Theoretical study on electronic and optical properties of mixed valence perovskite Cs2Au2 X 6 (X = Cl, Br, I)

We study the electronic and optical properties of the mixed valence perovskite Cs2Au2 X 6 (X = Cl, Br, I) using the fully relativistic all-electron calculations. We find that Cs2Au2 X 6 exhibits indirect fundamental band gaps although the differences between the indirect and direct band gaps are small. For the electric field of light perpendicular to the tetragonal c axis, Cs2Au2Br6 and Cs2Au2I6 exhibit the maximum absorption coefficient of about 20 × 104 cm−1 around the photon energy of 2 eV, which is considerably larger than that of CH3NH3PbI3. For the electric field of light parallel to the tetragonal c axis, the absorption coefficient of Cs2Au2I6 is comparable to that of CH3NH3PbI3 in the main part of the solar spectrum. Furthermore, we estimate the photovoltaic performance of Cs2Au2 X 6 employing the spectroscopically limited maximum efficiency as a metric and discuss its dependence on the film thicknes in detail.