Heterophase Engineering of Mechanochemically‐Synthesised All‐Inorganic Ternary Copper Iodide for Tuneable White Light Emission

ABSTRACT Ternary metal halides have recently emerged as promising optoelectronic materials due to their ambient stability and high photoluminescence quantum yields from the low dimensionality of electronic structures. Their strong exciton‐phonon coupling enables broadband self‐trapped exciton emission, advantageous for white light‐emitting diode (WLED) applications. Particularly, the all‐inorganic caesium copper iodide (Cs‐Cu‐I) system – comprising blue‐emitting 0D Cs 3 Cu 2 I 5 and yellow‐emitting 1D CsCu 2 I 3 phases – offers a broadband white light emission spectrum with facile chromaticity tunability and emission stability. However, mechanistic understanding of multiphase formation and corresponding multiscale structural information has been limited for Cs‐Cu‐I, as well as its influence on emission properties. Here, we employ mechanochemical synthesis (MCS) as a solvent‐free, time‐resolved platform to investigate solid‐state reaction pathways by quenching at defined milling times, enabling correlation of structural evolution with photophysical responses. We observe a kinetically accessible formation pathway of a 0D/1D heterophase with a physical interface, with interfacial charge transfer and defect passivation effects that enhance the weakly‐emissive 1D phase, from which we designed optimal synthesis routes for 0D/1D heterophase‐based colour‐conversion device applications. This work establishes a comprehensive structure‐emission framework for Cs‐Cu‐I heterophase system and provides design principles and practical protocols for developing interface engineering in ternary metal‐halide emitters in next‐generation optoelectronics.