Enhancing the X‑ray\nSensitivity of Cs<sub>2</sub>AgBiBr<sub>6</sub> Double Perovskite\nSingle Crystals through Cation\nEngineering

Owing to their outstanding\noptoelectronic properties, halide perovskite\n(HP) materials have been employed in a wide range of applications,\nincluding solar cells, light-emitting devices, and X-ray detectors.\nAmong them, lead-free double HPs are characterized by enhanced stability\nand reduced toxicity compared with lead-based alternatives. Cs<sub>2</sub>AgBiBr<sub>6</sub>, in particular, has emerged as a promising\ncandidate for direct X-ray detection. The detection sensitivity, on\nthe other hand, cannot yet compete with that of lead-containing perovskites.\nDeveloping schemes to improve X-ray detection efficiency is critical\nfor reducing radiation exposure in medical imaging applications. Here,\nwe investigate the potential of controlled doping and cation substitution\nwith either lanthanides or small organic cations to improve the X-ray\ndetection performance of Cs<sub>2</sub>AgBiBr<sub>6</sub>. Our findings\nreveal that by growing the perovskite in a slightly Bi-poor and Eu-rich\nenvironment, the X-ray sensitivity significantly increases 7-fold\n(from 17 to 120 μC Gy<sub>air</sub><sup>–1</sup> cm<sup>–2</sup>) and simultaneously improves the phototo-dark current\nratio (from 2.5 to 29). Additionally, Cs-site substitution with imidazolium\nremarkably enhances the sensitivity over 10-fold (180 μC Gy<sub>air</sub><sup>–1</sup> cm<sup>–2</sup>), and ammonium\nenhances the phototo-dark current ratio to 37. Terahertz photoconductivity\nmeasurements reveal a positive correlation between enhanced X-ray\nsensitivity and improved charge transport properties (e.g., increased\nscattering time and, thus, carrier mobility) by doping. This study\noutlines straightforward strategies for boosting X-ray detection and\nfundamental photoconductivity in lead-free double HP, with potential\nimplications for broader optoelectronic applications.