Lead‐Free Cesium Thulium Halide Perovskite Microcrystals for Near Ultraviolet Luminescence

Abstract Lead halide perovskites attract tremendous research attention due to excellent optoelectronic properties. However, realizing efficient near ultraviolet (NUV) luminescence with these materials is still a big challenge. Herein, a novel rare‐earth perovskite cesium thulium chloride (CsTmCl₃) with high crystallinity has been synthesized via a simple hot‐injection method. The obtained CsTmCl₃ microcrystals have a size distribution of around 1–5 µm, and demonstrate a highly efficient NUV emission at 337 nm with a full width at half maximum (FWHM) of 68 nm. The determined band gap of CsTmCl₃ microcrystals is ≈3.92 eV, which is supported by theoretical calculations. Moreover, a high photoluminescence quantum yield (PLQY) of up to 12% in NUV region has been achieved in such a lead‐free perovskite. The findings suggest that CsTmCl₃ perovskite microcrystal is a promising low‐toxic material for applications in NUV optoelectronic devices.