Achieving Near‐Unity Photoluminescence Quantum Yield in Lead‐Free Cerium (III) Halides Cs ₃ CeX ₆ (X = Cl, Br, I) for Multifunctional Applications

Abstract Rare‐earth metal halides have emerged as a compelling class of luminescent materials, offering an environmentally benign alternative to toxic lead‐based halides. This work reports the successful synthesis of Cs 3 CeX 6 (X = Cl, Br, I) with tunable emission and excellent photoluminescence quantum yield (PLQY) of Cs 3 CeCl 6 . Through controlled halogen substitution in the Cs 3 CeX 6 (X = Cl, Br, I) system, composition‐dependent emission redshift (Cl→Br→I) is observed. Among them, Cs 3 CeCl 6 achieves a photoluminescence quantum yield (PLQY) of 99.89%, significantly surpassing the 92.57% PLQY of Cs 3 CeBr 6 and the 7.53% PLQY of Cs 3 CeI 6 , meanwhile surpassing that of most previously reported lead‐free metal halides, highlighting its exceptional potential for efficient luminescent applications. Thermal stability analysis reveals that Cs 3 CeCl 6 maintains remarkable emission efficiency at elevated temperatures, retaining > 85% of its room‐temperature luminescence intensity at 438 K, compared to only 60% for Cs 3 CeBr 6 and a mere 10% for Cs 3 CeI 6 . The exceptional thermal robustness originates from their characteristically high activation energies for thermal quenching, which effectively suppress non‐radiative relaxation pathways. White light‐emitting diodes (LEDs) based on Cs 3 CeI 6 exhibit a remarkable color rendering index (R a ) of 95.1, in contrast to those based on Cs 3 CeCl 6 (R a = 77.6) and Cs 3 CeBr 6 (R a = 79.6); all WLEDs based on Cs 3 CeX 6 (X = Cl, Br, I) demonstrate excellent atmospheric stability. With near‐unity PLQY and remarkable thermal stability of Cs 3 CeCl 6 , coupled with the outstanding white LEDs performance of Cs 3 CeI 6 , Cs 3 CeX 6 (X = Cl, Br, I) compounds emerge as promising candidates for next‐generation environmentally friendly optoelectronic applications, particularly in advanced display technologies and solid‐state lighting systems. This work contributes to the advancement of eco‐friendly emitters with both high efficiency and practical utility in next‐generation optical technologies.