Non-toxic 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 bandgap blue emitters with high photoluminescence quantum yields (PLQY) 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 Cs3Cu2X5 and CsCu2X3 (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 Cs3Cu2I5 as the most promising material as it maintains blue luminescence centered at 442 nm with high PLQY (>40%) after several days in air (Cs3Cu2Cl5 shows significantly higher PLQY over 80% but is unstable in air). Based on this, we fabricated homogeneous and pinhole-free Cs3Cu2I5 thin films by thermal single-source vacuum deposition. Crystalline phase and photoluminescence are maintained in the thin films, validating that these low-toxicity materials can be synthesized and processed by fully solvent-free routes for a widespread implementation in optoelectronic devices. EXPERIMENTAL SECTIONMaterials. Cesium chloride (CsCl, > 99 %), cesium bromide (CsBr, > 99 %) and cesium iodide (CsI, > 99 %) were purchased from TCI.non-exposed and exposed powders; variation in PLE and PL wavelengths of Cs3Cu2X5 powders; XRD and PL spectra of solution processed thin-films; SEM image of thin-film by SSVD of Cs3Cu2I5.

Fully inorganic cesium lead halide perovskites, such as CsPbI2Br, show enhanced thermal stability compared to hybrid ones, and are being widely investigated as wide bandgap absorbers for tandem applications. Despite their simple stoichiometry, the preparation of highly crystalline and stable cesium lead halides thin-films is not trivial. In general, high efficiency solar cells based on solution-processed CsPbI2Br thin films are prepared with high temperature annealing or with the use of chemical additives. In this work we use solvent-free synthesis to investigate the formation of CsPbI2Br in bulk or in thin-film, via mechanochemical synthesis and multiple-source vacuum deposition, respectively. We demonstrate the importance of fostering halide alloying in the vacuum processing of inorganic lead halide perovskites, which can be attained either by using mixed halide precursors or by increasing the number of precursors (and hence deposition sources). These strategies lead to highly oriented perovskite films even at room temperature, with improved optoelectronics properties. We obtained promising power conversion efficiency of 8.3% for solar cells employing as-deposited perovskites (without any annealing) and of 10.0% for devices based on CsPbI2Br annealed at low temperature (150 °C). This study allowed us to highlight the most promising processes and strategies to further optimize the material deposition as well as the solar cells architecture. 2

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