Two‐Dimensional Cs ₂ AgIn _x Bi _{1‐ x} Cl ₆ Alloyed Double Perovskite Nanoplatelets for Solution‐Processed Light‐Emitting Diodes

Abstract Lead‐free double perovskites have emerged as a promising class of materials with potential to be integrated into a wide range of optical and optoelectronic applications. Herein, the first synthesis of 2D Cs ₂ AgIn _x Bi _{1‐ x} Cl ₆ (0 ≤ x ≤ 1) alloyed double perovskite nanoplatelets (NPLs) with well controlled morphology and composition is demonstrated. The obtained NPLs show unique optical properties with the highest photoluminescence quantum yield of 40.1%. Both temperature dependent spectroscopic studies and density functional theory calculation results reveal that the morphological dimension reduction and In–Bi alloying effect together boost the radiative pathway of the self‐trapped excitons of the alloyed double perovskite NPLs. Moreover, the NPLs exhibit good stability under ambient conditions and against polar solvents, which is ideal for all solution‐processing of the materials in low‐cost device manufacturing. The first solution‐processed light‐emitting diodes is demonstrated using the Cs ₂ AgIn _0.9 Bi _0.1 Cl ₆ alloyed double perovskite NPLs as the sole emitting component, showing luminance maximum of 58 cd m ⁻² and peak current efficiency of 0.013 cd A ⁻¹ . This study sheds light on morphological control and composition‐property relationships of double perovskite nanocrystals, paving the way toward ultimate utilizations of lead‐free perovskite materials in diverse sets of real‐life applications.