Benign methylformamidinium byproduct induced by cation heterogeneity inhibits local formation of δ-phase perovskites†

Efforts to enhance the efficiency and stability of formamidinium lead triiodide (FAPbI₃) perovskite solar cells (PSCs) have primarily focused on employing methylammonium chloride (MACl) as an effective additive. MACl significantly improves the crystallinity and lowers the δ-to-α phase transition temperature of FAPbI₃, thereby contributing to the remarkable efficiency of these solar cells. However, upon evaporation with deprotonation of MACl during annealing, the highly reactive methylamine leads to the formation of N-methylformamidinium (MFA⁺) cations. Despite their potential for significant influence on the properties of FAPbI₃ perovskites, the chemical and optoelectronic characteristics of MFA⁺ in FAPbI₃ remain poorly understood. This study investigates the unexplored role of MFA⁺ in FAPbI₃ perovskite with MACl incorporation through advanced nanoscale characterization techniques, including photo-induced force microscopy (PiFM), four-dimensional scanning transmission electron microscopy (4D-STEM), and wavelength-dependent Kelvin probe force microscopy (KPFM). We reveal that MACl induces compositional heterogeneities, particularly formamidinium (FA⁺) and MFA⁺ cation inhomogeneities. Surprisingly, MACl selectively promotes the formation of MFAPbI₃ at grain boundaries (GBs) and as clusters near GBs. Additionally, we confirm that MFAPbI₃ is a wide bandgap, and charge carriers are effectively separated at GBs and clusters enriched with MFAPbI₃. This is particularly interesting because MFAPbI₃, despite its crystal structural similarity to yellow phase δ-FAPbI₃, displays a high surface photovoltage, and does not deteriorate the solar cell performance. This study not only provides insights into the byproduct formation of MFA⁺ induced by local cation heterogeneity after employing MACl, but also guides a crucial perspective for optimizing formamidinium-based PSC design and performance.