Additive-Modulated Evolution of HC(NH₂)₂PbI₃ Black Polymorph for Mesoscopic Perovskite Solar Cells

Formamidinium lead triiodide (HC(NH₂)₂PbI₃ or FAPbI₃) is gaining increasing interest in the field of mesoscopic perovskite solar cells (PSCs) for its broader light absorption compared with the more widely studied CH₃NH₃PbI₃ (MAPbI₃). Because FAPbI₃ has two polymorphs ("black" α-FAPbI₃ and "yellow" δ-FAPbI₃) at ambient temperature, where α-FAPbI₃ is the desirable photoactive perovskite phase, it becomes particularly important to suppress the formation of the nonperovskite δ-FAPbI₃ for achieving high efficiency in FAPbI₃-based mesoscopic PSCs. In this study, we demonstrate that the judicious use of low-volatility additives in the precursor solution assists in the evolution of α-FAPbI₃ through the formation of non-δ-FAPbI₃ intermediate phases, which then convert to α-FAPbI₃ during thermal annealing. The underlying mechanism involved in the additive-modulated evolution of α-FAPbI₃ upon mesoporous TiO₂ substrates is elucidated, which suggests guidelines for developing protocols for the fabrication efficient FAPbI₃-based mesoscopic PSCs.