Nanotubes of TiO2 were inserted in working electrode of dye-sensitized solar cells. The surface states of TiO2 nanotubes and their influences on charge recombination in cell were studied. The results of the absorption spectra and X-ray photoelectron spectroscopy showed that Ti3+ species existed on TiO2 nanotube surfaces in the form of oxygen vacancy-Ti3+ states, and the analysis of the electrochemical impedance spectra of cells demonstrated that the electron recombination was aggravated in TiO2 nanotube-containing cell due to the insertion of TiO2 nanotubes in working electrodes. A possible reason was proposed. The inserted TiO2 nanotubes introduced oxygen vacancy-Ti3+ states into the band gap of TiO2, which could capture electrons validly and act as charge recombination centers, leading to the aggravated electron recombination in TiO2 nanotube-containing cell. The results here demonstrated that, although Ti3+ ions absorbing the visible light was of great significance in photocatalysis, the oxygen vacancy-Ti 3+ states, which existed on TiO2 nanotube surfaces, aggravated the electron recombination in dye-sensitized solar cells instead.

The Role of Oxygen Vacancy-Ti3+ States on TiO2 Nanotubes' Surface in Dye-Sensitized Solar Cells. Zhang, X., Tian, H., Wang, X., Xue, G., Tian, Z., Zhang, J., Yuan, S., Yu, T., Zou, Z.: Materials Letters, 2013, 100, 51-3