In Situ Degradation Pathway Analyses on Hybrid Perovskites with Mixed Cations and Anions

Multi-cation and multi-anion hybrid perovskites are one of the most attractive materials currently under investigation for tandem photovoltaic application because of the possibility they offer to finely tune the band gap, thus allowing the coupling with other semiconductors. If mixing different ions in the perovskite compositions could, on one side, give the possibility to create different useful materials with different properties, on the other side it could be detrimental to the hybrid perovskite stability. The presence of different ions leads to the formation of different polytypes that could open new pathways for the degradation of the prepared films. In this work, we have investigated the role of different polytypes on the degradation and stability of mixed hybrid perovskites by in situ X-ray diffraction analyses. We found that the use of even a small amount of methylammonium cations opens a new route for the degradation of the perovskite film triggering a more advanced degradation beyond the loss of the methylammonium cations. By in-situ analyses during focused thermal treatments, we unveil an interplay between the different polytypes triggered by temperature. In particular, we show how the 4H-polytype of the formamidinium lead iodide (FAPbI₃) is recovered with an annealing at 100°C in N₂ that restores the 3C-polytype. We propose the use of 1,3:2,4-bis-O-(4-methylbenzylidene) D-sorbitol (MDBS) additive as a remedy to increase the stability of the perovskite film even in presence of the methylammonium cation by making the perovskite films more compact thus reducing the surface/volume ratio.