Resolving Nanoscale Heterogeneities in Lead Halide Perovskites Through Low-Dose Concurrent 4D-STEM-EDX Mapping

Mixed-cation lead mixed-halide perovskites are promising materials for applications in photovoltaics; however, it has been suggested that they exhibit instabilities linked to nanoscale heterogeneity. Directly probing the origins of this heterogeneity requires characterization with nanoscale spatial resolution, making transmission electron microscopy (TEM) an essential tool. However, characterizing these materials is challenging due to their extreme sensitivity to electron irradiation. Here, we develop a low-dose, concurrent methodology using four-dimensional scanning transmission electron microscopy (4D-STEM) and energy-dispersive x-ray spectroscopy (EDX) in order to map both the chemical and structural architecture of a (FA₀.₈₃Cs₀.₁₇)Pb(I₀.₈Br₀.₂)₃ perovskite film without inducing damage. Our correlative analysis reveals a complex mosaic of coexisting crystal structures in this state-of-the-art LHP film. We establish a direct link between local chemical composition and crystal structure, showing that the formation of undesirable, photovoltaically inactive hexagonal polytypes is predominantly driven by local deficiencies in the stabilizing cesium cation. These findings provide crucial insight into one of the fundamental origins of structural instabilities in mixed composition perovskite thin-films, suggesting that achieving long-term device performance requires the development of fabrication routes that ensure compositional homogeneity at the nanoscale.