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    Adsorption of dyes on Sahara desert sand
    (Elsevier Science Bv, 2009) Varlikli, Canan; Bekiari, Vlasoula; Kus, Mahmut; Boduroglu, Numan; Oner, Ilker; Lianos, Panagiotis; Lyberatos, Gerasimos; Icli, Siddik
    Sahara desert sand (SaDeS) was employed as a mineral sorbent for retaining organic dyes from aqueous solutions. Natural sand has demonstrated a strong affinity for organic dyes but significantly lost its adsorption capacity when it was washed with water. Therefore, characterization of both natural and water washed sand was performed by XRD, BET, SEM and FTIR techniques. It was found that water-soluble kyanite, which is detected in natural sand, is the dominant factor affecting adsorbance of cationic dyes. The sand adsorbs over 75% of cationic dyes but less than 21% for anionic ones. Among the dyes studied, Methylene Blue (MB) demonstrated the strongest affinity for Sahara desert sand (Q(e) = 11.98 mg/g, for initial dye solution concentration 3.5 x 10(-5) mol/L). The effects of initial dye concentration, the amount of the adsorbent, the temperature and the pH of the solution on adsorption capacity were tested by using Methylene Blue as model dye. Pseudo-first-order, pseudo-second-order and intraparticle diffusion models were applied. It was concluded that adsorption of Methylene Blue on Sahara desert sand followed pseudo-second order kinetics. Gibbs free energy, enthalpy change and entropy change were calculated and found -6411 J/mol, -30360 J/mol and -76.58 J/mol K, respectively. These values indicate that the adsorption is an exothermic process and has a spontaneous nature at low temperatures. (C) 2009 Elsevier B.V. All rights reserved.
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    Adsorption of dyes on Sahara desert sand
    (Elsevier Science Bv, 2009) Varlikli, Canan; Bekiari, Vlasoula; Kus, Mahmut; Boduroglu, Numan; Oner, Ilker; Lianos, Panagiotis; Lyberatos, Gerasimos; Icli, Siddik
    Sahara desert sand (SaDeS) was employed as a mineral sorbent for retaining organic dyes from aqueous solutions. Natural sand has demonstrated a strong affinity for organic dyes but significantly lost its adsorption capacity when it was washed with water. Therefore, characterization of both natural and water washed sand was performed by XRD, BET, SEM and FTIR techniques. It was found that water-soluble kyanite, which is detected in natural sand, is the dominant factor affecting adsorbance of cationic dyes. The sand adsorbs over 75% of cationic dyes but less than 21% for anionic ones. Among the dyes studied, Methylene Blue (MB) demonstrated the strongest affinity for Sahara desert sand (Q(e) = 11.98 mg/g, for initial dye solution concentration 3.5 x 10(-5) mol/L). The effects of initial dye concentration, the amount of the adsorbent, the temperature and the pH of the solution on adsorption capacity were tested by using Methylene Blue as model dye. Pseudo-first-order, pseudo-second-order and intraparticle diffusion models were applied. It was concluded that adsorption of Methylene Blue on Sahara desert sand followed pseudo-second order kinetics. Gibbs free energy, enthalpy change and entropy change were calculated and found -6411 J/mol, -30360 J/mol and -76.58 J/mol K, respectively. These values indicate that the adsorption is an exothermic process and has a spontaneous nature at low temperatures. (C) 2009 Elsevier B.V. All rights reserved.
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    Bilayer heterojunction solar cell based on naphthalene bis-benzimidazole
    (Elsevier Science Sa, 2008) Erten, Sule; Icli, Siddik
    We have fabricated a bilayer heterojunction solar cell consisting of p-n active layers of naphthalenetetracarboxylic bis-benzimidazole, N-CON, and zinc phthalocyanine, ZnPc. Conjugated naphthalene derivative of naphthalene bis-benzimidazole was employed as an electron acceptor. Bilayer solar cells were produced by successive evaporation of zinc phthalocyanine, ZnPc, and naphthalene bis-benzimidazole, N-CON, on glass substrates coated with indium doped tin oxide, ITO/PEDOT: PSS/ZnPc/N-CON/Al. Photovoltaic characteristics of bilayer cell were measured under simulated AM 1.5 sun illumination (V(oc) = 0.50 V, J(sc) = 2.11 mA/cm(2), FF = 0.51, g = 0.54%). Efficiency of the cell was discussed in relation to energy gap and work functions of the thin layers. (c) 2007 Elsevier B.V. All rights reserved.
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    Biological investigation of I-131-labeled new water soluble Ru(II) polypyridyl complex
    (Pergamon-Elsevier Science Ltd, 2008) Ocakoglu, Kasim; Yildrim, Yeliz; Lambrecht, Fatma Yurt; Ocal, Jale; Icli, Siddik
    New [Ru(L1)(dcbpy)(NCS)(2)] complex was synthesized in a one-pot reaction starting from [RuCl2(p-cymene)](2), where the ligands (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine, L1 = dipyrido[3,2-a:2',3'-c]phenazine-11-ylcarbonyl) -sodium) are introduced sequentially. The resulting complex was characterized by IR, NMR, and elemental analysis. The complex was labeled with I-131. Biodistribution study of the complex was carried out using I-131-labeled [Ru(L1)(dcbpy)(NCS)(2)] complex. The biodistribution study performed with albino Wistar male rats has shown that the complex has high uptake in the lung, small intestine, fat, and spleen. (C) 2007 Elsevier Ltd. All rights reserved.
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    Carbazole-based organic dye sensitizers for efficient molecular photovoltaics
    (Elsevier Science Bv, 2010) Zafer, Ceylan; Gultekin, Burak; Ozsoy, Cihan; Tozlu, Cem; Aydin, Banu; Icli, Siddik
    Efficient dye sensitizers N,N'-dialkylcarbazolocarbazole derivatives BG-501 and BG-502 were synthesized and characterized. UV-vis, 1H NMR and CV were used for the structural characterization. The aim of the synthesis of these molecules is to improve some optical and electronic properties such as molar absorption coefficient, spectral coverage and electron injection properties of the sensitizers. Conjugated organic dyes exhibit higher molar extinction coefficients which allow harvesting more light in thinner semiconductor layer. Carbazole derivative dyes are well known in the literature with their thermal and photochemical stabilities and improved electron donor properties. We have achieved promising photovoltaic conversion efficiencies with new dyes BG-501 and BG-502 under standard conditions (AM1.5G, 100 mW. cm(-2) light intensity). The conversion efficiencies of solar cells are eta:3.18% and eta:2.49% with ionic liquid-based electrolyte for BG-501 and BG-502. respectively. (C) 2009 Elsevier B.V. All rights reserved.
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    Charge transfer through amino groups-small molecules interface improving the performance of electroluminescent devices
    (Elsevier Science Bv, 2016) Havare, Ali Kemal; Can, Mustafa; Tozlu, Cem; Kus, Mahmut; Okur, Salih; Demic, Serafettin; Demirak, Kadir; Kurt, Mustafa; Icli, Siddik
    A carboxylic group functioned charge transporting was synthesized and self-assembled on an indium tin oxide (ITO) anode. A typical electroluminescent device [modified ITO/TPD (50 nm)/Alq(3) (60 nm)/LiF (2 nm)/(120 nm)] was fabricated to investigate the effect of the amino groups-small molecules interface on the characteristics of the device. The increase in the surface work function of ITO is expected to facilitate the hole injection from the ITO anode to the Hole Transport Layer (HTL) in electroluminescence. The modified electroluminescent device could endure a higher current and showed a much higher luminance than the nonmodified one. For the produced electroluminescent devices, the I-V characteristics, optical characterization and quantum yields were performed. The external quantum efficiency of the modified electroluminescent device is improved as the result of the presence of the amino groups-small molecules interface. (C) 2016 Elsevier B.V. All rights reserved.
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    Comparative study of arylene bisimides substituted with imidazole side group for different dielectrics on the OFET application
    (Elsevier Science Sa, 2013) Tozlu, Cem; Erten-Ela, Sule; Singh, Th. Birendra; Sariciftci, N. Serdar; Icli, Siddik
    We analyzed the effect of polymeric dielectric with hydroxyl and hydroxyl-free group on current-voltage characteristics of organic thin film transistor by the use of benzimidazole-derived arylene bisimide derivatives. Polyvinyl alcohol (PVA) with polar group and benzocyclobutene (BCB) with non-polar group were used as solution processed dielectric materials to compare with each other in thin film transistor application. The hydroxyl group has a significant effect on turn-on voltage and turn-off current in depletion regime due to hydroxyl group. It is observed that the surface morphology is influenced by the chemical structure of polymeric dielectric concerning surface energy. The electron field effect mobility of both arylene bisimides is enhanced by decreased surface energy of dielectric. The highest mobility was obtained by employing naphthalene bis-benzimidazole as an active layer on both dielectrics compared with perylene bis-benzimidazole semiconductor. The electrical behaviors of these semiconductors are discussed in relation to gate dielectric surface properties. (c) 2013 Elsevier B.V. All rights reserved.
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    A comparative study of supported TiO2 as photocatalyst in water decontamination at solar pilot plant scale
    (Asme, 2006) Kus, Mahmut; Gernjak, Wolfgang; Fernandez Ibanez, Pilar; Malato Rodriguez, Sixto; Blanco Galvez, Julian; Icli, Siddik
    The degradation of gallic acid and imidacloprid with supported Millennium PC500 and Degussa P25 TiO2 is reported. TiO2 particles were immobilized using a titanium sol-solution and direct deposition on glass supports. The film characterization was done by x-ray diffraction, scanning electron microscopy, and atomic force microscopy, and degradation of organic compounds was monitored by high-performance liquid chromatography, total organic carbon analyzer and ion chromatography. The experiments were performed under sunlight in compound parabolic collector plants with flat supports inside the glass tubes. Photocatalytic activity of the films was compared and identified. Although sol-gel coatings had better mechanical properties, mineralization was observed to be approximately five times slower than paste-deposited films. Photoactivity of the films decreased with silver deposition due to the nature of the organic compounds. The rate constants were calculated to be between 2 x 10(-1) and 6 x 10(-2) ring m(2)/kJ for organic compounds, and 6 x 10(-2) and 6 x 10(-3) for total organic carbon.
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    Construction of a fuel cell system with DC power distribution for residential applications
    (Pergamon-Elsevier Science Ltd, 2011) Cetin, Engin; Yilanci, Ahmet; Ozturk, Harun K.; Kasikci, Ismail; Colak, Metin; Icli, Siddik
    In recent years, fuel cell systems have been popular as electricity production systems these are clean, environmental-friendly, modular, and independent from fossil fuels. In Pamukkale University Clean Energy House, Denizli-Turkey, 1.2 kWp hydrogen-fuel cell-DC micro grid system was set up to investigate the fuel cells with DC power distribution. In the system, electricity is generated by 1.2 kWp PEm fuel cell unit. Hydrogen is transformed to DC electrical energy by fuel cell in the case of its necessity and then, DC electricity produced by fuel cell is distributed via DC power box consisting of 2 DC/DC converters, circuit breakers and monitored by data acquisition equipments. DC electricity is consumed by DC loads such as 44 lamps, freezer/refrigerator, 2 fans, TV, vacuum cleaner and circulation pump. In this study, a fuel cell system with DC power distribution grid under different DC load conditions is investigated. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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    Conventional and microwave-assisted synthesis of ZnO nanorods and effects of PEG400 as a surfactant on the morphology
    (Elsevier Science Sa, 2009) Erten-Ela, Sule; Cogal, Sadik; Icli, Siddik
    Conventional and microwave-assisted synthesis of ZnO nanorods have been performed with and without using PEG400. ZnO nanorods were synthesized with 50-250 nm of diameter which depends on the used surfactant and methods. Surfactant effects of PEG400 on the size and morphology of ZnO nanorods were investigated. The microwave method was compared to the conventional heating method. Morphologies were investigated by using scanning electron microscopy (SEM). (c) 2008 Elsevier B.V. All rights reserved.
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    Dicationic bis-imidazolium molten salts for efficient dye sensitized solar cells: Synthesis and photovoltaic properties
    (Pergamon-Elsevier Science Ltd, 2009) Zafer, Ceylan; Ocakoglu, Kasim; Ozsoy, Cihan; Icli, Siddik
    New dicationic bis-imidazolium salts based ionic liquids were synthesized to develop new electrolytes to improve photovoltaic properties of dye sensitized solar cells. Various properties of electrolytes such as viscosities, ion diffusion coefficients, charge transfer resistances and photovoltaic properties were studied. Electrochemical impedance spectroscopy has been performed to investigate diffusion coefficients and charge transfer resistances. Influence of polarity and chain length on the photovoltaic performance, was investigated. A DSSC employing the K34 (butyl-1,4-bis(3-methylimidazolium iodide) gives an open-circuit voltage of 0.64 V, a short-circuit current of 17.11 mA/cm(2) and conversion efficiency of 5.60% under light intensity of 100 mW/cm(2) while the DSSC based on 1-butyl-3-methylimidazolium iodide which is a reference ionic liquid exhibited 5.64% efficiency due to the lowest viscosity, highest diffusion coefficient. (C) 2009 Elsevier Ltd. All rights reserved.
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    Dicationic bis-imidazolium molten salts for efficient dye sensitized solar cells: Synthesis and photovoltaic properties
    (Pergamon-Elsevier Science Ltd, 2009) Zafer, Ceylan; Ocakoglu, Kasim; Ozsoy, Cihan; Icli, Siddik
    New dicationic bis-imidazolium salts based ionic liquids were synthesized to develop new electrolytes to improve photovoltaic properties of dye sensitized solar cells. Various properties of electrolytes such as viscosities, ion diffusion coefficients, charge transfer resistances and photovoltaic properties were studied. Electrochemical impedance spectroscopy has been performed to investigate diffusion coefficients and charge transfer resistances. Influence of polarity and chain length on the photovoltaic performance, was investigated. A DSSC employing the K34 (butyl-1,4-bis(3-methylimidazolium iodide) gives an open-circuit voltage of 0.64 V, a short-circuit current of 17.11 mA/cm(2) and conversion efficiency of 5.60% under light intensity of 100 mW/cm(2) while the DSSC based on 1-butyl-3-methylimidazolium iodide which is a reference ionic liquid exhibited 5.64% efficiency due to the lowest viscosity, highest diffusion coefficient. (C) 2009 Elsevier Ltd. All rights reserved.
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    Effect of side chain substituents on the electron injection abilities of unsymmetrical perylene diimide dyes
    (Elsevier Sci Ltd, 2011) Dincalp, Haluk; Askar, Zuhal; Zafer, Ceylan; Icli, Siddik
    Three near-infrared (NIR) absorbing unsymmetrical perylene diimide D A D type dyes containing 6-undecanoxy as donor group were utilized in dye-sensitized nanocrystalline TiO(2) solar cells. Structure of the acceptor side of the molecules were improved by adding 4-[2-methyl-5-(cyanoacrylic acid)-3-thienyl]-phenyl (V), 3-carboxy-2-pyridil (VI) and 3-carboxy-2-pyrazyl (VII) moieties attached to one of the N-side of the dye. The relationship between the molecular structure of the acceptor sites of the dyes and the photovoltaic performances were discussed. Electrochemical measurements indicated that band gaps of the dyes were energetically favorable for electron injection from the excited state of the dyes to the conduction band of TiO(2) nanoparticles. However, three dyes gave lower conversion efficiency on DSSC applications. Strong electron-withdrawing nature of perylene core might not permit to transfer the photo-generated electrons to the carboxyl groups anchoring to TiO(2) surface, and then solar-to-electricity conversion efficiencies of the dyes were reduced. (C) 2011 Elsevier Ltd. All rights reserved.
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    Effect of TiO2 modification with amino-based self-assembled monolayer on inverted organic solar cell
    (Elsevier Science Bv, 2017) Tozlu, Cem; Mutlu, Adem; Can, Mustafa; Havare, Ali Kemal; Demic, Serafettin; Icli, Siddik
    The effects of surface modification of titanium dioxide (TiO2) on the performance of inverted type organic solar cells (i-OSCs) was investigated in this study. A series of benzoic acid derivatized self-assembled monolayer (SAM) molecules of 4'-[(hexyloxy) phenyl] amino-3,5-biphenyl dicarboxylic acid (CT17) and 4'-[1-naphthyl (phenyl) amino] biphenyl-4-carboxylic acid (CT19) were utilized to modify the interface between TiO2 buffer layer and poly-3 hexylthiophene (P3HT):[ 6,6]-phenyl C-61 butyric acid methyl ester (PC61BM) active layer having the device structure of ITO/TiO2/SAM/P3HT:PC61BM/MoO3/Ag. The work function and surface wetting properties of TiO2 buffer layer served as electron transporting layer between ITO and PC61BM active layer were tuned by SAM method. The solar cell of the SAM modified devices exhibited better performance. The power conversion efficiency (PCE) of i-OSCs devices with bare TiO2 electrodes enhanced from 2.00% to 2.21% and 2.43% with CT17 and CT19 treated TiO2 electrodes, respectively. The open circuit voltage (V-oc) of the SAM treated TiO2 devices reached to 0.60 V and 0.61 V, respectively, while the V-oc of untreated TiO2 was 0.57 V. The water contact angle of i-OSCs with CT17 and CT19 SAMs was also higher than the value of the unmodified TiO2 electrode. These results show that inserting a monolayer at the interface between organic and inorganic layers is an useful alternative method to improve the performance of i-OSCs. (C) 2017 Elsevier B.V. All rights reserved.
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    Electrical properties of SAM-modified ITO surface using aromatic small molecules with double bond carboxylic acid groups for OLED applications
    (Elsevier Science Bv, 2014) Can, Mustafa; Havare, Ali Kemal; Aydin, Hasan; Yagmurcukardes, Nesli; Demic, Serafettin; Icli, Siddik; Okur, Salih
    5-[(3-Methylphenyl)(phenyl)amino]isophthalic acid (5-MePIFA) and 5-(diphenyl)amino]Isophthalic acid (5-DPIFA) organic molecules were synthesized to form self-assembled monolayer on indium tin oxide (ITO) anode to enhance hole transport from ITO to organic hole transport layers such as TPD. The modified surface was characterized by scanning tunneling microscopy (STM). The change in the surface potential was measured by Kelvin probe force microscopy (KPFM). Our Kelvin probe force microscopy (KPFM) measurements showed that the surface potentials increased more than 100 mV with reference to bare indium tin-oxide. The results show that the threshold voltage on OLEDs with modified ITO is lowered significantly compared to OLEDs with unmodified ITO. The hole mobility of TPD has been estimated using space-charge-limited current measurements (SCLC). (C) 2014 Elsevier B.V. All rights reserved.
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    Electrochemical and optical properties of biphenyl bridged-dicarbazole oligomer films: Electropolymerization and electrochromism
    (Pergamon-Elsevier Science Ltd, 2009) Koyuncu, Sermet; Gultekin, Burak; Zafer, Ceylan; Bilgili, Hakan; Can, Mustafa; Demic, Serafettin; Kaya, Ismet; Icli, Siddik
    4,4'-Di(N-carbazoyl)biphenyl monomer (CBP) was synthesized and coated onto ITO-glass surface by electrochemical oxidative polymerization. Its CV shows two distinct one-electron and stepwise oxidation processes occurred at 1.29 and 1.61 V. By using this property, the monomer was electrochemically polymerized separately at these oxidation states and thus, two different oligomer films were obtained afterwards. Their spectro-electrochemical and electrochromic properties were also investigated. Switching ability of the oligomers was evaluated by kinetic studies upon measuring the percent transmittance (%T) at their maximum contrast point, indicating that these oligomers were found to be suitable material for electrochromic devices. (C) 2009 Elsevier Ltd. All rights reserved.
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    Electrochemical and optical properties of biphenyl bridged-dicarbazole oligomer films: Electropolymerization and electrochromism
    (Pergamon-Elsevier Science Ltd, 2009) Koyuncu, Sermet; Gultekin, Burak; Zafer, Ceylan; Bilgili, Hakan; Can, Mustafa; Demic, Serafettin; Kaya, Ismet; Icli, Siddik
    4,4'-Di(N-carbazoyl)biphenyl monomer (CBP) was synthesized and coated onto ITO-glass surface by electrochemical oxidative polymerization. Its CV shows two distinct one-electron and stepwise oxidation processes occurred at 1.29 and 1.61 V. By using this property, the monomer was electrochemically polymerized separately at these oxidation states and thus, two different oligomer films were obtained afterwards. Their spectro-electrochemical and electrochromic properties were also investigated. Switching ability of the oligomers was evaluated by kinetic studies upon measuring the percent transmittance (%T) at their maximum contrast point, indicating that these oligomers were found to be suitable material for electrochromic devices. (C) 2009 Elsevier Ltd. All rights reserved.
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    Electrochemical and optical properties of novel donor-acceptor thiophene-perylene-thiophene polymers
    (John Wiley & Sons Inc, 2008) Koyuncu, Sermet; Kus, Mahmut; Demic, Serafettin; Kaya, Ismet; Ozdemir, Eyup; Icli, Siddik
    In this study, donor-acceptor type thiophene-perylene-thiophene monomers were synthesized and polymerized by both oxidative polymerization using FeCl3 as catalyst and the electrochemical process. UV-vis, FTIR, H-1 NMR, and elemental analysis techniques were used for structural characterization. Thermal behaviors of these compounds were determined by using TGA system. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels and electrochemical and optical band gap values were calculated by using the results of cyclic voltammetry and UV-vis measurements, respectively. The number-average molecular weight (M-n), weight-average molecular weight (M-n), and polydispersity index (PDI) values of synthesized polymers were determined by size exclusion chromatography. Conductivity measurements of these polymers were carried out by electrometer by using a four-point probe technique. The conductivity was observed to be increased by iodine doping. (C) 2008 Wiley Periodicals, Inc.
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    Electrochromic and electroluminescent devices based on a novel branched quasi-dendric fluorene-carbazole-2,5-bis(2-thienyl)-1H-pyrrole system
    (Royal Soc Chemistry, 2011) Koyuncu, Sermet; Usluer, Ozlem; Can, Mustafa; Demic, Serafettin; Icli, Siddik; Sariciftci, Niyazi Serdar
    We report here the synthesis of a novel branched quasi-dendric system, 9,9'-(9,9'-dihexylfluorene-2,7-diyl)bis[3,6-bis(2,5-bis(2-thienyl) pyrrol-1-yl)carbazole], (FCSNS), in four steps, followed by coating onto an ITO-coated glass surface by an electropolymerization process to give a very stable cross-linked polymeric film (poly-FCSNS). The yellowish-green color of this film in its neutral state changed reversibly to black upon oxidation. An electrochromic device, assembled in the sandwich configuration [ITO/anodically coloring polymer (poly-FCSNS)//gel electrolyte//cathodically coloring polymer/(PEDOT)/ITO], exhibited a relatively short response time (about 1 s), a high redox stability, and a high coloration efficiency (1624 cm(2) C-1). In addition to electrochromic studies, organic light-emitting diode (OLED) work was also carried out using FCSNS. A multilayer OLED having a configuration of ITO/PEDOT:PSS/FCSNS/Alq(3)/LiF : Al was fabricated, and it showed a turn-on voltage of approximately 6 V and exhibited a bright green emission with a luminance of 3700 cd m(-2). The maximum luminous efficiency was found to be 2.0 cd A(-1) at 14 V and 11.75 mA cm(-2). The emitted light from the OLED device is green, and has the color coordinates of (x, y) (0.33, 0.54) according to CIE. Electrochemical and optical properties were also studied by using cyclic voltammetry, UV-Vis absorption and fluorescence spectroscopy, respectively.
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    Enhanced electroluminescence from MEH-PPV-POSS:CuInS2 nanocomposite based organic light emitting diode
    (Elsevier Science Sa, 2011) Saygili, Gamze; Ozsoy, Cihan; Oner, Ilker; Zafer, Ceylan; Varlikli, Canan; Icli, Siddik
    Polymer:quantum dot (QD) composites show enhanced optical and electronic properties. In this study, polymer light-emitting diodes (PLEDs) were fabricated and characterized using poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] end capped with polyhedral oligomeric silsesquioxanes (MEH-PPV-POSS) as a luminescent polymer host and copper indium disulfide (CuInS2) QDs as a dopant. The emitted light originates from MEH-PPV-POSS. Incorporation of CuInS2 QDs into polymer matrix until certain amounts improved the device performance in terms of electroluminescence (EL) intensity, luminance, current and power efficiency compared to that of the undopped device. The improvement is remarkable when the QD concentration is 0.3 wt% in the composite. We demonstrate that CuInS2 QDs provide a better balance of charge carriers and prevent the formation of polymeric aggregates. (C) 2010 Elsevier B.V. All rights reserved.