Reveal the Humidity Effect on the Phase Pure CsPbBr(3) Single Crystals Formation at Room Temperature and Its Application for Ultrahigh Sensitive X‐Ray Detector

Generally, growing phase pure CsPbBr(3) single crystals is challenging, and CsPb(2)Br(5) or Cs(4)PbBr(6) by‐products are usually formed due to the different solubilities of CsBr and PbBr(2) in the single solvent. Herein, the growth of high‐quality phase pure CsPbBr(3) perovskite single crystals at room temperature by a humidity controlled solvent evaporation method is reported first. Meanwhile, the room temperature phase transition process from three dimensional (3D) cubic CsPbBr(3) to two dimensional (2D) layered tetragonal CsPb(2)Br(5) and the detailed mechanism induced by humidity are revealed. Moreover, compared with the organic–inorganic perovskite, the prepared CsPbBr(3) single crystals are much more stable under high humidity, which satisfies the long‐term working conditions of X‐ray detectors. The X‐ray detectors based on CsPbBr(3) single crystals show a high sensitivity and a low detection limit of 1.89 μGy(air) s(–1), all of which meet the needs of medical diagnosis.