Solvent-Free Synthesis and Thin-Film Deposition of Cesium Copper Halides with Bright Blue Photoluminescence

Nontoxic alternatives to lead halide perovskites are highly sought after for applications in optoelectronics. Blue-luminescent materials are especially demanded as they could be used to prepare white light-emitting diodes, with important potential applications in lighting systems. However, wide band gap blue emitters with high photoluminescence quantum yields (PLQYs) are typically more difficult to obtain as compared to green- or red-emitting ones. Here, we prepared two series of inorganic cesium copper halides, with the general formulas Cs 3 Cu 2 X 5 and CsCu 2 X 3 (X = Cl, Br, I, and mixtures thereof) by dry mechanochemical synthesis at room temperature. X-ray diffraction demonstrates quantitative conversion of binary precursors into the desired ternary structures and good halide mixing in single-phase compounds. We identified Cs 3 Cu 2 I 5 as the most promising material as it maintains blue luminescence centered at 442 nm with a high PLQY (>40%) after several days in air (Cs 3 Cu 2 Cl 5 shows significantly higher PLQY, over 80% but is unstable in air). Based on this, we fabricated homogeneous and pinhole-free Cs 3 Cu 2 I 5 thin films by thermal single-source vacuum deposition. Crystalline phase and photoluminescence features are maintained in the thin films, demonstrating that these low-toxicity materials can be synthesized and processed by fully solvent-free routes for a widespread implementation in optoelectronic devices.