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

Two‐step‐fabricated FAPbI 3 ‐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 FAPbI 3 ‐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‐CsPbI 3 to perovskite α‐phase, which is energetically more favorable than the conventional pathways from PbI 2 . However, pinholes are formed due to the nonuniform nucleation with sparse δ‐CsPbI 3 crystals. Fortunately, incorporating GA + in the second step can not only promote the phase transition from δ‐CsPbI 3 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.