Two-dimensional lead-free perovskite Cs3Bi2I8.3Br0.7 single crystals with anisotropic ion migration and hard X-ray responses

Abstract

All-inorganic Bi-based perovskite single crystals have been regarded as one of the semiconductors for X-ray detection due to their high absorption coefficient and excellent carrier transport properties. Herein, the low-dimensional centimeter-sized Cs₃Bi₂I_8.3Br_0.7, Cs₃Bi₂I_8.5Br_0.5, and Cs₃Bi₂I₉ single crystals were successfully grown using the vertical Bridgeman method. With the ratio of Br/I increased to 0.7/8.3, the crystal structures of these homologues transform from zero-dimensional (0D) Cs₃Bi₂I₉ (space group: P6₃/mmc) to two-dimensional (2D) Cs₃Bi₂I_8.3Br_0.7 (space group: Pm1). The anisotropic properties of Cs₃Bi₂I_8.3Br_0.7 single crystals with a layered structure were systematically investigated. Despite a high sensitivity (7579 μC Gy_air⁻¹ cm⁻²) and a low detection limit (101.85 nGy_air s⁻¹) under a low electric field of 100 V mm⁻¹, the (20) plane device shows severe dark current baseline drift under a high electric field of 400 V mm⁻¹ due to the low ion activation energy of 241.24 meV. In compasion, the (001) plane device exhibits a stable and non-drifting dark current baseline and a high sensitivity of 1.46 × 10⁴ μC Gy_air⁻¹ cm⁻² at 25 °C under 400 V mm⁻¹ due to the high ion activation energy of 580.62 meV. As the temperature increases to 75 °C, the sensitivity of the (001) plane was further increased to 1.89 × 10⁴ μC Gy_air⁻¹ cm⁻². The anisotropic ion migration and X-ray detection performances are attributed to the difference of carrier transportation and ion activation energy along intra-layer and inter-layer directions. Our results show that anisotropic engineering is an efficient way for enhancing the X-ray detection performances of 2D Cs₃Bi₂I_8.3Br_0.7 single crystals.