First-Principles Investigation into the Interaction of H₂O with α-CsPbI₃ and the Intrinsic Defects within It

CsPbI₃ 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-CsPbI₃ 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 CsPbI₃, especially in humid environments, is the main obstacle to its further development. In this study, we studied the interaction of H₂O with α-CsPbI₃ 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, H₂O is expected to have a tendency to diffuse into the bulk once it adsorbs on the surface. Moreover, the intrinsic vacancy of V_Pb⁰ in the bulk phase can greatly promote H₂O insertion due to the rearrangement of two I atoms in the two PbI₆ octahedrons nearest to V_Pb⁰ and the resultant breaking of the Pb-I bond, which could promote the phase transition of α-CsPbI₃ in a humid environment. Moreover, H₂O adsorption onto V_I⁺¹ contributes to a further distortion in the vicinity of V_I⁺¹, which is expected to enhance the effect of V_I⁺¹ on the phase transition of α-CsPbI₃. Clarifying the interaction of H₂O with α-CsPbI₃ and the intrinsic defects within it may provide guidance for further improvements in the stability of α-CsPbI₃, especially in humid environments.