<p>The incorporation of fluorinated side-chains into M-series acceptors has been shown to enhance both the fill factor (FF) and power conversion efficiency (PCE) of the resulting photovoltaic devices. However, the effects of the fluorinated side-chain positions on the charge mobility and photovoltaic performance of M-series acceptors remain unexplored. Herein, we develop a partially fluorinated alkyl chain, 7-butyl-1,1,1,2,2-pentafluoro-octyl, which is incorporated either on the oxygen or nitrogen atoms of the conjugated backbone of the M-series acceptor, resulting in two new acceptors named O5F and N5F. Compared to the acceptor with fluorinated side chains on the oxygen atom (O5F), the acceptor with fluorinated side chains on the nitrogen atom (N5F) shows a closer π-π stacking distance and an enhanced charge mobility. As a result, the photovoltaic device based on PM6:N5F exhibits a PCE of 18.8% with an FF of 80.7% in comparison with the PM6:O5F-based counterpart, which shows a PCE of 17.8% with an FF of 79.2%. The PCE of 18.8% represents one of the highest PCE values reported to date for A-D-A-type small-molecule acceptors. More importantly, the PM6:N5F-based device shows significantly improved device stability with a <i>T</i>₈₀ lifetime (defined as the time required for the efficiency to degrade to 80% of its initial value) of 1084 h under one sun illumination compared to the PM6:O5F-based device (123 h).</p>