Growth And Characterization Of CH₃NH₃PbCl₃ And Mixed-halide Perovskite Photoelectric Crystals

The organic-inorganic hybrid perovskite materials CH3NH3PbXa(X= halide)have attracted great interest due to their superior optical and electrical properties.The solar cells based on these materials have the advantages of low cost,self-assembly and high power conversion efficiencies(PCEs),etc.However,many of their physical and chemical properties are not clear.In this paper,CH3NH3PbC13.CH3NH3PbI3-xClx、CH3NH3PbI3-Brx and CH3NH3PbBr3-yCly single crystals were grown by the solution method and their growth method,structure and optical properties were studied.The main research contents are as follows:(1)CH3NH3PbCl3 single crystals with the size about 4mm×3 mm×3 mm were grown by evaporation and inversion crystallization method,respectively.Powder X-ray diffraction(PXRD)of the obtained crystals powder showed that the crystals were of cubic structure,and their lattice constants were 5.6833 A and 5.6891 A,respectively.The optical properties were characterized by UV-VIS-NIR and photoluminescence spectroscopy.The results showed that the absorption edge of CH3NH3PbC13 crystal was about 423 nm,with the photoluminescence peak at 433 nm.the band gap at 2.97 eV.(2)CH3NH3PbI3-xClx crystal was grown by solution cooling method with CH3NH3I and PbCl2 as raw materials.Lead compounds and CH3NH3PbI3-xClx crystal were obtained with different CH3NH3I/PbCI2 ratios.Our results show that CH3NH3PbI3-xClx single crystal with very low chlorine content can be obtained by solution method.XPS result shows the content of chlorine in CH3NH3PbI3-xClx is about 0.91 at.%.The CHaNH3PbI3-Clx crystal grown is of tetragonal structure with the lattice constant a = b = 8.8165 A,c = 12.7920 A and the bandgap value of 1.57 eV.(3)Systematic halogen doping CH3NH3PbI3-xBrx and CH3NH3PbBr3-yCly bulk single crystals were grown with mixed organic solvents via the ITC(Inverse temperature crystallization)method.The result of X-ray diffraction measurement shows the lattice constants of CH3NH3PbI3-xBrx and CH3NH3PbBr3-yCly crystals change linearly with x(y)values.For CH3NH3PbI3-xBrx crystals,cubic to tetragonal phase transformation was observed where the iodine content reaches to a certain level.The comparison of UV-vis-NIR absorption and PL spectra of CH3NH3PbI3-xBrx and CH3NH3PbBr3-yCly crystals shows their absorption coefficients are contributed by direct and indirect bandgap transitions.Three kinds of mechanism were involved in their radiative recombination,including direct emission,indirect emission and direct-indirect coexisting emission.CH3NH3PbI3-xBrx(x=1.645 2.39,2.81,2.9)crystals were considered to be indirect band gap materials,since their band edges can be divided into two sections by a turning point.TG analysis shows the beginning decomposing temperature of CH3NH3PbI3-xBrx and CH3NH3PbBr3-yCly crystals decreases with the increasing x(y)values.