Dimensional and Optoelectronic Tuning of Lead‐free Perovskite Cs3Bi2I9−nBrn Single Crystals for Enhanced Hard X‐ray Detection

Abstract Inorganic Bi‐based perovskites have shown great potential in X‐ray detection for their large absorption to X‐rays, diverse low‐dimensional structures, and eco‐friendliness without toxic metals. However, they suffer from poor carrier transport properties compared to Pb‐based perovskites. Here, we propose a mixed‐halogen strategy to tune the structural dimensions and optoelectronic properties of Cs 3 Bi 2 I 9−n Br n (0≤n≤9). Ten centimeter‐sized single crystals are successfully grown by the Bridgman technique. Upon doping bromine to zero‐dimensional Cs 3 Bi 2 I 9 , the crystal transforms into a two‐dimensional structure as the bromine content reaches Cs 3 Bi 2 I 8 Br. Correspondingly, the optoelectronic properties are adjusted. Among these crystals, Cs 3 Bi 2 I 8 Br exhibits negligible ion migration, moderate resistivity, and the best carrier transport capability. The sensitivities in 100 keV hard X‐ray detection are 1.33×10 4 and 1.74×10 4 μC Gy air −1 cm −2 at room temperature and 75 °C, respectively, which are the highest among all reported bismuth perovskites. Moreover, the lowest detection limit of 28.6 nGy air s −1 and ultralow dark current drift of 9.12×10 −9 nA cm −1 s −1 V −1 are obtained owing to the high ionic activation energy. Our work demonstrates that Br incorporation is an effective strategy to enhance the X‐ray detection performance by tuning the dimensional and optoelectronic properties.