Abstract

Abstract The halide postdeposition treatment technique is a widely used strategy for mitigating defects in perovskite. However, when applied to mixed‐halide perovskites, it often leads to surface and internal halide heterogeneity, which compromises luminescence performance and spectral stability. In this work, blue mixed‐halide 3D perovskites are engineered with acetate (Ac⁻)‐rich surfaces to optimize the post‐treatment process and achieve halide homogeneity. The findings demonstrate that the strong interaction between surface Ac⁻ ions and Pb 2+ ions significantly reduces the formation of halide vacancy defects caused by the washing effect of isopropanol during post‐treatment. This defect reduction slows the infiltration of halide ions into the perovskite lattice, providing more time for surface reconstruction and minimizing the accumulation of introduced halide ions at the surface. As a result, a mild halide redistribution occurs, promoting the formation of a uniform mixed‐halide perovskite phase. This approach enabled the development of blue mixed‐halide 3D PeLEDs with a record external quantum efficiency of 19.28% (emission peak at 482 nm), comparable to state‐of‐the‐art blue reduced‐dimensional perovskite‐based PeLEDs. Additionally, the device demonstrated a narrowband and stable electroluminescence spectrum with a full width at half maximum (FWHM) of less than 16 nm.