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

Abstract

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 0.5mol.% 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 2mol.% respectively. DSSC with 1mol.% Mg doped TiO2 photoanodes (1Mg-TiO2) and 1mol.% 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.