Local Lattice Regulation in Cesium Copper Halide Phosphors with Record‐Breaking External Quantum Efficiency for WLED and X‐Ray Imaging Applications

ABSTRACT Copper‐based metal halides have attracted increasing attention due to their non‐toxicity, low cost, and high luminescence efficiency. Nonetheless, improving the external quantum efficiency (EQE) of copper‐based metal halides continues to be a major challenge, constraining their broader commercial application. In this study, the local lattice regulation strategy is proposed to significantly enhance the EQE of Cs 3 Cu 2 Cl 5 phosphors. Through Cd 2+ doping, the Cs 3 Cu 2 Cl 5 : Cd phosphor realizes a record EQE of 90.26%, coupled with excellent thermal stability (83%@368K), highlighting its potential for advanced academic research and applications. Through first‐principles calculations and various experimental tests, it is evident that the electronic structure regulation and the structure distortion effectively enhance light absorption and suppress strong exciton‐phonon interactions. Moreover, the film fabricated by Cs 3 Cu 2 Cl 5 : Cd phosphors exhibit favorable X‐ray imaging characteristics (spatial resolution of 5.6 lp/mm, detection limit of 4.48 µGys −1 , light yield of 52870 ph MeV −1 ), and the WLED based on Cs 3 Cu 2 Cl 5 : Cd phosphors exhibit excellent performance (Ra = 94.1, CCT = 5465), meeting the requirements of X‐ray medical diagnostics and full‐spectrum lighting. This work opens up an innovative avenue for exploring local lattice regulation to enhance the EQE of copper(I)‐based halide perovskites in the future.