Ultrabright Light Emission Properties of All-Inorganic and Hybrid Organic–Inorganic Copper(I) Halides

All-inorganic and hybrid copper(I) halides have recently emerged as candidate optical materials due to their extremely high light emission efficiencies, nontoxic and earth-abundant elemental compositions, and low-cost solution processability. Originally inspired and motivated by the research on the halide perovskites family, the development of copper(I) halide light emitters has been following its unique path. In this perspective, we discuss the distinct low-dimensional crystal structures of all-inorganic halides, which enable strong charge localization and formation of room-temperature self-trapped excitonic (STE) states, giving rise to largely Stokes-shifted light emission with near-unity quantum yields. Due to their unique electronic structures, all-inorganic copper halides are predominantly blue emitters with unusually weak tunability of their optical properties (e.g., halide substitution has a minimal impact on their photoluminescence properties). These shortcomings paved the way for the exploration of more robust and diverse families of hybrid organic–inorganic copper(I) halides, which are discussed next. Our discussions of crystal and electronic structures and optical properties of all-inorganic and hybrid Cu(I) halides are complemented by the reported uses of these materials in optoelectronic applications. Finally, we identify remaining fundamental questions, knowledge gaps, and practical challenges and outline several paths forward for addressing these issues, including through new materials discovery and in-depth studies of photophysics of Cu(I) halides and derivative materials.