Charged Defects in TiO₂Anatase: A Comparative Study of Hybrid DFT, GW and BSE to Explore Optical Properties

Titanium dioxide (TiO₂) anatase is one of the most abundant, functionally versatile oxide materials. However, owing to its large bandgap, the photo-absorption efficiency is limited only in the UV region. Doping-mediated modulation is one of the most pragmatic approaches in the pursuit to improve the photocatalytic and solar energy conversion efficiencies of TiO₂. We report here using state-of-the-art hybrid density functional and ab initio atomistic thermodynamics, the thermodynamic (meta-) stability of different non-metal dopants X (X= N, C, S, Se) as a function of charge state at realistic temperature and pressure. Knowing the most stable defects, we aim an accurate theoretical estimation of the optical properties of doped TiO₂. To do this, we have used many body perturbation theory viz. one particle Green's function (GW) method and higher order Green's function techniques Bethe-Salpeter Equation (BSE). Our calculations reveal the highly anisotropic nature of the doped TiO₂. The n-type doped TiO₂ is optically active in both x, z direction, whereas the p-type doped TiO₂ is optically inactive along z-direction. The most evident hallmark of the doped system is the appearance of absorption peaks at low energy below 3 eV, to give rise visible-light absorption.