Decisive influence of amorphous PbI2−x on the photodegradation of halide perovskites

Abstract

Capturing the photolysis process of archetypal perovskite materials (such as methylammonium lead triiodide, MAPbI₃) into metallic Pb⁰ is fundamentally important to understand the photodegradation pathway of organic–inorganic hybrid perovskites (OIHPs). In this study, the photodecomposition pathway of a MAPbI₃ film in an atmospheric argon (Ar) gas environment was quasi-in situ studied comprehensively. Under light illumination, the decomposition of the perovskite film is first triggered at the grain boundaries, along with an obvious generation of metallic lead (Pb⁰) nanoparticles. Then, the degradation sites extend into the inner MAPbI₃ grains. By calculating the growth kinetics of the Pb⁰ enriched nanoparticles, we can find that it strictly obeys an interfacial diffusion-controlled growth model. Atomic resolution high angle annular dark-field (HAADF) characterization confirms that the Pb⁰ enriched nanoparticles are with Pb@PbI_2−x (x ≈ 0.4) core–shell structures, while the interface between crystalline MAPbI₃ and amorphous PbI_2−x controls the photodegradation pathway of perovskite films. To the best of our knowledge, this is the first time to directly observe the photolysis process of perovskite films in a practical environment (Ar gas and light illumination), particularly with nano or even atomic-scale resolution. Furthermore, the discovery of the interfacial-controlled photodegradation pathway of MAPbI₃ into amorphous PbI_2−x and then into Pb@PbI_2−x aggregates has never been elucidated before.