New Donor-Acceptor Random Copolymers with Side-Chains of para-Substituted Dicyano-triphenylamine and Oxadiazole Derivatives : Synthesis, Properties, and Memory Device Applications
Date Issued
2009
Date
2009
Author(s)
Chen, Po-Cheng
Abstract
In recent years, polymer systems have attracted significant research interest for memory device applications due to their tunable electrical properties through molecular design and good processibility. The representative classes of polymer materials used for fabrication on memories are conjugated polymers, non-conjugated polymers (functional polyimide systems or polymers with specific pendent donor or acceptor moiety) and polymer nanocomposites (metal nanoparticle or fullerene embedded). However, the effects of polymer structure on the memory characteristics have not been fully explored yet. In this thesis, the homopolymers pendant with donors or acceptors and related copolymers containing of both donor and accepter units were synthesized and evaluated on memory devices. n chapter 2, the homopolymers with pendant para-substituted dicyano- triphenylamine (D, as donor) or three different oxadiazole derivatives (2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole (A1), 2-(4-vinylbiphenyl)-5-(4-phenyl)- 1,3,4-oxadiazole (A2), and 2-(4-vinylbiphenyl)-5-(4-ethoxyphenyl)-1,3,4-oxadiazole (A3), as electron acceptor) and also related random copolymers consisting of pendant donor and acceptor moieties (the ratio of donor:acceptor is 8:2, 5:5 and 2:8, respectively) were synthesized by Nitroxide-Mediated Living Free Radical Polymerization (NMP) and characterized by 1H NMR spectrum and element analysis (EA). The para-substituted dicyano groups on triphenylamine improve the stability without the dimerization. The side-chain acceptor A2 shows more conjugated length relative to A1 by introducing another benzene ring, and A3 system further enhances the electronic density by adding terminal electron donating moiety (-OEt). The two thin film absorbance peaks of copolymers at 353 nm and 308 ~ 332 nm belong to the corresponding homopolymers (PD at 355 nm, PA1 at 308 nm, PA2 at 330 nm, PA3 at 332 nm, respectively) and the relative absorbance of two peaks vary with donor and acceptor ratios. The band gaps calculated from absorption edge are in the range of 3.11 ~ 3.20 eV and increase slightly with the larger acceptor content. The HOMO and LUMO energy levels of the random copolymers with different donor and acceptor ratios obtained from cyclic voltammetry (CV) are almost the same with the corresponding HOMO level of donor (-5.63 eV) and LUMO levels of acceptor (-2.63 ~ 2.81 eV), respectively, mainly due to poor hybridization of adjacent donor and acceptor which could be considered as an isolated system. The electronic properties can be well-tuned through different donor-acceptor structures and ratios of the copolymers systems. n chapter 3, the memory device of the random copolymers and homopoymers are fabricated and measured in ambient atmosphere. Different solvents (N,N-dimethyl formamide (DMF) or chlorobenzene (CB)) are chosen for polymer thin film processing. The bistable memory behavior is conducted by a simple sandwich device configuration consisted of spin-coated polymer film between indium-tin oxide (ITO) bottom electrode and aluminum (A1) top electrode. All the polymers exhibit nonvolatile flash type of switching behavior. The memory devices have low turn-on threshold voltage below 1 V and exhibits long retention time of 11000 seconds. The mechanism of the switching behavior is based on filamentary conduction with space charge limited current (SCLC) theory. As the polymer films prepare from DMF, the OFF state current during erasing scan increases from 10-5 A to 10-2 A after several WRER cycles. These phenomena are further obviously demonstrated in the case of the larger acceptor contents of the random copolymers (5:5 and 2:8 donor:acceptor ratio) and homopolymers pendant with acceptor due to the presence of the binding heteroatom (N) on acceptor with Al which contributes to filament conductions. Introducing electron donating ethoxy substituent (A3) which enhance the binding ability of N atom of oxadiazole group makes the memories difficult to turn off and also keep them on the high conductance since the stable filament channel are formed. The memory can still restore to the initial OFF state if the erasing process scan more negatively which also means the enhanced threshold voltage (from -3 V to -5 V) as the acceptor ratio increases. The polymer surface prepared from the CB solution is smooth and the current elevation of OFF state during the erasing switch could not be found. The study demonstrates that pendent donor/acceptor structures and ratio of copolymer and processing condition on the switching characteristics of memory device are fully explored.
Subjects
memory
polymer
donor
acceptor
oxadiazole
triphynelamine
Type
thesis
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