Advances to Stabilize Photoactive Phase of FAPbI3 Perovskite

Recently, hybrid organic-inorganic perovskite materials have drawn widespread attention because of their outstanding optoelectrical properties (i.e., high absorption coefficient, long carrier diffusion distance), hence they are suitable light-absorbing materials for photovoltaic application. Among all perovskite materials, formamidinium lead iodide (FAPbI(3)) based solar cells exhibit impressive power conversion efficiency (PCE) at laboratory stage, showing great potential to compete with silicon-based solar cell. However, FAPbI(3) still suffers from poor phase stability which is the prior problem that needs to be addressed before its further commercialization. To be precise, the photoactive phase (alpha phase) is thermodynamically metastable at room temperature, which not only makes alpha phase tend to transform into photoinactive phase (delta phase), but also causes competitive crystallization between two phases during the film preparation process, making it hard to fabricate pure alpha-FAPbI(3) films. In our review, we summarized key factors that are vital for obtaining high-quality FAPbI(3) perovskite thin films and enhancing the stability of FAPbI(3) photoactive phase. First of all, precursor solution stability is of great importance since the conditions of precursor solution determine the nucleation and crystal growth process of perovskite. By introducing coordinating additives, using FAPbI(3) single crystal as raw material or applying co-solution strategy, the impurities formed by side reaction during precursor solution aging can be effectively suppressed, thus the stability of FAPbI(3) solution can be greatly prolonged. Second, the crystallization kinetics of FAPbI(3) have been systematically manipulated to obtain dense and large grain size perovskite films. Through introducing intermediate phase, regulating the surface energy, and retarding the crystal growth of FAPbI(3) in crystallization process, not only films without pinholes and fewer grain boundaries can be obtained, the pre-formed delta phase at room temperature can also be well-suppressed, thus high-quality alpha-FAPbI(3) films can be obtained. Third, how to thermodynamically enhance the phase stability of acquired FAPbI(3) film has been extensively studied. The Gibbs free energy of FAPbI(3) photoactive phase can be reduced through composition engineering, dimension engineering and external additives engineering, hence the phase transition barrier from alpha phase to delta phase has been significantly improved, which further enhance the phase stability of alpha-FAPbI(3). Lastly, we pointed out challenges of each method and proposed potential applications of mentioned strategies on improving the stability of all kinds of perovskite materials, thus further boost the commercialization of perovskite solar cell devices.