Interfacial energy-level mismatch and insufficient stability at electrode/organic semiconductor junctions remain major obstacles to the concurrent enhancement of efficiency and lifetime in organic solar cells (OSCs). Although self-assembled monolayers (SAMs) offer precise interfacial control, conventional single-component SAM hole transport layers (HTLs) often fail to balance charge selectivity and interfacial robustness. Here, we introduce a hybrid SAM strategy by incorporating a sulfur-containing additive, phenothiazine-based phosphonic acid (PTZ-Phos), into a carbazole-based 4PACZ SAM. The sulfur heteroatom effect enables synergistic optimization of the ITO/SAM and SAM/active-layer interfaces, leading to improved energy-level alignment, wettability, and suppressed non-radiative recombination. The resulting hybrid SAM exhibits a HOMO level of −5.31 eV, forming a low hole-extraction barrier of 0.17 eV with PM6, while the upshifted LUMO level of −3.52 eV increases the electron-blocking barrier to 0.58 eV PM6:Y6-based OSCs achieve a power conversion efficiency of 17.19%, with an open-circuit voltage of 0.830 V, a short-circuit current density of 27.33 mA cm−2, and a fill factor of 75.78%. Notably, the devices retain 80% of their initial efficiency after 1877 h of storage in a nitrogen atmosphere. This work demonstrates a scalable interfacial engineering strategy for high-efficiency, long-term-stable OSCs.