Effects of ionic radii of co-dopants (Mg, Ca, Al and La) in TiO2 on performance of dye-sensitized solar cells

Doping is a method whereby small amounts of impurities are introduced into the lattice of a material and is employed for improving the efficiency of dye-sensitized solar cells (DSSCs) via modification of the semiconductor material. Co-doping, which involves doping with more than one type of impurity, is typically used over doping owing to its unique effects on the short-circuit current and open-circuit voltage; the different types of added impurities can exert a synergy effect in enhancing the performance of DSSCs. Of particular interest is the study of the effects of the ionic radii of co-dopants on the performance of DSSCs that is not well understood to date. Thus, in this paper, TiO2 was co-doped with lanthanum and magnesium, calcium, or aluminium at respective contents of 0.5 mol%. The co-doped TiO2 and pure TiO2 powders were prepared by sol-gel. Mg-La co-doped TiO2-based DSSC displayed the highest efficiency (i.e., 6.60%) and Al-La co-doped TiO2-based DSSC displayed the lowest efficiency. Among the dopants studied herein, aluminium ion has the smallest ionic radius. Thus, aluminium ions could easily enter the lattice structure of TiO2, generating a high concentration of holes that led to the decrease of electron transport and photocurrent, hence lower DSSC efficiency.