ZnO multipods as promising nanostructured photoanodes for Dye-sensitized Solar Cell

Zinc oxide is a well known material for Dye-sensitized Solar Cell (DSC) application, being an interesting alternative to the most commonly used titanium dioxide. With respect to titanium dioxide, ZnO presents higher electron mobility and longer carrier lifetime, being these parameters very important in the fabrication of efficient DSC photoanodes. Moreover, zinc oxide is well known for its ability to easily grow in a wide variety of nanostructures, like nanoparticles, nanowires or nanorods, nanotubes, nanosheets, nanoplants and nanotetrapods. In this work, the fabrication and characterization of DSCs exploiting multipod ZnO-based photoanodes is proposed. The shape-controlled synthesis of these ZnO micro-particles was carried out by simple hydrothermal process through the reaction of zinc acetate salts with potassium hydroxide. Mild reaction conditions were used (70 °C) and wurtzite-like crystalline structure can be readily obtained after the reaction process. After dispersion of ZnO nanostructures in acetic acid-based solution, a 14 µm-thick ZnO layer acting as DSC photoanode was obtained. It is well known that the sensitization procedure in solvent-rich or acid environments with Ru-based dye with a COOH anchoring group is remarkably critical for ZnO. In particular, the presence of carboxylic acid binding groups can lead to dissolution of ZnO and precipitation of molecular Zn2+/dye complexes, resulting in reduced overall electron injection efficiency by the excited dye. The fine-tuning of the sensitization procedure is thus crucial to achieve photoconversion performances comparable with the one obtained for TiO2. With the goal of overcome this drawback and therefore achieve high efficiency from the cells fabricated with these nanostructured photoanodes, the optimization of the experimental conditions (N719 dye immersion time, sensitizing solution pH, dye concentration) was carried out by means of response surface methodology (RSM), a collection of statistical and mathematical techniques useful for developing, refining and improving products and processes. The proposed chemometric method is a very powerful tool for an accurate tuning of the experimental conditions with a reduced number of tests, and its applicability is not limited to the proposed field (sensitization of nanostructured ZnO) but can have a widespread interest in many aspects of DSC fabrication.