Improved efficiency by a fluorescent dye in red organic light-emitting devices based on a europium complex

By doping a fluorescent dye in the emissive layer, we realized high efficient red organic light-emitting diodes (OLEDs) based on a europium complex. The OLEDs realized by this method showed pure red emission at 612 nm with a full width at half maximum Of 3 nm. The Commission International de L'Eclairage Coordination keeps approximately the same as the emission of pure Eu3+. The maximum brightness and EL efficiency reached 2450 cd/m(2) at 20 V and 9.0 cd/A (6.0 lm/w) at a current density of 0.012 mA/cm(2), respectively. At the brightness of 100 cd/m(2), the current efficiency reached 4.4 cd/A.

Modeling of organic light-emitting diodes with graded concentration in the emissive multilayer

We model the electrical behavior of organic light-emitting diodes whose emissive multilayer is formed by blends of an electron transporting material, tris-(8-hydroxyquinoline) aluminum (Alq(3)) and a hole transporting material, N,N-'-diphenyl-N,N-'-bis(1,1(')-biphenyl)-4,4-diamine. The multilayer is composed of layers of different concentration. The Alq(3) concentration gradually decreases from the cathode to the anode. We demonstrate that these graded devices have higher efficiency and operate at lower applied voltages than devices whose emissive layer is made of nominally homogeneous blends. Our results show an important advantage of graded devices, namely, the low values of the recombination rate distribution near the cathode and the anode, so that electrode quenching is expected to be significantly suppressed in these devices.

Preparation of Pt/C catalyst with a new and simple organic sol method

It is reported for the first time that the Pt/C catalyst can be prepared with a new and simple organic sol method using SnCl2 as the reductant. It was found that the average size of the Pt particles in the Pt/C catalysts could be controlled with controlling the preparation conditions. The effect of the average sizes of the Pt particles in the Pt/C catalysts obtained with this method on the electrocatalytical activity of the oxidation of methanol was investigated.

Organic thin-film transistors having inorganic/organic double gate insulators

Bottom-contact organic thin-film transistors (BC OTFTs) based on inorganic/organic double gate insulators were demonstrated. The double gate insulators consisted of tantalum pentoxide (Ta2O5) with high dielectric constant (kappa) as the first gate insulator and octadecyltrichlorosilane (OTS) with low kappa as the second gate insulator. The devices have carrier mobilities larger than 10(-2) cm(2)/V s, on/off current ratio greater than 10(5), and the threshold voltage of -14 V, which is threefold larger field-effect mobility and an order of magnitude larger on/off current ratio than the OTFTs with a Ta2O5 gate insulator. The leakage current was decreased from 2.4x10(-6) to 7.4x10(-8) A due to the introduction of the OTS second dielectric layer. The results demonstrated that using inorganic/organic double insulator as the gate dielectric layer is an effective method to fabricate OTFTs with improved electric characteristics.

Organic thin-film transistors in sandwich configuration

Organic thin-film transistors (OTFTs) having source/drain electrodes sandwiched between copper phthalocyanine (CuPc) and cobalt phthalocyanine (CoPc) layers, CuPc/CoPc SC OTFTs, are investigated. Comparing their properties with that of CuPc-based top-contact OTFT, field-effect mobility increases from 0.04 to 0.11 cm(2)/Vs, threshold voltage shifts from -13.8 to -8.9 V, and the current on/off ratio maintains at a level of 10(5). A top-contact OTFT with a layer of CuPc and a layer of CoPc (10%)-CuPc mixture reveals that the combination of CuPc and CoPc enhances charge injection from the source electrode into the active layer and increases the off-state current. The sandwich configuration increases the field-effect mobility, reduce the threshold voltage, and improve the on/off ratio at the same time. Our results indicate that using a double-layer of active organic materials in sandwich configuration is an effective way to improve OTFT performance.

Study on the instability of organic field-effect transistors based on fluorinated copper phthalocyanine

The effects of positive and negative gate-bias stress on organic field-effect transistors (OFET) based on tantalum (Ta)/tantalum pentoxide (Ta2O5)/fluorinated copper phthalocyanine (F16CuPc) structure are investigated as a function of stress time and stress temperature. It is shown that gate-bias stress induces a parallel threshold voltage shift (DeltaV(T)) of OFETs without changes of field-effect mobility mu(EF) and sub-threshold slope (DeltaS). The DeltaV(T) is observed to be logarithmically dependent on time at high gate-bias appropriate to OFET operation. More importantly, the shift is directional, namely, be large shift under positive stress and almost do not move under negative stress. The threshold voltage shift is temperature dependent with activation energy of 0.51 eV We concluded that threshold voltage shift of the OFET with F16CuPc as active layer is due to charge trapping in the insulator in which trapped carriers have redistribution.

Fabrication, characterization and conductivity of organic-inorganic hybrid Langmuir-Blodgett films based on polyquinoline and rare earth-substituted heteropolymolybdate

The organic/inorganic hybrid Langmuir-Blodgett (LB) films were obtained by the compact organization of poly(1,2-dihydro-2,2,4-trimethyl)quinoline (PQ), octadecylamine (ODA) and rare earth-substituted heteropolymolybdates. They were characterized by surface pressure-area (pi-A) isotherms, absorption spectra, fluorescence spectra, atomic force microscope (AFM) and scanning tunneling microscopy (STM). The atomic force microscope revealed a granular surface texture of nanosized rare earth-substituted heteropolymolybdate. The scanning tunneling microscopy indicated that the hybrid LB films containing rare earth-substituted heteropolymolybdates had the better electrical conductivity than LB film of PQ/ODA.

Fabrication, structure and conductivity of hybrid organic/inorganic Langmuir-Blodgett films of polyquinoline/octadecylamine/rare earth-substituted heteropolyanion

Three kinds of hybrid organic/inorganic Langmuir-Blodgett films are obtained by the compact organization of poly (1, 2-dihydro-2,2,4-trimethyl)quinoline (abridged as PQ), octadecylamine(abridged as OA) and rare earth-substituted heteropolyanions [abridged as RE(PW11,)(2), RE=Ce-II, Eu-II, Gd-II] using the Langmuir-Blodgett technique. They are characterized by the pi-A isotherms, the absorption spectra, the fluorescence spectra and the atomic force microscope. The scanning tunneling microscopy shows that the conductivity of the hybrid LB films is much better after heteropolyanions having been incorporated in the films.

Photochromic inorganic-organic multilayer films based on polyoxometalates and poly(ethylenimine)

Novel photochromic inorganic-organic multilayers composed of polyoxometalates and poly(ethylenimine) have been prepared by the layer-by-layer (LbL) self-assembly method. The growth process, composition, surface topography, and photochromic properties of the multilayer films were investigated by UV-visible and Fourier transform infrared spectroscopy, atomic force microscopy, electrospin resonance (ESR), and X-ray photoelectron spectroscopy (XPS). Irradiated with ultraviolet light, the transparent films changed from colorless to blue. Moreover, the blue films showed good reversibility of photochromism, and could recover the colorless state gradually in air, where oxygen plays an important role in the bleaching process. On account of the ESR and XPS results, parts of W6+ in multilayers were reduced to W5+, which exhibited a characteristic blue; a possible photochromic mechanism can be speculated. This work provides basic guideline for the assembly of multilayers with photochromic properties.

Highly efficient green phosphorescent organic light-emitting diodes based on Cu-I complexes

Mononuclear Cu-I complexes with mixed ligands are used to fabricate green phosphorescent organic light-emitting diodes. The electroluminescence (EL) maximum at 524 nm coincides well with its photoluminescent (PL) spectrum in poly(methyl methacrylate) film (see Figure). A maximum current efficiency of 10.5 cd A(-1) at 105 cd m(-2) and a maximum brightness up to 1663 cd m(-2) are

Novel hole-transporting materials based on 1,4-bis(carbazolyl)benzene for organic light-emitting devices

Novel hole-transporting molecules containing 1,4-bis(carbazolyl)benzene as a central unit and different numbers of diphenylamine moieties as the peripheral groups have been synthesized and characterized. These compounds are thermally stable with high glass transition temperatures of 141-157 degreesC and exhibit chemically reversible redox processes. Their amorphous state stability and hole transport properties can be significantly improved by increasing the number of diphenylamine moieties in the outer part and by controlling the symmetry of the carbazole-based molecules. These compounds can be used as good hole-tran sporting materials for organic electroluminescent (EL) devices. The device performance based on tri- and tetra-substituted carbazole derivatives is comparable to that of a typical 4,4'-bis[N-(1-naphthyl)-N-phenylamino] biphenyl (NPB)-based device.

Preparation and the investigation of its electrochemical and photoluminescent characteristics of organic-inorganic hybrid multilayers containing europium-substituted heteropolytung state

Photoluminescent multilayers were fabricated by layer-by-layer deposition between europium-substituted heteropolytungstate K-13 [Eu(SiW11O39)(2)].28H(2)O (denoted ESW) and a cationic polymer of quaternized poly(4-vinylpyridine) partially complexed with osmium bis(2,2'-bipyridine) (denoted as QPVP-Os) on glassy carbon and quartz substrates. The resulting photoluminescent organic-inorganic hybrid multilayers were characterized by electrochemical impedance spectroscopy, UV-Vis absorption spectrometry, cyclic voltammetry and photoluminescence spectra. Electrochemical impedance spectroscopy, UV-Vis absorption spectrometry and cyclic voltammetry results demonstrated that the multilayers were regular growth each layer adsorption. The photoluminescent properties of the films at room temperature were investigated to show the characteristic Eu3+ emission pattern of D-5(0) --> (7) F-j.

Naphthyl end-capped quarterthiophene: A simple organic semiconductor with high mobility and air stability

An organic semiconductor that can be mass produced is synthesized by end-capping quaterthiophene with naphthyl units (NaT4). An organic thin-film transistor (OTFT, see figure) has been fabricated using this organic semiconductor, and exhibits stability under ambient conditions with a mobility of up to 0.40 cm(2) V-1 s(-1).

Heterojunction ambipolar organic transistors fabricated by a two-step vacuum-deposition process

Ambipolar organic field-effect transistors (OFETs) are produced, based on organic heterojunctions fabricated by a two-step vacuum-deposition process. Copper phthalocyanine (CuPc) deposited at a high temperature (250 degrees C) acts as the first (p-type component) layer, and hexadecafluorophthalocyaninatocopper (F16CuPc) deposited at room temperature (25 degrees C) acts as the second (n-type component) layer. A heterojunction with an interpenetrating network is obtained as the active layer for the OFETs. These heterojunction devices display significant ambipolar charge transport with symmetric electron and hole mobilities of the order of 10(-4) cm(2) V-1 s(-1) in air. Conductive channels are at the interface between the F16CuPc and CuPc domains in the interpenetrating networks. Electrons are transported in the F16CuPc regions, and holes in the CuPc regions. The molecular arrangement in the heterojunction is well ordered, resulting in a balance of the two carrier densities responsible for the ambipolar electrical characteristics. The thin-film morphology of the organic heterojunction with its interpenetrating network structure can be controlled well by the vacuum-deposition process.

Application of ceramic carbon materials for solid-phase extraction of organic compounds

Ceramic carbon materials were developed as new sorbents for solid-phase extraction of organic compounds using chlorpromazine as a representative. The macroporosity and heterogeneity of ceramic carbon materials allow extracting a large amount of chlorpromazine over a short time. Thus, the highly sensitive and selective determination of chlorpromazine in urine sample was achieved by differential pulse voltammograms after only 1-min extraction. The total analysis time was less than 3 min. In comparison with other electrochemical and electrochemi-luminescent methods following 1-min extraction, the proposed method improved sensitivity by about 2 and 1 order of magnitude, respectively. The fast extraction, diversity, and conductivity of ceramic carbon materials make them promising sorbents for various solid-phase extractions, such as solid-phase microextraction, thin-film microextraction, and electrochemically controlled solidphase extraction. The preliminary applications of ceramic carbon materials in chromatography were also studied.