Theoretical Study On The Defects Of Double Perovskite Structure Cs₂AgBiBr₆

Solar cells are considered to be one of the important directions to solve the future energy crisis.Therefore,research on low-cost and high-efficiency solar cells is essential for commercial production.Current studies have found that the perovskite structure halide has superior photoelectric properties,but traditional perovskite solar cells still have disadvantages such as environmental instability and lead toxicity.Inorganic lead-free halide double perovskite is a large class of compounds with the general formula A₂B⁺B³⁺X₆.The crystal structure of the double perovskite material A₂B⁺B³⁺X₆ is similar to that of the perovskite material ABX ₃.These double perovskite materials have good photoelectric properties,including long carrier life,good stability to air and moisture,and with the characteristics of low effective carrier mass,lead-free double perovskite materials represented by Cs₂AgBiBr₆ have great potential in the application of solar light absorption layers.In the production of semiconductor optoelectronic devices,defects are often introduced.Defects can be used as a suitable method to adjust the optoelectronic properties of optoelectronic materials.In recent years,double perovskite Cs₂AgBiBr₆ has attracted widespread attention as a new type of perovskite photoelectric material because of its non-toxicity and enhanced chemical and thermodynamic stability.In this paper,based on the calculation method of density functional theory,the influence of vacancy defects and impurity defects on the photoelectric properties of inorganic double perovskite Cs₂AgBiBr₆ is studied,the effects of these two different defects on the structure,electronic structure,optical properties,exciton binding energy and carrier mobility of the double perovskite Cs₂AgBiBr₆ material are discussed separately.The main work of this paper is as follows:(1)In order to understand the effect of vacancy defects on the optical and electrical properties of double perovskite Cs₂AgBiBr₆,based on density functional theory and supercell model,the single-atom vacancies of Cs,Ag,Bi and Br,and Cs-Br and Ag-Br atom pairs were systematically studied.The effect of vacancy on double perovskite Cs₂AgBiBr₆ crystal structure,electronic structure,light absorption,binding energy of carriers and excitons.It is found that the vacancy defects in the double perovskite Cs₂AgBiBr₆ cause less distortion of the crystal lattice;the vacancy defects cannot generate additional defect states in the band gap,and the Cs,Ag,Bi,Ag-Br and Cs-Br defects will change the band gap from The indirect band gap of the undoped system becomes the direct band gap,and the band gap value is reduced;the introduction of Cs,Ag,Bi,Ag-Br and Cs-Br vacancy defects enhances the light absorption capacity and causes the red shift of the absorption spectrum in the low energy region;in terms of the effective mass of electrons and holes,the introduction of Cs,Ag,Bi,Ag-Br and Cs-Br vacancy defects will produce unbalanced charge transport properties and increase exciton binding energy.The results of this work provide us with a new theoretical perspective on how vacancy defects affect the photoelectric properties of double perovskite Cs₂AgBiBr₆.(2)Doping is a relatively effective method to solve the structural stability of the inorganic double perovskite Cs ₂AgBiBr₆ solar cell and improve the photoelectric conversion efficiency.Based on the supercell model,the substitution of Rb for one of the Ag and Cs atoms in the double perovskite Cs₂AgBiBr₆supercell was studied,and the doping concentration was 6.25%.Tolerance factors and octahedral factors indicate that the Rb-doped perovskite system is stable;the impurity formation energy(energy to replace Ag and Cs is-5.37 and-3.00 eV)indicates the introduction of Rb doping Impurity is an exothermic process;Rb doping changes the indirect band gap(2.25 eV)of the double perovskite Cs₂AgBiBr₆ into a direct band gap(the band gap values of the substituted Ag and Cs systems are 2.27 and2.19 eV),and in the band gap no impurity state is introduced;doping also expands the absorption range of visible light,reduces(increases)the effective mass of electrons(holes),and increases the elastic constant.(3)The photoelectric properties of CH(NH₂)₂⁺(FA)doped perovskite solar cell material Cs Pb I₃ are highly sensitive to the orientation of organic cations.The organic-inorganic hybrid double perovskite(FA)₂Bi Cu I₆material has excellent optical and electrical properties.There are good prospects in the application of photovoltaic and optoelectronic devices.In order to understand the effect of FA doping on the photoelectric properties of double perovskite Cs₂AgBiBr₆,this paper studied the organic cation FA in five different molecular orientations to replace a Cs atom in the super unit cell,the corresponding atomic coordinates is(0.5,0.5,0.5).The influence of the introduction of FA impurities on the geometric structure,electronic structure,optical properties,exciton dissociation and carrier mobility of the double perovskite Cs₂AgBiBr₆ was explored and analyzed in detail.The geometric structure results show that FA doping increases the supercell volume of the double perovskite Cs_1.9375FA_0.0625AgBiBr₆by 0.41%,0.50%,0.50%,0.39%and 0.34%;the electronic structure shows that the band gap after FA doping is an indirect band gap,and the band gap value increases(the band gap value of Cs₂AgBiBr₆ is 2.25eV,and the band gap value of Cs_1.9375FA_0.0625AgBiBr₆ are 2.39,2.38,2.40,2.41 and 2.37eV),there is no impurity state in the band gap;the light absorption spectrum shows that in the energy range of 0-3eV and 3.9-4.6eV,it exhibits better light absorption performance than the pure phase double perovskite Cs₂AgBiBr₆;the introduction of FA increases the effective mass of electrons and reduces the effective mass of holes.As a result,the hole carrier mobility is greater than the electron carrier mobility.It is hoped that this research can provide a theoretical basis for the development of stable organic-inorganic hybrid halide double perovskite.