Degradation Study of Co-evaporated Methylammonium Tin Iodide

Hybrid organic-inorganic metal halide perovskites (HOIPs) have been under the spotlight
since they were first used for solar cell applications. Since then, the power conversion efficiency
of HOIP-based solar cells has increased significantly and the current record is 26.7% on the
laboratory scale [1], which is comparable to the record of more mature technologies such as
silicon solar cells. Furthermore, HOIP solar cells are a low-cost alternative that is relatively
easy to produce and can be easily adapted by changing the substrate or composition. The
major drawbacks of HOIP based solar cells are that they tend to degrade when exposed to
external stresses, and record efficiency devices contain lead (Pb). The latter may hinder the
commercialization of this technology, because of the toxicity of Pb, which is of great concern
to human health and the environment. A possible way to avoid Pb in HOIP-based solar
cells is to replace it with tin (Sn). Which is a less toxic substitute for Pb and it is placed
right above Pb in the periodic table, meaning that both elements have similar reactivity.
Furthermore, Sn-based perovskite solar cells were demonstrated and yielded the best results
in terms of stability and efficiency for Pb-free HOIPs. Nevertheless, Sn-based HOIPs are also
prone to degradation under external stimuli and present an additional challenge compared
to Pb-based perovskites: Sn is more prone to oxidation.
In this thesis, Sn-based HOIP, more specifically methylammonium tin iodide (MASnI3)
is synthesized using physical vapor deposition, as it is a solvent-free technique. The MASnI3
films were exposed in a controlled environment to different external stimuli, such as light,
water, and synthetic air in order to study the different degradation pathways. Light, air, and water have been shown to degrade MASnI3, but the degradation pathway is different for
each of them. First, light-induced degradation is shown to result in the formation of majorly
SnI2, while the organic component leaves the surface. Second, water-induced degradation
also mainly forms SnI2, but further degradation into SnO2 was observed. Third, synthetic
air-induced degradation is shown to form SnO and an intermediate phase that was assumed
to combine tin, iodine, and oxygen. Finally, the formation of SnI4 was not detected for any
of the stimuli, contrary to what is described in the literature.