Suppressed phase transition of Rb/K incorporated inorganic perovskite with a water-repelling surface
Inorganic cesium lead halide (CsPbI3) is a promising candidate for next-generation photovoltaic devices but photoactive α-phase CsPbI3 can rapidly transform to non-photoactive yellow δ-CsPbI3 in humid atmosphere. Here, we report that partial substitution of cesium by potassium or rubidium element can effectively improve phase stability against moisture by forming a water-repelling surface layer with Rb/K segregation. Using density functional theory, we found that the water-induced polarization, which triggers the PbI62- octahedra distortion and accelerate the phase transition, can be effectively alleviated after incorporating Rb/K elements. Further exploration of transition states suggests that Rb/K doped surface layers result in a higher activation barrier for water penetration. The electronic structure analysis further reveals that the barrier enhancement originates from the absence of the participation of inner 5p electrons in Rb/K-H2O binding, which induces much low energy barrier in pristine CsPbI3. Based on these improvements, doped perovskites remained major α-phase after direct exposition to ambient airs (RH~30%) for 5 hours while pristine CsPbI3 showed an irreversible degradation. With clarified mechanism of enhanced phase stability of Rb/K incorporation, we suggested such doping method as a promising strategy to be widely applied in the field of photovoltaic devices.