Determination of the True Lateral Grain Size in Organic-Inorganic Halide Perovskite Thin Films

In this letter, methylammonium lead iodide (MAPbI₃) thin films were examined via piezoresponse force microscopy (PFM) and nanoindentation (NI) to determine if long-range atomic order existed across the full width and depth of the apparent grains. From the PFM, the piezoelectric response of the films was strongly correlated with low-index planes of the crystal structure and ferroelastic domains in macroscale solution-grown MAPbI₃ crystals, which implied long-range order near the top surface. From the NI, it was found that the induced cracks were straight and extended across the full width of the apparent grains, which indicated that the long-range order was not limited to the near-surface region, but extended through the film thickness. Interestingly, the two MAPbI₃ processes examined resulted in subtle differences in the extracted electro-mechanical and fracture properties, but exhibited similar power conversion efficiencies of >17% in completed devices.