Abstract
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.
Highlights
Research on dyesensitized solar cells (DSSCs) usually based on nanocrystalline TiO2 has been extensively pursued, and the number of papers and patents published in this area has grown exponentially over the last ten years
It is known that C, S, F and N - doped titanium dioxide exhibit significantly higher photoactivity compared to pure TiO2, and among them, nitrogen is the best candidate for optimizing TiO2 with visible light [14]
A sufficient amount of work has been devoted to doping with a nitrogen of the anatase TiO2 cell, which showed that doping shifts the absorption edge to the region of lower energies and increases the absorption capacity of the material in the visible region by reducing the bandgap [14, 16-19]
Summary
Molecular modeling with different computational methods shows a potential application for elucidating physical, chemical, and biological properties for several systems and molecules [1-5]. Stable solar energy of AM 1.5 spectra cannot be converted, and it exhibits significant activity only when irradiated with ultraviolet light of less than 360 nm In this regard, many efforts have been directed to overcome the disadvantages and optimize the absorption capacity and photocatalytic properties of TiO2, especially in the visible light range. The possibility of using this approach is precisely due to the fact that the energy levels of such defects can be located in the band so that the absorption of light by defects (impurity absorption) is possible in the longer wavelength region [11] The efficiency of such an approach was shown in many works, but the activity depends on many parameters, such as the concentration of the dopant, the energy levels of the dopant in the titanium dioxide lattice, and the distribution of doping atoms [12]. Titanium dioxide doped in oxygen position has both shallow donor and acceptor levels above the conduction and valence bands, respectively, due to the oxygen vacancy [17] and TiO2 (in the structure of anatase and rutile) containing nitrogen in the interstices of the crystal lattice has isolated impurity levels in the bandgap
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