An environmentally friendly synthesis route: Low-temperature preparation of vacancy-ordered double perovskites Cs2SnX6 (X = Cl, Br, I) via ionic liquid

Lead-free vacancy-ordered double perovskites have emerged as promising materials for optoelectronic applications due to their environmentally friendly characteristics and exceptional properties. However, conventional synthesis methods often depend on toxic reagents and stringent conditions, limiting their large-scale synthesis and practical application. In this work, an environmentally friendly synthesis route was proposed for preparing vacancy-ordered double perovskites Cs2SnX6(X = Cl, Br, and I) with high crystallinity under low-temperature and ambient-pressure conditions. This method utilizes ion liquid (i.e., 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), 1-butyl-3-methylimidazolium bromide ([Bmim]Br) and 1-butyl-3-methylimidazolium iodide ([Bmim]I)) in combination with saturated aqueous solutions of ammonium halides as solvents, replacing traditional hydrogen halide acid or polar organic solvents. Experimental and characterization results demonstrate that the Cs2SnX6(X = Cl, Br, and I) possess high crystallinity, well-defined morphology, and improved thermal stability. These improvements are attributed to the hydrogen bonding interactions between ionic liquids and the perovskite precursors. Additionally, the halogen-rich environment provided by ionic liquids and ammonium halide salts facilitates defect passivation. Furthermore, this method is applicable to the synthesis of doped perovskite crystals, demonstrated by the successful synthesis of Bi-doped Cs2SnCl6crystals with a photoluminescence quantum efficiency of 12.73%. This study presents a novel strategy for synthesizing high-quality vacancy-ordered double perovskites and their doping or alloyed compounds.