Precise Control of Perovskite Crystallization Kinetics via Sequential A‐Site Doping

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. Here, the crystallization kinetics of a benchmark FAPbI₃ -based perovskite film with sequential A-site doping of Cs⁺ and GA⁺ is 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 the perovskite α-phase, but also eliminate pinholes via Ostwald ripening and enhanced grain boundary migration, thus boosting efficiencies of perovskite solar cells over 23%. This work demonstrates the unprecedented advantage of the two-step process over the one-step process, allowing a precise control of the perovskite crystallization kinetics by decoupling the crystal nucleation and growth process.