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

Formamidinium lead triiodide (HC(NH 2 ) 2 PbI 3 or FAPbI 3 ) is gaining increasing interest in the field of mesoscopic perovskite solar cells (PSCs) for its broader light absorption compared with the more widely studied CH 3 NH 3 PbI 3 (MAPbI 3 ). Because FAPbI 3 has two polymorphs (“black” α-FAPbI 3 and “yellow” δ-FAPbI 3 ) at ambient temperature, where α-FAPbI 3 is the desirable photoactive perovskite phase, it becomes particularly important to suppress the formation of the nonperovskite δ-FAPbI 3 for achieving high efficiency in FAPbI 3 -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-FAPbI 3 through the formation of non-δ-FAPbI 3 intermediate phases, which then convert to α-FAPbI 3 during thermal annealing. The underlying mechanism involved in the additive-modulated evolution of α-FAPbI 3 upon mesoporous TiO 2 substrates is elucidated, which suggests guidelines for developing protocols for the fabrication efficient FAPbI 3 -based mesoscopic PSCs.