Review of Stability Enhancement for Formamidinium-Based Perovskites

Organic-inorganic hybrid perovskites (OIHPs) are one of the hottest fields on account of their immense potential for photovoltaics. As one of the most promising OIHPs, formamidinium (FA)-based perovskites have been developed very fast in the past few years. The power conversion efficiency (PCE) has reached certified 24.2%, which is comparable with that of monocrystalline silicon solar cells. However, the easy formation of nonperovskite delta-phase formamidinium lead triiodide (FAPbI(3)) at a low temperature needs to be solved when fabricating a high-quality light absorber layer. Several strategies have been used to avoid the formation of delta-phase FAPbI(3) and improve phase stability in recent years such as tolerance factor adjustment, dimensional engineering, addictive processing, interfacial modification, defects passivation, and in situ growth. These approaches can enhance the phase stability to some extent; however, their contribution to long-term stability and especially their real mechanism is still unknown. Herein, the relationships among the tolerance factors, the structure of FAPbI(3), and the phase transition phenomenon are summarized. In addition, various methodologies and potential mechanisms for stabilizing alpha-phase FAPbI(3) at room temperature (RT) are discussed. In conclusion, a series of challenges in the popular processings of perovskite solar cells and their corresponding solutions that help achieve commercialization faster are summarized.