Browsing by Author "Workalemahu, Bantikassegn (PhD)"
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Item Current-Voltage Characterstics and Impedance Spectroscopy of Poly (3-(4'-(1/1, 4/1), 7/1-Trioxaoctyl) Phenyl) Thiophene) (PEOPT)(Addis Ababa University, 2000-06) Ayalew, Tesfaye; Workalemahu, Bantikassegn (PhD)This thesis is based on the study of the electronic properties of PEOPT in the form of AI/PEOPTIITO sandwich structure. An interface between aluminum contacts to PEOPT films were studied using complex impedance spectroscopy and current-voltage characteristic measurements. The current density-voltage curve is asymmetric and non-ohmic and shows Schottky barrier type rectification. The complex impedance spectra are semi-circles that show the existence of a region depleted of mobile charge carriers. An equivalent circuit consisting of one parallel RC circuit is envisaged as a model of the interfaceItem Electrical Properties of Junctions Between a Aluminum and Poly[3-(2'-ButyloxY-5'-(1'''- OXOOCTYL)Phenyl)Thiophene](Addis Ababa University, 2003-06) Yehulie, Mandefro; Workalemahu, Bantikassegn (PhD)The electrical properties of junctions between a low work function metal (Al) and poly[3-(2’- butyloxy-5’-(1’’’-oxooctyl)phenyl)thiophene] in the form of Al/polymer/ITO sandwich structures have been investigated by means of complex impedance spectroscopy, currentvoltage (I-V), and capacitance-voltage (C-V) characteristics. The current–voltage curve is non-ohmic and reveals Schottky barrier type of rectification. The C-V data together with I-V characteristics confirms the polymer is a p-type semiconducting polymer. The complex impedance spectroscopy shows the absence of an insulating interfacial resistive layer. Schottky device parameters have been calculated from I-V curve by applying the thermionic emission equation. As result, saturation current density ( ) 2 13 0 2.2 10 cm J A − = × , ideality factor (n) = 3.5, rectification ratio = 1761, built-in voltage = 0.96V, potential barrier = 1.11eV, dopant density 17 3 ( ) 4.83 10 − N = × cm a , and depletion width at zero bias voltage = 25.7nm have been deduced from our experimental dataItem Electrical Properties of Junctions between Aluminium and Pol Y(3-0ctylphenylthiophene) (POPT)(Addis Ababa University, 1999-07) Tessema, Wudyalew; Workalemahu, Bantikassegn (PhD)This thesis is based on the study of the electronic properties of poly(3-octylphenylthiophene) (POPT) in the form of AlJPOPTIITO sandwich stlucture prepared in our laboratory using a vacuum evaporation technique. The electronic properties of the sandwich structure were analyzed using cmrent- voltage characteristics and complex impedance spectroscopy. The optical absorption spectrum of spin coated thin films of POPT was measured using Perkin Elmer 1..19 UVNislNIR spectrophotometer which showed a forbidden energy gap of about 1.77eV and hence it belongs to the class of semiconducting organic materials. The I-V characteristics of the sandwich structure show a rectifying behavior. The complex impedance analysis exhibits a bias voltage dependent part of a single semicircle in the Cole-Cole plot. This is modeled by one parallel RC electrical circuit, usually obtained fi·om metal-semiconductor (MS) devicesItem Electrical Properties of Junctions between Aluminium and POLY (3-(2,5-Dioctylphenyl)-Thiophene) (PDOPT)(Addis Ababa University, 1998-06) Goro, Girma; Workalemahu, Bantikassegn (PhD)This thesis is based on the study of interface properties between a reactive metal and a neutral conjugated polymer. The polymer used is poly(3-(2,5-dioctylphenyl)-thiophene) (PDOPT). The techniques employed are absorption spectroscopy, current-voltage and complex impedance spectroscopy measurements. The absorption measurement reveals that the polymer, PDOPT, has an energy gap of about 2.02 eV and hence it belongs to the class of semiconducting materials. The I-V characteristic of AVPDOPTIITO structure shows symmetrical but non-ohmic curves. The complex impedance analysis exhibits a single semicircle whose diameter depends on the magnitude of the applied voltage. For smaller values of the applied voltage, both for the reverse and forward biases, the extrapolated diameter of the semicircle is larger. This value is consistent with the result of the I-V measurements. So this device, AVPDOPTIITO, may be considered as a metal-semiconductor device with no rectifying Schottky barrier formation at the AVPDOPT interface. The small value of current may be due to high bulk resistance of the polymer, as well as the probable formation of thick oxides of aluminiumItem Electrical Properties of Junctions Between Aluminuim and Poly[3-(2'-Octyloxy-5'-(1'''-Oxooctyl)Phenyl)Thiophene(Addis Ababa University, 2003-06) Homa, Bekele; Workalemahu, Bantikassegn (PhD)In this thesis, the property of the junction between aluminium and neutral conjugated polymer is studied. The conjugated polymer used is poly[3-(2'-octyloxy-5'-(1'''- oxooctyl)phenyl)thiophene] which is the derivative of polythiophene having 8-carbon atoms alkyl side chain which makes it different from other thiophene derivatives. The techniques employed are optical absorption spectrum, I-V characterization and impedance spectroscopy study. Optical spectroscopy study showed the optical band gap of the polymer is 2.03eV indicating the polymer is in the range of conventional inorganic semiconductor. Based on this result the theoretical background in studying the property of junction between aluminium and the polymer is that of conventional inorganic semiconductors. The I-V characteristics of Al/poly[3-(2'-octyloxy-5'-(1'''-oxooctyl)phenyl)thiophene]/ITO structure showed non-ohmic, non-rectifying and nearly symmetrical curves. The complex impedance analysis exhibited a semicircle whose diameter depends on the magnitude of the applied bias voltage; the diameters are small for larger bias voltages and increases for small intermediate bias voltages. This result is consistent with the I-V characteristics. Therefore, the junction between aluminum and the polymer forms no Schottky barrierItem Electrical Properties of Junctions between Aluminum & POLY [3-(2'-PENTYLOXY-5'-(1'''-OXOOCTYL) PHENYL)THIOPHENE](Addis Ababa University, 2003-06) Digafe, Hiwot; Workalemahu, Bantikassegn (PhD)This thesis is based on the study of the electronic properties of Junction between Al and poly[3-(2'-pentyloxy-5'-(1'''-oxoocty)phenyl)thiophene in the form of Al/poly[3-(2'-pentyloxy- 5'-(1'''-oxoocty)phenyl)thiophene]/ITO sandwich structure. The junction between aluminium contacts to poly[3-(2'-pentyloxy-5'-(1'''-oxoocty)phenyl]thiophene films was studied using current-voltage, capacitance-voltage and complex impedance spectroscopy techniques. The current density-voltage curve is asymmetric and non-ohmic and shows Schottky barrier type rectification. The complex impedance spectra are semi-circles that show the existence of a region depleted of mobile charge carriers. An equivalent circuit consisting of one parallel RC circuit is envisaged as a model of the interfaceItem Electrical Properties of Junctions between Aluminum and Poly [3-(2'-PentyloxY-5'-(1'''-OxooctyL) Phenyl)Thiophene](Addis Ababa University, 2003-06) Digafe, Hiwot; Workalemahu, Bantikassegn (PhD)This thesis is based on the study of the electronic properties of Junction between Al and poly[3-(2'-pentyloxy-5'-(1'''-oxoocty)phenyl)thiophene in the form of Al/poly[3-(2'-pentyloxy- 5'-(1'''-oxoocty)phenyl)thiophene]/ITO sandwich structure. The junction between aluminium contacts to poly[3-(2'-pentyloxy-5'-(1'''-oxoocty)phenyl]thiophene films was studied using current-voltage, capacitance-voltage and complex impedance spectroscopy techniques. The current density-voltage curve is asymmetric and non-ohmic and shows Schottky barrier type rectification. The complex impedance spectra are semi-circles that show the existence of a region depleted of mobile charge carriers. An equivalent circuit consisting of one parallel RC circuit is envisaged as a model of the interfaceItem Electronic Properties of Junctions between Aluminum and Poly(3-(2,5-dioctylphenyl) Thiophene) (PDOPT) thin films.(Addis Ababa University, 2001-06) Tizazu, Getachew; Workalemahu, Bantikassegn (PhD)This thesis is based on the study of the electrical properties of the Sckottky barriers formed between aluminum and spin-coated films of PDOPT in the form of AI/PDOPT/PEDOT-PSS/ITO and AIIPDOPT/ITO sandwich structures, respectively. From the I-V characteristic, the rectification ratio, diode quality factor and barrier height have been obtained for both AIIPDOPT/PEDOT-PSS/ITO and for AIIPDOPT/ITO structures. The Cole-Cole plot of the complex impedance spectra for AI/PDOPT/ITO exhibits part of a single bias voltage-dependant semicircle, which is a characteristic of most metal semiconductor junctions. The complex impedance spectra of AI/PDOPT/ PEDOTPSS/ ITO device showed two partially overlapping semicircles at reveres bias voltages which revealed the existence of thin insulating interfacial layer at metal/polymer junctionItem Photovoltaic Properties of Poly[3-(4-0ctylphenyl)-2,2'-Bithiophene] (PTOPT)(Addis Ababa University, 1998-06) Yadete, Tolessa; Workalemahu, Bantikassegn (PhD)The purpose of tlus study is to investigate the photovoltaic effect of the undoped poly[3-( 4-octylphenyl)-2,2'-bithiophene 1 (PTOPT) by performing the current density-voltage (J-V) measurements in the dark and under illumination. The Al/PTOPT junction forms a Schottky barrier. The semiconductor property of PTOPT was confirmed from optical absorption spectroscopy. The electrical and photovoltaic properties of an Al/PTOPT/ITO device were investigated by the J-V measurement in the dark and under illumination. From J-V in the dark, the reverse saturation current density, Jo = 6.7 x 1O-14Alcm2 was obtained. The diode ideality factor n and the barrier height <1\ are 4.0 and 1.2V, respectively. Optical absorption reveals that the band gap, Eg , of PTOPT is about 2.0eV. From photovoltaic characterization, the open-circuit voltage and the sholt-circuit current density of the device are Voc = O.75Vand Jsc = 2.5 x 1O-5Alcm2 , respectively. The effective area of the cell was 0.06cm'. The fill-factor and the power conversion efficiency obtained was 0.2 and 0.004% respectively. The illumination of the sample was made from the ITO side with a white light of intensity about 100m W/cm2Item Quantum efficiency and photovol talc §~ ~roperties of a photodiode made of ;/ poly (3-4(-octylphenyl)-2, 2'-bithiophene) (ptopt)(Addis Ababa University, 2000-10) Gadisa, Abay; Workalemahu, Bantikassegn (PhD)In this thesis we investigated the photovoltaic properties of poly [3-(4- octylphenyl)-2, 2'-bithiophene] (PTOPT) in the form of AI/PTOPT/ITO sandwich structure. The peaks of the spectral response of the device were found to match with the absorption spectrum of PTOPT for illumination from both the AI and ITO sides. However, the IPCE% obtained when the device was illuminated from the AI side is found to be lower than for illumination from the ITO side. The experimentally collected data on absorption and action spectrum are explained based on the theoretical model proposed by Gosh and Feng. The photovoltaic parameters were determined from the I-V curve recorded by illuminating the photodiode through the AI side with monochromatic light (500 nm) of intensity 7.4 JlW/cm2. No correction was done for reflection from the surface of the AI electrode. A power conversion efficiency of 0.7% and a fill factor of 0.25 were also obtained. The small value of the efficiency is due to the low mobility of the charge carriers of the photodiode. Moreover, the short circuit photocurrent of the device was found to vary with the incident light intensity. The observed variation of the photocurrent with the incident light irradiation (linear at low light intensities and non-linear at higher light intensities) will be explained using a simple kinetic model