Metastable Zone Engineering With Excess Halide for Low‐Defect MAPbBr ₃ Single Crystals with Ultra‐Low X‐Ray Detection Limit

ABSTRACT MAPbBr 3 single crystals (MPB SCs) are promising materials for X‐ray detection; however, their use in low‐dose imaging is limited by high dark current, low detectability, and noise that degrades image clarity. Under X‐ray irradiation, where electron–hole pair generation is inherently low, suppressing defects is critical to minimize losses and maximize charge collection. In this study, metastable zone engineering is employed by tuning the MABr:PbBr 2 ratio during inverse temperature crystallization (ITC) to produce defect‐suppressed MPB SCs. Excess MABr broadens and stabilizes the metastable zone, reducing nucleation events and enabling the growth of large, high‐crystallinity MPB SCs exceeding 10 mm within 11 h. These crystals exhibit electronic properties closer to intrinsic behavior, with prolonged carrier lifetimes of 371 ns and an enhanced mobility‐lifetime ( µτ ) product of 3.43 × 10 −3 cm 2 V −1 . Photoluminescence spectra reveal a distinct secondary peak from photon recycling, with intensity 1.25 times greater than the band‐edge peak, indicating efficient photon reabsorption due to reduced non‐radiative losses. Devices fabricated from excess MPB SCs achieve a record‐low detection limit of 0.47 nGy s −1 , high sensitivity (6.117 × 10 3 µC Gy air −1 cm −2 ), and high‐resolution X‐ray imaging, demonstrating strong potential for advanced medical and industrial applications.