Iodine modulates the MACl-assisted growth of FAPbI${}_3$ for high efficiency perovskite solar cells

The preferential growth of α-phase formamidinium perovskite (α-FAPbI${}_3$) 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 (MA${}^0$) and HCl can introduce a side reaction between MA${}^0$ and formamidinium (FA), undermining the compositional purity and phase stability of α-FAPbI${}_3$. In this study, it is demonstrated that addition of iodine (I${}_2$) into the FAPbI${}_3$ 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 I${}_2$ facilitates spontaneous growth of α-FAPbI${}_3$ prior to annealing, with an improved bottom morphology due to the formation of fewer byproducts. Perovskite solar cells derived from an I${}_2$-incorporated solution deliver a champion power conversion efficiency of 25.2% that is attributed to suppressed non-radiative recombination. (© 2024 The Authors. Advanced Energy Materials published by Wiley‐VCH GmbH)