Solvent-Dependent Role of SnF2 in DMSO-Free Tin Halide Perovskite Solar Cells

Abstract

SnF2 is widely used as an additive to suppress Sn2+ oxidation in tin perovskite solar cells (Sn-PVK-PVs). However, its effect differed between the two processing routes examined here. In this study, we evaluated the influence of SnF2 addition on dimethyl sulfoxide (DMSO)-free N,N-dimethylformamide (DMF):2-(methylamino)pyridine (2MAP)-based FASnI3 films in terms of optical properties, Sn oxidation state, crystal growth, charge transport, and device performance, and compared the results with those for conventional DMF:DMSO-based films. In the DMF:DMSO system, SnF2 addition improved the Urbach tail, suppressed the photoluminescence (PL) peak blueshift, prolonged the time-resolved PL (TRPL) lifetime, and increased the Sn2+ fraction in X-ray photoelectron spectroscopy (XPS) depth profiling. Thus, in DMSO-containing processes, SnF2 acts as an effective redox-stabilizing additive that suppresses Sn2+ oxidation. In contrast, in the DMF:2MAP system, the interior of the SnF2-free film was already Sn2+-rich, and the additional oxidation-suppression effect of SnF2 was limited. SnF2 addition was accompanied by reduced grain size, increased grain-boundary density, and higher dark current, together with lower short-circuit current density (Jsc), fill factor (FF), device performance, and initial operational output. These results demonstrate that the beneficial role of SnF2 is not universal and can be limited under the specific DMSO-free DMF:2MAP processing conditions employed here.