The Role of SnF₂ Additive on Interface Formation in All Lead-Free FASnI₃ Perovskite Solar Cells

Tin-based perovskites are promising alternative absorber materials for lead-free perovskite solar cells but need strategies to avoid fast tin (Sn) oxidation. Generally, this reaction can be slowed down by the addition of tin fluoride (SnF₂) to the perovskite precursor solution, which also improves the perovskite layer morphology. Here, this work analyzes the spatial distribution of the additive within formamidinium tin triiodide (FASnI₃) films deposited on top of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layers. Employing time-of-flight secondary ion mass spectrometry and a combination of hard and soft X-ray photoelectron spectroscopy, it is found that SnF₂ preferably accumulates at the PEDOT:PSS/perovskite interface, accompanied by the formation of an ultrathin SnS interlayer with an effective thickness of ≈1.2 nm.