Enhanced efficiency of dye-sensitized solar cells based on Mg and la co-doped TiO₂ photoanodes

The efficiency of Dye-Sensitized Solar Cells (DSSC) can be significantly enhanced by optimizing the concentration of surface trap states and by tailoring the band gap energy in the semiconductor oxides used as photoanodes. Magnesium (Mg) and Lanthanum (La) ions have been co-doped onto TiO₂ and used for the fabrication of photoanodes for DSSC. The doped TiO₂ particles were characterized by SEM, XRF, XRD, Raman, XPS and UV-vis diffuse reflection spectroscopy. The fabricated cells were characterized by I-V and EIS techniques. XRD and Raman measurements performed on the doped powders showed only peaks ascribed to pure anatase. The particle sizes were significantly reduced by doping with the double metal ions, La and Mg, as calculated by the Scherrer equation using the anatase XRD 101 peaks. Current - voltage measurements on the fabricated DSSC presented maximum photoelectric conversion efficiency (PCE) of 8.04% from cells with 0.5 mol.% Mg and La co-doped TiO₂ anodes (0.5MgLa-TiO₂), which gave a 20% improvement over cells with pure TiO₂ anodes that had 6.7% PCE. The DSSC performance reduced gradually to 6.67 and 5.45 as the concentration of the co-dopants, Mg and La increased to 1 and 2 mol.% respectively. DSSC with 1 mol.% Mg doped TiO₂ photoanodes (1Mg-TiO₂) and 1 mol.% La doped TiO₂ photoanodes (1La-TiO₂) reached 6.8% and 7.6% PCE, respectively, lower than that of 0.5MgLa-TiO₂. EIS analysis showed that the DSSC with co-doped TiO₂ anodes indicated an overall stronger resistance to charge recombination with the electrolyte than those with the single doped and pure TiO₂ photoanodes. 0.5MgLa-TiO₂ was found to be the optimum concentration for DSSC efficiency with reduced charge recombination. This is attributed to the elevation of free surface state energy and the presence oxygen vacancy (trap states) as compared to pure TiO₂.