Browsing by Subject "Zinc Oxide"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Electron Transfer Studies of Thiophene-Based Oligomers to Zinc Oxide Nanocrystals for Dye-Sensitized Solar Cells
    (2017-01) Oehrlein, Amanda
    Dye-sensitized solar cells (DSSCs) are an alternative to traditional solid-state solar cells which are currently used to harvest solar energy. DSSCs contain semiconductor nanomaterials and dye molecules to harvest light. Currently DSSCs are less efficient compared to solid-state cells and understanding the electron transfer rate between the dyes and nanomaterials can lead to improvements in efficieny. Oligothiophenes are model compounds for polythiophene, commonly used as light absorbers on organic bulk heterojunction solar cells, and have been studied in DSSCs. Oligothiophene-based dyes are bound to zinc oxide nanocrystals (ZnO NCs) to study the rate of electron transfer. Chapter two focuses on terthiophene dyes, an oligothiophene with three thiophenes, with either a carboxylate or phosphonate moiety at the 2-position which serves as an anchoring group to (ZnO NCs). Chapter three focuses on oligomer dyes with two to five thiophenes that were synthesized with a cyanoacrylate moiety, which serves as the anchoring group to ZnO NCs. Electronic absorption and fluorescence measurements, combined with reduction potentials, provided estimates of excited state potentials for all dyes. Static quenching of dyes’ fluorescence was observed when the dyes were bound to ZnO NCs. Ultrafast transient absorption spectroscopic experiments were used to probe the lifetimes of the dyes’ excited singlet state. In the presence of ZnO NCs, the disappearance of the singlet excited states of the dyes corresponds to the appearance of the spectroscopic signatures of the oxidized dyes. The rate of electron transfer from the dye to the nanocrystal was then found.
  • Thumbnail Image
    Item
    Synthesis of titanium dioxide and zinc oxide nanowires for excitonic solar cells.
    (2009-08) Boercker, Janice Elaine
    Dye-sensitized, quantum-dot sensitized, quantum-dot, and hybrid organic/inorganic solar cells are promising excitonic photovoltaic devices for the generation of low cost, carbon free energy. Wide-band gap semiconductor nanowire photoanodes have the potential to increase the efficiencies of these excitonic solar cells. Controlling and tailoring the dimensions of the nanowires (i.e. nanowire height, diameter, and planar number density) for each solar cell type is important for efficiency improvement. Obtaining such control will require a detailed and fundamental understanding of the nanowire growth process. Towards this end, the synthesis of TiO2 and ZnO nanowire films in aqueous solutions was studied. Anatase TiO2 nanowire films were grown on flexible titanium foil substrates using a three step hydrothermal synthesis. First, the top surface of the titanium foil was transformed to Na2Ti2O4(OH)2 nanotubes through hydrothermal oxidation in NaOH. Next, the Na2Ti2O4(OH)2 nanotubes were converted to H2Ti2O4(OH)2 nanotubes by ion exchange. Finally, the H2Ti2O4(OH)2 nanotubes were converted to polycrystalline anatase nanowires through a topotactic transformation. The film morphology evolution, crystal structure transformations, and growth mechanism were examined in detail. Dye-sensitized solar cells (DSSCs) were assembled from these TiO2 nanowire films. Transient photocurrent and photovoltage spectroscopies were used to measure the electron transport and recombination rates in these solar cells. Compared to TiO2 nanoparticle DSSCs the electron collection efficiency in the TiO2 nanowire DSSCs was increased due to decreased electron recombination. However, the electron transport in the nanowire DSSCs was similar to that of TiO2 nanoparticle DSSCs. The synthesis of ZnO nanowires from aqueous solutions of methenamine (HMT) and zinc nitrate hexahydrate on substrates was studied in detail. A ZnO nanowire growth mechanism was proposed which predicts that the precursor is a zinc-methenamine complex (Zn-HMT2+ and Zn-HMT-Zn4+) which reacts at the hydroxyl terminated nanowire surface. This growth mechanism was supported by examining the growth with several experimental techniques, kinetic modeling, and thermodynamic calculations. In addition, the ZnO nanowire film growth, on ZnO seeded substrates, was found to be mass transport limited. This results in an inverse relationship between the nanowire planar number density and the height and diameter of the nanowires. By stirring the solution the growth rate is increased by approximately a factor of four.
  • Thumbnail Image
    Item
    Zinc oxide nanoparticles: doping, Inkjet printing, and electron accepting from photoexcited porphyrin dyes
    (2013-06) Bierbaum, Andrew Joseph
    This research attempted to extend the useful applications of ZnO by investigating ZnO nanoparticles, doping ZnO nanoparticles, characterizing electron injection from dye molecules into ZnO nanoparticles, and depositing thin films of doped ZnO nanoparticles using inkjet printing. Chapter 1 describes research that produced particles ranging from 2.7 nm to 1 μm of undoped and doped ZnO. These particles were made using solution methods with zinc acetate and aluminum and gallium nitrate salts as dopants, and the particles were characterized by ultraviolet visible absorption, photoluminescence, infrared absorption, and transmission or scanning electron microscopy. The doped ZnO nanoparticles displayed optical signatures of doping in particles larger than 10 nm. This is significant because doping of nanoparticles is still not fully understood, and there are few examples of successfully doping nanoparticles. Chapter 2 describes the research done toward inkjet printing of ZnO films for potential use in a fully inkjet printed solar cell. The research aim was to produce a TCO film of ZnO using inkjet printing that had a bulk resistivity between 10-2 and10-3 Ω cm, a thickness between 0.1 and 1 μm, the highest transparency possible, and processed using conditions under 250 ºC. Film produced using solution methods including inkjet printing were characterized by four point probe ohmmeter, x-ray diffraction, ultraviolet visible absorption, visible microscopy, profilometry, and scanning electron microscopy. Inkjet printed films produced using nanoparticles did not meet the production requirements, but ii progress towards these goals are presented along with the successes and shortcoming of the methods used. Chapter 3 describes the research done on charge transfer from photoexcited porphyrin dyes into ZnO nanoparticles dispersions in methanol. The goal of this research was to further the understanding of the dye-semiconductor interaction and important electron transfer characteristics. Using a series of three porphyrin dyes and a range of particle sizes, the rate of electron transfer was investigated.