First-principles Calculation Of Halogen-containing Perovskite And Micron Crystal Luminescence Performance

Since the beginning of the 21st century,with the development of the economy,human society's demand for energy has been increasing.Traditional fossil energy reserves are limited,and the massive use of fossil energy will lead to environmental pollution problems,so it is extremely urgent to find altemative energy and new energy materials.Compare to organic-inorganic hybrid halide perovskites,all inorganic halide perovskites?CsPbBr? are more stable,which is popular material concerned by energy conversion materials researchers at this stage and has been widely used in the field of optoelectronic materials.At present,the research about CsPbBr series perovskites is mainly focused on three-dimensional perovskite CsPbBr₃ and zero dimensional perovskite Cs₄PbBr₆.There seems to be some controversies about the photoluminescence mechanism of Cs₄PbBr₆ in academic circles.Some scholars believe that Cs₄PbBr₆ itself can't emit light,the second phase?CsPbBr₃ quantum dot?in Cs₄PbBr₆ can.while the other consider that the luminescence phenomenon of Cs₄PbBr₆ is caused by its defect energy level.Besides,the study of the growth of Cs₄PbBr₆ crystal and its related properties are hindered due to the toxicity of anti solvent.In this thesis,the mechanical and optical properties of CsPbX₃ and Cs4PbX₆?X=Cl,Br,I? were disgussed by using first-principles calculation method firstly.The calculation of mechanical properties can provide theoretical support for the preparation of optical components,and the calculation of optical properties is helpful to explore the luminescence mechanism of Cs₄PbBr₆.Then CsPbBr₃ and Cs₄PbBr₆ single crystals were prepared by using ethanol as antisolvent.The luminescence and laser properties of Cs₄PbBr₆ microcrystals were studied.The main research works and key results are summarized as follows:According to the first principle,the bandgaps of CsPbX₃ and Cs₄PbX₆?X=Cl,Br,I? is calculated by using CASTEP module of Materials Studio software.The density of state,absorption spectrum and mechanical properties of the two crystal structures were analyzed,they are both P-type direct gap semiconductors,the bandgap decreases with the increase of halogen atom radius,the bandgaps originate from the transition of p-orbital of halogen atom to p-orbital of Pb atom.The analysis of elastic constant and elastic modulus shows that CsPbX₃ is obviously anisotropic,and the anisotropy of CsPbX₃ increases with the increase of the radius of halide ions.It can determin that the plasticity of CsPbX₃ is better than Cs₄PbX₆ by compare their G/B value.Compared to CsPbX₃,the anisotropy of Cs₄PbX₆ is not obvious,and it is brittle,the brittleness of Cs₄PbX₆ increases with the increase of the atomic number of halogens.CsPbBr₃ and Cs₄PbBr₆ single crystals of micron scale?or millimeter scale? were obtained by using anhydrous ethanol as the antisolvent instead of dichloromethane?highly toxic? with different antisolvent addition methods,the relationship between the addition methods and the morphology of the single crystal was discussed in this paper.The dynamic equilibrium processes of the precursor solution,saturation process,critical crystal nucleus,precipitation,growth of crystal nucleus,and dissolution of crystal nucleus by precursor have an important influence on the lamellae,blocky and the grains with no obvious crystal surface characteristics.The pure phase CsPbBr₃ and Cs₄PbBr₆ were tested by UV-Vis spectrophotometer.CsPbBr₃ has no luminescence phenomenon under the excitation of UV continuous wave band.Cs₄PbBr₆ has luminescence phenomenon at 525nm wavelength under the excitation of UV.The calculated bandgap of Cs₄PbBr₆ is contrary to the phenomenon.The change of energy band structure caused by the introduction of -OH into Cs₄PbBr₆ crystal structure is simulated after considering the hydroxyl in both water and absolute alcohol,the results show that-OH does form a 2.6eV sub-bandgap in the band structure of Cs₄PbBr₆,which also explains why Cs₄PbBr₆ with 3.43eV bandgap exist the luminescence phenomenon of at 525nm.The laser properties of Cs₄PbBr₆ hexagonal grains at different positions and different times were studied by the excitation of two-photon pumped sapphire laser?800nm,45fs,1kHz?.At the edge and the center of the crystal,the excitation time is 0-57min,and the excitation emission intensity and wavelength?525 nm? remain stable.In the central position along the c-axis direction,a parallel crystal surface is provided.When the laser threshold value is 153.5?J/cm²,the excitation wavelength is 525nm laser,and the formula ??=?²/?2n_gL? is used to judge that the excitation cavity mode is Fabry Perot mode.