Tin halide perovskites are a less toxic alternative to lead-based materials in perovskite solar cells, but the oxidation of Sn(II) to Sn(IV) in air introduces an additional degradation pathway that limits the commercial viability of these materials. Efforts to improve long-term stability through materials engineering requires not only detailed mechanistic pictures of both oxygen- and moisture-induced degradation but also an understanding of how the two are coupled. We have found that <i>in situ</i> X-ray absorption spectroscopy (XAS) at the Sn K-edge provides a precise measure of the average valency of the bulk material, allowing oxidation kinetics to be measured on time scales ranging from minutes to days. For the particular case of formamidinium tin(II) iodide perovskite thin films, the kinetics are consistent with an area contraction model, allowing us to compare rate constants directly for varying film morphologies and conditions. Differences in oxidation rates are apparent under dry and humid conditions, underscoring the importance of isolating oxidation, hydration, and mechanical degradation when performing such measurements. Complementary optical absorption and X-ray diffraction studies further demonstrate the challenge of obtaining comparable data from techniques lacking the element specificity and oxidation state sensitivity of XAS. Finally, <sup>119</sup>Sn Mössbauer spectroscopy will be presented as a lab-based technique capable of providing information on both valency and speciation throughout oxidation.

Real-time investigation of Sn(II) oxidation in lead-free halide perovskites by X-ray absorption and Mössbauer spectroscopy

ACS Fall 2021

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