Browsing by Author "Mamuye, Tesfaye"

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    Characterization of Alternating Polyfluorene Green-6 and [6,6]-Phenyl C61-Butyric Acid Methyl Ester(Pcbm) based Bulk Heterojunction Solar Cell
    (Addis Ababa University, 2006-07) Mamuye, Tesfaye; Tessema, Genene (PhD)
    Photovoltaic device based on blend of alternating poly uorene green-6 (APFO Green- 6) and [6,6]-phenyl C61-butyric acid methyl ester(PCBM) in a sandwich structure of ITO/PEDOT-PSS/APFO Green-6:PCBM/Al was fabricated. The device was characterized by analyzing the current density-voltage measurements under dark and illumination as well as by impedance spectroscopy. The observed J-V curve under dark agree well with the trap-controlled space charge limited transport theory. The main solar cell parameters recorded at room temperature under 80mA/cm2 white light intensity are: VOC = 547mV, JSC = 0.314mA/cm2, with power conversion ef- ciency 0.107 %, and ll factor, of 0.5. The complex impedance analysis exhibits a bias voltage dependent whose Cole-Cole plot is a single semicircle. This is modelled by one parallel RC circuit, usually obtained from metal-semiconductor(MS) device
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    The Study of Strongly Correlated Electronic Systems using Dynamical Mean field Theory
    (Addis Ababa University, 2016-06) Mamuye, Tesfaye
    The physics of strongly correlated electronic systems has attracted much attention because of its unique and distinct properties like Mott metal-to-insulator transition, giant magneto-resistance, superconductivity. While the density functional theory (DFT), the standard approach to electronic band calculations, is able to describe properties of weakly correlated systems but is known to have serious limitation in the description of strongly correlated systems, the dynamical mean field theory (DMFT) has become an established method for the treatment of strongly correlated systems. In this dissertation, we study the properties of correlated electronic systems near Mott metalto- insulator transitions using DMFT, which maps Hubbard like lattice model in infinite coordination number to a single site impurity Anderson model with the self-consistent conditions. We implement the continuous-time hybridization expansion (CT-HYB) version of continuous-time quantum Monte Carlo (CT-QMC) for the impurity model to investigate the Mott metal-to-insulator in the Hubbard model on Bethe lattice without symmetry braking. At half filling a gap opening transition is found to occur as the interaction strength is increased beyond critical value. We extend our analysis to the inclusion of retarded interaction (electron-phonon interaction) in the frame work of Hubbard-Holstein model. We implement the CT-HYB in the presence of retarded interaction for the impurity model. The effect of the inclusion of retarded interaction to Hubbard model is to shift the Mott metal-to-insulator to a large critical values as compared to the pure Hubbard model. The interplay of electron-electron and electron-phonon interaction near the Mott metal-to-insulator transition are discussed