Impact of SnF Addition on the Chemical and Electronic Surface Structure of CsSnBr.

We report on the chemical and electronic structure of cesium tin bromide (CsSnBr) and how it is impacted by the addition of 20 mol % tin fluoride (SnF) to the precursor solution, using both surface-sensitive lab-based soft X-ray photoelectron spectroscopy (XPS) and near-surface bulk-sensitive synchrotron-based hard XPS (HAXPES). To determine the reproducibility and reliability of conclusions, several (nominally identically prepared) sample sets were investigated. The effects of deposition reproducibility, handling, and transport are found to cause significant changes in the measured properties of the films. Variations in the HAXPES-derived compositions between individual sample sets were observed, but in general, they confirm that the addition of 20 mol % SnF improves coverage of the titanium dioxide substrate by CsSnBr and decreases the oxidation of Sn to Sn while also suppressing formation of secondary Br and Cs species. Furthermore, the (surface) composition is found to be Cs-deficient and Sn-rich compared to the nominal stoichiometry. The valence band (VB) shows a SnF-induced redistribution of Sn 5s-derived density of states, reflecting the changing Sn/Sn ratio. Notwithstanding some variability in the data, we conclude that SnF addition decreases the energy difference between the VB maximum of CsSnBr and the Fermi level, which we explain by defect chemistry considerations.