First-principles calculations of structural, electronic, and optical properties of double perovskites Cs2Sn1- x B x I6 (B = Si, Ge; x = 0, 0.25, 0.50, 0.75, 1)

Highlights: • The effect of Si/Ge doping on the material properties of Cs₂SnI₆ is studied. • The structural stability of Cs₂SnI₆ is reduced by doping Si or Ge. • Cs₂SnI₆ shows strong optical absorption coefficient in the visible light region. In this study, the structural stability, electronic and optical properties of all-inorganic mixed-metal double perovskites Cs₂Sn_1-xB_xI₆ (B = Si, Ge; x = 0, 0.25, 0.50, 0.75, 1) have been explored by using first-principles calculations. Our calculated results show that Cs₂SnI₆ shows excellent stability, while Cs₂SiI₆ and Cs₂GeI₆ are unstable. Moreover, the structural stability of Cs₂SnI₆ is reduced by doping Si or Ge. The predicted band gaps of the Cs₂Sn_1-xSi_xI₆ systems are in the range of 1.1–1.3 eV. The band gaps of the Cs₂Sn_1-xGe_xI₆ systems decrease gradually with the increasing concentration of Ge doping. Cs₂SnI₆ exhibits the highest optical absorption coefficient in the visible light region among the Cs₂Sn_1-xSi_xI₆ systems. The Cs₂Sn_1-xGe_xI₆ systems show stronger absorption coefficients in the range of 500–1000 nm compared to the pure Cs₂SnI₆ perovskite. Based on our calculated results, Cs₂SnI₆ is expected to become the most promising candidate material for perovskite solar cells.