Dual-Armed <i>N</i>-Methoxy Triphenyl-Aminated Phenothiazine-Linked Benzodithiophene Hole Transport and Interface Passivation Materials for Perovskite Solar Cells

The efficient and stable operation of perovskite solar cells (PSCs) under ambient conditions remains a key challenge for their commercialization, with surface defects and interfacial instability being major performance bottlenecks. This study introduces two organic molecules, RK1 and RK4, designed to function as both interfacial passivation agents and hole transport materials (HTMs), to address these challenges through a unified dual-role approach. These molecules were synthesized via N-methoxy triphenyl-aminated phenothiazine units, coupled with sulfur-rich methoxy benzodithiophene units in donor−π–donor (D1–π–D2) and donor−π–donor−π–donor (D1–π–D2–π–D1) configurations and named as RK1 and RK4, respectively. As evidenced by steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) studies, these molecules enable efficient hole extraction, and as confirmed by X-ray photoelectron spectroscopy (XPS) studies, they effectively passivate defects such as undercoordinated Pb2+ ions by coordination interaction. Devices employing RK1 and RK4 as interfacial layers achieved power conversion efficiencies (PCEs) of 17.28 and 18.52%, respectively, with RK4 as a standalone HTM reaching a PCE of 16.82%. In addition to the superior performance, RK1- and RK4-passivated devices exhibited better stability with unencapsulated devices retaining up to 78% of their initial PCE after 888 h and up to 58% after 1536 h, compared to the spiro-OMeTAD devices, which degraded rapidly just within 216 h. The durability and better device performance is attributed to the enhanced hydrophobicity of RK1 and RK4, which minimizes moisture ingress and ion migration, coupled with their ability to improve crystallinity and suppress nonradiative recombination. By integrating multisite defect passivation, optimized energy level engineering, enhanced hole mobility, and hydrophobicity, this work offers a promising strategy for dual-role organic materials, advancing PSCs toward scalable and commercially viable solutions.