AbstractThe preferential growth of α‐phase formamidinium perovskite (α‐FAPbI₃) at low temperatures can be achieved with the incorporation of chloride‐based additives, with methylammonium chloride (MACl) being the most common example. However, compared to other less‐volatile chloride additives, MACl only remains in the growing perovskite film for a short time before evaporating during annealing, primarily influencing the early stages of film formation. In addition, evaporation of MACl as methylamine (MA⁰) and HCl can introduce a side reaction between MA⁰ and formamidinium (FA), undermining the compositional purity and phase stability of α‐FAPbI₃. In this study, it is demonstrated that addition of iodine (I₂) into the FAPbI₃ precursor solution containing MACl suppresses the MA‐FA side reaction during annealing. Additionally, MACl evaporation is delayed owing to strong interaction with triiodide. The added I₂ facilitates spontaneous growth of α‐FAPbI₃ prior to annealing, with an improved bottom morphology due to the formation of fewer byproducts. Perovskite solar cells derived from an I₂‐incorporated solution deliver a champion power conversion efficiency of 25.2% that is attributed to suppressed non‐radiative recombination.