The preparation of porous metal oxide semiconductor films at low temperature (< ca. 150 °C) is of high interest in view of the production of (flexible) plastic-based dye-sensitized solar cells (DSSC), which would allow reductions in weight and costs. In case of ZnO, porous films electrodeposited at temperatures as low as 70 °C have shown superior performance in DSSC even if compared to nanoparticular films sintered at high temperature [1]. Recently we were also able to directly electrodeposit crystalline TiO₂ films consisting of rutile with a small amount of anatase. The films are anodically electrodeposited from TiCl3-solution at 80 °C using surfactants, e.g. sodium dodecyl sulfate (SDS), as structure-directing additives to generate a porous structure [2].
Before the use in DSSC the additives have to be removed in order to obtain pores with sufficient accessibility. Various solvents for the desorption of SDS, e.g. ethanol and aqueous solutions of HCl and KOH, were tested, of which the latter was the most successful. Furthermore, water vapor treatment in an autoklav at temperatures of up to 150 °C led to an improved crystallinity of the anatase phase, while the rutile phase stays unaltered. This texture change lead to a significantly higher dye loading in comparison to untreated films (see Figure) and a corresponding increase in efficiency.
[1] T. Oekermann, T. Yoshida et al., J. Phys. Chem. B 109 (2005) 12560.
[2] K. Wessels, A. Feldhoff, M. Wark, J. Rathousky, T. Oekermann, Electrochem. Solid-State Lett. 9 (2006), C93.