Fabrication and Characterization of Low Dimensional Metal Oxides and Sulfides for Solar Energy Conversion

Increasing energy needs and the environmental damage caused by carbon dioxide (CO2) accumulation in the atmosphere due to fossil fuel burning are the two most pressing issues in today’s world. A fast transition to renewable sources-based energy system could address these issues. Solar photovoltaics (PV) and solar photoelectrochemical (PEC) water splitting are two promising renewable energy conversion technologies. Nevertheless, the PV and PEC technologies must offer affordable power and fuels, respectively, for making an impact in the energy market. The primary objective of this dissertation work was to develop and characterize novel nanostructured semiconductors for efficient solar energy conversion. Considering the potential for efficient solar energy conversion, material abundance, environmental compatibility and economic viability, tungsten oxide and copper tin sulfide were selected for the investigation. We employed a colloidal synthesis method to obtain CTS sol. Electrochemical anodization, a low cost and scalable method, was used to fabricate nanostructured tungsten oxide. A major invention that emerged from the dissertation work was the growth of tungsten oxide nanotubes using anodic oxidation. Except for an unconfirmed work, no study had ever shown the fabrication of WO3 nanotubes of length in the micrometer scale. We identified the fabrication conditions favorable for growing ordered nanoporous and nanotube array films of WO3 in a wide thickness range. We carried out a comprehensive investigation of the effects of additives, oxidizers and solvent composition in the electrolyte and other synthesis conditions on the growth of anodic nanostructures of WO3. The photoanodes fabricated using this material for PEC water splitting showed ~75% improvement in the photocurrent compared to the highest reported for a W/WO3 photoanode. CTS thin films were fabricated using a newly developed sol synthesis method. The sol was highly air stable. Performance of CTS solar cells employing WO3 films as window layers was compared with sol TiO2 based CTS cells. Although the PV performance was not highly impressive, the study showed that CTS and WO3 nanotube could be promising for future photovoltaics. The dissertation discusses the details of the development processes of these new materials and their properties relevant to devices for solar energy conversion.