Homogenous FACsPbI ₃ Perovskite Solar Cells Enabled by a Seed‐Facilitated Cation Exchange Strategy

While cesium incorporation into perovskites to form mixed‐cation perovskites (e.g., FACsPbI 3 ) can potentially improve the thermal stability, the effect is usually limited by the phase segregation resulting from cation inhomogeneity within FACsPbI 3 . Here, we successfully solve this problem by employing a novel seed‐facilitated cation exchange strategy, enabling homogeneous FACsPbI 3 films for efficient and stable photovoltaics. In this case, FAPbI 3 films are deposited on top of a substrate layer formed by Cs x PbI x+2 perovskite seeds, which regulate FACsPbI 3 crystal nucleation and growth. Among different Cs x PbI x+2 seeds, 0D Cs 4 PbI 6 is demonstrated to be the most effective in promoting a homogeneous Cs distribution by facilitating cation exchange and suppressing the formation of the yellow non‐perovskite phase. Based on the homogeneous FACsPbI 3 perovskite films, we achieve a 5 cm × 5 cm minimodule with a certified efficiency of 22.45%. In addition, our seed‐facilitated cation exchange strategy also modifies the buried interface between the perovskite layer and electron transport layer, making it possible to develop high‐quality flexible perovskite solar cells with a certified stabilized power output efficiency of 23.36%.