First-Principles Investigation into the Interaction of HO with α-CsPbI and the Intrinsic Defects within It.

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