The Organic Flash Cycle (OFC) is proposed as a vapor power cycle that could potentially increase power generation and improve the utilization efficiency of renewable energy and waste heat recovery systems. A brief review of current advanced vapor power cycles including the Organic Rankine Cycle (ORC), the zeotropic Rankine cycle, the Kalina cycle, the transcritical cycle, and the trilateral flash cycle is presented. The premise and motivation for the OFC concept is that essentially by improving temperature matching to the energy reservoir stream during heat addition to the power cycle, less irreversibilities are generated and more power can be produced from a given finite thermal energy reservoir. In this study, modern equations of state explicit in Helmholtz energy such as the BACKONE equations, multi-parameter Span-Wagner equations, and the equations compiled in NIST REFPROP 8.0 were used to accurately determine thermodynamic property data for the working fluids considered. Though these equations of state tend to be significantly more complex than cubic equations both in form and computational schemes, modern Helmholtz equations provide much higher accuracy in the high pressure regions, liquid regions, and two-phase regions and also can be extended to accurately describe complex polar fluids. Calculated values of saturated liquid and vapor density and vapor pressure were then compared to values listed in the NIST Chemistry WebBook to ensure accuracy for the temperature range of interest. Deviations from the NIST WebBook were typically below 1%; a comparison of first law efficiencies for an ideal basic Rankine cycle yielded less than 0.4% difference between calculations using the Helmholtz-explicit equations of state and NIST REFPROP. Also by employing the BACKONE and Span-Wagner equations, the number of potential aromatic hydrocarbon and siloxane working fluids that are appropriate for high and intermediate temperature applications is expanded considerably.

A theoretical investigation on the OFC is conducted using the aforementioned Helmholtz-explicit equations of state for 10 different aromatic hydrocarbon and siloxane working fluids for intermediate temperature finite thermal energy reservoirs (~300^oC). Results showed that aromatic hydrocarbons to be the better suited working fluid for the ORC and OFC due to less "drying" behavior and also smaller turbine volumetric flow ratios resulting in simpler turbine designs. The single flash OFC is shown to achieve utilization efficiencies that are comparable to the optimized basic ORC (~0.63) which is used as a baseline. It is shown that the advantage of improved temperature matching during heat addition was effectively negated by irreversibilities introduced into the OFC during flash evaporation. Several improvements to the basic OFC are proposed and analyzed such as introducing a secondary flash stage or replacing the throttling valve with a two-phase expander. Utilization efficiency gains of about 10% over the optimized basic ORC can be achieved by splitting the expansion process in the OFC into two steps and recombining the liquid stream from flash evaporation prior to the secondary, low pressure, expansion stage. Results show that the proposed enhancements had a more pronounced effect for the OFC using aromatic hydrocarbon working fluids (5-20% utilization efficiency improvement) than for siloxane working fluids (2-4%). The proposed modifications were aimed towards reducing irreversibility in flash evaporation; it was observed for siloxanes that the primary source of irreversibility was due to high superheat at the turbine exhaust because of the highly "drying" nature of the fluid. Though an order of magnitude analysis, results showed that the OFC and ORC to require similar heat transfer surface areas.

For low temperature thermal energy reservoirs (80-150^oC) applicable to non-concentrated solar thermal, geothermal, and low grade industrial waste heat energy, alkane and refrigerant working fluids possess more appropriate vapor pressures. The optimized single flash OFC was again shown to generate comparable power per unit flow rate of the thermal energy reservoir than the optimized basic ORC. With some of the previously proposed design modifications though, the OFC can produce over 60% more power than the optimized ORC. For low temperature applications, the minimum temperature difference between streams in the heat exchanger, or pinch temperature, becomes an important design parameter. Reduction of the pinch temperature even slightly can yield significantly higher gains in power output, but will also increase required heat exchanger surface area and subsequently capital costs.

A high-level design of a liquid-fluoride salt (NaF-NaBF4) cooled solar power tower plant is presented; liquid-fluoride salt is used rather than current molten nitrate salts to increase the receiver temperature and subsequently allow for higher efficiency gas power cycles to be used. Graphite or direct energy storage in the salt itself is proposed. The power block component of this heliostat-central receiver plant is a combined cycle system consisting of a topping Brayton cycle with intercooling, reheat, and regeneration and a bottoming low-temperature modified OFC. The combined cycle is designed with dry cooling in mind, such that operation in desert climates are more suitable. The combined cycle design is shown to increase power block efficiencies by 6%-8% over the Brayton cycle with intercooling, reheat, and regeneration alone. An estimated 30% annual average total solar-to-electric conversion efficiency is possible with this system design, which is comparable to some of the most efficient high temperature solar power tower designs to date. Theoretically, power block efficiencies over 60% are possible; however, emission losses from the isothermal central receiver would limit the plant's operational temperature range. Results show that for high efficiency solar power towers to be realized, high temperature non-isothermal, or partitioned, receivers operating efficiently above 1000oC are necessary. Other potential areas of renewable energy system integration for the OFC include a co-generation solar thermal-photovoltaic system that employs highly concentrated, densely packed photovoltaic cells using single-phase or two-phase cooling. The thermal energy absorbed by that coolant could then be used as the working fluid in a separate OFC to further produce power in co-generation with the concentrated photovoltaics.

Photoelectrochemistry and its associated technologies show unique potential to facilitate the large-scale production of solar fuels - those energy-rich chemicals obtained through conversion processes driven by solar energy, mimicking the photosynthetic process of green plants. The critical component of photoelectrochemical devices designed for this purpose is the semiconductor photoelectrode, which must be optically absorptive, chemically stable, and possess the required electronic band alignment with respect to the redox couple of the electrolyte to drive the relevant electrochemical reactions. After many decades of investigation, the primary technological obstacle remains the development of photoelectrode structures capable of efficient and stable conversion of light with visible frequencies, which is abundant in the solar spectrum. Metal oxides represent one of the few material classes that can be made photoactive and remain stable to perform the required functions. The unique range of functional properties of oxides, and especially the oxides of transition metals, relates to their associated diversity of cation oxidation states, cation electronic configurations, and crystal structures.

In this dissertation, the use of metal oxide films, nanomaterials, and heterostructures in photoelectrodes enabling the solar-driven oxidation of water and generation of hydrogen fuel is examined. A range of transition- and post-transition-metal oxide material systems and nanoscale architectures is presented. The first chapters present results related to electrodes based on alpha-phase iron(III) oxide, a promising visible-light-active material widely investigated for this application. Studies of porous films fabricated by physical vapor deposition reveal the importance of structural quality, as determined by the deposition substrate temperature, on photoelectrochemical performance. Heterostructures with nanoscale feature dimensionality are explored and reviewed in a later chapter, which describes the methodologies to combine the unique and complimentary functional properties of dissimilar oxides to optimize the water photo-oxidation process Experimental results based on an iron(III) oxide-tungsten(VI) oxide system show enhancements associated with the heterostructure, which may indicate the presence of unexpected minority carrier dynamics, as observed additionally by ultrafast transient absorption spectroscopy.

Next, a new conceptual framework for the design of solar water oxidation photoelectrodes based on the spatially inhomogeneous doping of wide-bandgap metal oxide nanostructures is introduced and experimentally verified. It is found that optical absorption and electronic conduction can be decoupled and optimized by spatially segregating the functional impurity species that facilitate their associated physical processes. In the final chapters of this dissertation the electronic structures of key oxide-oxide interfaces, relevant to the operation of efficient photoanodes, are examined using synchrotron-based soft x-ray spectroscopy. These studies indicate that the interfacial regions of electrodes possess distinct electronic structures, which deviate in terms of orbital character and occupancy from those of their constituent bulk oxides. These observations inform methodology to address certain operational deficiencies associated with the use of metal oxides for solar energy conversion applications.

Pulsed laser deposition (PLD) is a physical vapor deposition technique for thin film fabrication. Compared with other techniques, pulsed laser deposition technique has advantages such as stoichiometry, flexibility, versatility, lower deposition temperature, ability to grow metastable materials. Because of these advantages, pulsed laser deposition has been widely used in materials research. In this dissertation, pulsed laser deposition has been used to grow thin films for solid oxide fuel cell, light-emitting diode, and solar cell applications.

Firstly, yttria-stabilized zirconia (YSZ) and cerium dioxide (CeO2) thin films are deposited in oxygen-deficient environments; their properties are compared to those deposited in oxygen-rich environments. Oxygen-deficient films are highly (001)-oriented, which corresponds to a surface that is expected to be forbidden based on Tasker's theoretical calculation of stoichiometric ionic crystals. A model considering non-stoichiometry-induced surface relaxation and surface atomic density is proposed to explain the orientation phenomenon observed under oxygen-deficient deposition conditions. This model is consistent with previous experimental results for indium tin oxide (ITO), SnO2 and NiO thin films deposited under similar conditions. Detailed studies of the preferred orientation of these oxygen-deficient ionic crystals are of direct relevance to the fabrication of films for use in solid oxide fuel cells.

Secondly, undoped, Cu-doped, Se-enriched, Cu2Se-doped, Ag-doped, Ag2Se-doped, and nitrogen-doped ZnSe films have been grown on fused quartz substrates by pulsed laser deposition. It is found that adding a small amount (~2 mol%) of Cu2Se can significantly improve crystallinity and (111) texturing of ZnSe films. While the other films are highly resistive, Cu2Se-doped ZnSe films are p-type conducting with hole concentrations of ~ cm-3 and resistivity of ~0.098 ohm*cm (compared with previous reports of ~ cm-3 and 0.75 ohm*cm, respectively). The successful heavy p-type doping of ZnSe films is attributed to substitution of Zn atoms with Cu while limiting selenium-vacancy-associated compensating defects with additional selenium. Nitrogen doping has turned ZnSe films more favorable to wurtzite structures. Two newly observed Raman peaks at 555 cm-1 and 602 cm-1 are assigned to N local vibrational modes of hexagonal ZnSe structures. The nitrogen-doped ZnSe films are not conductive, which might be due to compensating defects arising from the presence of native defects or other impurities. This work is of importance to solve doping difficulties and contact problems of wide-bandgap semiconductors.

Finally, batch growth of thin films by pulsed laser deposition has been tried. Using the natural temperature gradient, films with different deposition temperatures have been fabricated together. With change of deposition temperatures, ZnSe films are shown to have problems associated with crystalline defects, selenium loss, or phase separation. ZnSe films with improved crystallinity and no phase separation have been achieved using a 16 mol% Se enriched target. Multi-plume pulsed laser deposition has been proposed and discussed. With directionality of PLD plumes and non-uniformity of PLD films, this system is supposed to be more suitable for high-throughput compound thin film fabrication, which makes it very promising for efficient materials optimization and exploration. High-throughput fabrication of compound thin films has been successfully achieved using this system.

Highly-mismatched alloys: ZnO1-xTex and GaN1-xSbx are discussed within the context of finding the suitable material for a cost-effective Si-based tandem solar cell (SBTSC). SBTSC is an attractive concept for breaking through the energy conversion efficiency theoretical limit of a single junction solar cell. Combining with a material of 1.8 eV band gap, SBTSC can theoretically achieve energy conversion efficiency > 45%.

ZnO and GaN are wide band gap semiconductors. Alloying Te in ZnO and alloying Sb in GaN result in large band gap reduction to < 2 eV from 3.3 eV and 3.4 eV respectively. The band gap reduction is majorly achieved by the upward shift of valence band (VB). Incorporating Te in ZnO modifies the VB of ZnO through the valence-band anticrossing (VBAC) interaction between localized Te states and ZnO VB delocalized states, which forms a Te-derived VB at 1 eV above the host VB. Similar band structure modification is resulted from alloying Sb in GaN. ZnO1-xTex and GaN1-xSbx thin films are synthesized across the whole composition range by pulsed laser deposition (PLD) and low temperature molecular beam epitaxy (LT-MBE) respectively. The electronic band edges of these alloys are measured by synchrotron X-ray absorption, emission, and the X-ray photoelectron spectroscopies. Modeling the optical absorption coefficient with the band anticrossing (BAC) model revealed that the Te and Sb defect levels to be at 0.99 eV and 1.2 eV above the VB of ZnO and GaN respectively. Electrically, ZnO1-xTex is readily n-type conductive and GaN1-xSbx is strongly p-type conductive. A heterojunction device of p-type GaN0.93Sb0.07 with n-type ZnO0.77Te0.93 upper cell (band gap at 1.8 eV) on Si bottom cell is proposed as a promising SBTSC device.

The reliable production of clean, inexpensive energy to meet the growing global demand remains one of the most significant challenges to modern society. As high-technology, high power-consumption machines and devices proliferate and as underserved populations around the globe come "on the grid," energy demand will significantly expand. Photovoltaic (PV) technologies represent one of the most direct paths to harvesting solar energy, converting solar energy directly into electricity with no moving parts, no pollution, and no greenhouse gas emissions. These technologies often utilize materials that may not exist in sufficient abundance to meet large-scale (TW) deployment demands. For instance, the highly successful Cu(In,Ga)Se2 material system has achieved module efficiencies up to 15% [1] but contains In and Se, which are relatively scarce and expensive. Shafarman reports that the availability of In would limit total installed capacity to 0.1TW [2], far below the 10 - 20 TW required to meet global energy demand. Moreover, CIGSe devices typically involve processing with highly toxic H2Se gas [2].

Therefore, significant research has recently been devoted to discovering and developing PV devices that use non-toxic, inexpensive, earth-abundant elements. Based on the success of the Cu(In,Ga)Se2 material system, the development of the electronically analogous Cu2ZnSnS4 material system represents a natural next step. CZTS replaces the scarce Group III (In/Ga) elements with more abundant Group II (Zn) and Group IV (Sn) elements. Sulfur also replaces Selenium, allowing less-toxic processing conditions. With a direct band gap of 1.5eV and high optical absorption > 104cm-1, CZTS represents an ideal absorber layer for conventional single-junction solar cells [3]. Further, it is hoped that CZTS, which shares similar crystal structure and electronic attributes with CIGSe, may also share a similar resilience in its opto-electronic properties to grain boundaries and to stoichiometric deviations that has afforded CIGSe devices much success [2].

Devices with moderately high efficiencies of 12.6% have been fabricated using Se-alloyed CZTSeS absorber layers [4]. In general, however, the fabrication of high-quality CZTSeS absorber layers has been extremely challenging, due to the narrow phase formation region for CZTSeS [3, 5], elemental and compound volatility [3, 6-8], and phase decomposition at the surface and back contacts [9, 10]. Despite the moderately high device efficiencies achieved, all aspects of CZTS device design merit continued investigation. Scalable and reliable fabrication methods that are capable of producing phase-pure, void-free CZTSeS absorber layers have yet to be demonstrated. Such advancements will require better understanding of the crystal formation processes and defect behavior of CZTSeS. The work in this dissertation aims to improve understanding of the growth processes of CZTS thin films and to provide a pathway to fabrication of high-quality absorber layers for use in photovoltaic devices. Reliable growth of high-quality CZTS thin films will enable investigation of the fundamental properties of CZTS, as well as device and interface behavior, necessary to improve device efficiency.

This dissertation investigated the growth behavior of CIGSe and CZTS thin films developed for use as photovoltaic absorber layers. The CIGSe thin films were fabricated by pulsed laser deposition, and the CZTS thin films were fabricated by pulsed laser deposition and co-electrodeposition methods. The effects of deposition and annealing parameters on the film properties were systematically investigated, critical growth parameters identified, and optimized fabrication conditions recommended. In pulsed laser deposited CIGSe thin films, annealing in sulfur background significantly improved the electronic quality of the films, reducing carrier concentrations and increasing Hall mobilities. In pulsed laser deposition of CZTS thin films, proper adjustment of the laser fluence and sputtering target composition enabled films with the desired stoichiometries to be deposited at room temperature. A two-step temperature profile involving a long dwell at low temperature and short dwell at the crystal formation temperature yielded the most-stable films with the optimum structural properties. Proposed growth mechanisms and optimized fabrication processes are presented.

To study the deposition mechanisms active in the co-electrodeposition of S-containing CZTS precursors, cyclic voltammagrams were compared with varying potential depositions. Reduction mechanisms and peak assignments are proposed. Films deposited using a fabrication process from literature demonstrated rough, powdery morphologies with Zn-poor, Sn-rich compositions. The morphology was attributed to diffusion-limited growth mechanisms with significant hydrogen co-deposition. However, removing tartaric acid from the deposition bath dramatically reduced the roughness and increased the uniformity of the films. To further improve film composition and morphology, bath component concentrations were systematically modified to yield Cu-poor, Zn-rich films. Optimum bath compositions were successfully identified, although a delayed onset of damaging hydrogen evolution reaction was observed during growth. Stopping the deposition before its onset limited the damage caused by this reaction, but films were limited to small thicknesses. Stirring, increasing bath concentration, and galvanostatic control were investigated as possible methods to increase the film thickness before the onset of this reaction. Optimized co-electrodeposition processes were identified, with linearly-swept potential and pulsed stirring methods representing the most promising processes. Similar to the pulsed laser deposited CZTS films, a two-step temperature profile yielded the most-stable films with the optimum structural properties.

Finally, co-electrodeposition of metallic CZT precursors was investigated. The as-deposited films were highly compact, uniform and free of damage, and the metal ratios in the film could be directly controlled by modifying the metal sulfate concentration ratios in the bath. Varying bath concentrations were investigated, in order to determine the effect on film thickness and composition. Optimized co-electrodeposition processes were identified. After sulfurization, films demonstrated ideal compositions and structures but also significant phase segregation, with the film laterally divided into regions of large-grained CZTS and small-grained ZnS phases. Annealing in Argon at low temperature prior to sulfurization significantly improved the film homogeneity, although a non-negligible density of pinholes remained. It is unclear if the lack of S-content in the precursors, the loss of sulfur overpressure, or both, contributed to the formation of voids and pinholes.

The work in this dissertation provides a comprehensive foundation on which to further improve the deposition of co-electrodeposited S-containing and metallic precursors. The growth of high-quality CZTS precursors was demonstrated and a pathway to additional optimization recommended. With minor adjustments in the electrodeposition process, and improved monitoring and control of the sulfur overpressure, it is believed that very high-quality CZTS films can be reliably fabricated using a two-step co-electrodeposition and sulfurization process. Notably, the co-electrodeposition method represents a simple fabrication method that utilizes low-toxicity components, complies with existing plating technologies, and provides high throughput. The demonstration of such a process would represent a significant step forward in the development of the earth-abundant, low-toxicity CZTS material system.

Efficient hydrogen storage is required for fuel cell vehicles to be competitive with those driven by internal combustion engines. Current methods of storage such as compressed gas and liquid hydrogen cannot meet this standard, so novel hydrogen storage materials such as metal hydrides are required. No simple metal hydride meets the required specifications. Research is required to find new materials or improve existing materials.

This thesis describes the research practices necessary to achieve legitimate and repeatable results in laboratories across the world. Examples of experiments using these techniques are presented, such as a high throughput technique to optimize materials systems with up to three components such as calcium borohydride with titanium catalyst and magnesium hydride with nickel and aluminum as destabilizing elements and catalysts. Thin films composed of gradients of each material were deposited by sputtering, creating a single thin film sample covering all potential material combinations. Optical properties of the samples under hydrogen pressure were monitored to identify the regions with largest and fastest hydrogen uptake. In the Ca-B-Ti system, titanium did not sufficiently catalyze the borohydride formation reaction at low temperature. Substantial hydrogen uptake was shown in the Mg-Ni region of the Mg-Ni-Al films. Al did not participate in the reaction at low temperature.

Further investigation of the role of catalysts and destabilizing elements in improving hydrogen storage performance through X-ray Absorption and Emission Spectroscopy measurements of the Mg-Ni system during hydrogenation is presented. Typical X-ray spectroscopy measurements use a synchrotron radiation source and require ultra high vacuum conditions. For these experiments we designed a chamber which can be inserted into a vacuum chamber allowing in situ measurements of a sample under hydrogen pressure, providing information on the role of Ni in hydrogen absorption of Mg-Ni mixtures.

Finally, another technique for improving hydrogen storage performance is presented which focuses on promising materials studied using the high throughput technique. TiO₂ powder was ball milled together with NaBH₄, and gravimetric analysis shows a 50% improvement in the kinetics of the hydrogen desorption reaction and a reduction in desorption temperature of 60 ^oC.

This dissertation is focused on the development and characterization of a novel class of solid-state nano-structured composites for hydrogen storage based on silica aerogel. It is organized sequentially around experiments conducted to fabricate, optimize and characterize silica aerogel and the composites for hydrogen storage. First, the basics of nano-structured media, silica aerogel technology and solid-state hydrogen storage are introduced. Next, the fabrication and optimization of silica aerogel for hydrogen storage is described in detail. The key result is that varying fabrication parameters can improve the physical properties of the resultant silica aerogel in the context of hydrogen storage. The fabrication of solid-state nano-structured composites using chemical vapor infiltration is then discussed. A series of experiments is used to parameterize the fabrication process, which results in a collection of parameters that minimize variation and structural damage in the composites. Silica aerogel and the composites are then physically characterized using transmission electron microscopy, X-ray diffraction and porosimetry in order to investigate their nano-structuring.

An overview of hydrogen storage characterization and two innovations that improve the accuracy and efficiency of hydrogen storage characterization of low-bulk density media like silica aerogel and the composites are then presented. Finally, the innovations are applied to silica aerogel and the composites to characterize their hydrogen storage performance. Silica aerogel and the composites are found to outperform the most common benchmark in physisorption media, and one composite in particular shows unique hydrogen storage performance.

Organic light-emitting diodes (OLEDs) have made tremendous technological progress in the past two decades and have emerged as a top competitor for next generation light-emitting displays and lighting. State-of-the-art OLEDs have been reported in literature to approach, and even surpass, white fluorescent tube efficiency. However, despite rapid technological progress, efficiency metrics must be improved to compete with traditional inorganic light-emitting diode (LED) technology. Organic materials possess specialized traits that permit manipulations to the light-emitting cavity. Overall, as demonstrated within, these modifications can be used to improve electrical and optical device efficiencies. This work is focused at analyzing the effects that nanopatterned geometric modifications to the organic active layers play on device efficiency.

In general, OLED efficiency is complicated by the complex, coupled processes which contribute to spontaneous dipole emission. A composite of three sub-systems (electrical, exciton and optical) ultimately dictate the OLED device efficiency. OLED electrical operation is believed to take place via a low-mobility-modified Schottky injection process. In the injection-limited regime, geometric effects are expected to modify the local electric field leading to device current enhancement. It is shown that the patterning effect can be used to enhance charge carrier parity, thereby enhancing overall recombination. Current density and luminance characteristics are shown to be improved by OLED nanopatterning from both the model developed within and experimental techniques.

Next, the optical enhancement effects produced by the nanopatterned array are considered. Finite-difference time-domain (FDTD) simulations are used to determine positional, spectral optical enhancement for the nanopatterned device. The results show beneficial effects to the device performance. The optical enhancements are related to the reduction in internal radiative quenching (improved internal quantum efficiency) and improvement in light extraction (improved outcoupling efficiency). Furthermore, the electrical model is used to construct a positional radiative efficiency map that when combined with the optical enhancement reveals the overall external quantum efficiency enhancement.