Electrodeposition is a convenient method for the preparation of TiO₂ films on conductive substrates, e.g. for dye-sensitized solar cells (DSSC). A recently proposed method [1] employs basic Ti(IV)-alkoxide solutions containing hydroquinone, which is oxidized to benzoquinone, generating protons that induce the precipitation of Ti(IV)-oxo-hydroxide. Benzoquinone is incorporated into the films, providing conducting pathways and thereby promoting film growth. Calcination of the films results in formation of crystalline TiO₂ and removal of the benzoquinone, leading to pore formation. Utilization of other proton suppliers (e.g. 2-methyl-hydroquinone) allows for pore size variation [2].
We have now studied the influence of the calcination temperature on the DSSC performance of the films. Higher temperatures lower the porosity of the films by grain coarsening, decreasing the dye loading capacity, whereas the electron collection efficiency increases as confirmed by intensity modulated photocurrent / photovoltage spectroscopy. Due to these contrary effects the efficiency reaches its optimum at a calcination temperature of 500 °C (Figure 1a). The main limiting factor in DSSC performance for these films was found to be the low film thickness of less than 1 µm. Since thicker films deposited in a single step tend to be unstable, we developed a layer-by-layer deposition method with intermediate calcination steps, by which the efficiency could be increased (Figure 1b).

[1] S. Sawatani, T. Yoshida, T. Ohya, T. Ban, Y. Takahashi, H. Minoura, Electrochem. Solid State Lett. 8 (2005) C69.
[2] K. Wessels, M. Maekawa, J. Rathousky, T. Yoshida, M. Wark, T. Oekermann, Micropor. Mesopor. Mater., in press.