A dye-sensitized solar cell (DSSC) is mainly composed of a dye-adsorbed semiconductor (photoanode), an electrolyte, and a counter electrode. When incident light is illuminated, excited electrons from the dye are injected into the semiconductor. The electrons migrate through the semiconductor and attain to an external circuit. However, charge recombination can occur in the semiconductor layer, and it leads to efficiency diminution of a DSSC. Therefore, various investigations have been reported on modification of the semiconductor to minimize charge recombination.
Meanwhile, we tried to modify a TiO₂ electrode with a metal-doped TiO₂. Pure TiO₂ generally known as an n-type semiconductor was gained via hydrothermal process. In addition, we prepared 0.25 mol% Cr-doped TiO₂ via similar procedure to the TiO₂ synthesis above. The resulting TiO₂s were sequentially deposited on a FTO (F-doped tin oxide) substrate by using a polymer binder and calcined at 500 ^oC for 30 min.
According to I-V measurement (Fig. 1), the double-layered electrode worked as a species of diode, thus we confirmed that Cr-doped TiO₂ became a p-type semiconductor and a p-n homojunction was successfully formed at the interface of the TiO₂ layers. This p-n homojunction was expected not only to enhance charge transport but to also repress charge recombination since electrons would be forced to transfer to the direction in accordance with electric field appearing in the junction. Consequently, the DSSC adopting this p-n homojunction as a photoanode exhibited improved overall conversion efficiency by ca. 18 % with respect to that employing pure TiO₂.