Phenothiazine-Based Self-Assembled Monolayers with Tailored Dipole Moments for Enhanced Energy Level Alignment in Perovskite Solar Cells

The use of self-assembled monolayers (SAMs) as hole-transport layers (HTLs) in inverted perovskite solar cells (iPSCs) has attracted significant attention because of their simple synthesis, tunable energy levels, and potential to enhance charge extraction at the interface. In this study, three phenothiazine-based SAMs─PTZ3PA, SPTZ3PA, and Br-SPTZ3PA─were rationally designed and synthesized with targeted functionalization using sulfone (−SO2) and bromine (−Br) substituents to modulate interfacial energy alignment and enhance charge transport via induced molecular dipole moments. The calculated dipole moments of PTZ3PA, SPTZ3PA, and Br-SPTZ3PA were 2.11, 5.26, and 6.25 D, respectively. These findings reveal that the introduction of electron-withdrawing groups such as sulfone and bromine markedly increase the molecular dipole moment, which is expected to influence the vacuum level at the interface and effectively tune the work function of the ITO electrode. Among the synthesized SAMs, PTZ3PA exhibited the highest device performance in iPSCs, achieving a power conversion efficiency (PCE) of 20.18%. This result validated our functional group engineering strategy and illustrated the potential for dipole moment modulation through the molecular design of SAMs. Collectively, this work offers critical insights into the structure–function relationships of SAM-based HTLs and provides a foundational framework for the development of high-performance iPSCs.