Preparation and Characterization of Zinc Oxide Photoelectrodes for Aqueous Grätzel Solar Cells

This work focuses on the preparation and characterization of zinc oxide based photoanodes conceived for application in water-based dye-sensitized solar cells (DSSCs). The results achieved, combined with the use of biosourced electrolyte components, open up a variety of very interesting features in the scenario of sustainable, low-cost and easy processable aqueous DSSCs. Nowadays, one of the main challenges in the field of DSSCs is to replace the commonly used organic electrolyte solvents, which are highly volatile, flammable, toxic and sensitive to moisture/water contaminations with water, the universal, greenest and cheapest solvent. In this work, water was used as the unique solvent to dissolve the redox couples in the electrolyte system; moreover, in order to improve the long-term stability of devices, a biosourced jellifying agent was added, namely carboxymethylcellulose. In the framework of photoanode development, the most commonly used titanium dioxide (TiO2) was replaced by zinc oxide (ZnO). ZnO has similar semiconducting characteristics and some additional advantages, such as enhanced electron mobility and a variety of possible nanostructures with peculiar morphological characteristics, which can be exploited to tailor the photoanode architecture. In particular, three different microstructures (i.e., desert roses, multipods and microwires) were obtained through hydrothermal synthesis in order to maximize, at the same time, the surface area of the semiconducting layer and the electron transfer rate. This target was reached with the desert rose morphology that, among others, was confirmed as the most suitable photoanode also by photovoltaic measurements. At the same time, in order to tailor as desired the photoanode architecture, the ZnO dissolution caused by prolonged contact with the acidic groups present in the electrolyte/dye solution was taken into account. Indeed, the optimization of the sensitizing conditions, i.e. immersion time of the electrode and co-adsorbent concentration (chenodeoxycholic acid, CDCA) in the dye solution, was carried out through a chemometric approach in order to limit/overcome this issue. In particular, this method allowed to identify the optimal manufacturing conditions in 1 h of sensitizing time and in a molar ratio of 1:50 between dye and CDCA concentrations, through which an efficiency of 0.34% was achieved.