Finite Size Effects on Light-Induced Correlated Ionic and Electronic Transport in Mixed Halide Perovskites

High Resolution Image Download MS PowerPoint Slide Mixed halide perovskites often undergo reversible phase segregation under illumination, yet the exact underlying mechanism and the material properties affecting this process remain unclear. By combining time-resolved photoluminescence (TRPL) with in situ X-ray diffraction (XRD) under illumination, we show that segregation kinetics and the thermodynamic limit of segregation under illumination in MAPbI 1.5 Br 1.5 are not intrinsically linked. The segregation rate increases linearly with the defect density inferred from TRPL. In contrast, the equilibrium extent of segregation is independent of defect density but instead decreases with reducing crystallite size down to a critical, finite-size threshold of ∼40 nm, below which segregation is suppressed. Defect passivation using the ionic liquid BMIMBF 4 slows the segregation kinetics but does not affect equilibrium limits. These findings establish crystallite size as a thermodynamic constraint and defects as kinetic mediators, outlining considerations for designing photostable mixed halide perovskites.