Shedding Light on the Birth of Hybrid Perovskites: A Correlative Study by In Situ Electron Microscopy and Synchrotron-Based X-ray Scattering

Lead halide perovskites (LHPs) have emerged as promising candidates for a broad range of optoelectronic devices because of their unique physical properties. Methylammonium lead iodide (MAPI) perovskite has been the most commonly studied LHP due to its very promising optoelectronic properties. One of the main explored pathways for obtaining MAPI perovskites is the synthesis by ligand-assisted reprecipitation (LARP); however, this method is not totally understood from a phenomenological point of view. In this study, we took advantage of the development of a series of advanced in situ techniques to bring new insights into the pathway of this process that leads to MAPI perovskites from a precursor solution. First, we monitored the nucleation and growth processes of a solvated intermediate phase obtained via LARP by correlating local information obtained by a direct visualization of the reaction medium using liquid-phase transmission electron microscopy (TEM) and more global information brought by synchrotron-based X-ray scattering measurements. This combined analysis, in real time and under representative conditions of the synthesis method, allowed us to decipher the structural evolution of the emerging phases, from amorphous and roughly spherical objects in the early stages of the process toward elongated ribbon-like morphology particles after a few seconds, through a crystallization process. Second, we followed in situ the transition of the as-obtained solvated intermediate phase toward the final phase of MAPI perovskite during a thermal treatment at 80 °C, using in this case a combination of gas-phase TEM, in image and diffraction modes, and temperature-resolved X-ray diffraction. We provided direct and unprecedented evidence of the fragmentation of the crystals, simultaneously to the occurrence of the structural transformation between the intermediate and final phases. Our approach, which involved also challenging development of new correlative methods, demonstrates the high interest of such an in situ correlative study for a better understanding of the synthesis and properties of new emerging materials including LHPs.