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.