Effects of the partially fluorinated side-chain positions on the charge mobility and photovoltaic performance of M-series acceptors
Incorporating fluorinated side-chains into M-series acceptors enhances the fill factor (FF) and power conversion efficiency (PCE) of organic solar cells (OSCs). However, the impact of fluorinated side-chain positions on charge mobility and photovoltaic performance remains unexplored. Here, we synthesize a partially fluorinated alkyl chain, 7-butyl-1,1,1,2,2-pentafluoro-octyl, and attach it to either the oxygen or nitrogen atoms of the M-series acceptor backbone, yielding two new acceptors, O5F and N5F. Compared to O5F, N5F exhibits closer π-π stacking and higher charge mobility. Consequently, PM6:N5F-based devices achieve a PCE of 18.8% with an FF of 80.7%, surpassing PM6:O5F counterparts (PCE 17.8%, FF 79.2%). The 18.8% PCE is among the highest reported for A-D-A-type small-molecule acceptors. Notably, PM6:N5F devices show significantly improved operational stability, with a T80 lifetime of 1084 hours under one-sun illumination, versus 123 hours for PM6:O5F. This work demonstrates that positioning partially fluorinated side-chains on nitrogen atoms optimizes intermolecular packing and carrier transport, enhancing both efficiency and stability. It underscores the potential of partially fluorinated side-chains in designing high-performance NFAs.