Structural, elastic, electronic and optical properties of lead-free halide double perovskite Cs2AgBiX6 (X = Cl, Br, and I)

In search of the lead-free photovoltaic materials that could be used as the potential alternative of the front runner hybrid perovskite CH3NH3PbI3, we have employed first-principles calculations to investigate the structural, elastic, electronic and optical properties of lead-free inorganic halide double perovskite materials Cs2AgBiX6 (X = Cl, Br and I). The optimized lattice parameters are in good agreement with experimental values (for X = Cl and Br) and we predict lattice parameter for not synthesized yet Cs2AgBiI6 to be 12.27 Å. Among the double perovskites under investigation, the Cs2AgBiBr6 composition has the highest elastic constants and elastic modulli values which indicate the large response to uniaxial strain with high incompressibility and hardness of the bromide composition compared to other two compositions. We believe that almost equal sized alternate octahedral in bromide composition reproduce an incompressible and hard structure. All the three halide compositions have indirect nature of the band gap with the high dispersive nature of valence band maximum and conduction band minimum and results in smaller values of effective masses. The energy band gap decreases by replacing smaller Cl ion to larger I ion in Cs2AgBiX6 composition and as a result the tendency of the red shift in all the optical spectra is observed.