Molecular-driven lead-free halide photoferroelectric solid solution for high-temperature sensitive self-powered x-ray detection.

Halide double perovskite ferroelectrics have recently emerged as an environmentally friendly candidate in radiation detection, photovoltaics, and optoelectronic memory devices due to their unique spontaneous polarization and semiconductor properties. However, it is a huge challenge to achieve efficient carrier transport and high Curie temperature (Tc) in double perovskite ferroelectrics, owing to the limitations of twisted frameworks and structural construction. Here, we present a high-Tc heterovalent metal solid solution double perovskite ferroelectric, (4Br2FBZ)2CsAgBiBr7 (4Br2FBZ-CAB, 4Br2FBZ = 4-bromo-2-fluorobenzylammonium), through a strategy of cation-engineered symmetry regulation. The metal ions Ag and Bi transitioned from an alternating arrangement to a mixed-site solid solution pattern, effectively optimizing the electronic band structure to enhance carrier transport. Meanwhile, 4Br2FBZ-CAB exhibits a high Tc of 484 kelvin, substantially expanding the family of high-Tc lead-free ferroelectrics. Benefiting from its remarkable photoelectric and ferroelectric properties, a high self-powered x-ray detection sensitivity of 273.8 microcoulomb per gray per square centimeter (μC Gyair-1 cm-2 ) was achieved at room temperature. In addition, this exceptionally high Tc enables the detector to operate self-powered at high temperatures, resulting in an excellent sensitivity of 896.9 μC Gyair-1 cm-2 and a low detection limit of 13.9 nGyair s-1 at 425 kelvin. This work opens an avenue for expanding the family of high-Tc lead-free perovskite ferroelectrics with superior semiconducting properties.