Research On Doping Inorganic Double Perovskite Luminescent Materials

Lead halide perovskite materials can be applied in photovoltaic devices such as solar cells,light-emitting diodes（LED）because of their excellent photoelectric properties like high fluorescence quantum yield（PLQY）,high light absorption coefficient,high carrier mobility,high defect tolerance,direct and continuous adjustable band gap.However,lead halide perovskite materials face two problems in practical application:poor stability and strong toxicity.Thus,researchers are trying to develop stable lead-free perovskites.In 2017,a new lead-free perovskite,Cs₂Ag InCl₆,replacing two Pb²⁺with one Ag⁺and one Bi³⁺,was first reported.Cs₂Ag InCl₆is reported to be a direct bandgap semiconductor material with long carrier lifetime and wide photoemission spectrum,which is beneficial for its application in the field of WLED.However,the practical application of Cs₂Ag InCl₆faces two major problems:（1）The traditional synthesis methods of Cs₂Ag InCl₆are liquid-phase methods,which are time-consuming,high energy consumption,low yield and waste generation,which are not suitable for large-scale industrial production;（2）Due to parity forbidden transition,the PLQY of Cs₂Ag InCl₆is low,less than 0.1%.To solve the above problems,this paper developed a pure solid phase ball milling method to synthesize Cs₂Ag InCl₆double perovskite,which does not need to add auxiliary reagents,and is highly efficient and pollution-free.At the same time,the luminescence properties and lattice structure of Cs₂Ag InCl₆double perovskite were regulated and optimized by ionization doping method.The luminescence mechanism of Cs₂Ag InCl₆double perovskite was investigated by X-ray diffraction,spectral detection and other methods.The research contents of this paper are as follows:1.The PLQY of double perovskite Cs₂Ag InCl₆was increased by codoping Na⁺and Bi³⁺through solid phase ball milling.The doping of Na⁺and Bi³⁺can optimize the luminescence properties of Cs₂Ag InCl₆,increasing PLQY from 1.1%to 32.4%.Spectral analysis results show that the wide spectral luminescence of Cs₂Ag InCl₆belongs to the self-trapped exciton（STE）,which is derived from the Jahn-Teller distortion formed by the[Ag Cl₆]octahedron.Na⁺doping can break parity forbidden transition induced by inversion symmetry of Cs₂Ag InCl₆lattice structure.Bi³⁺doping can reduce the lattice defects of Cs₂Ag InCl₆,improve the crystallinity and weaken the non-radiative recombination.The WLED can be prepared by packaging the Na⁺and Bi³⁺doped Cs₂Ag InCl₆on the commercial ultraviolet（UV）chip.The color temperature of the WLED device is 2953 K and the color coordinate（0.44,0.41）is in line with the international standard of warm white light.Meanwhile,the color rendering index reaches 88.7,which reflects the potential of double perovskite Cs₂Ag InCl₆in the field of white light lighting.2.The effect of Al³⁺partial substitution of In³⁺on the luminescent properties of Cs₂Ag In_0.9Bi_0.1Cl₆was studied.The results of structural characterization show that Al³⁺partially replaces In³⁺into the Cs₂Ag In_0.9Bi_0.1Cl₆lattice.The spectral study shows that the band gap of the product decreases from 2.95 e V to 2.90 e V,and the emission spectrum redshifts from 620 nm to 650 nm.Density functional theory（DFT）theoretical calculation results show that the conduction band minimum（CBM）and valence band maximum（VBM）of Al³⁺doping products are flat,which is conducive to the formation of STE.This study proves that Al³⁺doping can achieve the regulation of Cs₂Ag InCl₆luminescence center wavelength,to achieve the purpose of regulating Cs₂Ag InCl₆luminescence performance.3.The effect of K⁺partial substitution of Ag⁺on the structure and luminescence properties of Cs₂Ag InCl₆was studied.The results show that when K⁺doping is less than 60%,K⁺mainly replaces Ag⁺,which causes lattice expansion of Cs₂Ag InCl₆,eliminates inversion symmetry of lattice structure,breaks parity forbidden transition,and enhances photoluminescence intensity of Cs₂Ag InCl₆.The optimal doping ratio is 40%,the center wavelength of Cs₂Ag_0.6K_0.4InCl₆is 640 nm,the half-height width is 180 nm,the average fluorescence lifetime is 29.2 ns and the PLQY reaches to 10.5%.When the doping ratio of K⁺exceeds 60%,K⁺begins to replace the position of Cs⁺,which results in the presence of cubic phase Cs_2-xK_1+x-yAg_yInCl₆and monoclinic phase Cs_2-xK_1+xInCl₆.These products are dominated by non-radiative recombination due to strong electron-phonon coupling.