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

Abstract Surface Engineering and the effective management of lead iodide (PbI 2 ) are pivotal for improving the performance of two‐step perovskite solar cells. While significant efforts have been devoted to the conversion process of 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, we thoroughly investigate the often overlooked PbI 2 residual and its detailed impact on device performance. Importantly, oversized residual PbI 2 clusters have been observed to accumulate at the bottom surface of perovskite, significantly hindering the device performance. To address this issue, we develop a new strategy involving 1‐butyl‐3‐methylimidazolium acetate (BMIMAc) ionic liquid (IL) additives and find 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. Interestingly, 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 contrast to the pristine sample. Furthermore, the incorporation of ILs into PbI 2 efficiently slows down the perovskite crystallization process by binding with PbI 2 , resulting in high‐quality perovskite films with larger grain sizes and reduced PbI 2 content. 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 following the IL treatment. Leveraging effective PbI 2 management and precise regulation of the perovskite crystallization process, the champion devices achieve a power conversion efficiencie (PCE) of 25.06% with long‐term stability. This article is protected by copyright. All rights reserved