Bromine-functionalized carbazole derivatives in perovskite precursors: defect passivation for enhanced perovskite photovoltaics

Carbazole phosphonic acid self-assembled monolayers (SAMs) have emerged as an effective strategy to enhance the performance of perovskite solar cells (PSCs) via interface engineering. While previous studies have mainly focused on their application at the hole transport layer (HTL), their roles within the perovskite layer itself remain underexplored. In this work, two carbazole-based SAM molecules, 2-(9H-carbazol-9-yl)ethylphosphonic acid (2PACz) and 2-(3,6-dibromo-9H-carbazol-9-yl)ethylphosphonic acid (Br-2PACz), were directly incorporated into perovskite precursors to investigate their influence on film formation and electronic structure. Compared with the control film, the incorporation of both 2PACz and Br-2PACz improved film uniformity and morphology, and significantly prolonged carrier lifetimes, indicating suppressed non-radiative recombination and reduced defect density. Notably, Br-2PACz shows a more pronounced effect: the bromine substituent enhances the electron-withdrawing character of the molecule, leading to a deeper highest occupied molecular orbital (HOMO) level and pronounced modulation of the interfacial electronic structure. As a result, the energy-level alignment at the perovskite/electron-transport interface is optimized, facilitating charge extraction and defect passivation. These synergistic effects contribute to higher power conversion efficiency (PCE). This work demonstrates an additive-engineering approach based on carbazole SAMs to control perovskite crystallization and interfacial energetics, providing a promising pathway toward high-efficiency PSCs.