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. The incorporation of iodine (I2) is shown to modulate the methylammonium chloride (MACl)‐assisted growth of α‐phase formamidinium perovskite (α‐FAPbI3), with a delayed release of volatile MACl and suppressed cation side reaction. The optimized device delivers an efficiency of 25.2% with an impressive fill factor (FF) of 84.2%, which is attributed to suppressed non‐radiative recombination.