Two-dimensional optical excitations in the mixed-valence Cs2Au2I6 fully inorganic double perovskite

In the continuous quest for better performing materials in photovoltaics, here we investigate the electronic and optical properties of the mixed-valence Cs2Au2I6 fully inorganic double perovskite. In particular, building on previous experimental and theoretical analyses, we calculate the quasiparticle and excitonic features of this appealing and overlooked material using a combined fully ab initio approach based on density functional and many-body perturbation theories. The interest in this material is associated with a charge transfer transition from the valence band to an intermediate band that are constituted by the same atom (Au) but in a different oxidation state. The bandgap calculated with the GW method is direct and the value of 1.35 eV is of considerable interest for solar-harvesting applications. The importance of including both local field effects and electron-hole interaction in the calculation of the optical properties of the material is clearly demonstrated by solving the Bethe-Salpeter equation. Furthermore, a quasi-2D exciton spatial localization is elucidated, a feature that correlates well with the in-plane Au(i) -> Au(iii) charge transfer excitation.