Metal oxides are a very important class of materials with a wide range of photovoltaic applications. Transparent conducting oxides (TCOs) are the primary front contact materials used in thin film solar cells. Identification of methods for reducing the resistivity of these materials would have significant benefits. Development of p-type TCOs would provide alternative back contact materials and could enable further development of technologies such as bifacial, window and multijunction cells. A series of studies into these areas is presented in this work. Aluminium doped zinc oxide (AZO) is a well-known n-type TCO consisting entirely of Earth-abundant materials. Targets were manufactured from AZO powder, which was synthesised using a patented emulsion detonation process developed by Innovnano S.A. All films showed good optical transmission. Resistivity was found to decrease with both increasing time and temperature up to 300 degree C. Temperatures above 300 degree C were found to be detrimental to film formation, with increasing amounts of damage to the crystal structure and consequent increases in the resistivity. The effect of alloying molybdenum oxide with molybdenum nitride through reactive sputtering in a mixed oxygen-nitrogen atmosphere was investigated. All alloys were found to show p-type behaviour. Resistivity was found to improve with increased nitrogen content, in contrast to optical transmission, which reduced. A selection of compositions were deposited onto CdTe cells as back contacts. These cells showed an increase in efficiency with increasing nitrogen content. Work function was found to increase with increasing oxygen content, but all work functions were low. Resistivity was shown to correlate strongly with efficiency, caused by a corresponding increase in cell voltage. This implies that to form an ohmic contact on CdTe with p-type materials, work function may be less important than resistivity. The copper oxides are p-type, but uses are limited by the narrow band gaps. Cupric oxide was chosen for investigation and for alloying with other oxides with the aim of increasing the band gap. It was found that temperature and deposition environment have significant impacts on sputtered cupric oxide (CuO) films, with low temperatures and high oxygen environments producing the lowest resistivities. Extrinsic sodium doping was found to reduce the resistivity by up to four orders of magnitude. High oxygen content sodium-doped films were found to have carrier concentrations two orders of magnitude higher than that of indium tin oxide.

Abstract In this study, copper oxide thin films prepared by DC reactive magnetron sputtering using a Cu target were studied. By changing the oxygen partial pressure ratios and sputtering power and deposition temperatures during sputtering, we obtained copper oxide thin films with different properties. The structures of copper oxide thin films were characterized by glancing incident angle X-ray diffraction. Clear crystal orientation at (002) plane were observed at 50% and 60% oxygen partial pressure ratio. The preferred orientation at (111) plane were observed with heating substrate to 200¢J. The optical and electrical properties of cupric oxide thin films were measured by UV-VIS spectrophotometer and four-point probe system. The cupric oxide thin films deposited with heating substrate to 200¢J exhibited the resistivity of 0.77£[-cm and optical band gap of 1.57 eV. Keywords¡G cupric oxide, thin film, magnetron sputtering, band gap

The dye sensitized solar cell (DSSC) is a promising low-cost technology alternative to conventional solar cell in certain applications. A DSSC is a photo-electrochemical photovoltaic device, mainly composed of a working electrode, a dye sensitized semiconductor layer, an electrolyte and a counter electrode. Sunlight excites the dye, producing electrons and holes that can be transported by the semiconductor and electrolyte to the external circuit, converting the sunlight into an electrical current. A material that could be useful for DSSCs is the nanoscale cupric oxide, which can act as a p-type semiconductor and has interesting properties such as low thermal emittance and relatively good electrical properties. The goal of this project was to synthesize and characterize CuO nanoparticles using three different methods and look into each products potential use and efficiency in DSSCs. The particles were synthesized using two different solution based chemical precipitation methods and a flame spray pyrolysis method, yielding nanostructures with different compositions, structures and sizes ranging from ~20 to 1000 nm. The nanoparticle powder synthesized by the flame spray pyrolysis route was tested as semiconductor layer in the working electrode of the DSSC. Current-voltage measurements presented low solar conversion efficiencies with a reversed current, meaning that the cupric oxide cells did not work in a desirable way. Further studies of the cupric oxide synthesis and its suitability in DSSCs are needed to increases the future possibilities for gaining well working p-type DSSCs with higher efficiencies.

Since the beginning of the Industrial Revolution, the global energy consumption has been continuously increasing, supplied mainly by coal, oil and natural gases. Unfortunately, this consumption is linked to the emission of greenhouse gasses such as CO2 to the atmosphere. For this reason, it is extremely important to look for sustainable and renewable energy sources in order to replace the commonly used fossil fuels. Within the different types of renewable energy sources, solar energy holds by far the largest potential capacity. In this respect, artificial photosynthesis is a promising technology not only to harvest solar energy, but also as a means of storage by producing energy-rich chemical fuels such as H2 from water. The main components of photoelectrochemical water splitting devices are the semiconductor light absorber photoelectrodes and the electrolyte. Chapter 1 reviews the fundamental aspects of photoelectrochemical water splitting and overviews the physics and electrochemistry of semiconductor materials. The second chapter describes the methodologies and techniques employed throughout the thesis. The experimental results are reported from Chapter 3 to 8, focusing on the development and further optimization of two photoelectrodes, concretely hematite and cupric oxide, besides the design and fabrication of tandem cells for standalone water splitting. In the case of hematite photoanodes, the main efforts have focused on its doping to enhance carrier density and mobility as a way of diminishing recombination. The major drawback present in cupric oxide photoelectrodes is their instability against photocorrosion, for this reason, research has focused on protecting them, both by impregnation and adsorption methodologies. Finally, a tandem cell composed by a hematite photoanode and a cupric oxide photocathode was developed. It is worth noting that a polymer electrolyte membrane (PEM) was employed as to facilitate upscaling and diminish the corrosion observed employing the typical acidic or basic liquid electrolytes.

No description available.

Nanostructured materials especially, one-dimensional (1D) nanostructures have unique physical, chemical, mechanical properties and are the building blocks for a range of nanoscale devices. The procedure employed for the synthesis of nanostructures involves the use of sophisticated instruments or rigorous chemical reactions. The motivation of our work is to develop a strategy that is simple, cost effective and applicable to a host of oxide materials. Nanostructures of various oxides have been grown from the metal as the source material. 1D ZnO nanostructures have been obtained by simply heating Zn metal in ambient air at temperatures below 600 °C. The nanostructures grow on the surface of the source material and the morphology is controlled by monitoring the curvature of the source material. This technique has an added advantage that neither any catalyst nor any gas flow is required. Tetrapods of ZnO are obtained when Zn is heated above 700 °C in ambient air. It has been shown that the morphology and the aspect ratio (length-to-diameter ratio) of the tetrapods depend on the temperature and the temperature gradient. Photoluminescence studies reveal good optical quality ZnO nanostructures. The technique employed to synthesize 1D ZnO nanostructures has been checked for other oxides. The temperature required for the synthesis of Ga2O3 nanostructures is 1200 °C. Many researchers have shown that Ga2O3 emits in the blue-green region. A red emission is required to get the impression of white light which has been seen for nitrogen doped Ga2O3. As the temperature is very high and Ga is heated in ambient air, unintentional nitrogen doping of 1D Ga2O3 nanostructures is obtained which is the reason for white light emission. The morphology of Ga2O3 nanostructures has been controlled by monitoring the curvature of the starting material as is the case of ZnO. Similar technique has also been employed for the synthesis of CuO nanostructures. The morphology is temperature dependent and 1D CuO nanostructures are obtained when the synthesis temperature is between 400 and 600 C. Possible growth mechanisms have been proposed for all these oxide materials. The entire thesis is based on the results discussed above. It has been organized as follows: Chapter 1 deals with the introduction to nanostructures, importance of 1D nanostructures, the specific applications of different morphologies, materials that are widely explored in the synthesis of nanostructures and different approaches to the synthesis of nanostructures. Growth mechanisms like VLS, VS and SLS are briefly discussed. A brief review on the basic physical properties, applications and different morphologies of ZnO, Ga2O3 and CuO is outlined with emphasis to the various synthesis techniques. Finally the aim and scope of the present work is discussed. Chapter 2 describes the experimental setup used for the synthesis and the basic principles of characterization techniques like x-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), energy dispersive spectrum (EDS), electron energy loss spectroscopy (EELS), photoluminescence (PL), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), UV-Visible spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and thermogravimetric analysis (TGA). Chapter 3 deals with the synthesis of 1D ZnO nanostructures with different morphologies such as nanoneedles, nanorods, nanobelts from Zn powder/granule. The growth process is found to be different from the conventional VS mechanism. The advantage and the versatility of the method is emphasized. In this method, neither a catalyst nor any gas flow is required for the synthesis of oxide nanostructures. Depending upon the Zn powder or Zn granules as the starting material different nanostructures of ZnO have been synthesized. The as-synthesized materials are characterized by XRD, SEM, HRTEM, EDS, TGA and Raman spectroscopy and the results are discussed. Chapter 4 describes the controlled growth of ZnO tetrapods and the influence of temperature and temperature gradient on the growth process. Though there are several methods to synthesize ZnO tetrapods and it has been established that ZnO tetrapods can be synthesized by heating Zn in air, it is advantageous to grow tetrapods of different morphologies with different lengths. The large scale synthesis of ZnO tetrapods by heating Zn in air ambient is discussed in this chapter. The key parameters that control the diameter, length, and morphology of tetrapods are identified. It is shown that the morphology and dimensions of the tetrapods depend not only on the vaporization temperature but also on the temperature gradient of the furnace. The influence of vaporization temperature and growth temperature on the morphology of the tetrapods is discussed elaborately. Chapter 5 explains the one-step synthesis of nitrogen doped Ga2O3 nanostructures of different morphologies and the different growth mechanisms. The experimental method employed for the synthesis of nanostructures is simple and is different from the other reported methods. Neither any catalyst/substrate preparation nor any gas flow is required for the synthesis of Ga2O3 nanostructures. The synthesis involves the heating of molten Ga at high temperatures. Single crystalline monoclinic phase of nitrogen-doped Ga2O3 nanorods, nanobelts and nanoneedles are obtained by this method. The morphology is controlled by monitoring the curvature of the Ga droplet which is achieved by using different substrates. Possible growth processes of different morphology have been proposed. Chapter 6 includes some surprising results on the white light emission of Ga2O3 nanorods. High synthesis temperature generates a high vapor pressure suitable for the growth of Ga2O3 nanorods, creates oxygen vacancy and incorporates nitrogen from the ambient. The oxygen vacancy is responsible for the bluish-green emission, while nitrogen is responsible for the red emission. As a consequence, white light emission is observed from Ga2O3 nanorods when irradiated with UV light. The interesting point is that neither post-treatment of the nanorods nor size control is required for white light emission. Chapter 7 describes the synthesis of CuO nanostructures by heating Cu foil in air ambient. This is an attempt to check whether the synthesis technique employed for ZnO and Ga2O3 is applicable to other oxides. The as-synthesized CuO nanostructures are characterized by XRD, SEM, HRTEM, EDS, TGA, UV-visible, FTIR and the results are discussed. Chapter 8 gives the conclusions and the overall summary of the thesis.

In this work, experimental and theoretical approaches are applied to the study of chemical reaction dynamics. In Chapter 2, two applications of transition state theory are presented: (1) Application of microcanonical transition state theory to determine the rate constant of dissociation of C2F3I after π∗ ← π excitation. It was found that this reaction has a very fast rate constant and thus is a promising system for testing the statistical assumption of molecular reaction dynamics. (2) A general rate constant expression for the reaction of atoms and molecules at surfaces was derived within the statistical framework of flexible transition state theory. In Chapter 4, a computationally efficient TDDFT approach was found to produce useful potential energy surface landscapes for application to non-adiabatic predissociative dynamics of the molecule CS2 after excitation from the ground state to the singlet C-state. In Chapter 5, ultrafast experimental results of excitation of CS2 to the predissociative neutral singlet C-state is presented. The bandwidth of the excitation laser was carefully tuned to span a two-component scattering resonance with each component differently evolving electronically with respect to excited state character during the quasi-bound oscillation. Scalar time-resolved photoelectron spectra (TRPES) and vector time-resolved photoelectron angular distribution (TRPAD) observables were recorded during the predissociation. The TRPES yield of photoelectrons was found to oscillate with a quantum beat pattern for the photoelectrons corresponding to ionization to the vibrationless cation ground state; this beat pattern was obscured for photoelectron energies corresponding to ionization from the vibrationally excited CS2 cation. The TRPAD data was recorded for two general molecular ensemble cases: with and without a pre-excitation alignment laser pulse. It was found that in the case of ensemble alignment (Chapter 6), the “molecular frame” TRPAD (i.e. TRMFPAD) was able to image the purely valence electronic dynamics of the evolving CS2 C-state. The unaligned ensemble TRPAD observable suffers from excessive orientational averaging and was unable to observe the quantum beat. Engineering efforts were also undertaken to eliminate scattered light background signal (Chapter 7, Appendix A) and improve laser stability as a function of ambient pressure (Appendix B) for TRMFPAD experiments. / Thesis (Ph.D, Chemistry) -- Queen's University, 2012-09-11 22:18:20.89