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

CsPbI3 possesses three photoactive black phases (alpha, beta, and gamma) with perovskite structures and a non-photoactive yellow phase (delta) without a perovskite structure. Among these, alpha-CsPbI3 exhibits the best performance. However, it only exists at high temperatures and it tends to transform into the delta 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 alpha-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-Pb(0) 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-Pb(0) and the resultant breaking of the Pb-I bond, which could promote the phase transition of alpha-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 alpha-CsPbI3. Clarifying the interaction of H2O with alpha-CsPbI3 and the intrinsic defects within it may provide guidance for further improvements in the stability of alpha-CsPbI3, especially in humid environments.