Elimination of Yellow Phase: An Effective Method to Achieve High Quality HC(NH₂ )₂ PbI₃ -based Perovskite Films

Formamidinium lead iodide-based (FAPbI₃ ) perovskite is widely used in the field of photovoltaics, owing to its suitable bandgap (ca. 1.45 eV) and better thermal stability. FAPbI₃ has two polymorphs (black α-FAPbI₃ and yellow δ-FAPbI₃ ) at ambient temperature. The yellow δ-FAPbI₃ , which has no photoactivity, has a chain-like structure that likely hinders electron transport and reduces photovoltaic performance. However, pure-phase black α-FAPbI₃ without any yellow phase is difficult to obtain and the underlying mechanism of the phase transition is rarely investigated. In this study, a facile bi-additive method (BA method) has been developed to completely eliminate the yellow δ-FAPbI₃ phase by inducing a phase transition from δ-FAPbI₃ to α-FAPbI₃ . HI and Pb(SCN)₂ were employed as dual additives. Based on the investigation of the annealing time and temperature, we determined that the BA method can induce the phase transition and enhance the stability of α-FAPbI₃ . Owing to the enhanced crystallization as well as uniform morphology of the BA film, the perovskite solar cells (PSCs) exhibited an increased power conversion efficiency (PCE). Furthermore, the optimal devices displayed excellent stability and maintained over 80 % of initial PCE after aging for 400 h in air. This work provides a new insight into the fabrication of high-quality pure α-FAPbI₃ perovskite films and makes high efficiency photovoltaic devices a reality.