Understanding the Surface Chemistry of Tin Halide Perovskites

The role of tin fluoride in defining the complex surface chemistry of tin halide perovskites (THP) is investigated. It is shown that oxygen is found on the surface of tin perovskite thin films even if prepared under a virtually inert environment; however, the presence of SnF2 strongly affects the chemical nature of the found species. Oxygen primarily binds to tin in the form of SnO2 only when SnF2 is added to the precursor solution, while it preferentially binds to carbon and hydrogen in pristine materials. Thanks to the spatial mapping of both the local chemical environment and photoluminescence, it is shown that pristine films have a higher accumulation of iodine at the grain boundaries while the addition of SnF2 allows for preserving the perovskite phase and reducing chemical and optical heterogeneities. Finally, SnF2 does not help in avoiding nor slowing down the degradation of the perovskite film when exposed to ambient air and oxidation occurs on the whole THP-grain surface. These results provide insightful guidance toward understanding oxidation in THPs and elucidate its detrimental effect on the material's properties. The influence of SnF2 on the chemical and optoelectronic properties of tin triiodide perovskite surface is investigated. It is found that fluorine mediates the formation of a thin SnO2 surface layer. Pristine films are chemically heterogeneous with iodine segregated at grain boundaries, while SnF2-treated films show improved homogeneity which also results in improved photoluminescence efficiency. image