Organic solar cells (OSCs) offer advantages in flexibility, low cost, and solution processability, but the instability of conventional hole transport layers (HTLs) remains a major limitation. However, carbazole-based Self assembled monolayers (SAMs) often exhibit poor packing and photooxidative degradation. In this study, a non-halogenated phenothiazine-based SAM, [(4-(10H-phenothiazin-10-yl)butyl)phosphonic acid] (PZPA), is introduced to improve hole extraction and interfacial stability. The sulfur-containing phenothiazine core downshifts the HOMO level, enhances charge delocalization, and enables the formation of compact and ordered monolayer on ITO. Devices based on PM6:Y6BO achieve a power conversion efficiency of 18.2% with excellent thermal and UV stability, showing only 1.09% degradation under prolonged illumination. Moreover, PM6:L8-BO devices reach a PCE of 19.7%, highlighting phenothiazine-derived SAMs as efficient interfacial materials for high-performance OSCs.