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Increasing the efficiency of organic solar cells by photonic and electrostatic-field enhancements

Description: Organic photovoltaic (OPV) technology is an attractive solar-electric conversion paradigm due to the promise of low cost roll-to-roll production and amenability to flexible substrates. Power conversion efficiency (PCE) exceeding 7% has recently been achieved. OPV cells suffer from low charge carrier mobilities of polymers, leading to recombination losses, higher series resistances and lower fill-factors. Thus, it is imperative to develop fabrication methodologies that can enable efficient optical absorption in films thinner than optical absorption length. Active layers conformally deposited on light-trapping, microscale textured, grating-type surfaces is one possible approach to achieve this objective. In this study, 40% theoretical increase in photonic absorption over flat OPVs is shown for devices with textured geometry by the simulation results. For verifying this theoretical result and improving the efficiency of OPVs by light trapping, OPVs were fabricated on grating-type textured substrates possessing t pitch and -coat PV active-layer on these textured substrates led to over filling of the valleys and shunts at the crest, which severely affected the performance of the resultant PV devices. Thus, it is established that although the optical design is important for OPV performance but the potential of light trapping can only be effectively tapped if the textures are amenable for realizing a conformal active layer. It is discovered that if the height of the underlying topographical features is reduced to sub-micron regime (e.g. 300 nm) and the pitch is increased to more than a micron (e.g. 2 μm), the textured surface becomes amenable to coating a conformal PV active-layer. The resultant PV cells showed 100% increase in average light absorption near the band edge due to trapping of higher wavelength photons, and 20% improvement in power conversion efficiency as compared with the flat PV cell. Another factor that severely limits the performance of OPVs is recombination of charge carriers. ...
Date: November 3, 2012
Creator: Nalwa, Kanwar
Partner: UNT Libraries Government Documents Department

Cellulose and cellobiose: adventures of a wandering organic chemist in theoretical chemistry

Description: The energies arising from the rotation of free hydroxyl groups in the central glucose residue of a cellulose crystalline assembly, calculated using RHF, DFT, and FMO2/MP2 methods, will be presented. In addition, interactions of this central glucose residue with some of the surrounding residues (selected on the basis of the interaction strengths) are analyzed. The mechanism of acid-catalyzed hydrolysis of cellobiose, which is the repeating unit of cellulose. Energies corresponding to the different steps of this mechanism calculated using RHF and DFT are compared with those previously reported using molecular dynamics calculations and with experimental data.
Date: April 3, 2012
Creator: Baluyut, John
Partner: UNT Libraries Government Documents Department

Control of Laser Plasma Based Accelerators up to 1 GeV

Description: This dissertation documents the development of a broadband electron spectrometer (ESM) for GeV class Laser Wakefield Accelerators (LWFA), the production of high quality GeV electron beams (e-beams) for the first time in a LWFA by using a capillary discharge guide (CDG), and a statistical analysis of CDG-LWFAs. An ESM specialized for CDG-LWFAs with an unprecedented wide momentum acceptance, from 0.01 to 1.1 GeV in a single shot, has been developed. Simultaneous measurement of e-beam spectra and output laser properties as well as a large angular acceptance (> {+-} 10 mrad) were realized by employing a slitless scheme. A scintillating screen (LANEX Fast back, LANEX-FB)--camera system allowed faster than 1 Hz operation and evaluation of the spatial properties of e-beams. The design provided sufficient resolution for the whole range of the ESM (below 5% for beams with 2 mrad divergence). The calibration between light yield from LANEX-FB and total charge, and a study on the electron energy dependence (0.071 to 1.23 GeV) of LANEX-FB were performed at the Advanced light source (ALS), Lawrence Berkeley National Laboratory (LBNL). Using this calibration data, the developed ESM provided a charge measurement as well. The production of high quality electron beams up to 1 GeV from a centimeter-scale accelerator was demonstrated. The experiment used a 310 {micro}m diameter gas-filled capillary discharge waveguide that channeled relativistically-intense laser pulses (42 TW, 4.5 x 10{sup 18} W/cm{sup 2}) over 3.3 centimeters of sufficiently low density ({approx_equal} 4.3 x 10{sup 18}/cm{sup 3}) plasma. Also demonstrated was stable self-injection and acceleration at a beam energy of {approx_equal} 0.5 GeV by using a 225 {micro}m diameter capillary. Relativistically-intense laser pulses (12 TW, 1.3 x 10{sup 18}W/cm{sup 2}) were guided over 3.3 centimeters of low density ({approx_equal} 3.5 x 10{sup 18}/cm{sup 3}) plasma in this experiment. A statistical analysis of the CDG-LWFAs ...
Date: December 3, 2007
Creator: Nakamura, Kei
Partner: UNT Libraries Government Documents Department

A measurement of the 2 neutrino double beta decay rate of Te-130 in the CUORICINO experiment

Description: CUORICINO was a cryogenic bolometer experiment designed to search for neutrinoless double beta decay and other rare processes, including double beta decay with two neutrinos (2{nu}{beta}{beta}). The experiment was located at Laboratori Nazionali del Gran Sasso and ran for a period of about 5 years, from 2003 to 2008. The detector consisted of an array of 62 TeO{sub 2} crystals arranged in a tower and operated at a temperature of #24;10 mK. Events depositing energy in the detectors, such as radioactive decays or impinging particles, produced thermal pulses in the crystals which were read out using sensitive thermistors. The experiment included 4 enriched crystals, 2 enriched with {sup 130}Te and 2 with {sup 128}Te, in order to aid in the measurement of the 2{nu}{beta}{beta} rate. The enriched crystals contained a total of #24;350 g {sup 130}Te. The 128-enriched (130-depleted) crystals were used as background monitors, so that the shared backgrounds could be subtracted from the energy spectrum of the 130- enriched crystals. Residual backgrounds in the subtracted spectrum were fit using spectra generated by Monte-Carlo simulations of natural radioactive contaminants located in and on the crystals. The 2{nu}{beta}{beta} half-life was measured to be T{sup 2{nu}}{sub 1/2} = [9.81{+-} #6;0.96(stat){+-} 0.49(syst)]#2;x10{sup 20} y.
Date: November 3, 2011
Creator: Kogler, Laura
Partner: UNT Libraries Government Documents Department

Mutiscale Modeling of Segregation in Granular Flows

Description: Modeling and simulation of segregation phenomena in granular flows are investigated. Computational models at different scales ranging from particle level (microscale) to continuum level (macroscale) are employed in order to determine the important microscale physics relevant to macroscale modeling. The capability of a multi-fluid model to capture segregation caused by density difference is demonstrated by simulating grain-chaff biomass flows in a laboratory-scale air column and in a combine harvester. The multi-fluid model treats gas and solid phases as interpenetrating continua in an Eulerian frame. This model is further improved by incorporating particle rotation using kinetic theory for rapid granular flow of slightly frictional spheres. A simplified model is implemented without changing the current kinetic theory framework by introducing an effective coefficient of restitution to account for additional energy dissipation due to frictional collisions. The accuracy of predicting segregation rate in a gas-fluidized bed is improved by the implementation. This result indicates that particle rotation is important microscopic physics to be incorporated into the hydrodynamic model. Segregation of a large particle in a dense granular bed of small particles under vertical. vibration is studied using molecular dynamics simulations. Wall friction is identified as a necessary condition for the segregation. Large-scale force networks bearing larger-than-average forces are found with the presence of wall friction. The role of force networks in assisting rising of the large particle is analyzed. Single-point force distribution and two-point spatial force correlation are computed. The results show the heterogeneity of forces and a short-range correlation. The short correlation length implies that even dense granular flows may admit local constitutive relations. A modified minimum spanning tree (MST) algorithm is developed to asymptotically recover the force statistics in the force networks. This algorithm provides a possible route to constructing a continuum model with microstructural information supplied from it. Microstructures in ...
Date: August 3, 2007
Creator: Sun, Jin
Partner: UNT Libraries Government Documents Department

Detecting Molecular Properties by Various Laser-Based Techniques

Description: Four different laser-based techniques were applied to study physical and chemical characteristics of biomolecules and dye molecules. These techniques are liole burning spectroscopy, single molecule spectroscopy, time-resolved coherent anti-Stokes Raman spectroscopy and laser-induced fluorescence microscopy. Results from hole burning and single molecule spectroscopy suggested that two antenna states (C708 & C714) of photosystem I from cyanobacterium Synechocystis PCC 6803 are connected by effective energy transfer and the corresponding energy transfer time is {approx}6 ps. In addition, results from hole burning spectroscopy indicated that the chlorophyll dimer of the C714 state has a large distribution of the dimer geometry. Direct observation of vibrational peaks and evolution of coumarin 153 in the electronic excited state was demonstrated by using the fs/ps CARS, a variation of time-resolved coherent anti-Stokes Raman spectroscopy. In three different solvents, methanol, acetonitrile, and butanol, a vibration peak related to the stretch of the carbonyl group exhibits different relaxation dynamics. Laser-induced fluorescence microscopy, along with the biomimetic containers-liposomes, allows the measurement of the enzymatic activity of individual alkaline phosphatase from bovine intestinal mucosa without potential interferences from glass surfaces. The result showed a wide distribution of the enzyme reactivity. Protein structural variation is one of the major reasons that are responsible for this highly heterogeneous behavior.
Date: August 3, 2007
Creator: Hsin, Tse-Ming
Partner: UNT Libraries Government Documents Department

Wave propagation in photonic crystals and metamaterials: Surface waves, nonlinearity and chirality

Description: Photonic crystals and metamaterials, both composed of artificial structures, are two interesting areas in electromagnetism and optics. New phenomena in photonic crystals and metamaterials are being discovered, including some not found in natural materials. This thesis presents my research work in the two areas. Photonic crystals are periodically arranged artificial structures, mostly made from dielectric materials, with period on the same order of the wavelength of the working electromagnetic wave. The wave propagation in photonic crystals is determined by the Bragg scattering of the periodic structure. Photonic band-gaps can be present for a properly designed photonic crystal. Electromagnetic waves with frequency within the range of the band-gap are suppressed from propagating in the photonic crystal. With surface defects, a photonic crystal could support surface modes that are localized on the surface of the crystal, with mode frequencies within the band-gap. With line defects, a photonic crystal could allow the propagation of electromagnetic waves along the channels. The study of surface modes and waveguiding properties of a 2D photonic crystal will be presented in Chapter 1. Metamaterials are generally composed of artificial structures with sizes one order smaller than the wavelength and can be approximated as effective media. Effective macroscopic parameters such as electric permittivity {epsilon}, magnetic permeability {mu} are used to characterize the wave propagation in metamaterials. The fundamental structures of the metamaterials affect strongly their macroscopic properties. By designing the fundamental structures of the metamaterials, the effective parameters can be tuned and different electromagnetic properties can be achieved. One important aspect of metamaterial research is to get artificial magnetism. Metallic split-ring resonators (SRRs) and variants are widely used to build magnetic metamaterials with effective {mu} < 1 or even {mu} < 0. Varactor based nonlinear SRRs are built and modeled to study the nonlinearity in magnetic metamaterials and the ...
Date: December 3, 2009
Creator: Wang, Bingnan
Partner: UNT Libraries Government Documents Department

Absorption and emission properties of photonic crystals and metamaterials

Description: We study the emission and absorption properties of photonic crystals and metamaterials using Comsol Multiphysics and Ansoft HFSS as simulation tools. We calculate the emission properties of metallic designs using drude model and the results illustrate that an appropriate termination of the surface of the metallic structure can significantly increase the absorption and therefore the thermal emissivity. We investigate the spontaneous emission rate modifications that occur for emitters inside two-dimensional photonic crystals and find the isotropic and directional emissions with respect to different frequencies as we have expected.
Date: August 3, 2007
Creator: Peng, Lili
Partner: UNT Libraries Government Documents Department

Concurrences in the U. S. Supreme Court

Description: Thesis written by a student in the UNT Honors College discussing different types of opinions within the United States Supreme Court, with an emphasis on the history and practice of concurrences.
Date: April 3, 2000
Creator: Watson, Takiesha
Partner: UNT Honors College

Ultrafast dynamics of electrons at interfaces

Description: Electronic states of a thin layer of material on a surface possess unique physical and chemical properties. Some of these properties arise from the reduced dimensionality of the thin layer with respect to the bulk or the properties of the electric field where two materials of differing dielectric constants meet at an interface. Other properties are related to the nature of the surface chemical bond. Here, the properties of excess electrons in thin layers of Xenon, Krypton, and alkali metals are investigated, and the bound state energies and effective masses of the excess electrons are determined using two-photon photoemission. For Xenon, the dependence of bound state energy, effective mass, and lifetime on layer thickness from one to nine layers is examined. Not all quantities were measured at each coverage. The two photon photoemission spectra of thin layers of Xenon on a Ag(111) substrate exhibit a number of sharp, well-defined peaks. The binding energy of the excess electronic states of Xenon layers exhibited a pronounced dependence on coverage. A discrete energy shift was observed for each additional atomic layer. At low coverage, a series of states resembling a Rydberg series is observed. This series is similar to the image state series observed on clean metal surfaces. Deviations from image state energies can be described in terms of the dielectric constant of the overlayer material and its effect on the image potential. For thicker layers of Xe (beyond the first few atomic layers), the coverage dependence of the features begins to resemble that of quantum well states. Quantum well states are related to bulk band states. However, the finite thickness of the layer restricts the perpendicular wavevector to a discrete set of values. Therefore, the spectrum of quantum well states contains a series of peaks which correspond to the various allowed values ...
Date: May 3, 1999
Creator: McNeill, Jason D.
Partner: UNT Libraries Government Documents Department

Authenticated group Diffie-Hellman key exchange: theory and practice

Description: Authenticated two-party Diffie-Hellman key exchange allows two principals A and B, communicating over a public network, and each holding a pair of matching public/private keys to agree on a session key. Protocols designed to deal with this problem ensure A (B resp.)that no other principals aside from B (A resp.) can learn any information about this value. These protocols additionally often ensure A and B that their respective partner has actually computed the shared secret value. A natural extension to the above cryptographic protocol problem is to consider a pool of principals agreeing on a session key. Over the years several papers have extended the two-party Diffie-Hellman key exchange to the multi-party setting but no formal treatments were carried out till recently. In light of recent developments in the formalization of the authenticated two-party Diffie-Hellman key exchange we have in this thesis laid out the authenticated group Diffie-Hellman key exchange on firmer foundations.
Date: October 3, 2002
Creator: Chevassut, Olivier
Partner: UNT Libraries Government Documents Department