Stabilization of Black Perovskite Phase in FAPbI₃ and CsPbI₃

Although halide perovskites allow a great versatility, the application on single-absorber solar cells restricts significantly the number of available materials. In this context, CsPbI3 and FAPbI3 (FA, formamidinium) present a huge potential for the inorganic approach with enhanced stability and narrow bandgap, respectively. However, for these materials, Cs+ and FA+ are relatively too small and too big to stabilize the perovskite black phase at room temperature, both presenting a nonphotoactive yellow phase as the most stable phase. This fact limits dramatically their application and also helps in the understanding of the main research lines in the halide perovskite photovoltaic field in the quest for the stabilization of FAPbI3. In this Perspective, we present an overview of different strategies for the stabilization of the perovskite black phase of these two materials. We evaluate the stability approaches envisioning efficient and stable materials, with a particular focus on the positive and limiting aspects of the exploitation of low dimensionality and chemi-structural mechanisms.