Unveiling the Role of Cesium in Halide Perovskite Single Crystal for Stable and Ultrasensitive X‐Ray Detection

Abstract Metal halide perovskites have been demonstrated to be the promising X‐ray detection materials, among which MAPbI 3 is expected for high‐performance large‐area X‐ray detector integration due to its strong X‐ray absorption and solution processible at low temperature for industrial‐grade large‐size single crystal (SC). However, the commercial viability of MAPbI 3 SC X‐ray detectors remains challenging due to its poor intrinsic stability, large dark current, and significant ion migration. Herein, inorganic Cs + ions are designed to incorporate into the MAPbI 3 SC and the effects of Cs + on crystal structure, defect state, band structure, ion migration, and carrier transport in the SCs are systematically unveiled. The experimental results show that the incorporation of Cs + ion reduces defect density, inhibits ion migration, improves carrier mobility, and increases resistivity. Therefore, detectors fabricated on the SC with Cs + ions show high detection sensitivity of 49847 µC Gy −1 cm −2 , low detection limit of 3.1 nGy s −1 , short response raise time of 150 µs, and superior long‐term operating stability under continuous X‐ray irradiation and bias. The rare combination of these figure of merits enables the detector to achieve high‐definition X‐ray imaging, confirming that this work provides a new strategy for designing stable and sensitive X‐ray detectors.