Tuning the Self-Trapped Emission: Reversible Transformation to 0D Copper Clusters Permits Bright Red Emission in Potassium and Rubidium Copper Bromides

Ternary copper halides are promising materials for lighting and displays, but red emission has yet to be reported among this class of materials due to the difficulty in tuning the self-trapped exciton (STE) emission. Here, we report lead-free hybrid organic–inorganic ternary copper halides A6(DMSO)12[Cu8Br13][Cu4Br4(OH)(H2O)] (ACB-DMSO, A = K, Rb) synthesized as single crystals and microcrystalline suspensions. These materials emit in the red portion of the visible spectrum with a high photoluminescence quantum yield (PLQY) of up to 75%. The changes in the emission spectrum are caused by the solvent-induced transformation from the STE emitter A2CuBr3 to phosphorescent ACB-DMSO which reversibly transforms blue-emitting 1D copper chains to red-emitting 0D copper clusters. K6(DMSO)12[Cu8Br13][Cu4Br4(OH)(H2O)] (A = K) can be used as the red-emitting phosphor for red light-emitting didoes (LEDs), and by adding blue and green emitting cesium copper halides, an all copper-based white LED with a high color rendering index (CRI) over 97% is achieved.