Surface Chemistry of Tin Halide Perovskites and Its Effects on Optoelectronic Properties

Tin halide perovskites have recently garnered attention as low-bandgap materials for photovoltaic and light emitting applications.^[1,2] They exhibit self-p doping, which significantly impacts the optoelectronic properties.^[3,4] Additionally, the facile oxidation of Sn²⁺ to Sn⁴⁺ further enhances p-doping within the bulk and introduces non-radiative recombination centers on the surface.^[5] Through a combination of photoemission optical spectroscopy and microscopy techniques, we investigate the chemical and optoelectronic heterogeneity, as well as the oxidation process, of tin perovskite processed without and with the addition of SnF₂.
We investigate the role of SnF₂ in determining the complex surface chemistry of tin halide perovskites. We show that oxygen is present on the surface of tin perovskite thin films even in the absence of exposure to ambient air. However, the presence of SnF₂ strongly affects the chemical nature of the found species. Specifically, fluorine acts as scavenger for Sn⁴⁺ species, effectively capturing available oxygen, which then preferentially binds to tin in the form of SnO₂ only when SnF₂ is added to the precursor solution. Conversely, without the additive, oxygen is mainly due to adventitious species. We thus highlight that the dominance of a single chemical state in the XPS Sn core level does not exclusively indicate Sn²⁺ species in the perovskite form but could also indicate the formation of superficial SnO₂. Through spatial mapping of both the local chemical environment with X-ray Photoemission Electron Microscopy (XPEEM) and photoluminescence mapping, we show that pristine films exhibit a higher accumulation of iodide at the grain boundaries. However, the addition of SnF₂ facilitates the preservation of the perovskite phase and reduces both chemical and optical heterogeneities.
Furthermore, we show that exposing the films to ambient air results in analogous surface degradation, regardless of the presence of the additive. We distinguish the process of bulk oxidation and the impact on the optoelectronic properties: an initial increase in the doping density is observed, followed by the increase in long-lived deep defects.

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2. F. Yuan, G. Folpini, T. Liu, U. Singh, A. Treglia, J. W. M. Lim, J. Klarbring, S. I. Simak, I. A. Abrikosov, T. C. Sum, A. Petrozza, F. Gao, Nat Photonics 2024, DOI 10.1038/s41566-023-01351-5.
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