Strategic Modulation of Phenothiazine-Based Self-Assembled Monolayers for Optimized Energy-Level Alignment and Stability in Perovskite Solar Cells

Inverted perovskite solar cells (PSCs), a promising photovoltaic technology, require effective hole-selective contacts─often provided by self-assembled monolayers (SAMs)─to achieve high performance and stability. We systematically engineered five phenothiazine-based SAMs (MeO- to CN-2EPT) by varying their terminal substituents to tune their molecular dipole moments and highest occupied molecular orbital (HOMO) energy levels, thereby precisely controlling the interfacial energetics. Our study revealed a critical trade-off between charge extraction efficiency and stability. Br-2EPT demonstrated optimal energetic alignment with the perovskite valence band maximum (VBM = -5.68 eV), yielding the highest device power conversion efficiency (PCE of 19.9%) and minimal recombination losses. Conversely, CN-2EPT showed the strongest chemical interaction via its cyano functionality, leading to superior defect passivation and outstanding long-term stability (retaining 97.8% PCE after 100 h of MPP tracking and 95% after 70 days of thermal aging). However, the deeper HOMO energy level of CN-2EPT created a hole-extraction barrier, significantly limiting its fill factor (FF) and ultimate efficiency. These results highlight the importance of interfacial dipole engineering in SAMs and underscore that the critical design principle for next-generation hole-selective layers is the judicious combination of strong passivating groups with a HOMO energy level that closely matches the perovskite valence band to maximize both efficiency and long-term durability.