Synthesis and characterizations of structure and physical properties of halide perovskite APbX3 (A = CH3NH3, Cs; X = I, Br, Cl) single crystals

Halide perovskites have quickly emerged as most promising materials in the forefront of photovoltaic and optoelectronic devices, exhibiting superb optoelectronic properties with facile solution synthesis. However, most perovskite-based devices are still fabricated of polycrystalline films, which exhibit inferior carrier properties and low chemical stability compared to single-crystal counterparts. Furthermore, single crystals with low defect concentration and free-grain boundaries provide an ideal platform for gaining insights into the intrinsic properties which have not been thoroughly understood yet. Therefore, study of the single crystals is essential from fundamental and practical aspects, and it constitutes the main objective of this thesis. Here, various types of single crystals, including hybrid halide perovskites CH3NH3PbX3 (X = Cl, Br, & I) and all-inorganic halide perovskites (CsPbX3), are successfully grown and characterized by X-ray diffraction, absorption and photoluminescence spectroscopy, polarized light microscopy (PLM), and dielectric, polarization measurements. A novel, efficient technique is designed and applied to grow the halide perovskite crystals at room temperature. The solubility, nucleation, and growth mechanisms by the high-temperature and room-temperature growths are investigated, and different growth pathways are proposed. The characteristic twin domains of these crystals are investigated by PLM, and their crystal structures and phase transitions are accurately determined. A side-centered orthorhombic symmetry, derived from a distorted monoclinic unit cell, is experimentally discovered for the intermediate phase of MAPbX3 (X= Br, Cl) from the domain analysis based on optical crystallography. The existence of ferroelectricity in halide perovskites has remained debatable. The domain response under an external electrical field and stress reveals that these crystals are not ferroelectric but ferroelastic, opening a new view to improve the performance and stability through stress-mediated domain engineering. Lastly, comprehensive studies of dielectric and polarization properties of the crystals are performed over a broad range of temperature and frequency, leading to the finding of "incipient ferroelectricity" in MAPbCl3 which becomes the first example of order-disorder materials exhibiting such a behaviour. This work provides a novel and effective route to grow halide perovskite single crystals, and a better understanding of their physical properties, contributing to the acceleration of the development of crystals-based optoelectronic devices with superior performance and enhanced stability.