Halide perovskite materials exhibit promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25.5 % in single-junction devices and 29.5% in tandem devices. Formamadinium (FA) lead iodide (FAPbI₃) and FA-rich perovskites are preferred for photovoltaic applications, but their widespread adoption is hindered by the rapid degradation of the desirable corner-sharing cubic (3C) phase to an undesirable wide bandgap, face-sharing, hexagonal (2H) phase under ambient conditions. Alloying FA with small amounts of Cs⁺ and methylammonium (MA) on the A site cation of the ABX₃ perovskite structure has proven a promising strategy for stabilizing photoactive perovskite phases. For example, photovoltaic devices fabricated with compositions such as Cs_0.05FA_0.78MA_0.17Pb(I_0.83Br_0.17)₃ (triple cation) perovskites have achieved high device efficiencies with greatly enhanced reproducibility and ambient stability[1]. Recently, there has also been renewed interest in methods to stabilize pure FAPbI3 through strategies such as the incorporation of MA via treatment with methylammonium thiocyanate vapour[2], addition of formamidinium formate[3] or methylammonium formate[4].

The mechanism of improved stability obtained from these approaches is generally explained as either originating from a tuning of the Goldschmidt tolerance factor towards the perfect cubic perovskite structure via cation mixing in the case of Triple Cation perovskites[1,5], by templating growth of the corner-sharing cubic structure in stabilized-FAPbI₃ perovskites[2,4], or by reducing intrinsic defect density[3]. The key tenet in all these explanations is that the final photoactive perovskite material, regardless of stabilization approach, is a cubic perovskite structure and that this is the structure that should be pursued for optimal stability and performance. Here, we reveal that contrary to conventional wisdom, this is not the case. In this talk using scanning electron diffraction (SED) we reveal the nanoscale structural origins of improved material stability in FA-rich perovskites and stabilized thin films of α-FAPbI₃, two of the most promising candidates for commercial PV applications[6]. Further, we will describe how these nanoscale stability mechanisms are related to our recent reports of performance limiting trap clusters in high-performing perovskite films[7]. Together, our talk will help answer questions such as “What are the nanoscale origins of instability in perovskite materials and devices?”, “how important is phase purity for performance?”