Browsing by Author "Admassie, Shimelis"

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    Electrochemical and Optical Studies of Electronically Conducting Polymers for Use in Electronic Devices
    (Addis Ababa University, 2006-05) Admassie, Shimelis; Solomon, Theodros (PhD); Yohannes, Teketel (PhD)
    The electrochemical and optical properties of a series of low band gap alternating polyfluorene copolymers and polythiophene derivatives with quinoxaline units were characterized optically and electrochemically for use in polymer solar cells. The polymers have band gaps ranging from 2.1-0.8 eV. A structure-property correlation, which sheds light on the design of efficient devices, was observed. Oxadiazole-containing polymers were also optically and electrochemically characterized for polymer/polymer blend devices. Most of the polymers gave good electrochemical reversible reduction behavior with very high ionization potential (> 6.1 eV) and very high electron affinities (3.5-3.6 eV). A series of alkoxyphenyl-substituted fluorene units together with different amounts of a hole-transporting triphenylamine-substituted fluorene units designed for light-emitting devices were characterized. All polymers show high photoluminescence efficiency and light emission in the blue spectral region. Electrochemical studies shows more pronounced reversible behaviour and hence, improved holetransport, as the ratio of the hole-transporting unit is increased. The electroluminescence quantum efficiencies of the devices increase six times going from P10 to P11. Compared with P11, polymers P12 and P13 show lower efficiencies in devices. These findings indicate the presence of an optimal polymer composition, where balance between the charge-carrier mobilities has been reached. Anthrancene and benzothiadiazole containing polyfluorene copolymers (P14-P16) were also characterized. Polymers P15 and P16 show high photoluminescence efficiency while polymer P15 does not show any significant light emission up to 8.0 V. The results also show the need for balance of electron and hole injection and transport in polymer light emitting diodes. The photovoltaic properties of devices made using a highly conducting polymer electrode, from vapor-phase polymerized poly(3,4-ethylenedioxy)thiophene (VPP PEDOT) on glass substrate as an anode and a polyfluorene copolymer, APFO-3, mixed with PCBM as the active layer. The device performance was compared with that of devices made with PEDOT-PSS on glass substrates. The device performance of solar cells constructed from the VPP PEDOT made on a flexible and transparent substrate as a cathode material and ITO/ PEDOT:PSS films supported on glass substrates as anode were characterized. The devices have shown good electrical and optical responses. The electrochromic polymer PEDOT-PSS spin coated on ITO/glass was patterned with soft lithographic method in order to diffract the incident light, and thereby modify absorption of light by the film to improve the electrochromic efficiency of the polymer. The absorbance peak at around 610 nm was found to be much higher in the patterned PEDOT-PSS film than the one observed in the unpatterned film. Values of coloration efficiencies varying from 107 to 174 cm2/C were obtained for three different unpatterned PEDOT-PSS films, where as for three different patterned PEDOT-PSS films higher values ranging from 204 to 371 cm2/C were found. These increased values of the electrochromic efficiencies are attributed to diffraction. The electrochromic behaviours of some polythiophenes were also investigated in bilayer system. The solvatochromic and thermochromic behaviors of phenyl-substituted polythiophenes were studied. The pristine polymers, upon dissolution in chloroform, exhibited blueshifted absorption. The solid films of the polymers showed significant blue-shifted as well as red-shifted absorptions when heated. While the addition of methanol to the chloroform solutions of the polymers caused dramatic chromic changes and development of red-shifted spectra for many of the polymers investigated, the symmetrically phenyl-substituted and sterically hindered polymer (polymer P1) does not show significant changes. These chromic behaviors have been examined in terms of substituent effects and attempt has been made to explain these effects by calculating the energy barrier for rotation to a planar structure using the HF SCF method and 3-21G* basis set