Role of Octahedron Alloying in Photodynamics of Lead‐Free Halide Double Perovskite Nanoplatelets

Abstract The lead‐free halide double perovskites have attracted great interest owing to their unique photophysical properties. Among them, the luminescence mechanisms of the Cs₂AgInCl₆ crystallites are still under debate. A hot‐injection method is developed to synthesize novel hollow and spatially symmetric Cs₂AgInCl₆ nanoplatelets (NPLs) and study the effect of the cation/octahedron alloying on their photodynamics by using the experimental characterizations in conjunction with the density functional theory calculations. The results reveal that the pure Cs₂AgInCl₆ NPLs exhibit wide‐band and double‐peaked photoluminescence originating separately from free and self‐trapped excitons. The Ag⁺/Na⁺ and In³⁺/Bi³⁺ alloying, that is, the partial substitution of the [AgCl₆]⁵⁻ and [InCl₆]³⁻ octahedra by [NaCl₆]⁵⁻ and [BiCl₆]³⁻ octahedra, leads to crystal symmetry breaking and strongly enhanced exciton localization. As a result, the self‐trapped exciton luminescence becomes predominant in the yielded Cs₂AgInCl₆:NaBi nanoplatelets, and the quantum yield is highly improved to 10.8%. These results improve the understanding of the role of octahedron alloying in the photophysics of the double perovskite nanocrystals and pave the way for their applications in solid‐state lighting.