Self energy and excitonic effect in (un)doped TiO2 anatase: a comparative study of hybrid DFT, GW and BSE to explore optical properties

Abstract

TiO₂ anatase has significant importance in energy and environmental research. However, the major drawback of this immensely popular semi-conductor is its large bandgap of 3.2 eV. Several non-metals have been doped experimentally for extending the TiO₂ photo-absorption to the visible region. Providing in-depth theoretical guidance to the experimentalists to understand the optical properties of the doped system is therefore extremely important. We report here using a state-of-the-art hybrid density functional approach and many body perturbation theory (within the framework of GW and BSE) the optical properties of p-type (S and Se doped) and n-type (N and C doped) TiO₂ anatase. The anisotropy present in non-metal doped TiO₂ plays a significant role in the optical spectra. The p-type dopants are optically active only for light polarized along the xy direction, whereas the n-type dopants are optically active when light is polarized along the xy and z directions in the low energy region. We find that dopants give rise to new absorption peaks at low energy below 3 eV (i.e. the visible spectral region). This helps to improve the opto-electronic and solar absorption properties. All the calculations are well validated with respect to the available experimental observations on pristine TiO₂ anatase.