Proceedings of Online Meetup - Beyond Lead Halide Perovskites: Syntheses and Applications of Metal Halide Semiconductors (MABP)
Publication date: 23rd April 2020
Room-temperature ion migration under external electric field is one of the main origins of the current-voltage hysteresis commonly observed in solar cells and light emitting diodes fabricated using halide perovskites. The resulting mixed ionic-electronic conductivity in lead halide perovskites is now finding novel applications in resistive switching1. In the perovskite structure the octahedra are corner sharing in axial as well as equatorial position but by moving to their layered two-dimensional (2D) analog structures, possibilities open up in constraining the ion conduction pathways along the intralayer. Furthermore, the structural requirements on the metal cation size are relaxed in these 2D structures allowing the synthesis of lead-free compositions. In this study, we utilize the solvent acidolysis crystallization technique to in-situ form the organic cation (methylammonium, MA+) in order to grow halide-deficient methylammonium copper halide crystals of MA2CuCl4 and MA2CuClxBr4-x. X–ray powder diffraction (XRPD) and pair distribution function (PDF) using synchrotron radiation were applied to determine the average and local crystal structure of both samples at room temperature. Though both samples show that halogen vacancies are concentrated in the equatorial positions of the copper halide octahedra, the pure chloride sample adopt a monoclinic structure with a corrugated octahedral planes while the mixed halide composition crystallize in an orthorhombic structure with flatter planes. Furthermore, fitting of the PDF data reveals a clear anisotropy in the thermal vibrations of the chlorine ions in the mixed halide sample. We find that these local structure variations connects to the hysteresis observed in the current-voltage curves.
1. Choi, J., Han, J. S., Hong, K., Kim, S. Y. & Jang, H. W. Organic–Inorganic Hybrid Halide Perovskites for Memories, Transistors, and Artificial Synapses. Adv. Mater. 30, 1–21 (2018).
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