Browsing by Author "Hsieh, BR"

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Charge transport in doped organic semiconductors
(2003-08-15)
Shen, YL
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Diest, K
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Wong, MH
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Hsieh, BR
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Dunlap, DH
We report an unusual transition in the conductivity of an organic semiconductor upon doping: For low doping levels, the conductivity of N,N,N-',N-'-tetra-p-tolyl-4-4(')-biphenyldiamine dispersed polycarbonate increases with doping in a nearly linear fashion, and shows an activation energy of 0.2 eV. At high doping levels, a superlinear increase of conductivity with doping is observed, and the activation energy decreases, reaching a low of 0.12 eV. This behavior is understood in terms of broadening of the transport manifold due to enhanced disorder coming from the dopants.
Controlling hole-transport in aluminum tris (8-hydroxyquinoline), Alq(3)-based organic light emitting diodes to improve the device lifetime by an oxidized transport layer
(2004-06-15)
Mathai, MK
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Papadimitrakopoulos, F
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Hsieh, BR
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應用化學系
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Department of Applied Chemistry
A salt containing polymer, called an oxidized transport layer (OTL), was investigated for hole injection and transport into Alq(3)-based multilayered organic light emitting diode (OLED) devices. The OTL comprises an aryldiamine containing hole transport polymer binder with a corresponding low molecular weight radical cation salt. We demonstrate herein that the OTL behaves like a tunable resistor for holes, and its hole-transport properties can be controlled by the salt concentration and thickness of the OTL. Based on a careful analysis of the current density-voltage (J-V) device characteristics as a function of the above parameters, electron/hole currents were balanced to minimize oxidative degradation of Alq(3). It was found that an OLED device (ITO/OTL/NPB/Alq(3)/CsF/Al) with a 5000 Angstrom thick OTL at 5% salt concentration operated with a half-life exceeding 1000 h at a constant current of 10 mA/cm(2). Similar devices with 2.5% and 10% salt doping showed an order of magnitude lower half-life attributed to unbalanced carrier concentrations. Moreover, by demonstrating that the majority of the field drops across the Alq(3) layer, the doping level as opposed to OTL thickness was established as the primary contributor controlling hole transport in these OLEDs. This beneficial behavior is, however, observed only above a certain OTL thickness in the vicinity of 5000 Angstrom. (C) 2004 American Institute of Physics.
Fluorescent conjugated polymer films as TNT chemosensors
(2004-08-13)
Chang, CP
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Chao, CY
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Huang, JH
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Li, AK
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Hsu, CS
Three prototypical light emitting conjugated polymers, two poly(p-phenylene vinylenes) (MEH-PPV and DP10-PPV) and a poly(diphenylacetylene), were examined for chemosensor applications to detect explosive compounds such as 2,4,6-trinitrotoluene (TNT) and 2,4- and 2,6-dinitrotoluene (DNTs). All polymer thin films showed high fluorescence quenching sensitivity towards TNT and DNTs, indicating that a wide range of emissive conjugated polymers are potentially useful chemosensor materials for detecting landmines. The relative sensitivity of fluorescence quenching of the polymers by the analytes has been rationalized in terms of the vapor pressure of the analytes, the solubility parameters of the polymers and the analytes, and the relative energy levels of the polymers. (C) 2004 Elsevier B.V. All rights reserved.
New soluble poly(2,3-diphenylphenylene vinylene) derivatives for light-emitting diodes
(2005-04-22)
Yang, SH
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Chen, JT
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Li, AK
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Huang, CH
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Chen, KB
Soluble poly(2,3-diphenylphenylene vinylene) (DP-PPV) derivatives, poly(2,3-diphenyl-5-(4-heptyloxy-4'-oxytrimethylenediphenyl)phenylene vinylene) (P1), poly(2,3-diphenyl-5-[4-(4-pentylcyclohexyl)phenoxyl-propyl-p-phenylene vinylene) (P2) and poly(2,3-diphenyl-5-(2-(1, 4, 5-triphenyl-1H-2-imidazoloyl)-1-oxytrimethylene phenyl) phenylene vinylene) (P3), were synthesized. P1 and P2 contain liquid crystal side groups; while P3 contains a charge transport group. The polymers showed solid state UV-Visible absorption and photoluminescence in the range of 370-380 and 480-510 nm, corresponding to blue green emission. Similar to many other reported systems, the solution fluorescence spectra of P1 and P2 showed slight solvent dependency and blue-shifted with respect to those of the films. These were not the case for P3 which showed little solvent dependency and little blue shift in solution. Two-layer light-emitting devices with a simple ITO/PEDOT/polymer/Ca (Al) configuration were fabricated and characterized. (c) 2004 Elsevier B.V. All rights reserved.
Novel poly(2,3-biphenyl-1,4-phenylenevinylene) derivatives containing long branched alkoxy and fluorenyl substituents: Synthesis, characterization, and their applications for polymer light-emitting diodes
(2005-10-18)
Chen, KB
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Li, HC
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Chen, CK
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Yang, SH
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Hsieh, BR
Two series of poly(2,3-Biphenyl-1,4-phenylenevinylene) (DP-PPV) derivatives containing long branched alkoxy chains or fluorenyl substituents were synthesized. The branched alkoxy groups were introduced to enhance the solubility for spin-coating processes. Bulky fluorenyl groups with two hexyl chains on the 9-position were introduced to increase steric hindrance and prevent close packing of the main chain. By controlling the feed ratio of different monomers during polymerization, DP-PPV derivatives with high molecular weights were obtained. All synthesized polymers possess high glass transitions temperatures and thermal stabilities. The maximum photoluminescent emissions of the thin films are located between 498 and 564 nm. Cyclic voltammetric analysis reveals that the band gaps of these light-emitting materials are in the range from 2.4 to 2.5 eV. A double-layer electroluminescent device with the configuration of ITO/PEDOT/polymer 2/Ca/Al exhibited a low turn-on voltage (4.0 V), a very high external quantum efficiency (3.39 cd/A), and a high brightness (16 910 cd/m(2)).
Poly(2,3-diphenylphenylene vinylene) derivatives having liquid crystalline side groups
(2000-09-01)
Li, AK
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Yang, SS
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Jean, WY
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Hsu, CS
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Hsieh, BR
Poly(2,3-diphenylphenylene vinylene) (DP-PPV) derivatives with pendant mesogene groups were synthesized. A biphenyoxyl or cyclohexylphenoxy moiety serves as the rigid mesogen core, a methylene chain as the spacer, and a alkyl or alkoxy group as the flexible terminal group. The optical properties of the polymers were investigated by UV-vis and photoluminescent spectroscopies. The polymer films emitted green-blue photoluminescence and electroluminescence. The phase transition behaviors of the polymers were characterized by a polarized optical microscope and a differential scanning calorimeter. The monomers did not show mesophases, while the polymers exhibited a nematic phase around 170-190 degrees C. The aligned films exhibited dichroism in absorption and emission spectra, implying that the polymers have potential for linearly polarized LED application.