Impact of SnF₂ Addition on the Chemical and Electronic Surface Structure of CsSnBr₃

Abstract

We report on the chemical and electronic structure of cesium tin bromide (CsSnBr3) and how it is impacted by the addition of 20 mol % tin fluoride (SnF2) 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 % SnF2 improves coverage of the titanium dioxide substrate by CsSnBr3 and decreases the oxidation of SnII to SnIV 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 SnF2-induced redistribution of Sn 5s-derived density of states, reflecting the changing SnII/SnIV ratio. Notwithstanding some variability in the data, we conclude that SnF2 addition decreases the energy difference between the VB maximum of CsSnBr3 and the Fermi level, which we explain by defect chemistry considerations.

Unveiling Roles of Tin Fluoride Additives in High‐Efficiency Low‐Bandgap Mixed Tin‐Lead Perovskite Solar Cells

“…Tin fluoride (SnF 2 ), as the most commonly used additive, has been first used in Pb‐free Sn‐based PSCs, [ 30,38–44 ] in which SnF 2 can effectively inhibit the generation of Sn vacancies, and thus significantly reduce the hole density [ 30,39,45,46 ] by means of SnF 2 itself, [ 30,38,39,42,43 ] SnF 2 ·3FACl (FA = formamidinium), [ 47 ] (SnF 2 (DMSO)) 2 (DMSO = dimethyl sulfoxide), [ 48 ] etc. Inheriting the great success of SnF 2 in Sn‐based PSCs, most of existing low‐ E g mixed SnPb PSCs have employed SnF 2 as one indispensable additive and exploited the functions of the additional additives, but no detailed studies have been reported to comprehensively investigate the roles of such extremely important SnF 2 in both mixed SnPb perovskite absorbers and devices, especially in efficient low‐ E g mixed SnPb PSCs.…”

Impact of Tin Fluoride Additive on the Properties of Mixed Tin‐Lead Iodide Perovskite Semiconductors

“…[17,18,47,48] Despite SnF 2 being so widely used, its effects on mixed tinlead perovskites have not previously received comprehensive investigation. Studies on neat-tin perovskites with no alloying of metal cations have explored the influence of SnF 2 on optoelectronic and structural properties of these materials, [49,50] and compared the effects of 0, 5, and 10 mol% addition of SnF 2 . [40] However, these investigations did not consider mixed tin-lead compositions.…”

Recent progress toward highly efficient tin‐based perovskite (ASnX3) solar cells

“…used hard X‐ray photoelectron spectroscopy (HAXPES) to indicate that 20 mol% SnF 2 addition increases CsSnBr 3 coverage on the compact TiO 2 (c‐TiO 2 ) substrate confirmed by the vanishing of Ti 3s, 3p, and O 2s shallow core levels of the CsSnBr 3 /c‐TiO 2 samples (Figure 12E). [ 100 ] SnF 2 plays a role in the crystal nucleation process due to the least solubility in common solvent among SnX 2 (X = F, Cl, Br, I) series. [ 101 ] It promotes Sn‐based perovskite nucleation and enables homogeneous crystal growth with full coverage.…”