Browsing by Author "Tessema, Genene (PhD)"
Now showing 1 - 9 of 9
Results Per Page
Sort Options
Item 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) deviceItem Intensity Dependence of the Photovoltaic Parameters of Organic Bulk-Heterojunction Solar Cell(Addis Ababa University, 2007-05) Gebre, Berket; Tessema, Genene (PhD)As the evidence of global warming continues to build up, it is becoming clear that we will have to find ways to produce electricity with out the release of carbondioxide and other green house gases. Fortunately, we have renewable energy sources which neither run out nor have any significant harmful effects on our environment. Harvesting energy directly from the sunlight using photovoltaic (PV) technology is being widely recognized as an essential component of future global energy production. In this thesis, intensity dependence and a brief over view regarding photovoltaic properties of polymer based bulk heterojunction solar cell is presented and discussedItem Junction Studies between Aluminum and Polymer, Poly[2-Methoxy-5-(30,70- Dimethyloctyloxy)-1, 4-Phenylene Vinylene], Mdmo-Ppv and Mdmo-Ppv Blended With C60(Addis Ababa University, 2005-06) Nigussa, Zelalem; Tessema, Genene (PhD)This thesis is based on the experimental investigation of the junction between reactive low work function metal, Aluminum and Conjugated polymer, MDMO-PPV. The polymer is blended with C60 and the composition were also made with pure polymer. The techniques employed are absorption spectroscopy, current-voltage characteristics measurement and complex impedance spectroscopy. The absorption measurement shows that MDMO-PPV is a semiconductor with energy band gap of about 2.0032eV. The current density-voltage curve is found to be asymmetric and non-ohmic and shows Schottky barrier type recti¯cation. The recti¯cation ratio of fullerene blended MDMO-PPV is found to be 100 whereas that of pure MDMO-PPV is between 10 and 20, and more diode characteristics are observed in the case of fullerene blended polymer. The complex impedance spectra are single semi-circle displaced from the origin that show the existence of mobile charge depleted regions with some contact resistance. The radius of the semi-circle is found to depend on the bias of the junctionItem Photovoltaic and charge Carrier Transport Parameters of PTOPT(Addis Ababa University, 2007-08) Gebremichael, Bizuneh; Tessema, Genene (PhD)Low cost and easy processability of organic electronic and photonic devices, which is made from conductive polymer make them to be called a 'noble materials'. The rise of the idea of self healing materials, especially polymeric materials[21], seems to make a remarkable change on the short lifetime of the device which is made of organic polymers. As a consequence, a rapid and de nite analysis of different physical phenomenons in polymeric devices is expected from the theoretical as well as the the experimental physics. In this thesis, the photovoltaic and charge transport properties of the photovoltaic cells made from poly[3-(4-octylphenyl)-2,2'-bithiophene](PTOPT) was studied. A Schottky diode is prepared from a single layer polymer sandwiched in Al and PEDOT:PSS/ITO electrode. That is Al/PTOPT/PEDOT:PSS/ITO layered device structure. By applying a forward bias voltage, the current value for different voltage is measured and found that a rectifying contact is formed. Then, under AM 1.5 (one sun) illumination, the J-V data was collected and a power conversion ef ciency of 0.35% is recorded. Furthermore, dark current-voltage measurement is made by reversing the polarity of the electrodes. The symmetric nature of the J-V semi-logarithmic plot reveals that there is a unipolar charge injection in both sides of the electrodes. As the consequence, from the space charge limited region of the J-V data, we studied the electric eld dependence of the hole transport in the device and we determined the zero eld mobility ("$#&%('*),+) -/.102+435)768 93:0,; <>=@? BA ) and the eld activation factor (CD%E'GFH+0I3J.K02+- L76M N3:0I; O7'P? A /67RQ)Item Photovoltaic Properties of Bulk-Heterojunction Solar Cell by Enhancing Absorption Band in Polyfulorene Copolymer Blend(Addis Ababa University, 2007-08) Ashagre, Solomon; Tessema, Genene (PhD)In this thesis, the photovoltaic properties of the blended polymer of APFO-G6 have been investigated based on enhancing its absorbance. The structure of the blended polymer was ITO/PEDOT:PSS/D75:APFO-G6:PCBM/Al. The electrical properties were studied using Current Voltage (J-V) characteristics and Impedance spectroscopy (IS) measurement. The optical absorption spectrum was measured using UV/VIS/NIR Spectrometer. The J-V plot shows the structure has photovoltaic properties.The absorbtion spectrum shows the structure absorbed as needed in the extended visible region of the spectrum. Fill factor and the ef ciency was improved due to increment of absorbance. And according to impedance Spectroscopy measurement, the Cole- Cole plot shows the depletion resistance and capacitance depend on the bias voltageItem Photovoltaic Properties of Polymer based Bulk Heterojunction Solar Cell Composed of Poly[3-(2,5-Dioctyle-Pheneyl) Thiophene](Pdopt) and (PCBM)(Addis Ababa University, 2006-07) Masresha, Tizazu; Tessema, Genene (PhD)The Theme of this thesis is to characterize polymer based photovoltaic cell using the J-V measurements and impedance spectroscopy. Absorption spectra of pure (neutral) poly[3-(2,5-dioctyle-pheneyl) thiophene](PDOPT) study showed that the optical band gap of this polymer about 2eV, indicating that it is in the range of inorganic semiconductors. The sandwich structure ITO/PEDOT:PSS/PDOPT:PCBM/Al were utilized to fabricate the device. All the devices which are tested in this work, are bulk heterojunction that are composed of the p-type , PDOPT and n-type fullerene derivative, (phenyl-(6,6’)-c61)-butyric acid methyl ester ( PCBM )blend. The J-V measurements under both illumination dark indicated that the typical diode rectification property with rectification ratio 1.5× 103 at ( ±1.8V ) Such a device of blend ratio 1:2 (PDOPT:PCBM) expected to show a photovoltaic behavior with a power conversion efficiency about 0.028 % under 100mWcm−2Item The Study of Porphyrins Confinement in Nafion Membrane and Photovoltaic Properties of a Bulk Heterojunction Solar Cell(Addis Ababa University, 2007-03) Hailu, Girma; Tessema, Genene (PhD)With the development of strong laser sources for different applications which in some instances has extremely high intensity, optical power limiters are needed to provide human eye and optical detector protection. So, a smart ultra fast optical power limiter that is transparent for low incident intensities and blocks the output at high input intensities is required. The operation of optical power limiters is based on the phenomena of nonlinear absorption (NLA). Different Porphyrins have been reported to have NLA property which put them as potential candidate for application as optical power limiters. In this paper, we have investigated NLA and optical limiting properties of five different types of Porphyrins by using the Z-scanning technique. The Porphyrins were confined into Nafion membrane in order to protect them from possible degradation. The results of the experiments clearly show the fascinating NLA and optical power limiting properties of the Porphyrins under investigation. By fitting the Z-scan results with the theoretical fitting function, the nonlinear absorption coefficients (¯) of all the Porphyrins with different concentrations for each of them is determined. Besides, their linear absorption coefficients are obtained by considering low intensity regimes from the data and applying Beer’s low of linear absorptionItem Temperature-Dependent,; Hyperfine Interaction at the 111CD Nucleus in Silicon(Addis Ababa University, 2005-06) Senbeta, Teshome; Tessema, Genene (PhD)Impurity atoms (indium and tellurium ) were incorporated in silicon through an implantation process. The samples were annealed to remove radiation damage. The formation of Cd-Te complex and the temperature dependence of the quadrupole interaction frequency ( QIF) were studied with help of perturbed 7 — 7 angular correlation ( PAC) technique. The observed pairing between Cd and Te was due to the interaction between acceptors and donor atoms in silicon. This explains how the presence of Te influences the lattice site of the Cd atoms. The unique quadrupole interaction frequency UQ = 444M H z was associated to this complex formation. The strong quadrupole interaction frequency, 444MHz at room temperature, decreases to 388MHz at 7U 0°C' which is associated to the state (Item Variation of the 111 _CD - P Population in Silicon with Annealing Temperature.(Addis Ababa University, 2007-08) Yirdaw, Temesgen; Tessema, Genene (PhD)Studying the nature of defects in semiconductor helps to minimize the undesired behaviors they contribute. In this study, the perturbed angular correlation method has used to understand the formation of the substitutional %$'&)(+* pairs in silicon. T 0/2h1e pairs are characterized by a unique quadrupole interaction frequency of ,- . MHz, that suggest a strong interaction between acceptor and donor atoms in an elemental semiconductor silicon. The variation of the nearest neighbor * %$'&3( population with the annealing temperature is discussed. Finally, the binding energy and ratios of free and trapped states around the complex are determined using the fraction of the probe atom that form the pairs at annealing temperature intervals of 700C-100C C-1000C