A novel approach to titania nanowire arrays as photoanodes of back-illuminated dye-sensitized solar cells

Titania nanowire arrays have been deposited on Ti foils through direct oxidizing the Ti substrate with aqueous hydrogen peroxide solutions containing melamine and nitric acid, and the applicability of such nanowire arrays to back-illuminated dye-sensitized solar cells studied in parallel with titania nanotube arrays on Ti foils. The low-temperature nitrogen adsorption measurement reveals that the film with nanowires 25 nm in diameter and 1 μm in length possesses a BET specific surface area of 59.0 m2 g−1, a value much larger than 26 m2 g−1 calculated for the nanotube with an inner diameter of 80 nm, an outer diameter of 120 nm and a total length of 3 μm. Assuming an indirect transition between band gaps, the nanowire film exhibits a bandgap of 3.1 eV, slightly larger than that of 3.0 eV for the nanotube one. A detailed electrochemical study suggests that, in comparison with the nanotube film, the nanowire one exhibits much lower saturated photocurrent and poorer conductivity under the Xe-lamp irradiation. However, when utilized to construct back-side illuminated dye-sensitized solar cells, the cell with the 2 μm-thick nanowire photoanode possesses significantly higher efficiency than the one with the 3 μm-thick nanotube photoanode. The relatively high energy conversion efficiency is contributed to the high specific surface area and the unique mesoporous structure of the titania nanowire arrays, which favors the adsorption of dye molecules.