Spontaneous low-temperature crystallization of α-FAPbI₃ for highly efficient perovskite solar cells

Formamidinium lead triiodide (HC(NH₂)₂PbI₃ or FAPbI₃) is a promising light absorber for high-efficiency perovskite solar cells because of its superior light absorption range and thermal stability to CH₃NH₃PbI₃ (MAPbI₃). Unfortunately, it is difficult to fabricate high-quality FAPbI₃ thin films to surpass the MAPbI₃-based cells due to easily forming unwanted but more stable yellow δ-phase and thus requiring high annealing-temperature for wanted photovoltaic-active black α-phase. Herein, we reported a novel low-temperature fabrication of highly crystallized α-FAPbI₃ film exhibiting uniaxial-oriented nature with large grain sizes up to 2 μm. First-principles energetic calculations predicted that this novel deposition should be ascribed to the formation of a high-energy metastable two-dimensional (2D) intermediate of MAFAPbI₃Cl followed by a spontaneous conversion to α-FAPbI₃. The ions exchange reaction during this MAFAPbI₃Cl-FAPbI₃ conversion account for the perovskite film uniaxial-oriented grown along the (1 1 1) direction. This large-grain and uniaxial-oriented grown α-FAPbI₃ based solar cells exhibited an efficiency up to 20.4% accompanying with low density-voltage (J-V) hysteresis and high stability.