Formation and stabilization of metastable halide perovskite phases for photovoltaics

Halide perovskites are semiconductors that have impacted the photovoltaic field in single and multijunction devices. Among them, metastable FAPbI3-based (FA, formamidinium) and CsPbI3-based perovskites are frontrunner materials for perovskite solar cells. However, they face the same challenge; their photoactive black perovskite phases spontaneously transform into the yellow non-photoactive δ phases at room temperature. Synthesizing and stabilizing the pure perovskite phase has been a great challenge, leading to frequent updates of the reported record efficiency and stability for organic-inorganic hybrid perovskite and all-inorganic perovskite photovoltaics. In this perspective, we first discuss the material properties of metastable perovskites FAPbI3 and CsPbI3 and later review the most relevant strategies currently employed for obtaining and stabilizing these perovskite phases. Finally, we summarize the thermodynamic changes taking place during transition processes and outlook on the potential applications and scalability of these metastable perovskites.