Degradation mechanisms in mixed-cation and mixed-halide CsxFA1-xPb(BryI1-y)3 perovskite films under ambient conditions

Multicomponent perovskites of the type CsxFA1-xPb(BryI1-y)3 are good candidates for high efficient perovskite and tandem solar cells. In this work the degradation mechanims of these multicomponent films were investigated and our results show that the degradation is a complex process, with the formation of a number of intermediates and lead-based products. In-situ X-ray diffraction analysis carried out in the first stages of the degradation indicate that differently from MAPbI3 perovskite, the degradation of these multicomponent films begins with the formation of hexagonal polytypes as intermediates, which in turn are converted to hydrated phases. The initial steps of the degradation were also monitored for the first time by in-situ environmental scanning electron microscopy (ESEM) with 75% humidity. In-situ ESEM images show that the degradation has its beginning at the “valleys” of the wrinkled morphology found in these films, possibly because of smaller grain size in these regions. XPS analysis confirm that the hydrated perovskite films continue to react with the environment, leading to the formation of metal hydroxide, carbonate, and oxide as final products. Our results also indicate that the degradation mechanism are highly dependent on the Cs concentration and Br content providing a guide for choosing the best compositions for efficient, but more environmentally stable solar cells.