Atomic-scale microstructure of metalhalide perovskite

INTRODUCTION Hybrid metal halide perovskites are highly favorable materials for efficient photovoltaic and optoelectronic applications. The mechanisms behind their impressive performance have yet to be fully understood, but they likely depend on atomic-level properties that may be unique to these perovskites. Atomic-resolution transmission electron microscopy is well suited to provide new insights but is challenging because of the highly beam-sensitive nature of hybrid perovskites. RATIONALE We used low-dose scanning transmission electron microscopy (STEM) imaging to determine the microstructure of thin hybrid perovskite films. Thermally evaporated thin films of formamidinium and methylammonium lead triiodide (FAPbI3 and MAPbI3, respectively) were examined on ultrathin carbon-coated copper TEM grids to reveal the nature of boundaries, defects, and decomposition pathways. RESULTS Using low-dose low-angle annular dark field (LAADF) STEM imaging, we obtained atomic-resolution micrographs of FAPbI3 films in the cubic phase. We found that prolonged electron irradiation leads to a loss of FA+ ions, which initially causes the perovskite structure to change to a partially FA+-depleted but ordered perovskite lattice, apparent as light-and-dark checkered patterns in STEM images. Further electron beam exposure leads to the expected deterioration to PbI2 as the final decomposition product. We propose that the observed intermediate checkered pattern is triggered by an initially random, beam-induced loss of FA+, followed by subsequent reordering of FA+ ions. The discovery of this intermediate structure explains why the perovskite structure can sustain significant deviations from stoichiometry and recovers remarkably well from damage. We further revealed the atomic arrangement at interfaces within the hybrid perovskite films. We found that PbI2 precursor remnants commonly encountered in hybrid perovskite films readily and seamlessly intergrow with the FAPbI3 and MAPbI3 lattice and can distort from their bulk hexagonal structure to form a surprisingly coherent transition boundary, exhibiting low lattice misfit and strain. We observed PbI2 domains that nearly perfectly follow the surrounding perovskite structure and orientation, which suggests that PbI2 may seed perovskite growth. These observations help to explain why the presence of excess PbI2 tends not to impede solar cell performance. Images of FAPbI3 grain boundaries further revealed that the long-range perovskite structure is preserved up to the grain boundaries, where sharp interfaces are generally present, without any obvious preferred orientation. Near-120° triple boundaries are most commonly observed at the intersection of three grains, which we generally found to be crystallographically continuous and associated with minimal lattice distortion. Finally, we identified the nature of defects, dislocations, and stacking faults in the FAPbI3 lattice. We discovered dislocations that are dissociated in a direction perpendicular to their glide plane (climb-dissociated), aligned point defects in the form of vacancies on the Pb-I sublattice, and stacking faults corresponding to a shift of half a unit cell, connecting Pb-I columns with I– columns rather than with FA+ columns. CONCLUSION Our findings provide an atomic-level understanding of the technologically important class of hybrid lead halide perovskites, revealing several mechanisms that underpin their remarkable performance. The highly adaptive nature of the perovskite structure upon organic cation loss yields exceptional regenerative properties of partly degraded material. The observation of coherent perovskite interfaces with PbI2 explains the barely diminished optoelectronic performance upon such precursor inclusions, while sharp interfaces between perovskite grains grant a benign nature. Such atomically localized information enables the targeted design of methods to eliminate defects and optimize interfaces in these materials.