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

Formamidinium lead triiodide (HC(NH2)(2)PbI3 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 CH3NH3PbI3 (MAPbI(3)). Because FAPbI(3) has two polymorphs ("black" alpha-FAPbI(3) and "yellow" delta-FAPbI(3)) at ambient temperature, where alpha-FAPbI(3) is the desirable photoactive perovskite phase, it becomes particularly important to suppress the formation of the nonperovskite delta-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 alpha-FAPbI(3) through the formation of non-delta-FAPbI(3) intermediate phases, which then convert to alpha-FAPbI(3) during thermal annealing. The underlying mechanism involved in the additive-modulated evolution of alpha-FAPI(3) upon mesoporous TiO2 substrates is elucidated, which suggests guidelines for developing protocols for the fabrication efficient FAPbI(3)-based mesoscopic PSCs.