Two series of well-defined brush polymers bearing a triazole moiety on each bristle were prepared from the click chemistry reactions of a poly(glycidyl azide) (PG) and a poly(4-azidomethylstyrene) (PS) with alkyne derivatives. The thin-film morphologies and properties, especially electrical memory performances, of these triazole-containing brush polymers were investigated in detail. The brush polymers with a triazole ring substituted with an alkyl or alkylenylphenyl group in the bristle exhibited only dielectric characteristics. By contrast, the other brush polymers bearing a triazole ring substituted with phenyl or its derivatives with a longer π-conjugation length in the bristle demonstrated excellent unipolar permanent memory behaviors with low power consumption, high ON/OFF current ratios and high stability and reliability under ambient air conditions. Furthermore, their memory type could be tuned to p-or n-type by the incorporation of an electron-donating or -accepting group into the phenyl unit linked to the triazole moiety. Overall, this study presents the first demonstration of the azide-alkyne click chemistry synthesis of triazole moieties with substituent(s) that exhibit a resonance effect; this approach is a very powerful synthetic route to develop electrical memory polymers suitable for the low-cost mass production of high-performance, polarity-free programmable memory devices.
A series of novel
π-conjugated copolymers based on 2,2′-bis(1,3,4-thiadiazole)
(BTDz) have been developed. Among them, the BTDz-based donor–acceptor
alternating copolymer with the (E)-1,2-di(3-(2-ethylhexyl)thiophene)vinylene
donor unit (PBTDzTV) exhibited a high solubility and high crystallinity.
PBTDzTVs favorably self-assembled, forming face-on and edge-on multibilayer
structures in thin nanoscale films. The relative volume fractions
of these structures varied depending on the polymer’s molecular
weight. The higher molecular weight polymer formed a higher volume
fraction of the face-on structure; in particular, the polymer with
a 26.6 kDa of number-average molecular weight made only the face-on
structure. The device performance was improved as the polymer molecular
weight and the volume fraction of the face-on structure increased.
The bulk-heterojunction photovoltaic device based on PBTDzTV:PC71BM demonstrated the high power conversion efficiency (PCE)
of 8.04% when the device was fabricated with the highest molecular
weight polymer having the face-on structure.
The mechanism behind electrical memory behavior of carbazole-containing polyimides (PIs) in nanoscale thin films was investigated. For this investigation, a series of poly(3,3'-dihydroxy-4,4'-biphenylene-co-3,3'-bis(N-ethylenyloxycarbazole)-4,4'-biphenylene hexafluoro-isopropylidenedi-phthalimide)s (6F-HAB-HABCZn PIs) with various compositions was synthesized as a model carbazole-containing polymer system. The thermal properties, band gaps, and molecular orbital levels of the PIs were determined. Furthermore, the chemical compositions, as well as the nanoscale thin film morphologies and electron densities, were analyzed, providing detailed information on the population and positional distribution of carbazole moieties in thin films of the PIs. PI Devices were fabricated with aluminum electrodes and tested electrically. The PI thin film layers in the devices exhibited electrically permanent memory behavior, which was driven by trap-limited space-charge limited conduction and ohmic conduction. The permanent memory characteristics were found to be attributed to the incorporated carbazole moieties rather than from the other chemical components. Furthermore, the memory characteristics depended significantly on the population and positional distribution of carbazole moieties in the PI layer, as well as the film thickness. Considering that the backbone is not conjugated, the present results collectively indicate that the electrical switching behavior of the PI films is driven by the carbazole moieties acting as charge traps and a hopping process using the carbazole charge-trap sites as stepping-stones.
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