Two‐Step Perovskite Solar Cells with > 25% Efficiency: Unveiling the Hidden Bottom Surface of Perovskite Layer

Abstract While significant efforts in surface engineering have been devoted to the conversion process of lead iodide (PbI 2 ) into perovskite and top surface engineering of perovskite layer with remarkable progress, the exploration of residual PbI 2 clusters and the hidden bottom surface on perovskite layer have been limited. In this work, a new strategy involving 1‐butyl‐3‐methylimidazolium acetate (BMIMAc) ionic liquid (IL) additives is developed and it is found that both the cations and the anions in ILs can interact with the perovskite components, thereby regulating the crystallization process and diminishing the residue PbI 2 clusters as well as filling vacancies. The introduction of BMIMAc ILs induces the formation of a uniform porous PbI 2 film, facilitating better penetration of the second‐step organic salt and fostering a more extensive interaction between PbI 2 and the organic salt. Surprisingly, the oversized residual PbI 2 clusters at the bottom surface of the perovskite layer completely diminish. In addition, advanced depth analysis techniques including depth‐resolved grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) and bottom thinning technology are employed for a comprehensive understanding of the reduction in residual PbI 2 . Leveraging effective PbI 2 management and regulation of the perovskite crystallization process, the champion devices achieve a power conversion efficiency (PCE) of 25.06% with long‐term stability.