Titanium dioxide (TiO2) polymorphs have recently gained a lot of attention in dye-sensitized solar cells (DSSCs). The brookite polymorph, among other TiO2 polymorphs, is now becoming the focus of research in DSSC applications, despite the difficulties in obtaining it as a pure phase experimentally. The current theoretical study used different nonmetals (C, S and N) and (C-S, C-N and S-N) as dopants and co-dopants, respectively, to investigate the effects of mono-doping and co-doping on the electronic, structural, and optical structure properties of (210) TiO2 brookite surfaces, which is the most exposed surface of brookite. The results show that due to the narrowing of the band gap and the presence of impurity levels in the band gap, all mono-doped and co-doped TiO2 brookite (210) surfaces exhibit some redshift. In particular, the C-doped, and C-N co-doped TiO2 brookite (210) surfaces exhibit better absorption in the visible region of the electromagnetic spectrum in comparison to the pure, S-doped, N-doped, C-S co-doped and N-S co-doped TiO2 brookite (210) surfaces.

The most popular semiconductor in dye-sensitized solar cells (DSSCs) is titanium dioxide (TiO2) because of its low cost, non-toxicity, and good stability. However, the DSSCs still have low efficiency due to the low light absorption of TiO2 in the visible region. Understanding the properties of TiO2 can contribute to improving the efficiency of DSSCs. In this study, we use density functional theory to investigate the electronic and optical properties of TiO2 brookite (210) surface mono-doped and co-doped with 4d transition metals, silver, and molybdenum. Our results show that the band gap energy of brookite (210) surface is 3.514 eV, which reduces to 1.143 eV and 0.183 eV when doped with Ag and Mo, respectively. However, doping with both Ag and Mo yielded a band gap of 0.387 eV. The results suggest the presence of Ag and Mo 4d states below the conduction band minimum, which could be responsible for the narrowing of the band gap on brookite (210) surface. Both mono-doped and co-doped brookite (210) surfaces have higher visible light absorbance compared to the undoped brookite (210) surface and extends to the near-infrared region.