A Deformable Additive on Defects Passivation and Phase Segregation Inhibition Enables the Efficiency of Inverted Perovskite Solar Cells over 24%

Abstract The defects and phase segregation in perovskite will significantly reduce the performance and stability of perovskite solar cells (PSCs). In this work, a deformable coumarin is employed as a multifunctional additive for formamidinium–cesium (FA‐Cs) perovskite. During the annealing process of perovskite, the partial decomposition of coumarin passivates the Pb²⁺, iodine, and organic cation defects. Additionally, coumarin can affect colloidal size distributions, resulting in relatively large grain size and good crystallinity of target perovskite film. Hence, the carrier extraction/transport can be promoted, trap‐assisted recombination is reduced, and energy levels are optimized in target perovskite films. Furthermore, the coumarin treatment can significantly release residual stress. As a result, the champion power conversion efficiencies (PCEs) of 23.18% and 24.14% are obtained for Br‐rich (FA_0.88Cs_0.12PbI_2.64Br_0.36) and Br‐poor (FA_0.96Cs_0.04PbI_2.8Br_0.12) based devices, respectively. The flexible PSCs based on Br‐poor perovskite exhibit an excellent PCE of 23.13%, one of the highest values for flexible PSCs reported to date. Due to the inhibition of phase segregation, the target devices exhibit excellent thermal and light stability. This work provides new insights into the additive engineering of passivating defects, stress relief, and inhibition of phase segregation of perovskite films, offering a reliable method to develop state‐of‐the‐art solar cells.