| Summary: | CsPbI<sub>3</sub> possesses three photoactive black phases (α, β, and γ) with perovskite structures and a non-photoactive yellow phase (δ) without a perovskite structure. Among these, α-CsPbI<sub>3</sub> exhibits the best performance. However, it only exists at high temperatures and it tends to transform into the δ phase at room temperature, especially in humid environments. Therefore, the phase stability of CsPbI<sub>3</sub>, especially in humid environments, is the main obstacle to its further development. In this study, we studied the interaction of H<sub>2</sub>O with α-CsPbI<sub>3</sub> and the intrinsic defects within it. It was found that the adsorption energy in the bulk is higher than that on the surface (−1.26 eV in the bulk in comparison with −0.60 eV on the surface); thus, H<sub>2</sub>O is expected to have a tendency to diffuse into the bulk once it adsorbs on the surface. Moreover, the intrinsic vacancy of V<sub>Pb</sub><sup>0</sup> in the bulk phase can greatly promote H<sub>2</sub>O insertion due to the rearrangement of two I atoms in the two PbI<sub>6</sub> octahedrons nearest to V<sub>Pb</sub><sup>0</sup> and the resultant breaking of the Pb–I bond, which could promote the phase transition of α-CsPbI<sub>3</sub> in a humid environment. Moreover, H<sub>2</sub>O adsorption onto V<sub>I</sub><sup>+1</sup> contributes to a further distortion in the vicinity of V<sub>I</sub><sup>+1</sup>, which is expected to enhance the effect of V<sub>I</sub><sup>+1</sup> on the phase transition of α-CsPbI<sub>3</sub>. Clarifying the interaction of H<sub>2</sub>O with α-CsPbI<sub>3</sub> and the intrinsic defects within it may provide guidance for further improvements in the stability of α-CsPbI<sub>3</sub>, especially in humid environments.
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