Suppressing Phase Instability and Iodide Expulsion in Mixed-Halide Perovskites via Directed Hole Transfer and Defect Management.

Bandgap tunability in mixed-halide perovskite CsPb(BrI) nanocrystals (NCs) makes them appealing for multijunction solar cells and tunable optoelectronics. However, light-induced halide migration and phase segregation, primarily driven by hole trapping and surface defects, limit their practical utility. Holes oxidize iodide ions, while defects facilitate their diffusion, leading to phase segregation and iodide expulsion. This work addresses these issues through a dual strategy: directional hole extraction and robust defect passivation. Hole-accepting moieties such as ferrocene carboxylic acid (FcA) and (dimethylaminomethyl)ferrocene (FcAm) act as guided sinks for photogenerated holes, preventing their interaction with iodide and thus suppressing iodide expulsion and enhancing photostability. Additionally, dual defect passivation using a zwitterionic mixture of oleic acid (OA) and oleylamine (OAm) reduces light-induced spectral shifts by mitigating defect-driven halide migration. A bromide-excess treatment employing thionyl bromide-modified CsPbBr NCs (CPBSOBr), followed by FcAm integration, further enhances stability through hydrogen bonding between protonated FcAm and bromide ions, efficiently quenching holes and minimizing iodide loss. This study highlights the pivotal role of surface engineering in stabilizing mixed-halide perovskite NCs under prolonged light exposure. By providing fundamental insights into halide migration and phase segregation mitigation strategies, the design of photostable perovskite materials is advanced for high-performance photovoltaic and optoelectronic applications.