Dual‐Action Sulfonium‐Amino Acid Additive Enabling Crystallization Regulation and Defect Passivation for High‐Performance Carbon‐Electrode Printable Mesoscopic Perovskite Solar Cells

The solution‐based processing of halide perovskites typically yields films of limited crystallinity, plagued by abundant grain boundaries and surface defects that degrade device performance. To address this, we introduce multifunctional DL‐methionine‐S‐methyl sulfonium chloride (DMSC) as a dual‐action additive for the perovskite precursor, which enables high‐quality crystallization and efficient defect passivation, substantially boosting the performance of hole‐conductor‐free, printable mesoscopic perovskite solar cells (p‐MPSCs) with carbon electrodes. The distinct functional groups of DMSC (–NH 2 , –COOH, and sulfonium) coordinate strongly with the perovskite components. This interaction refines the colloidal particle size distribution in the precursor solution, retards uncontrolled nucleation, and promotes subsequent crystal growth, ultimately enhancing the perovskite crystallinity within the mesoporous scaffold. Moreover, DMSC effectively passivates defect sites by interacting with Pb and I species, which suppresses non‐radiative recombination and enhances stability against aging. Consequently, p‐MPSCs incorporating DMSC achieve a champion power conversion efficiency of 22.5%, an encouraging advance over the 20.6% efficiency of control devices. Furthermore, encapsulated DMSC‐based p‐MPSCs retain 95% of their initial efficiency after 1000 h of maximum power point tracking under continuous one‐sun illumination at 55 ± 5°C and 55 ± 5% relative humidity. This work establishes an effective dual‐action additive strategy for achieving high‐quality perovskites towards high‐performance p‐MPSCs.