Browsing by Author "Nguyen, T. P."

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Defect states investigation in poly(2-methoxy,5-(2 ' ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV)
(2006-07-26)
Nguyen, T. P.
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Le Rendu, P.
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Gaudin, O.
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Lee, A. J. T.
;
Jackman, R. B.
in this study, application of the charge based Deep Level Transient Spectroscopy (Q-DLTS) has been extended to poly(2-methoxy,5-(2ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) single layer diodes, whose cathode consisted of aluminium and anode of indium tin oxide (ITO). The results reveal a broad Q-DLTS spectrum, with at least two maxima, consistent with a complex distribution of trap states. Only the Q-DLTS maximum with the shortest relaxation time could be fully resolved over the temperature range used here (100-300 K), yielding activation energies and capture cross sections in the ranges 0.3-0.4 eV and 10(-20) 10(-18) cm(2), respectively. It will be shown that this energy level is likely related to a majority-carrier (hole) trap, consistent with a Poole-Frenkel injection mechanism and p-type doping of the MEH-PPV film. The origin of the second (non-resolved) Q-DLTS peak will also be discussed. (c) 2005 Elsevier B.V. All rights reserved.
Green emitting polyhedral oligomeric silsesquioxanes/poly(phenylene vinylene) derivative materials for highly efficient organic light emitting diodes (OLEDs)
(2008-06-01)
Lee, C. W.
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Josse, Y.
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Hsu, C. H.
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Nguyen, T. P.
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應用化學系
Hybrid materials composed of polyhedral oligomeric silsesquioxanes (POSS) and a polyphenylene vinylene (PPV) derivative were fabricated and characterized by optical and surface spectroscopy. The hybrid thin films have similar optical properties as the pristine polymer but contributions of the POSS are evidenced in infrared spectroscopy. Organic light emitting diodes using the hybrid material as an emitter clearly showed an improvement of the electrical characteristics and the efficiency of the devices as compared to those using the pristine polymer. The effects of POSS incorporation to the polymer are discussed via the obtained results.
Hybrid nanocomposites for optical applications
(2009-10-01)
Nguyen, T. P.
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Lee, C. W.
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Hassen, S.
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Le, H. C.
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應用化學系
Incorporating inorganic particles into conjugated polymer matrices is an area of current interest in the fields of optoelectronics and solar energy. The hybrid nanocomposites exhibit interesting physical properties thanks to good optical properties of polymers and to high carrier mobility of inorganic semiconductors. A judicious combination of organic/inorganic can therefore provide materials of low cost, ease processing, high stability, with specific electrical and optical properties. In the present study, we briefly review the composite materials that have been successfully utilized in the field of optoelectronics and photovoltaic conversion. We shall describe in particular a family of nanocomposites using polyhedral oligomeric silsesquioxanes (POSS) of general formula (RSiO(3/2))(n) where R is an organic group as a core. The composites are made by grafting functional polymer groups to the core, which allows the control of their optical properties. Such composites have high mechanical resistance and stability because of the special structure of the core. For illustration, we present a study of polyfluorene (PF)/POSS materials used as an active layer in organic light emitting diodes, with improved performance as compared to those using polymer only, and we discuss the role of the particles in the transport and emission processes in the devices studied. (C) 2009 Elsevier Masson SAS. All rights reserved.
Investigation of defects in organic semiconductors by charge based Deep Level Transient Spectroscopy (Q-DLTS)
(2009)
Nguyen, T. P.
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Renaud, C.
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Le Rendu, P.
;
Yang, S. H.
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應用化學系
We report the results of measurements of traps in light emitting devices using a new derivative of poly(phenylene vinylene) (PPV) as an active material by the charge based Deep Level Transient Spectroscopy (Q-DLTS) technique. Diodes of structure Indium Tin Oxide (ITO)/PEDOT:PSS/poly(2-ethylhexyl)surfanyl-5- methoxy phenylene vinylene (MEH-S-PPV)/M with M = Al and M = Ca/Al were investigated by measurements of current-voltage-luminance characteristics. From analysis of these characteristics, evidence of charge trapping in devices was demonstrated. The trap parameters were then determined from Q-DLTS measurements, which were carried out on the samples as a function of the charging time, the applied voltage and the temperature. Five trap levels of activation energy in the range [0.-0.6 eV] and of density of order of 10(17) cm(-3) were identified in diodes with Ca/Al cathode. Electron (one level) and hole (four levels) traps were then clearly distinguished by performing measurements in hole-only devices. Trapping processes are discussed and tentatively proposed to performance of the light emitting diodes studied. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Investigation of the optical properties of polyfluorene/ZnO nanocomposites
(2011-04-01)
Zou, J. P.
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Le Rendu, P.
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Musa, I.
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Yang, S. H.
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Dan, Y.
Polyfluorene (PF) and its derivatives are very promising candidates for organic light emitting diodes (OLEDs) in lighting applications because of their high photoluminescence and electroluminescence efficiencies. Recent investigations of potential materials for OLEDs have shown that introducing n-type inorganic nanoparticles into conjugated polymers is efficient to produce stable and high performance devices. In this study, composite thin films made by incorporation of zinc oxide (ZnO) nanoparticles into a PF derivative have been prepared and their optical properties have been investigated. The prepared thin films were stored in different media (in air, in vacuum, in the dark or exposed to light) in order to study environmental influences on the material stability. Analysis of spectral data obtained from infrared (IR), Raman, UV-vis, and photoluminescence (PL) measurements shows a large enhancement in luminescence for polymer nanocomposites while using high nanoparticle concentrations (within a limit of 10% ZnO). Time-resolved PL performed on those nanocomposite films corroborated the above result: it indicated that the light-emission enhancement can be explained by efficient energy transfer from nanoparticles to the polymer chains and increase of the chain separation distance. In addition, the nanocomposites were found to be more stable than pristine polymer films whatever the storage conditions were used. It was confirmed by IR analysis that incorporation of nanoparticles into polymers prohibited the formation of fluorenonyl groups in PF chains, which was identified as the main cause of the degradation of the polymer under photo-oxidation. (C) 2011 Elsevier B.V. All rights reserved.
Investigations of organic light emitting diodes with CdSe(ZnS) quantum dots
(2008-02-15)
Lee, C. W.
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Chou, C. H.
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Huang, J. H.
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Hsu, C. S.
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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.
Study of trap states in polyfluorene based devices by using TSC technique
(2008-08-30)
Renaud, C.
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Huang, C. H.
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Lee, C. W.
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Le Rendu, P.
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Nguyen, T. P.
The trap states in poly(9,9-dihexylfluorene-co-N,N-di(9,9-dihexyl-2-fluorenyl)-N-phenylamine) (PF-N-Ph) based light emitting diodes have been investigated by using the thermally stimulated current (TSC) technique in the temperature range of 90-320 K. The studied structure consisted of indium-tin-oxide/polyethylene-dioxythiophene: polystyrene-sulfonate/PF-N-Ph/Al. Four traps centers denoted as A, B, C, and D trap types have been identified with densities in the range of 10(16)-10(17) cm(-3). Study of the dependence of TSC characteristics on the device polarity suggested that the A, C and D type traps are electron traps while the B type traps are hole traps. They can be described by Gaussian distributions centered on mean trap levels. (c) 2008 Elsevier B.V. All rights reserved.
Study of traps in polydiacetylene based devices using TSC technique
(2006-12-01)
Renaud, C.
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Huang, C. H.
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Zemmouri, M.
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Le Rendu, P.
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Nguyen, T. P.
Trap parameters in poly(1-(3,4-difluorophenyl)-2-(4-pentylcyclohexylphenyl)acetylene) (PDPA-2F) based devices have been investigated by using the thermally stimulated current (TSC) technique. The device structure is ITO-PEDOT-(PDPA-2F)-M, where M stands for the cathode metal (Al, Ca/Al, and Au). The results reveal at least three TSC peaks in devices denoted as peaks A, B and C. Comparing trap parameters in ITO-PEDOT-(PDPA-2F)-Au hole-only device and ITO-PEDOT-(PDPA-2F)-Ca Al (Al) bipolar devices, we assigned A and B trap types to hole-like traps and C type traps to electron-like traps. The trap densities are in the range of 10(15)-10(17) cm(-3) and the trap levels are 0.12 eV (A type traps), 0.36 eV (B type traps), and 0.25 eV (C type traps).