Rapid Synthesis of Bright, Shape‐Controlled, Large Single Crystals of Cs3Cu2X5 for Phase Pure Single (X = Br, Cl) and Mixed Halides (X = Br/Cl) as the Blue and Green Components for Printable White Light‐Emitting Devices

Abstract Ternary copper halides have garnered significant interest for their bright, high quantum yield emission stemming from the radiative decay of self‐trapped excitons (STEs). Cesium copper halides have shown promise for use in optoelectronics, including light‐emitting devices (LEDs) for lighting and displays. To date several synthetic procedures for Cs 3 Cu 2 X 5 (X = Cl, Br, and mixed Br/Cl) have been proposed for making nanocrystals, microcrystals, or polycrystalline thin films. Here, a synthetic method for making large single crystals (SCs) with millimeter dimensions in less than 30 min is presented. Phase pure mixed halide SCs are also produced and in‐depth structural analysis has been performed for the first time, definitively showing the site preferences for mixing chloride into the pure Cs 3 Cu 2 Br 5 structure. Quantum yields for SCs of X = Cl and Br are 100% and 27% respectively, with long lifetimes and strong evidence of STE emission. This synthesis can be used to produce white light UV‐downconversion LEDs using ternary copper halides as the blue and green components along with the commercial red phosphor K 2 SiF 6 :Mn 4+ . These devices give a Commission Internationale de l'Éclairage (CIE) coordinate of (0.3327, 0.3342) and color rendering index of 90% at a color temperature of ≈5500 K.