Investigation of Structural and Optoelectronic Properties of N-Doped Hexagonal Phases of TiO2 (TiO2-xNx) Nanoparticles with DFT Realization: OPTIMIZATION of the Band Gap and Optical Properties for Visible-Light Absorption and Photovoltaic Applications

The geometric structure, electronic and optical properties of N-doped TiO2 (TiO2-xNx) were studied within the framework of density functional theory. The effective electron-electron exchange correlation functional and the modified Becke-Johnson potential were used to calculate electronic and optical properties. The calculated optical parameters and the density of electronic states indicate that the TiO2-xNx (0.06 <= x <= 0.25) system has a property favorable for application in solar cells. The calculated structural characteristics show that the size of these systems increases with the increasing concentration of additives. The electronic properties of N-doped TiO2 show that the bandgaps tend to decrease, and some 2p states of N atoms are located inside the bandgap, which leads to a decrease in the photon energy of the transition and absorption of visible light. As a result, the bandgap effectively decreases with doping concentration increase, while the absorption is effectively improved due to the extended absorption range, both ultraviolet, visible, and infrared range of light emission. It was found that the optimal concentration of nitrogen doping (12.5 at.%) noticeably increases the absorption capacity; hence, the conversion efficiency of TiO2 in the visible region of radiation and effectively reduces the bandgap from 3.2 to 2.4 eV. However, any further increase in concentration does not lead to an additional improvement of the absorption capacity despite the change in the bandgap, which is in good agreement with the existing experimental data. These superior characteristics make N-doped TiO2 a promising material for low-cost, high-efficiency solar cells for the mass market.