Lead free double perovskites of A₂’B’B’’X₆ (A=Cs⁺ ,B’=Cu⁺, Ag⁺, Na⁺  ;B’’ =Bi³⁺,Sb³⁺,In³⁺; X =F^-,Cl^-,Br^-,I^-)  architecture have created enthusiasm among the photovoltaic research  community. Especially, Cesium based all inorganic perovskite, Cs₂AgBiX₆ has shown promising optoelectronic properties due to their optimal bandgaps, long diffusion length, high absorption coefficient etc. First all the Cs₂AgBiX₆ (X=F, Cl, Br, I) structures were optimized at the PBE level.  Then the optoelectronic properties of Cs₂AgBiX₆ (X=F, Cl, Br, I) were investigated employing Density Functional Theory (DFT).  Structural, electronic and optical properties of Cs₂AgBiX₆ cubic Fm-225 and tetragonal phase I4/m were obtained using the VASP code. Generalized Gradient Approximation (GGA) and Hybrid functional method (HSE06) were used to analyze the ground state properties of optimal Cs₂AgBiX₆ structures. Among all the Cs₂AgBiX₆ (X=F, Cl, Br, I)  structures, the Cubic phase of Cs₂AgBiBr₆ had seen to have the highest absorption coefficient along with prominent electronic features that are favourable for optoelectronic applications. Thus, the cubic Cs₂AgBiBr₆ was selected for gradual substitution of other halide ions.  The Br^- ions were partly replaced with Cl^-  and I^- ions in direct substitutions. Electronic and optical properties were investigated by employing GGA and HSE06 methods for these mixed halide compounds of Cs₂AgBiBr_6-xCl_x and Cs₂AgBiBr_6-xI_x  where x=1,2,3,4,5. We found out that the phase of the Cs₂AgBiBr₆ changed from cubic phase to tetragonal (x=5) when the chloride and iodide substitutions were done. It is also noted that these  mixed compounds exhibited indirect band gap nature regardless of the halide composition (different x values) and the band gap of Cs₂AgBiBr₆ could be tuned by Cl and I doping. Our in-depth analysis shows that Cs₂AgBiBr₆ and their mixed halides have the potential to become active double perovskite materials for PV applications. 

Keywords: Density functional Theory, Cs₂AgBiX₆, mixed halides, optoelectronic properties