Electrochemical Properties of Transparent Conducting Films of Tantalum-Doped Titanium Dioxide

Highly conducting, optically transparent Ta-doped TiO2 (anatase) thin films are grown on ordinary soda-lime glass substrates by pulsed-laser deposition. They exhibit quasi-reversible cyclic voltammograms of Fe(CN)63−/4− and dimethylviologen redox couples, mimicking the electrochemical activity of F-doped SnO2 (FTO). Hence, our Ta-doped titania films can prospectively replace FTO, e.g. in homo-junction dye-sensitized and perovskite solar cells. However, these films are idle for the Ru(bpy)32+ oxidation, which is attributed to the space-charge barrier. The flatband potential of our Ta-doped TiO2 is comparable to that of an undoped reference film and/or of the pristine anatase single-crystal electrode. Our films show photoelectrochemical activity upon irradiation with UV light at potentials positive to flatband. The UV-photocurrents decrease proportionally to the increase of Ta-content. The Li-insertion ability analogously decreases with increasing Ta-content. This is attributed to the positive charge of Ta5+ cations which occupy the Ti4+ sites in anatase lattice and thus impede the Li+-transport. Consistent with the quasi-metallic nature of our films, the Li-extraction peak in cyclic voltammograms shows no cut at larger potentials.