Fluorine ion induced phase evolution of tin-based perovskite thin films: structure and properties.

To study the effect of fluorine ions on the phase transformation of a tin-based perovskite, CsSnI (F) films were deposited by using thermal vacuum evaporation from a mixed powder of SnI, SnF and CsI, followed by rapid vacuum annealing. The color evolution, structure, and properties of CsSnI F films aged in air were observed and analyzed. The results showed that the colors of the films changed from black to yellow, and finally presented as black again over time; the unstable B-γ-CsSnI F phase transformed into the Y-CsSnI F phase, which is then recombined into the CsSnI F phase with the generation of SnO in air.