The effect of solution process control on the formation of the α-FAPbI^sub 3^ perovskite: FAPbI^sub 3^ versus MAPbI^sub 3^ solar cells

This study aims at the fabrication of hole-conductor-free perovskite solar cells by the sequential deposition method. To develop the cells, MAPbI3 (CH3NH3PbI3) and FAPbI3 (HC(NH2)2PbI3) perovskites were used as light-absorbing materials. It is difficult to achieve a black α-FAPbI3 perovskite due to the unwanted yellow δ-FAPbI3 phase that is usually first formed at a low temperature and then converted into a black α-FAPbI3 perovskite at a high annealing temperature. A high annealing temperature can also lead to the uncontrollable degradation of the α-FAPbI3 perovskite. Here, we report the effect of various temperatures of a dipping solution of formamidine iodide (FAI) in isopropanol on the formation of black α-FAPbI3 perovskites from the initial stage. The effects of various annealing temperatures on the surface morphology and the optical properties of the perovskite layer are also studied. The optimum temperatures of the FAI solution and annealing for making an FAPbI3 perovskite were 80 °C and 170 °C respectively. A stable black FAPbI3 perovskite layer was formed in these optimum conditions. Moreover, the short-circuit current density and the efficiency of the fabricated FAPbI3 and MAPbI3 perovskite solar cells were approximately the same.