Study of Heterogeneity across Cs_x(CH₃NH₃)_1-xPbBr₃ Halide Perovskite Crystals with XPS Imaging and Small-Area Spectra

Interest in halide perovskites (HaPs) is motivated by the combination of superior optoelectronic properties, ease of synthesis, and a surprisingly low density of electrically active defects. HaPs possess high chemical sensitivity, especially those having an organic cation at their A position (AMX₃). X-ray photoelectron spectroscopy (XPS) is a surface technique with sensitivity that goes down to a single atomic layer and provides unique information that relates the elemental composition with the chemical and electronic states of the elements in the material. Our study focuses on XPS imaging in combination with selected small-area spectra and uses aged (3 years old) solution-grown single crystals of mixed A-cation Cs_xMA_1-xPbBr₃ (MA = CH₃NH₃⁺) HaPs as a candidate for investigating intracrystal heterogeneity. With XPS, we followed the variations in chemical composition by measuring crystals at different regions down to 50 μm diameter of the samples. By comparing the surface of the crystals with their cross-section, we found significant changes in the Cs⁺ and Br^- concentrations, which increase toward the interior of the crystal. Contrarily, concentrations of carbon and nitrogen predominate on the top surface and especially at crystal edges, which form a partial covering of the crystals beyond the visible crystal boundaries, something that is not seen by electron microscopy analysis and shows the advantage of the XPS for measurements of light elements. Besides demonstrating the utility of the XPS technique, this compositional heterogeneity within the Cs_xMA_1-xPbBr₃ crystals reveals novel insights into the complex chemical nature of what may be seen as uniform single crystals and brings crucial information for their understanding.