Deciphering Reaction Products in Formamidine-Based Perovskites with Methylammonium Chloride Additive

The fabrication of formamidinium lead iodide (FAPbI₃) perovskite solar cells (PSCs) involves the addition of methylammonium chloride (MACl) to promote low-temperature α-phase formation and grain growth. However, as the added MACl deprotonates and volatilizes into methylamine (MA⁰) and HCl for removal, MA⁰ can chemically interact with formamidinium (FA⁺), forming methyl formamidinium (MFA⁺) as a byproduct. Despite its significance, the chemical interactions among FAPbI₃ perovskites, MACl additives, and their byproducts remain poorly understood. Our findings reveal that the FA⁺ and MA⁰ reaction primarily yields a mixture of cis/trans-N-methyl formamidinium iodide (MFAI) isomers, with cis-MFAI prevailing as the dominant species. Moreover, MFAI subsequently reacts with PbI₂ to yield fully formed cis-MFAPbI₃ 2H-phase perovskite. We elucidated the effects of MFAI on the crystal growth, phase stability, and band gap of formamidine-based perovskites through the growth of single crystals. This research offers valuable insights into the role of these byproducts in influencing the efficiency and long-term stability of future PSCs.