Preparation of (CH₃NH₃)₃Bi₂I₉ Thick Film via Mist Deposition Method for X-Ray Detection

X-ray imaging has been widely applied in medical diagnosis and non-destructive testing of industrial products. In sensitive X-ray detectors, lead halide perovskites are emerging as absorption layer materials. However, the toxicity of lead limits its applications. Furthermore, European Union laws restrict the use of lead in electrical and electronic equipment through the Restriction of Hazardous Substances Directive, thereby limiting the commercial application of lead halide perovskites. Recently, methylammonium bismuth iodide (MA₃Bi₂I₉) was demonstrated as a candidate for a lead-free X-ray detector with high sensitivity because of its high attenuation coefficients and large mobility-lifetime product. Zheng et al. [1] illustrated highly sensitive X-ray detection using solution-processed MA₃Bi₂I₉ single crystals with a sensitivity of 10,620 μC Gy_air⁻¹ cm⁻², which was 500 times higher than the conventional direct-conversion detectors made with amorphous Se.<br/>A direct fabrication of thick MA₃Bi₂I₉ films (approximately 100 μm) on the thin-film transistor (TFT) arrays is required to apply MA₃Bi₂I₉ films for imaging devices. However, a fabrication method that suffices these requirements is yet to be established. Herein, we report the fabrication of MA₃Bi₂I₉ films using the mist deposition method, which can deposit thick films directly on the TFT arrays. In this method, a precursor solution is atomized through ultrasonic vibration and transported to the film deposition section by gas flow. These mist particles were sprayed onto a heated substrate, and the solvent was evaporated to obtain a MA₃Bi₂I₉film. We have also reported the fabrication of CsPbBr₃[2] and Cs₂AgBiBr₆[3] thick films (approximately 100 μm) in our previous studies using the mist deposition method.<br/>In this conference, we report the fabrication of MA₃Bi₂I₉ thick films with a thickness of 110 μm. The precursor solution was prepared by dissolving MAI and BiI₃ (in a molar ratio of 3:2) in dimethylformamide. The effect of the precursor solution concentration on the crystal orientation and morphology of the films was investigated. In addition, we report the application of these thick films in X-ray detection. The MA₃Bi₂I₉ based X-ray detectors exhibited a sensitivity of 27.6 μC Gy_air^-1 cm⁻² to X-ray photons of 70 kV_p an electric field of 455 V mm⁻¹.<br/>References<br/>[1] X. Zheng et al., J. Energy Chem., vol. 49, pp. 299–306, (2020) [2] Y. Haruta et al., Appl. Phys. Express, vol. 12, no. 8, p. 085505, (2019) [3] Y. Haruta et al., Cryst. Growth Des., vol. 21, pp. 4030–4037, (2021)