Surface structure and electronics of all-inorganic halide perovskites CsPbI₃:A first-principles study of surface reconstructions

The all-inorganic halide perovskite CsPbI₃ is emerging as a promising perovskite solar cell (PSC) material with a power conversion efficiency of 18%. To further improve the efficiency and stability of CsPbI₃-based PSCs, better knowledge of the materials surface and interface properties is required. In this work, we use density-functional theory and atomistic thermodynamics to calculate the surface phase diagrams of the (001) surfaces of CsPbI₃ in the Pm-3m (α), P4/mbm and Pnma (γ) phases. We consider different add-atom and missing-atom reconstructions and investigate different growth regimes. For stable surface reconstructions in the surface phase diagram, we then calculate the electronic structure. Our results indicate that surfaces with CsI terminations, including the clean surface, are more stable. Surfaces with missing Pb atoms exhibit a unipolar self-doping behaviour with Cs and I atoms inducing transition energy levels in the valence and conduction bands, respectively. Even though the removal and addition of CsI and PbI₂ complexes leads to considerable surface reconstructions, they remain electronically neutral and do not donate charge to the valence or conduction band.