Rb 2 CuX 3 (X = Cl, Br): 1D All‐Inorganic Copper Halides with Ultrabright Blue Emission and Up‐Conversion Photoluminescence

All‐inorganic metal halides such as Cs 4 PbX 6 and CsPbX 3 (X = Cl, Br, and I) are attracting global attention owing to their promise in optoelectronic applications. However, the presence of the toxic heavy metal lead (Pb) in these materials is a major concern. Here, a family of nontoxic high‐efficiency blue‐emitting all‐inorganic halides Rb 2 CuX 3 (X = Br and Cl) is reported; the compounds exhibit 1D crystal structures featuring anionic∞ 1 [ C u X 3 ] 2− ribbons separated by Rb + cations. The measured record high photoluminescence quantum yield values range from 64% to 100% for Rb 2 CuBr 3 and Rb 2 CuCl 3 , respectively. Furthermore, the measured emission linewidths are quite narrow with full width at half maximum values of 54 and 52 nm for Rb 2 CuBr 3 and Rb 2 CuCl 3 , respectively. Single crystals of Rb 2 CuCl 3 demonstrate an anti‐Stokes photoluminescence signal, shown for the first time for Pb‐free metal halides. The discovery of highly efficient narrow blue emitters based on a nontoxic and inexpensive metal copper paves a way for the consideration of low‐cost and environmentally friendly copper halides for practical applications.