Alkyl side chain engineering for difluorinated benzothiadiazole flanked non-fullerene acceptors toward efficient polymer solar cells

Side chain engineering plays a vital role in fine tuning of non-fullerene acceptors for the contemporary organic solar cells. In this work, we compare central alkyl side chains in the modification of photophysical and photovoltaic properties of difluorinated benzothiadiazole flanked non-fullerene acceptors. With a hexyl side chain, the resulting non-fullerene H-FFBR possesses more compact intermolecular interaction, leading to narrower bandgap, higher-lying HOMO level and stronger self-aggregation, if compared to O-FFBR with octyl side chain. For the photovoltaic performances, with PTB7-Th as the donor polymer, the O-FFBR-based solar cells exhibit power conversion efficiency of 9.4%, which is higher than that of 9.1% for the H-FFBR-based devices. Bulk-heterojunction morphologies of the O-FFBR- and H-FFBR-based active layers were compared by atomic force microscopy (AFM) and transmission electron microscope (TEM), revealing that O-FFBR is likely to give lower surface roughness and smaller phase separation. Our work demonstrates that varying alkyl side chain of a non-fullerene acceptor is valuable to tune the bulk-heterojunction morphology and resulting photovoltaic performance.

Automated summary

This study reveals that modifying alkyl side chains of non-fullerene acceptors can adjust the photovoltaic performance, and the non-fullerene acceptor with hexyl side chains exhibits more compact intermolecular interactions compared to that with octyl side chains. Devices utilizing octyl side chains achieve higher power conversion efficiency in comparison to those using hexyl side chains.