Recent Advances on the Strategies to Stabilize the alpha-Phase of Formamidinium Based Perovskite Materials

Perovskite solar cells (PSC) are considered promising next generation photovoltaic devices due to their low cost and high-power conversion efficiency (PCE). The perovskite material in the photovoltaic devices plays the fundamental role for the unique performances of PSC. Formamidinium based perovskite materials have become a hot-topic for research due to their excellent characteristics, such as a lower band gap (1.48 V), broader light absorption, and better thermal stability compared to methylammonium based perovskite materials. There are four phases of perovskite materials, named the cubic alpha-phase, tetragonal beta-phase, orthorhombic gamma-phase, and delta-phase (yellow). Many research focus on the transition of alpha-phase and delta-phase. alpha-Phase FA-based perovskite is very useful for photovoltaic application. However, the phase stability of alpha-phase FA-based perovskite materials is quite poor. It transforms into its useless delta-phase at room temperature. This instability will lead the degradation of PCE and the other optoelectronic properties. For the practical application of PSC, it is urgent to understand more about the mechanism of this transformation and boost the stability of alpha-Phase FA-based perovskite materials. This review describes the strategies developed in the past several years, such as mixed cations, anion exchange, dimensions controlling, and surface engineering. These discussions present a perspective on the stability of alpha-phase of FA-based perovskite materials and the coming challenges in this field.