules in organic solar cells is critical to improve light absorption, charge mobility, and exciton diffusivity for high photovoltaic efficiency. However, how to effectively control the molecular orientations is challenging and still unclear. Here, the molecular self-assembly and orientation formation are elucidated for a series of acceptor-donor-acceptor (A-D-A) small-molecule acceptors (SMAs) by atomistic molecular dynamics simulations. The results indicate that the A-D-A molecules tend to assemble via the end-group pi-pi stacking at the liquid-vapor interfaces during solvent evaporation, which is beneficial to enhance horizontal and face-on orientations. Remarkably, the order of both horizontal and face-on orientations in the films shows a good linear correlation with the number of pi-pi stacking among all the A-D-A SMAs. The pi-pi stacking is found to be cooperatively controlled by both the end-group pi-pi interaction and side-chain steric hindrance. This work provides the rationalization for the different molecular orientations in a wide range of A-D-A SMAs and is useful to improve the horizontal and face-on orientations for high-performance organic photovoltaics.