A Highly Efficient and Stable Blue‐Emitting Cs5Cu3Cl6I2 with a 1D Chain Structure

Abstract In the field of photonics, alkali copper(I) halides attract considerable attention as lead‐free emitters. The intrinsic quantum confinement effects originating from low‐dimensional electronic structure lead to high photoluminescence quantum yields (PLQYs). Among them, Cs 3 Cu 2 I 5 is the most promising candidate, satisfying both high PLQY and air stability. In this study, a strategy to explore a new material meeting these requirements through the use of the mixed‐anions of I − and Cl − is proposed. The expectation is maintained that the large difference in ionic radii between them likely results in the formation of a novel compound. Consequently, Cs 5 Cu 3 Cl 6 I 2 with a 1D zigzag chain structure is discovered. This material exhibits blue emission (≈462 nm) with a near‐unity quantum yield of 95%. An electronic structure calculation reveals that the localized nature of the valence band maximum is crucial in obtaining efficient self‐trapped exciton emission. Moreover, the iodine‐bridged 1D connectivity significantly enhances the chemical stability of Cs 5 Cu 3 Cl 6 I 2 , compared with the pure chloride phase. The present findings provide a new perspective for developing air‐stable alkali copper(I) halides with highly efficient luminescence.