Enhancing α-FAPbI₃ Crystallization and Photovoltaic Performance through Inhibiting MFA Formation

Methylammonium chloride (MACl) additive is almost irreplaceable in high-performance formamidinium (FA) perovskite photovoltaics. However, the byproduct of methyl formamidinium (MFA⁺) from the reaction of MA⁰ and FA damages the compositional purity and phase stability of α-FAPbI₃. The addition of iodine (I₂) to the FAPbI₃ precursor has been reported to inhibit the formation of the byproduct MFA⁺. Here, we systematically investigate the effect of MAI on perovskite films and devices by using MAI to replace MACl and I₂. The results demonstrate that the addition of MAI produces more I₃^- in the perovskite precursor, which inhibits the reaction between MA and FA and thus blocks the formation of MFA⁺. Meanwhile, MFA⁺ formation is reduced due to the delayed MACl evaporation caused by its strong interaction with I₃^-, facilitating the growth of α-FAPbI₃ with an improved bottom morphology. It eliminates unreacted PbI₂, forming a homogenized phase, and facilitates ordered growth along the (111) facet, enhancing charge transport and increasing the open-circuit voltage (V_OC). The optimized device shows a 2% improvement in PCE, with the V_OC increasing from 1.050 to 1.103 V. Additionally, the target device retains 97% of initial performance after 5495 min operation under maximum power point tracking, compared to 82.3% after 2000 min for the control device. This work provides insights into inhibiting the formation of MFA⁺ byproducts induced by the MA-FA side reaction following the introduction of MACl.