Side-Chain Triphenylamine and Oxadiazole Derivatives Donor-Acceptor Random Copolymers: Syntheses, Properties, nd Memory Device Application
Date Issued
2009
Date
2009
Author(s)
Yang, Guei-Yu
Abstract
As an emerging area in organic electronics, polymer memories have become an active research topic in recent years, because they are likely to be an alternative or supplementary technology to the conventional memory fabrication Polymer containing electron donor and acceptor is a spectacular research objective due to the tunable optoelectronic properties and better processibility, which leads to great potential on device application. Therefore, this thesis mainly focuses on the synthesis and characterizations of polymers with pendant electron donor and acceptor moiety combined with their applications on memory device.n the first part of this thesis (Chapter 2), Nitroxide-Mediated Radical Polymerization (NMRP) was applied to synthesize the homopolymers with pendant triphenylamine (TPA as electron donor) or three different oxadiazole derivatives (A1, A2, and A3 as electron acceptor), and also related random copolymers consisting of pendant donor and acceptor moiety, including poly(4-vinyltriphenylamine-γ-2-phenyl-5- (4-vinylphenyl)1,3,4-oxadiazole) (TPA-A1), poly(4-vinyltriphenylamine-γ-2-(4-vinyl- biphenyl)-5-(4-phenyl)-1,3,4-oxadiazole) (TPA-A2), and poly(4-vinyltriphenylamine- γ-2-(4-vinyl biphenyl)-5-(4-ethoxyphenyl)-1,3,4-oxadiazole) (TPA-A3). By changing the donor:acceptor ratio (8:2, 5:5, and 2:8, respectively), nine targeted random copolymers are successfully synthesized and characterized by 1H NMR spectrum and element analysis (EA). All the polymers have number average high molecular weight (>104) with polydispersity index (PDI) of 1.15 ~ 1.45, and soluble in common organic solvents such as chloroform, THF and chlorobenzene (CB). The shifted absorption band of these copolymers were shown as the increasing acceptor content, and the band gaps calculated from absorption edge are in the range of 3.36 ~ 3.67 eV. The poor orbital hybridization between adjacent donor and acceptor moiety, where each pendent group could be considered as an isolated one, causes the deviations between the band gaps obtained from the cyclic voltammetry (CV) and optical spectrum. Furthermore, adding acceptor moiety to copolymers will shift the λmax to longer wavelength in fluorescence spectra. The different side-chain donor/acceptor structures and ratios which influence the electronic and optoelectronic properties were systematically investigated.n the second part of this thesis (Chapter 3), both synthesized homopolymers and copolymers were used to evaluate the memory applications. The memory device of synthesized polymers with the configuration of ITO/Polymers/Al shows nonvolatile and rewritable switching current-voltage (I-V) characteristics. The turn-on threshold voltage is located around 1.5 V, and the memory device exhibits a high on/off current ratio of 104 and long retention time of 104 second in ambient atmosphere. The mechanism of the switching behavior is based on filamentary conduction with space charge limited current (SCLC) theory. The slope of logarithmic I-V curves enhances from 2 to 3.2 with increasing acceptor content which explains that the presence of the strong coordinate atom (N) on acceptor unit with Al reveals the filament formation behavior. Moreover, the rough polymer thin film surface prepared from low boiling point solvent (THF) shows WORM-like electrical memory switching since it becomes difficult to turn-off during the several cycles. The present study suggested the pendent electron donor and acceptor random copolymers have potential application on memory devices. The relationship between the chemical structure, electronic properties, morphologies and device performance were also established in this study.
Subjects
triphenylamine
oxadiazole
NMRP,flash,filament
Type
thesis
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