Progress in in situ TEM investigations of halide perovskites

Halide perovskites (HPs) have found wide-ranging applications in photovoltaic and optoelectronic devices, achieving remarkable success due to their unique crystal structure and properties. Given the sensitivity of perovskite materials to external stimuli, it is crucial to understand the intrinsic changes in structure and chemical composition during operational conditions. This understanding could assist researchers in exploring new strategies to enhance the photoelectrical properties and stability of these materials. While many in situ methods, such as in situ X-ray diffraction and in situ photoluminescence, have been employed to investigate the properties of perovskite materials in real-time, in situ transmission electron microscopy (TEM) stands out as an unparalleled technique for observing subtle changes at the micro and even atomic scale. In this review, we summarize recent advancements in studying HPs using in situ TEM. We first introduce studies on the crystallization process of HP crystals through in situ TEM observation, and then categorize research works on the degradation process of HPs driven by different external stimuli, including electron beam, heat, electrical bias, light, and ambient atmosphere. Finally, we highlight several challenges that still need to be addressed in the future. This review aims to present a thorough summary of the existing research and lay the groundwork for future inquiries in this captivating area.