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. The impact of the commonly used tin fluoride (SnF2) additive in Sn‐based perovskite solar cells on the poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/perovskite interface are analyzed. SnF2 is found to preferably precipitate at this interface where it forms a SnS interlayer of approximately 1.2 nm thickness induced by a chemical reaction with sulfur‐containing groups at the PEDOT:PSS surface.