Two-step fabricated FAPbI3-based perovskites have attracted increasing attention because of their excellent film quality and reproducibility. However, the underlying film formation mechanism remains mysterious. In this work, the crystallization kinetics of a benchmark FAPbI3-based perovskite film with sequential A-site doping of Cs+ and GA+ has been revealed by in situ X-ray scattering and first-principles calculations. Incorporating Cs+ in the first step induces an alternative pathway from δ-CsPbI3 to perovskite α-phase, which is energetically more favorable than the conventional pathways from PbI2. However, pinholes are formed due to the nonuniform nucleation with sparse δ-CsPbI3 crystals. Fortunately, incorporating GA+ in the second step can not only promote the phase transition from δ-CsPbI3 to perovskite α-phase, but also eliminate pinholes via Ostwald ripening and enhanced grain boundary migration, thus boosting efficiencies of PSCs over 23%. This work demonstrates an unprecedented advantage of two-step process over one-step process, allowing a precise control of the perovskite crystallization kinetics by decoupling the crystal nucleation and growth process.
Manuscript by Qin and Xue et al. was accepted by Advanced Materials, 2020, 2004630, https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202004630.