Tunable Properties Of Lead-free Halide Double Perovskites:A Theoretical Study

The discovery of lead-free halide double perovskites(LHDPs)has provided an efficient way to search air-stable and environmentally benign solar cell absorbers.However,the performances of LHDPs based solar cells are lower than those of organic-inorganic hybrid perovskite based solar cells.Strain engineering and pressure regulation has been regarded as two effective way to tune the properties of materials.Deeply understanding the effect of strain and pressure on the physical properties of LHDPs is vital for designing high performance solar cells.In this dissertation,the turnable optical,electrical,transport,mechanical and thermal properties of double perovskites Cs₂AgInX₆(X=Cl,Br,I)and Cs₂AgBiX₆(X=Cl,Br,I)by strain and pressure are studied by using VASP software and the first principles calculation based on density functional theory.1.Firstly,this work comparatively investigated the effect of pressure on the structure and photoelectric performance of Cs₂AgInX₆(X=Cl,Br,I)and Cs₂AgBiX₆(X=Cl,Br,I).Analysis shows that although Cs₂AgB"X₆ possess similar cubic structures,while the bond length and strength characters of Cs₂AgInX₆ are different to those of Cs₂AgBiX₆,which will influence their optical and electrical properties.Upon undergoing pressure,the stablility of Cs₂AgB"X₆ is improved and without phase-transition.In addition,the band gap of Cs₂AgInX₆ decreases first and then enlarged,while the band gap of Cs₂AgBiX₆ shifts down monotonously with the increasing pressure.2.Secondly,this work investigated the elastic and thermal properties of Cs₂AgB"X₆(B"=In,Bi;X=Cl,Br,I)under different pressures.Results show that the elastic constants of Cs₂AgInX₆ are lower than those of Cs₂AgBiX₆,and irrespective of pressure,while opposite characters are observed for the G and B under high pressure.Compared to Cs₂AgBiX₆,Cs₂AgInX₆ may be more suitable to flexible device due to the higher shear and bulk moduli,and they also show better anisotropy.In addition,the thermal conductivities of Cs₂AgBiX₆(X=Cl,Br)and Cs₂AgBiX₆(X=Cl,Br,I)are 0.41,0.32,0.37,0.29,0.23 W·m-1·K-1,respectively,and can be enlarged by pressure and surpass that of lead halide perovskite.Moreover,all the Cs₂AgB"X₆(B"=In,Bi;X=Cl,Br,I)possess the same saturated heat capacity of about 249.3 J·mol-1·K-1.Both the heat capacity and the coefficient of thermal expansion increase sharply with temperature and then reach saturation values,but the saturated thermal expansion of Cs₂AgB"X₆(B”=In,Bi;X=Cl,Br,I)are not the same3.Finally,the electrical,optical and transport properties of Cs₂AgInX₆(X=Cl,Br)were studied,and the uniaxial strain method was used to regulate these properties.The calculated band gaps of Cs₂AgInCl₆ and Cs₂AgInBr₆ are 0.65 and 0.58 eV,respectively,and the band gaps of Cs₂AgInCl₆ are reduced negligibly by strain,different to that of strained CS2AgInBr₆.The contribution degrees of Ag-dz2,dx2-y2 and In-dz2,dx2-y2 on the band edges of Cs₂AgInX₆(X=Cl,Br)are inconformity,resulting in a direct-to-indirect transition for Cs₂AgInBr₆ at 4%compressive strain.Moreover,the effective electron mass and electron mobility of Cs₂AgInCl₆(0.27 m0,976.79 cm2.V-1·s-1)are slightly lower than those of Cs₂AgInBr₆(0.39 m0,997.06 cm2·V-1·s-1),and the mobility of Cs₂AgInCl₆ is reduced by strains,while that of CS2AgInBr₆ is enhanced(reduced)by compression(tension).In addition,the absorption coefficients of Cs₂AgInX₆(X=Cl,Br)are deteriorated negligibly by strain.