Theoretical study of Zr doping on the stability, mechanical, electronic and optical properties of Cs2TiI6

In recent years, all-inorganic lead-free double perovskites have attracted great attention due to their excellent optoelectronic properties. In this work, we have investigated the structural stability, mechanical, electronic and optical properties of Cs2Ti1-xZrxI6 (x = 0, 0.25, 0.5, 0.75, 0.875, 1) based on first-principles calculations for the first time. With the increase of Zr doping concentration, the structural stability of Cs2TiI6 is improved. Moreover, these mixed Cs2Ti1-xZrxI6 systems are predicted to be stable according to the phonon spectrum calculation results. Our calculated results suggest that these Zr-doped double perovskites are brittle materials. When the value of x changes from 0 to 0.875, the band gap variation of Cs2Ti1-xZrxI6 is only 0.26 eV. However, the optical absorption coefficients are apparently decreased after Zr doping. The mixed Cs2Ti1-xZrxI6 systems have suitable band gaps for single-junction solar cells. The band gap of Cs2ZrI6 is predicted to be 1.92 eV, which is an ideal material for tandem solar cell. Our study is expected to explore their future applications in perovskite solar cells.