Mixed A-site engineering is an emerging strategy to overcome the difficulties in realizing high-quality perovskite films together with high ambient stability. Particularly, the α-FACsPbI3-based hybrid perovskites have been considered as a promising candidate for solar cell applications. However, the degradation mechanism of α-FACsPbI3 hybrid perovskites induced by light illumination remains unclear. Here, the illumination-caused instability of α-FACsPbI3 hybrid perovskites is investigated using various surface detection technologies, including photoelectron spectroscopy, scanning electron microscopy, and grazing incidence X-ray diffraction. The experimental findings reveal that the A-site vacancies arise from the migration of Cs+ cations from the perovskite surface into the bulk under light illumination, while their content is dependent on the light energy. The visible light enlarges the crystal lattice on the perovskite surface, leading to the Cs+ cation migration along with the lattice distortion of the PbI64- octahedron and phase separation. However, the ultraviolet light further causes a stronger interaction between FA+ and [PbI6]4-, leading to the partial decomposition of [PbI6]4- into Pb0 and I-. These results enrich the photodegradation mechanism, guiding the design of efficient and stable perovskite solar cells through surface passivation to suppress the Cs+ cation migration and to increase the octahedron dissociation energy.
Keywords: in situ PES; light illumination; perovskite; stability; vacancy.