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ABSTRACT: The bistable electroactive polymer is a new smart material capable of large strain, rigid-to-rigid actuation. At the rubbery state of the polymer heated to above its glass transition, stable electrically-induced actuation is obtained at strains as large as 150%. Electromechanical instability can be effectively overcome by the formation of interpenetrating polymer networks. An application as a refreshable Braille display is demonstrated.
Advanced Materials 09/2012; · 13.88 Impact Factor
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ABSTRACT: White polymer phosphorescent light-emitting diodes have been fabricated employing silver nanowire (AgNW)-polymer composite substrates. Solution processing is used to form all the layers in the devices, except for the cathode, for potentially low fabrication cost. The current efficiency in the front viewing direction is 20.3 cd A(-1) for devices comprising two complementary dopants (bis[(4,6-difluorophenyl)-pyridinato-N,C(2)] (picolinato)Ir(iii) (FIrpic) with sky blue emission and bis(1-phenylisoquinoline)(acetylacetonate) (Ir(piq)) for red emission). The efficiency is 42.3 cd A(-1) at 4000 cd cm(-2) for devices comprising the three primary colors (FIrpic, Ir(piq), and tris(2-(4-tolyl)pyridinato-N,C(2)) (Ir(mppy)(3)), with green emission). These values are, respectively, 35% and 41% higher than those of control devices fabricated on indium tin oxide (ITO) coated on glass substrates. The enhanced current efficiency is attributed to enhanced light out-coupling thanks to light scattering by the silver nanowires embedded in the transparent composite electrode.
Physical Chemistry Chemical Physics 08/2012; 14(41):14249-54. · 3.57 Impact Factor
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ABSTRACT: Stretchable transparent composites have been synthesized consisting of a silver nanowire (AgNW) network embedded in the surface layer of a crosslinked poly(acrylate) matrix. The interpenetrating networks of AgNWs and the crosslinked polymer matrix lead to high surface conductivity, high transparency, and rubbery elasticity. The presence of carboxylic acid groups on the polymer chains enhances the bonding between AgNWs and the polymer matrix, and further increases the stretchability of the composites. The sheet resistance of the composite electrode increases by only 2.3 times at 50% strain. Repeated stretching to 50% strain and relaxation only causes a small increase of the sheet resistance after 600 cycles. The morphology of the composites during reversible stretching and relaxation has been investigated to expound the conductivity changes.
Nanotechnology 08/2012; 23(34):344002. · 3.98 Impact Factor
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ABSTRACT: A new compliant electrode-based on silver nanowire-polymer composite has been developed. The composite electrode has low sheet resistance (as low as 10 Ω/sq), remains conductive (10(2) -10(3) Ω/sq) at strains as high as 140%, and can support Joule heating. The combination of the composite and a bistable electroactive polymer produces electrically-induced, large-strain actuation and relaxation, reversibly without the need of mechanical programming.
Advanced Materials 03/2012; 24(10):1321-7. · 13.88 Impact Factor
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ABSTRACT: Blue, green, and red electrophosphorescent polymer light-emitting diodes have been fabricated on silver nanowire-polymer composite electrode. The devices are 20%-50% more efficient than control devices on ITO/glass and exhibit small efficiency roll-off at high luminances. The blue PLEDs were repeatedly bent to 1.5 mm radius concave or convex with calculated strain in the emissive layer approximately 5% (tensile or compressive).
Advanced Materials 11/2011; 23(46):5563-7. · 13.88 Impact Factor
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Advanced Materials 10/2011; 23(38):4453-7. · 13.88 Impact Factor
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ABSTRACT: A photoassisted corona poling technique (PACP) was applied to a binary chromophore system that was composed of YLD-124 and DR1-co-PMMA to enhance the poling efficiency of an electrooptic (EO) polymer device. The DR1 chromophores as a photoaddressable host material were resonantly irradiated by a polarized pumping green laser beam at a wavelength of 532 nm which is within its molecular absorption band. This process caused the DR1 chromophore's reorientation via a cis-trans photoisomerization process. The coupled YLD-124 system's dipoles were then poled with the assistance of a static electrical field applied to the EO core layer. In this letter, we report the experimental demonstration of the photoassisted corona poling scheme in a binary chromophore system that improved Mach-Zehnder optical modulator performance by lowering its V <sub>π</sub>. It was shown that the device's V <sub>π</sub> was reduced by 21%-23% compared to a regular device that was only corona poled without the green light assistance.
IEEE Photonics Technology Letters 08/2011; · 2.19 Impact Factor
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Advanced Materials 07/2011; 23(34):3989-94. · 13.88 Impact Factor
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ABSTRACT: Solution processed highly efficient blue electrophosphorescent polymer light-emitting diodes have been fabricated containing an oligo(ethylene oxide) (PEO-DME) additive to enhance the electroluminescence efficiency. The addition of PEO-DME at 5–10 wt % effectively lowers both the electron and hole injection barriers. The maximum current efficiency obtained was 26.5 cd/A at an emission brightness of 2500 cd/m2. The high performance is due to (i) improved charge carrier injection at the interface resulting from specific interfacial interactions between PEO-DME and aluminum and (ii) improved charge carrier transporting ability and high electric field resulting from PEO-DME doping.
Applied Physics Letters 05/2011; 98(20):201110-201110-3. · 3.84 Impact Factor
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ABSTRACT: A linearized dual parallel Mach-Zehnder modulator (DPMZM) based on electro-optic (EO) polymer was both fabricated, and experimentally used to suppress the third-order intermodulation distortion (IMD3) in a coherent analog fiber optic link. This optical transmitter design was based on a new EO chromophore called B10, which was synthesized for applications dealing with the fiber-optic communication systems. The chromophore was mixed with amorphous polycarbonate (APC) to form the waveguide's core material. The DPMZM was configured with two MZMs, of different lengths in parallel, with unbalanced input and output couplers and a phase shifter in one arm. In this configuration each of the MZMs carried a different optical power, and imposed a different depth of optical modulation. When the two optical beams from the MZMs were combined to generate the transmitted signal it was possible to set the IMD3 produced by each modulator to be equal in amplitude but 180° out of phase from the other. Therefore, the resulting IMD3 of the DPMZM transmitter was effectively canceled out during two-tone experiments. A reduction of the IMD3 below the noise floor was observed while leaving fifth-order distortion (IMD5) as the dominant IMD product. This configuration has the capability of broadband operation and shot-noise limited operation simultaneously.
Optics Express 04/2011; 19(8):7865-71. · 3.59 Impact Factor
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ABSTRACT: We demonstrate that conjugated polymers are able to efficiently host blue and white electrophosphorescence if the conjugated polymer has both high triplet energy level (ET) and high-lying HOMO energy level. A novel conjugated polymer host (PmPTPA) is developed by attaching triphenylamine unit to poly(m-phenylene) backbone. The poly(m-phenylene) backbone endows PmPTPA an ET as high as 2.65 eV, which is sufficiently high to prevent triplet energy back transfer. The tethering triphenylamine unit leads to the HOMO energy level of −5.35 eV for PmPTPA and facilitates hole injection. As the result, blue phosphorescent polymer light-emitting diodes (PPLEDs) based on PmPTPA exhibit the luminance efficiency of 17.9 cd/A and external quantum efficiency of 9.3%. White PPLEDs with blue, green and red phosphorescent dopants dispersed in PmPTPA show the luminance efficiency of 22.1 cd/A and external quantum efficiency of 10.6%. For both the blue and white PPLEDs based on the conjugated polymer host PmPTPA, the EL performance are fairly comparable to those of the state-of-the-art nonconjugated polymer host, poly(vinyl−carbazole) (PVK). These results indicate that conjugated polymers are suitable host materials for PPLEDs with all emission colors.
03/2011;
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ABSTRACT: A poly(ethylene oxide) (PEO) oligomer capped with methacrylate end groups can conduct ionic species for the formation of p−i−n junction in polymer light-emitting electrochemical cells (LECs). The polymerization of the methacrylate during the junction formation can mitigate the nonuniformity of the doping frontiers and enhance the uniformity and stability of the junction. Sandwich-structured LECs employing this ionic conductor exhibit both high electroluminescent efficiency and stability. Luminous efficiency greater than 15 lm/W has been obtained using a yellow light-emitting poly(1,4-phenylene vinylene) derivative (Super-Yellow by Covion). In accelerated lifetime testing, the brightness dropped to 75% of the maximum 2200 cd/m2 of luminance (T75%) in 102 h under constant current operation; this corresponds to a lifetime (T75%) of 27 000 h at 100 cd/m2 of peak brightness. Both the efficiency and lifetime are comparable to the best-performing polymer OLEDs optimized for commercial applications.Keywords: polymer light-emitting electrochemical cells; p−i−n junction; organic electronics; super yellow; ionic conductors; OLEDs; doping
02/2011;
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ABSTRACT: Shape-memory polymer light-emitting diodes (PLEDs) using a new silver nanowire/polymer electrode are reported. The electrode can be stretched by up to 16% with only a small increase in sheet resistance. Large deformation shape change and recovery of the PLEDs to various bistable curvatures result in minimal loss of electroluminescence performance.
Advanced Materials 02/2011; 23(5):664-8. · 13.88 Impact Factor
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ABSTRACT: A poly(m-phenylene) derivative tethering carbazole unit (PmPCz) is reported to have a triplet energy level (ET) as high as 2.64 eV, sufficient to host blue electrophosphorescence. To our best knowledge, this is the first conjugated polymer with ET higher than that of state-of-the-art blue phosphorescent dopant, iridium(III) bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic). PmPCz exhibits good thermal stability and excellent miscibility with the blue phosphorescent dopant, FIrpic. Owing to the high ET and good miscibility, blends of PmPCz with FIrpic show no triplet energy back-transfer and exhibit emission exclusively from FIrpic, even at FIrpic concentration as low as 1 wt %. Single-layer blue phosphorescent polymer light-emitting diode based on the blend exhibits a luminance efficiency of 4.69 cd/A.
11/2010;
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ABSTRACT: Electrophosphorescent polymers are an important class of light-emitting materials that combine high electroluminescence efficiency of organometallic phosphohrescent complexes and solution processability of polymers. This review surveys the progress in the design, synthesis, characterization, photoluminescence and electroluminescence of electrophosphorescent polymers reported in the recent literatures. It focuses on polymers with phosphorescent guest covalent bonded to the polymers as a repeating unit in the main chain, a side chain, or polymer chain termini. The polymer backbones either have an extended π-electron conjugation or are non-conjugated. Important electrophosphorescent polymers and their performance in light emitting diodes are described.
Current Organic Chemistry 10/2010; 14(18):2133-2144. · 3.06 Impact Factor
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ABSTRACT: Soft silicone films have garnered a great deal of interest for use in dielectric elastomer transducers due to their excellent properties, including high elongation to rupture, low viscoelasticity, and broad application temperature range. However, silicone films generally have higher stiffness and lower dielectric strength than VHB acrylic elastomers, which limits the achievable actuation strain. Devices based on silicone dielectric elastomers always experience high rates of premature dielectric failure when operated at high strains. The premature failure is characterized by the loss of functionality or mechanical rupture of the material when operated below the material's dielectric strength and elongation to rupture. The use is reported of ultrathin coatings of single-walled carbon nanotubes (SWNTs) as the compliant electrodes, which can overcome the issue of premature failure. The self-clearing of the SWNT electrodes in the event of localized dielectric breakdown improves the apparent dielectric strength of the material by isolating the regions of reduced dielectric strength. The actuators may be operated at higher than 50% area strain with reasonably long lifetimes. High strains were measured between −40 and 80°C and in a broad frequency range up to 100 Hz. The fault tolerance introduced by the SWNT electrodes should broaden the application scope of silicone dielectric elastomers.
International Journal of Smart and Nano Materials. 03/2010; 1(1):40-52.
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Polymer International 02/2010; 59(3):384 - 390. · 1.90 Impact Factor
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ABSTRACT: Solid-state, white light-emitting electrochemical cells (LECs) based on a fluorene−oxadiazole copolymer have been successfully demonstrated. The copolymer has a π-conjugated backbone consisting of 75 mol % fluorene and 25 mol % 5,5′-diphenyl-2,2′-bi-1,3,4-oxadiazole. 2-(2-(2-Methoxyethoxy)ethoxy)ethyl attached on the 9-C of the fluorenes was introduced to promote ionic conductivity necessary for the operation of LECs. The copolymer, synthesized by Suzuki coupling reactions, has a number-average molecular weight of 8700 and polydispersity of 2.41. The LECs were fabricated by spin-coating the copolymer admixed with lithium trifluoromethanesulfonate onto indium−tin oxide (ITO) substrate coated with poly(ethylenedioxythiophene)−poly(styrenesulfonic acid) (PEDOT:PSS). The opposite electrode was vapor evaporated aluminum. The LEC devices showed a white electroluminescence with CIE coordinates of (0.24, 0.31). The emission spectrum consisted of emissions from both the individual polymer chains and excimers. The white LECs are promising for solid-state lighting devices with a simple device structure and low-cost fabrication.
01/2010;
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ABSTRACT: A number of materials have been explored for their use as artificial muscles. Among these, dielectric elastomers (DEs) appear to provide the best combination of properties for true muscle-like actuation. DEs behave as compliant capacitors, expanding in area and shrinking in thickness when a voltage is applied. Materials combining very high energy densities, strains, and efficiencies have been known for some time. To date, however, the widespread adoption of DEs has been hindered by premature breakdown and the requirement for high voltages and bulky support frames. Recent advances seem poised to remove these restrictions and allow for the production of highly reliable, high-performance transducers for artificial muscle applications.
Macromolecular Rapid Communications 01/2010; 31(1):10-36. · 4.60 Impact Factor
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ABSTRACT: Polymer light emitting devices were fabricated by roll lamination using single-walled carbon nanotubes as both anode and cathode. The devices exhibited a low turn-on voltage of 3.8 V, high brightness of 1400 cd/m2 at 10 V and maximum efficiency of 2.2 cd/A at 480 cd/m2. The devices are also highly transparent and exhibited very high flexibility. No failure was observed after bending the devices down to 2.5 mm radius.
Applied Physics Letters 11/2009; 95(20):203304-203304-3. · 3.84 Impact Factor
