<p>Currently, the polymer PM6 stands out as one of the most efficient donor polymers in the field of organic solar cells, and many researchers are dedicated to further enhancing its photovoltaic performance. Among various strategies, the end-capping modification was a simple and effective way to minimize the undesired end-capping impurity groups and to narrow the molecular weight distribution of the final polymer donors. In this study, we systematically investigated the substitution effect of H, F, and Cl of the end-capping naphthyl group of the PM6 polymer on the photovoltaic performance. Compared to the naphthyl and fluoro-naphthyl group, the chloro-naphthyl moiety possesses a larger dipole moment, thereby enhancing the interaction between chloro-naphthyl end-capped PM6 with the small molecule acceptor, L8-BO, significantly reducing the π-π stacking distances between molecules and enhancing charge-carrier mobilities. Devices based on PM6-Cl:L8-BO exhibited appropriate phase separation, optimal molecular orientation, and minimal charge recombination. Consequently, the PM6-Cl:L8-BO-based device achieved an outstanding power conversion efficiency of 18.07%, with simultaneous enhancements in short-circuit current density (<i>J</i>_SC) and fill factor (FF). This was much higher than those of PM6-F:L8-BO-based (17.60%) and PM6-H:L8-BO-based (16.87%) devices. These results demonstrated that the modulation of end-capped atoms in PM6 could significantly improve the photovoltaic performance of organic solar cells and provide guidance for the design and synthesis of high- performance polymers.</p>