Near‐Unity PLQY of Cs₃Cu₂X₅ (X = Cl, Br) for High‐Efficiency White Light‐Emitting Diodes with Exceptional Color Quality

Abstract Metal halide perovskites and their derivatives have emerged as highly promising materials for next‐generation optoelectronic devices, owing to their intrinsic defect tolerance and exceptional electrical and optical properties. Among these, lead‐free copper(I)‐based halide perovskite derivatives, Cs₃Cu₂X₅ (X = Cl, Br, I) (CHPs), have garnered significant attention as environmentally friendly and stable alternatives to lead‐based perovskites. In this study, a cost‐effective and sustainable synthesis route for Cs₃Cu₂Cl₅ and Cs₃Cu₂Br₅ powders is developed, which exhibit strong green (≈526 nm) and blue (≈458 nm) emissions, and achieve remarkable photoluminescence quantum yields (PLQYs) of 100% and 92%, respectively. Cs₃Cu₂X₅ (X = Cl, Br) powders are incorporated into 3D‐printed structures, exhibiting excellent transparency and color stability. Furthermore, white LEDs are fabricated by using the green‐emitting Cs₃Cu₂Cl₅ and blue‐emitting Cs₃Cu₂Br₅ and a yellow‐emitting double perovskite (PLQY≈ 73%), resulting in devices with an exceptionally high color rendering index (CRI) of up to 98 and tunable correlated color temperatures (CCTs) ranging from 3864 to 9677 K, closely mimicking natural white light. Beyond solid‐state lighting, the superior optical performance and stability of Cs₃Cu₂X₅ (X = Cl, Br) powders open new avenues for their application in photovoltaics and other optoelectronic devices.