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.