Browsing by Author "Hsu, C. S."

Now showing 1 - 8 of 8

Electron transport and electroluminescent efficiency of conjugated polymers
(2009-01-01)
Tseng, S. R.
;
Chen, Y. S.
;
Meng, H. F.
;
Lai, H. C.
;
Yeh, C. H.
The relation between charge balance and electroluminescent efficiency for polymers is studied systematically in this work. Light-emitting diodes of several kinds of polymers with very different efficiency are compared. The electron and hole currents are measured in the uni-polar devices and fitted by the theoretical model to get carrier mobilities. The universal features of the high efficiency polymers are that the electron and hole mobilities are comparable and the electron current is larger than the hole current due to the higher hole barriers. The electron current is more crucial than hole current to determine the efficiency in the bipolar light-emitting diode. The purity of polymers and low electron injection barriers both play significant roles in producing stronger electron currents. These results give the physical rules for designing new polymers and the device structures to achieve highly electroluminescent devices. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.
Investigations of organic light emitting diodes with CdSe(ZnS) quantum dots
(2008-02-15)
Lee, C. W.
;
Chou, C. H.
;
Huang, J. H.
;
Hsu, C. S.
;
Nguyen, T. P.
In this study, we report results on investigation of bilayer light emitting diodes made of organic capped CdSe(ZnS) core/shell type nanocrystals and poly [2-phenyl-3-(9,9-dihexyl-fluoren-2-yl) phenylene vinylene]-co-[2-methoxy-5-(2'-ethylhexyloxy) phenylene vinylene] (FP-PPV-co-MEH-PPV) electroluminescent co-polymer. Light emitting diodes of structure: indium-tin-oxide (ITO)/polyethylene dioxythiophene: polystyrene sulfonate (PEDOT:PSS)/FP-PPV-co-MEH-PPV/Ca/Al devices have been fabricated and studied. The co-polymer device emits a yellow light with a maximum brightness of 3949 cd/m(2) and a maximum external quantum efficiency of 0.27 cd/A at 10 V. Incorporation of CdSe(ZnS) quantum dots into the active polymer film resulted in an increase in device brightness, which reached 8192 cd/m(2) and in external quantum efficiency, which became 1.27 cd/A at 7 V with a lower turn-on voltage. The results indicate that CdSe(ZnS) quantum dots improved significantly the emission of the devices by modifying the injection and transport of the charge carriers. We suggest a non-uniform dispersion of QDs in the co-polymer layer with carrier blocking process by the particles, which produced a balanced charge distribution inside the co-polymer, and thus increased the emission efficiency. (c) 2007 Elsevier B.V. All rights reserved.
Surface-induced ordering effect in one antiferroelectric liquid crystal compound
(2009-03-01)
Pan, LiDong
;
Wang, Shun
;
Hsu, C. S.
;
Huang, C. C.
;
應用化學系
Null transmission ellipsometer was employed to study the surface-induced tilt transition of an antiferroelectric liquid crystal compound in the bulk smectic-A (SmA) temperature range. The data of the surface tilt transition can be fitted with an extended mean-field model. These results indicate the transition is near a mean-field tricritical point. The surface layers' tilt directions are found to be in different planes several degrees above the bulk SmA-SmC alpha* transition.
Surface-Surface Interaction in Smectic Liquid Crystal Films
(2012-01-11)
Pan, LiDong
;
Hsu, C. S.
;
Huang, C. C.
;
應用化學系
;
Department of Applied Chemistry
Null transmission ellipsometry was employed to study the field induced transition of the surface arrangements in freestanding films of smectic liquid crystals. The interlayer interaction between the two surfaces obtained from the threshold voltage for the transition is found to be antiferroelectric and is quasilong ranged. The possible microscopic origins of the measured interaction and its relevance to the interlayer interaction in antiferroelectric liquid crystal materials are discussed.
Synthesis of ethanol-soluble few-layer graphene nanosheets for flexible and transparent conducting composite films
(2011-07-22)
Nguyen, D. D.
;
Tai, N. H.
;
Chueh, Y. L.
;
Chen, S. Y.
;
Chen, Y. J.
We report a facile method of preparing few-layer graphene nanosheets (FLGs), which can be soluble in ethanol. Atomic force microscopy and high-resolution transmission electron microscopy studies reveal that FLGs have average thicknesses in the range of 2.6-2.8 nm, corresponding to 8-9 layers. A graphene/nafion composite film has a sheet resistance of 9.70 k Omega/sq at the transmittance of 74.5% (at 550 nm) while the nafion film on polyethylene terephthalate has a sheet resistance of 128 k Omega/sq at transmittance of 90.0%. For the cycling/bending test, almost no change in resistance was exhibited when the film was bent at an angle up to 140 degrees, and no obvious deviation in resistance could be found after 100 bending cycles was applied. In addition, an FLGs-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) composite layer was demonstrated as the effective hole transporting layer to improve the hole transporting ability in an organic photovoltaic device, with which the power conversion efficiency was enhanced from 3.10% to 3.70%. The results demonstrated the promising applications of FLGs on graphene-based electronics, such as transparent electrode and flexible conducting film.
Thickness Dependent Phase Behavior of Antiferroelectric Liquid Crystal Films
(2009-10-30)
Pan, LiDong
;
Wang, Shun
;
Hsu, C. S.
;
Huang, C. C.
;
應用化學系
Free standing films of a liquid crystal compound with simple surface enhanced order were studied. The resultant phase diagram demonstrates that (1) the short helical pitch smectic-C-alpha* phase disappears below a film thickness of 10 layers, and (2) the temperature window of a distorted 4 layer smectic-C-FI2* phase increases dramatically upon decreasing film thickness. The experimental findings were attributed to the reduced dimensionality and enhanced surface effects in thin films. The results of the smectic-C-alpha* phase are consistent with what have been reported for helically ordered magnetic thin films, with a noticeable difference due to the opposite effect of the surface on ordering in the two systems.
Triplet exciton energy transfer in polyfluorene doped with heavy metal complexes studied using photoluminescence and photoinduced absorption
(2006-12-01)
Liao, H. H.
;
Meng, H. F.
;
Horng, S. F.
;
Lee, W. S.
;
Yang, J. M.
Modulated photoinduced absorption and photoluminescence are used to study triplet-to-triplet Dexter energy transfer in Ir-complexes/polyfluorene blend systems. There is no Dexter energy transfer for red iridium (III)bis[2-(9,9-dibutylfluorenyl)-1-isoquinoline(acetylacetonate)] (DFIr) and red iridium (III) bis([2-(2-benzo-thienyl)pyridinatoN,C3] acetyl-acetonate) (BtpIr) dopants. Although green iridium(III)tris[2-(4-tolyl)pyridinato-N,C2] [Ir(mppy)(3)] has no triplet confinement in polyfluorene host, it has clear evidence for Dexter energy transfer. Aggregation and dopant lifetime are shown to significantly affect the energy transfer. The presence of Dexter transfer implies the possibility to harvest triplet excitons of polyfluorene in polymer light-emitting diodes even without carrier trapping and triplet exciton confinement.
Unique pitch evolution in the Smectic-C-alpha(*) phase
(2007-08-17)
Liu, Z. Q.
;
McCoy, B. K.
;
Wang, S. T.
;
Pindak, R.
;
Caliebe, W.
Employing resonant x-ray diffraction, we observed unique pitch evolutions in the smectic-C-alpha* phase in mixtures of two antiferroelectric liquid crystals. Our results show that the pitch in this phase continuously evolves across 4 layers, contradicting a theoretical model that predicts that the smectic-C-FI2* phase intervenes in the smectic-C-alpha* phase. The phase sequences we found can be explained by another model that includes one type of long-range interaction among smectic layers.