Cubic structure of the mixed halide perovskite CH₃NH₃PbI_3-xCl_xvia thermal annealing

A methodology has been developed to obtain a cubic structure of the mixed halide perovskite CH₃NH₃PbI_3-xCl_x that involves thermal annealing of a vacuum-deposited perovskite layer. In this process, a tetragonal-cubic transition of the mixed halide perovskite CH₃NH₃PbI_3-xCl_x has been observed using a variety of characterization techniques, e.g., X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical and Hall effect measurements. According to the XRD, the tetragonal structure of CH₃NH₃PbI_3-xCl_x changed into the cubic phase when the temperature increased to above the transition temperature, at which point all iodine atoms move towards the central axis while PbI₆ rotates around the C-N bond. After annealing, the stable cubic structure of the mixed halide perovskite CH₃NH₃PbI_3-xCl_x has a smaller band gap and a better optical absorption than the original tetragonal structure. Meanwhile, the microstructure has shown an increase of grain size after annealing, which could be given the fastest mobility of 13.5 cm² V^-1 S^-1, yielded from the Hall effect measurements. The potential benefits of the stable cubic structure and large grain size in the mixed halide perovskite CH₃NH₃PbI_3-xCl_x are to improve the device performance of solar cells from the viewpoint of bandgap engineering and crystalline quality.