AbstractOrganic photovoltaics (OPVs) have recently achieved efficiencies of over 19% and are well underway toward practical applications. However, issues concerning operational stability remain a major challenge ahead of OPV commercialization. Here, when replacing the conventional hole‐transporting layer PEDOT:PSS with a self‐assembled monolayer of [2‐(3,6‐dichloro‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (3,6‐Cl‐2PACz) or [2‐(4,5‐dichloro‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (4,5‐Cl‐2PACz) it is found that the T₈₀ lifetime of PM6:BTP‐eC9‐based devices can be improved from ~100 to ~470 and over 800 h, respectively. The power conversion efficiency is also improved from 17.29% to 18.17% and 18.67%, respectively. The improved performance and prolonged photostability in 4,5‐Cl‐2PACz‐based devices stem from the stabilized vertical distribution of donor and acceptor components, reducing the energetic disorder and thus alleviating non‐radiative recombination losses. It is further found that the surface energy of 4,5‐Cl‐2PACz‐modified substrates stays constant under prolonged illumination due to the improved intrinsic photostability of 4,5‐Cl‐2PACz, supporting the robust active layer morphology. Applying 4,5‐Cl‐2PACz in a ternary device of PM6:BTP‐eC9:L8‐BO‐F delivered an efficiency of 19.05% and a T₈₀ lifetime over 1140 h.