First-principles study on properties of lanthanide (Ln)-doped TiO2 (Ln = Ce, Nd and Er) for dye-sensitized solar cells using density functional theory / Mohd Hazrie Samat

In dye-sensitized solar cell (DSSC), titanium dioxide (TiO2) acts as a support for dye. However, TiO2 alone in DSSC has low light absorption in ultraviolet (UV) spectrum which causes low efficiency of DSSC. Thus, modification of TiO2 by doping with lanthanide (Ln) elements have been used as photoanodes in DSSC. However, an understanding on the structural, electronic and optical properties of Ln-doped TiO2 in DSSC is not fully understood. Therefore, the first-principles study on the properties of TiO2 and Ln-doped TiO2 using density functional theory (DPT) were investigated with special reference to their role as photoanodes in DSSC. The structural parameters of TiO2 in rutile, anatase and brookite phases are different because of the difference in atomic position of titanium (Ti) and oxygen (0) atoms. The large cell volume of anatase offers more space for dye loading capability which is desirable for inducing higher light harvesting. The DPT plus Hubbard U (DFT+U) yields a good electronic properties and phase stability of TiO2 compared to DPT. The high band gap of anatase describes its high photovoltage and the density of states (DOS) displays the hybridization between Ti 3d and O 2p states. The electron distribution and chemical bonding nature of TiO2 were explained from the electron density map while the Mulliken population analysis presents the net charge (e) of Ti and O atoms. The band gap of anatase TiO2 at 3.2 eV makes it sensitive only in UV light which neglects the light absorption in longer wavelength spectrum. Hence, the effect of TiO2 doping with Ln using cerium (Ce), neodymium (Nd) and erbium (Er) were determined. The Ln-doped TiO2 has lower band gaps compared to pure TiO2 due to the presence of impurity energy levels (IELs) from Ln 4/states. The presence of Ln 4/states in Ln-doped TiO2 can be seen from the DOS. Among the Ln-doped Ti Oz, the shifting of light towards longer wavelength spectrum is from Nd-doped TiO2. For further study on Nd-doped TiO2, the properties of novel materials from different Nd concentrations in TiO2 with stoichiometry Ti1-xNdxO2 at x = 0, 0.0625, 0.125 and 0.18750 were investigated. As the Nd concentration increases, the light absorption edge shifts towards longer wavelength spectrum, indicating efficient light harvesting to boost performance of DSSC. Overall, the first-principles study from the deepest atomic level in this work can clarify the doping effects in TiO2 and may improve the understanding on DSSC.