Synergistic Effect of H⁺ and I⁻ Oxidation Enables Long‐Term Stability of the Precursor Solutions and Enhanced Performance of FA‐Dominated Perovskite Solar Cells

Abstract The role of MA in FAMA mixed perovskite remains far from being fully understood, due to the intricate chemical evolutions in the precursor solutions. Adjusting the content of MA in FAMA mixed perovskite arbitrarily remains a great challenge. This study elucidates a synergistic effect between H + and I − oxidation which helps to reduce the content of MA in FA‐dominated perovskite. Briefly, excessive H + boosts the balanced and rapid assembly of MA and FA components in FA‐dominated perovskite and hampers unfavorable chemical evolution induced by nucleophilic reaction between MA and FA in the precursor solution, while I − oxidation accelerates the in situ crystallization of perovskite. Leveraging this synergistic effect, centimeter‐scaled FA x MA (1‐x) PbI 3 single crystals with arbitrarily adjustable values of x are successfully fabricated. In addition, peroxyacetic acid is introduced as the additive, enabling the blade‐coated FA 0.9 MA 0.1 PbI 3 perovskite solar cells (PSCs) to achieve an impressive efficiency of 23.7%. The efficiency achieved here is among the highest values for blade‐coated PSCs with FA content exceeding 90% so far. The optimized solution developed in this study achieved exceptional stability, allowing it to be stored under air conditions for over 2 months without significant degradation in cell efficiency. This outcome satisfies the requirements for commercialization.