Decisive influence of amorphous PbI on the photodegradation of halide perovskites

Capturing the photolysis process of archetypal perovskite materials (such as methylammonium lead triiodide, MAPbI 3 ) into metallic Pb 0 is fundamentally important to understand the photodegradation pathway of organic-inorganic hybrid perovskites (OIHPs). In this study, the photodecomposition pathway of a MAPbI 3 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 (Pb 0 ) nanoparticles. Then, the degradation sites extend into the inner MAPbI 3 grains. By calculating the growth kinetics of the Pb 0 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 Pb 0 enriched nanoparticles are with Pb@PbI 2− x ( x 0.4) core-shell structures, while the interface between crystalline MAPbI 3 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 MAPbI 3 into amorphous PbI 2− x and then into Pb@PbI 2− x aggregates has never been elucidated before. Amorphous PbI 2− x is the critical intermediate phase during the step-by-step decomposition from MAPbI 3 to metallic Pb 0 , and it dominates the interfacial-controlled photodegradation pathway of the perovskite film.