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.