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

Formamidinium lead triiodide (HC(NH2)(2)PbI3 or FAPbI(3)) is a promising light absorber for high-efficiency perovskite solar cells because of its superior light absorption range and thermal stability to CH3NH3PbI3 (MAPbI(3)). Unfortunately, it is difficult to fabricate high-quality FAPbI(3) thin films to surpass the MAPbI(3) based cells due to easily forming unwanted but more stable yellow delta-phase and thus requiring high annealing-temperature for wanted photovoltaic-active black alpha-phase. Herein, we reported a novel low-temperature fabrication of highly crystallized alpha-FAPbI(3) film exhibiting uniaxial-oriented nature with large grain sizes up to 2 mu m. 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 MAFAPbI(3) Cl followed by a spontaneous conversion to alpha-FAPbI(3). The ions exchange reaction during this MAFAPbI(3) Cl-FAPbI(3) conversion account for the perovskite film uniaxial-oriented grown along the (1 1 1) direction. This large-grain and uniaxial-oriented grown alpha-FAPbI(3) based solar cells exhibited an efficiency up to 20.4% accompanying with low density-voltage (J-V) hysteresis and high stability. (C) 2019 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.