Engineered Transition Metal Chalcogenides for Photovoltaic, Thermoelectric, and Magnetic Applications

This work focuses on the development of ternary and quaternary chalcogenide compounds featuring transition metal cations through careful engineering of the electronic and thermal transport as well as magnetic properties by traditional solid-state doping techniques and novel template structure synthesis methods for improvements in thermoelectric performance, diluted magnetic semiconductors, and photovoltaic conversion. Presented here is an innovative low-temperature batch synthesis that was developed to create hexagonal nanoplatelets of thermoelectrically interesting CuAgSe. This process utilized room temperature ion exchange reactions to convert cubic Cu2-xSe nanoplatelets into CuAgSe by replacing a portion of the Cu+ ions with Ag+ while maintaining the morphology of the nanoplatelet. This simple reaction process offers an energy efficient and versatile strategy to create interesting materials with superior thermoelectric performance. An investigation of the thermal and electronic transport of CuAl(SxSe1-x)2 solid solutions was also conducted. While these compounds yielded low thermal conductivity, they also exhibited low electronic conductivity. Doping with transition metals Ag, Hf, and Ti further reduced the thermal conductivity below 1 W/mK; however, most exciting was the determination that the thermal transport of the system could be modified by doping at the Al3+ site without affecting the electronic structure of the system, potentially leading to the use of CuAl(SxSe1-x)2 as a heavily doped thermoelectric material. The effect of local carrier concentration in the diluted magnetic semiconductor FeSb2Se4 was studied by substitution of In3+ for Sb3+. Using systematic Rietveld refinement, it was determined that In3+ resides in the semiconducting layer of the structure for concentrations of x ≤ 0.1, and the magnetic layer for x > 0.1. The increase in local carrier concentration has an appreciable effect on the electronic and magnetic properties of the material in a predictable manner based on the concentration of In3+. Lastly, two new perovskite-like selenides were developed using low-pressure synthesis methods, needle-like SrHfSe3 and distorted perovskite BaHfSe3. The optical band gap of SrHfSe3 was experimentally determined to be 1.15 eV by doping of Sb3+ for Sr2+, and 1.6 eV for BaHfSe3, both in the ideal range for visible light absorption. Thus, these new materials are intriguing candidates for thin-film photovoltaic applications.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137119/1/nmoroz_1.pd

State-of-the-Art of Quantum Dot System Fabrications

The book "State-of-the-art of Quantum Dot System Fabrications" contains ten chapters and devotes to some of quantum dot system fabrication methods that considered the dependence of shape, size and composition parameters on growth methods and conditions such as temperature, strain and deposition rates. This is a collaborative book sharing and providing fundamental research such as the one conducted in Physics, Chemistry, Material Science, with a base text that could serve as a reference in research by presenting up-to-date research work on the field of quantum dot systems

Properties of the Back Contact Interface for Non-Vacuum Deposited Precursor-Based Cu(In,Ga)Se₂ Solar Cells

In this work two possibilities for steps on the way towards cost-efficient solar cells are presented. Non-vacuum processes for the absorber fabrication are implemented and an idea how thinner absorber layers can still show relatively high conversion efficiencies is substantiated. In order to investigate the interface properties of absorbers built with non-vacuum processes a basic non-vacuum selenisation process has been developed initially. The selenisation set-up has been designed and built up and the corresponding process has been optimized. As a next step the molybdenum selenide formation and the influence of the changed selenium partial pressure and the precursor layers on it has been investigated: Both the molybdenum fabrication process (especially sputter pressure) and the selenisation (especially substrate temperature) were found to influence the MoSe2 formation. For sputtered precursors MoSe2 was formed during the selenisation. For doctor bladed precursors made from metal salts with ethylcellulose no molybdenum selenide formation has been observed. During the formation of the CIGS absorber layer a carbon layer was left between absorber and back contact which protected the molybdenum. Finally the influence of this carbon layer on the solar cell properties has been investigated. For thin absorber layers an additional thin carbon layer between back contact and absorber material was beneficial. Especially very thin layers showed significantly higher open circuit voltages. Most likely a lower recombination at the back contact is a key factor for the higher voltages in samples with a carbon layer at the back contact. Simulations with the Solar Cell Capacity Simulator (SCAPS) back up this assumption

Magnesium based materials for hydrogen based energy storage: Past, present and future

Magnesium hydride owns the largest share of publications on solid materials for hydrogen storage. The “Magnesium group” of international experts contributing to IEA Task 32 “Hydrogen Based Energy Storage” recently published two review papers presenting the activities of the group focused on magnesium hydride based materials and on Mg based compounds for hydrogen and energy storage. This review article not only overviews the latest activities on both fundamental aspects of Mg-based hydrides and their applications, but also presents a historic overview on the topic and outlines projected future developments. Particular attention is paid to the theoretical and experimental studies of Mg-H system at extreme pressures, kinetics and thermodynamics of the systems based on MgH2, nanostructuring, new Mg-based compounds and novel composites, and catalysis in the Mg based H storage systems. Finally, thermal energy storage and upscaled H storage systems accommodating MgH2 are presented

Nanostructured Materials for Energy Storage and Conversion

The conversion and storage of renewable energy sources is key to the transition from a fossil-fuel-based economy to a low-carbon society. Many new game-changing materials have already impacted our lives and contributed to a reduction in carbon dioxide emissions, such as high-efficiency photovoltaic cells, blue light-emitting diodes, and cathodes for Li-ion batteries. However, new breakthroughs in materials science and technology are required to boost the clean energy transition. All success stories in materials science are built upon a tailored control of the interconnected processes that take place at the nanoscale, such as charge excitation, charge transport and recombination, ionic diffusion, intercalation, and the interfacial transfer of matter and charge. Nanostructured materials, thanks to their ultra-small building blocks and the high interface-to-volume ratio, offer a rich toolbox to scientists that aspire to improve the energy conversion efficiency or the power and energy density of a material. Furthermore, new phenomena arise in nanoparticles, such as surface plasmon resonance, superparamegntism, and exciton confinement. The ten articles published in this Special Issue showcase the different applications of nanomaterials in the field of energy storage and conversion, including electrodes for Li-ion batteries and beyond, photovoltaic materials, pyroelectric energy harvesting, and (photo)catalytic processes

Thermoelectric Transport Properties of Novel Nanoscaled Materials via Homemade and Commercial Apparatus Measurements

Thesis advisor: Cyril P. OpeilThermoelectric (TE) materials are of broad interest for alternate energy applications, specifically waste heat applications, as well as solid-state refrigeration. The efficiency of TE materials can be improved through either the enhancement of the Seebeck coefficient and electrical conductivity, or through the reduction of the thermal conductivity, k, specifically the lattice portion of thermal conductivity, klatt. Nanostructuring has been proven to reduce klatt and therefore increase efficiency. The inability to accurately model the lattice and electronic contributions to k makes optimizing the reduction of klatt difficult. This work demonstrates that the lattice and electronic contributions to k in nanostructured materials can be directly measured experimentally by separating the contributions using magnetic field. We use this technique along with other characterization techniques to determine the effects of doping Ce, Sm, and Ho into Bi88Sb12. Along with enhancing the efficiency of the material, TE devices must be thermally stable in the temperature range of operation. Therefore we also study the effects of temperature cycling, annealing, oxidation, and diffusion barriers on TE devices. These studies are accomplished through both homemade and commercially available measurement equipment.Thesis (PhD) — Boston College, 2013.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics

Improvement of Thermoelectric Properties Through Manipulation of their Microstructure: the Effect of Graphene Reinforcement

Environmental changes and extreme climate-related events are mainly attributed to greenhouse gas (GHG) emissions and are becoming a growing concern. The reported scientific evidence, highlighting such interrelationships, has convinced researchers to look for clean energy sources and improve operational efficiencies, and capture and convert the waste heat into electricity. Since almost two-thirds of energy is converted to heat and wasted, the recovery of waste heat will boost savings in fossil fuel consumption as an abundant source of energy. In this regard, thermoelectric (TE) compounds can be employed to convert the waste heat into electricity, thereby increasing the efficiencies of energy generating operations. Such an approach is even applicable to renewable energy (RE) sources. However, the applications of the thermoelectric converters necessitate the development of advanced, efficient thermoelectric materials with a high level of thermomechanical stability. This doctoral research project aims to develop and modify thermoelectric compounds by manipulating their microstructure and improving their mechanical properties by reinforcement with graphene nanoplates (GNPs). To the best of our knowledge, there is no specific report in the open literature to determine the reinforcing effects of graphene nanofillers (e.g., GNPs) on thermoelectric products. There is a lack of a comprehensive assessment in the scientific and industrial communities in evaluating the advantages and drawbacks of GNPs, as the reinforcing agent on TE compounds. In this dissertation, to assess the performance of the GNPs, three potential thermoelectric compounds, namely MnTe, CoVSn, and CuSbTe2, have been investigated. These designated compounds address the requirements for covering an extended working temperature range from low to high, examining various crystal structures (e.g., Chalcogenides and half-Heusler), and developing environmentally-friendly (i.e., lead-free) TE products. The bulk samples with the addition of small quantities of GNPs (0.25, 0.5, 0.75, and 1 wt. %) were synthesized using powder metallurgy and fabricated by spark plasma sintering (SPS). The thermoelectric factors, magnetic behavior, microstructure, and mechanical properties of the samples were evaluated and analyzed. Grain growth inhibition is the main consequence of the reinforcing GNPs, which results in an enhancement in the thermoelectric and mechanical characteristics of the nominated TE products. Scattering of electrical carriers and phonons due to the precipitation of the reinforcing GNPs in the matrix, thus providing a higher density of microstructural boundaries, improves the thermoelectric properties. Furthermore, microstructural manipulation, such as crystal/particle size reduction caused by the segregation of the reinforcing GNPs as a second phase in the matrix, enhances the mechanical characteristics of TE compounds, for example, the fracture toughness () and hardness.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 202

Synthesis and characterization of nanostructured materials

In addition to technological motivations, nanomaterials are interesting for basic scientific investigation because their properties reside in the largely unexplored realm between molecules and bulk solids. The controlled synthesis of these materials, by methods that permit their assembly into functional nanoscale structures, lies at the core of nanoscience and nanotechnology. Here, controlled synthesis refers to a process of collective nanostructure growth where the pertinent attributes such as location, size, orientation, and composition as well as the electrical, mechanical, and chemical properties of the individual elements can be predetermined by the choice of the growth conditions and the preparation of the growth substrate. This dissertation work furthers the understanding of the mechanisms by which synthesis conditions affect the morphology, composition, and crystal structure of nanostructured materials with the objective of achieving greater control over the synthesis process. Three types of systems are investigated in depth: vertically aligned carbon nanofibers (grown by plasma-enhanced chemical vapor deposition), catalytic alloy nanoparticles (sputter-deposited, carbon-encapsulated), and tungsten nanowires (grown by electron-beam-induced deposition). The effects of growth parameters on the resulting nanostructure properties are characterized by methods including high-resolution transmission electron microscopy, electron diffraction, and chemical spectroscopy

Advances and Development of All-Solid-State Lithium-Ion Batteries

Lithium-ion battery technologies have always been accompanied by severe safety issues; therefore recent research efforts have focused on improving battery safety. In large part, the hazardous nature of lithium-ion batteries stems from the high flammability of liquid electrolytes. Consequently, numerous researchers have attempted to replace liquid electrolytes with nonflammable solid electrolytes in order to avoid potential safety problems. Unfortunately, current solid electrolytes are incapable of performing as effectively as liquid electrolytes in lithium-ion batteries due to inferior electrochemical capabilities. While some \u22all-solid-state\u22 batteries have found niche application, further technological advancement is required for large scale replacement of liquid-based batteries. The goal of this research is to develop all-solid-state batteries that can outperform liquid batteries and understand the mechanisms that dictate battery operation and behavior. This involves fabrication of highly conducting solid electrolytes, production and analyzation of batteries employing state-of-the-art electrode materials, and generation of high power and high energy density lithium batteries. In this dissertation, the first objective was to manufacture highly conducting solid electrolytes that are stable in contact with lithium metal. Numerous characterization techniques were used to gain understanding of physical and chemical properties of solid electrolytes, as well as mechanisms for fast ion conduction. A new process for production of highly conducting and stable solid electrolytes is developed and materials are used to evaluate performance of electrodes in an all-solid-state construction. The second objective of this work was to research the performance of both positive and negative electrodes incorporating solid electrolyte. Evaluation of electrochemical results allowed for a good understanding of reaction mechanisms taking place within composite battery materials and at electrolyte/electrode interfaces. Variation of solid electrolyte make-up and composite electrode architecture reveals numerous advantages of solid state batteries over liquid batteries. The third and final objective of this work was to demonstrate high energy/power density all-solid-state rechargeable batteries. Electrode materials with fast lithium diffusion and kinetic properties were investigated. Alteration of particle size and testing temperature exposed the capability of solid state batteries to achieved high performance, comparable to that of liquid electrolyte batteries