Intrinsic defects on α, γ and δ-CsPbI3 (001) surfaces and implications for the α/γ to δ phase transition

Abstract

Compared with traditional organic–inorganic hybrid perovskites, CsPbI₃ is considered to be a better solar photovoltaic absorption material. However, under environmental conditions, it will undergo the phase transition from the α phase to the γ phase and finally to the non-perovskite phase (δ), especially in a humid environment. Considering the important role of surface intrinsic defects in the phase transition process, we investigated the intrinsic defects on α, γ and δ-CsPbI₃ (001) surfaces by first-principles calculations based on density functional theory (DFT). The formation energy of most defects on the surface is similar to that in the bulk in all three phases except for V_Pb and V_I. The formation energy of V_Pb and V_I on the α-CsPbI₃ (001) surface is significantly increased, and the formation energy of V_Pb on the γ-CsPbI₃ (001) surface is also increased, due to the relaxation and distortion of the surface Cs and the Pb–I octahedron. The formation energy of interstitial defects on the α-CsPbI₃ (001) surface is the lowest due to the remaining large dodecahedral void, even though the Pb–I octahedron distortion has largely enhanced the stability of the α-CsPbI₃ (001) surface. The formation energy of V_Cs is the lowest in all three phases, indicating that Cs ions in CsPbI₃ are indeed flexible in CsPbI₃. The results are expected to provide a theoretical basis and guidance for the stability improvement of all-inorganic halide perovskites especially in a humid environment.