Synthesis and Characterization of Novel Transition Metal Chalcogenide Phases for Energy Storage, Energy Conversion and Optoelectronics

Abstract

Today’s energy needs are primarily provided by fossil fuels, which are harvested from the earth. Consuming fossil fuels to provide energy for civilization releases products into the atmosphere that contribute to climate change. Ongoing efforts to combat the existential crisis which climate change presents many of the emerging and commercialized technologies for solar, thermoelectric and battery applications involve transition metal chalcogenides. Some of the materials used for these applications are expensive and rare, such as gallium, vanadium and indium, or have no merits towards environmental stewardship, such as cadmium and lead. Thus, the purpose of this work is to further the ongoing effort to discover and develop new materials which are able to meet or exceed benchmarks for their application. This work focuses on the development of various metal chalcogenide material systems featuring d-block transition metals selected for their contribution to alter structure and properties. Various thermal, electronic and optical properties can be changed through substitution or doping with additional elements to affect to the base composition or as part of a gradient composition series. After an extensive description of experimental methods which describe the associated materials synthesis, processing and characterization techniques in chapter 2, chapter 3 explores the Cu4-xLixS2 phases for their contribution as further evidence in the formation of lithiated copper sulfide phases as part of the intercalation reaction before being converted to the binaries copper and lithium sulfide. Chapter 4 documents the development of Cu4TiSe4, a novel material with potential for thin-film photovoltaic technologies with its band gap in the range where the solar spectrum is the most bountiful (Eg,indirect = 1.16 eV, Eg,direct = 1.34 eV), an outstanding optical absorbance ( > 10-4 cm-1) outperforming commercially successful materials in the solar spectrum, and suitable for thin-film fabrication. Chapter 5 describes a brief study in utilizing elemental substitution in Cu4TiSe4 to alter the band gap by replacing sulfur into the selenium site. In this study, the amount of selenium which may be substituted without deviating from the parent Cu4TiSe4 structure is 16 % at and the direct band gap is alterable from 1.34 eV to 1.64 eV as determined from conducting tauc analysis on the diffuse reflectance spectra. The last experimental work in Chapter 6 covers the development of a chemical substitution series between the end compounds Cu3NbS4 and Cu3NbSe4. Through powdered x-ray diffraction of the series, it was found that for substituting less than 25% of the sulfur with selenium, the powdered patterns more closely resembled Cu3NbS4 and with shift which may see further development and application in optoelectronic devices such as LEDs. Finally, Chapter 7 provides further guidance in the research which this thesis may serve as a springboard for the development of ultra-high efficiency, low-cost, environmentally friendly and thin photovoltaics as well as mention other characterization methods which are necessary to diagnose and elucidate complications.