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 (VOC). The optimized device shows a 2% improvement in PCE, with the VOC 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.