ATOMIC AND ELECTRONIC STRUCTURES OF LEAD-FREE HALIDE PEROVSKITES PROBED WITH X-RAY ABSORPTION SPECTROSCOPY

Lead-based halide perovskites, and in particular hybrid organic–inorganic systems such as MAPI, have attracted significant attention in recent decades due to their promising optoelectronic properties. However, the poor air stability of hybrid perovskites and the high toxicity of lead hinder their large-scale implementation. In this work, I explore different ways to overcome these limitations, beginning with the replacement of the organic cation to enhance the stability of lead-based perovskites, and subsequently addressing the substitution of the toxic metal cation through either an isovalent approach, with Sn(II), or a heterovalent approach, with monovalent and trivalent cations to obtain double perovskites. After laboratory characterization, all the bulk and nanosized materials were studied with synchrotron radiation methods, especially X-ray absorption spectroscopy (XAS). XAS is an ideal complement to diffraction methods for probing the local structure of materials: nevertheless, despite its potential and the ever-growing body of literature on halide perovskites, the application of XAS to these compounds has so far remained relatively limited and with varying degrees of success. Both the near-edge and the extended regions of the XAS spectra were modeled quantitatively, providing information on oxidation state and local symmetry bond length and disorder. These spectra were complemented by HERFD also in the tender X-ray range, which enhances the energy resolution and reveals subtle features not accessible through conventional XAS. The experimental data were eventually modeled with ab initio simulations, allowing for the refinement of the local structural model and leading to accurate assessment of distortions and structural variations induced by cation substitution.