Iodine Modulates the MACl-Assisted Growth of FAPbI₃ for High Efficiency Perovskite Solar Cells

The 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.