X. T. Tao

Efficient organic red electroluminescent device with narrow emission peak

We report a red fluorescent material of 3-(dicyanomethylene)-5,5-dimethyl-1(4-dimethylamino-styryl)cyclohexene (DCDDC) and its use as emitter or dopant in tris-(8-hydroxyquinoline) aluminum based electrolumiescent (EL) devices. The DCDDC was synthesized by a simple “one-pot” reaction and shows bright red photoluminescence with a peak wavelength at 650 nm. EL properties of double-layer light-emitting diodes of indium tin oxide (ITO)/poly(N)-vinylcarbazole (PVK):N,N′-diphenyl-N,N-bis(3-methylphenyl)1,1–biphenyl-4,4′-diamine (TPD)/DCDDC/Mg:Ag and ITO/PVK:TPD/DCDDC(1%):Alq3/Mg:Ag using DCDDC as red emitters or dopants were characterized. The device with 1% DCDDC-doped Alq3 as emitting layer has a turn-on voltage of 5 V, and the highest luminance of 5600 cd/m2 is obtained at 15 V. The EL efficiency of the device at 100 cd/m2 is in the range of 1.1 l m/W, and the highest EL efficiency is 1.6 l m/W. The devices emit in the red region, and the full width at half maximum of the EL spectra is 70 nm.

Lithium tetra-(8-hydroxy-quinolinato) boron for blue electroluminescent applications

We report the optical and electroluminescent properties of the organometallic complex lithium tetra-(8-hydroxy-quinolinato) boron (LiBq4). The complex was prepared by the reaction of lithium borohydride with 8-hydroxyquinoline and shows absorption cutoff wavelength of 410 nm and bright blue photoluminescence with a peak wavelength at 466 nm. The ionization potential of LiBq4 was determined to be ∼5.6 eV by a photoemission apparatus. Electroluminescence (EL) properties of three-layer light-emitting diodes (LED) of ITO/PVK:TPD/LiBq4/Alq3/Mg with LiBq4 as the active layer were characterized. The EL spectra of the three-layer device are similar to the PL spectrum of the single layer LiBq4 but with peak wavelength at 475 nm. The results indicate that boron complexes are potential blue emitters for LED applications.

Application of emulsion of dense carbon dioxide in electroplating solution with nonionic surfactants for nickel electroplating

Abstract An electroplating reaction using nickel was carried out in an emulsion of dense carbon dioxide (CO2) and an electroplating solution with nonionic surfactants. The current efficiency and electrical resistance values were measured as a function of CO2 volume fraction in the emulsion with three kinds of surfactants. These results show that dense CO2 beyond the critical point of CO2 is effective to form an emulsion for the electroplating reaction. The nonionic surfactant octa(ethylene oxide) dodecyl ether, two kinds of poly (ethylene oxide)-b-poly (propylene oxide) with molecular weights of 745 and 950, respectively, were employed for emulsification. Moreover, a hydrophilic CO2-philic balance of the surfactants strongly influences on the stability of the emulsion. On the basis of the electrical conductivity measurements, the emulsion was classified into a CO2 in water (C/W) type with the CO2 volume fraction ranging from 0 to 80%. Compared to electroplating from the only solution, higher quality nickel films have been prepared by this method. The nickel films that were produced have a higher uniformity, a smaller grain size (sub-100 nm), and a significantly higher Vickers hardness.

Metal complex polymer for second harmonic generation and electroluminescence applications

We report the second harmonic generation and electroluminescent (EL) properties of a soluble metal complex polyurethane (PU). The PU was prepared by the reaction of a zinc Schiff base with 4,4′-diphenylmethane-diisocyanate. The polymer film has been effectively poled under a corona field and its linear and nonlinear optical (NLO) properties were characterized. The results indicated that the NLO effects of the polymer are mainly originated in the distorted coordination tetragonals formed by the central zinc atoms and coordination atoms. The polymer shows strong photoluminescence under a ultraviolet-lamp illumination and can be used as a luminescent material for EL devices.We report the second harmonic generation and electroluminescent (EL) properties of a soluble metal complex polyurethane (PU). The PU was prepared by the reaction of a zinc Schiff base with 4,4′-diphenylmethane-diisocyanate. The polymer film has been effectively poled under a corona field and its linear and nonlinear optical (NLO) properties were characterized. The results indicated that the NLO effects of the polymer are mainly originated in the distorted coordination tetragonals formed by the central zinc atoms and coordination atoms. The polymer shows strong photoluminescence under a ultraviolet-lamp illumination and can be used as a luminescent material for EL devices.

Novel main-chain poly-carbazoles as hole and electron transport materials in polymer light-emitting diodes

We report the use of substituted main-chain poly-carbazoles as hole and electron transporting polymers for electroluminescence (EL) applications. The polymers are soluble in common organic solvents and high quality thin films are obtained by spin coating method. A single layer of hole or electron transfer devices with indium tin oxide anode and aluminum cathode shows current densities up to 40–50 mA/cm2, respectively, but no visible emission from either layer. Double-layer structures combining the hole and electron transfer polymers show strong emission originated from the electron transfer layer. The peak of double-layer EL spectrum is about 30 nm redshifted to the peak of corresponding photoluminescence spectrum, which may be due to the formation of an exciplex between the two layers.

Tetravalent tin complex with high electron affinity for electroluminescent applications

We report the synthesis and electroluminescent properties of the organometallic complex diphenyltin bis-hydroxyquinoline (Sph2q2). The complex was prepared by the reaction of diphenyltin dichloride with 8-hydroxyquinoline and shows an absorption cutoff wavelength of 460 nm and bright photoluminescence with a peak wavelength at 508 nm. The ionization potential of Sph2q2 was determined to be ∼6.0 eV by photoemission apparatus, and the electron affinity is estimated to be 3.3 eV. Sph2q2 was used as an emitting and electron injection layer in bilayer devices. The electroluminescence (EL) properties and charge injection properties of Sph2q2 are characterized. Sph2q2 is easily soluble in common organic solvents and further modification of its properties is possible. The results indicate that tetravalent tin complexes are potential emitting and charge injection materials for EL applications.

Phase-matched second-harmonic generation in poled polymers by the use of birefringence

Green light has been observed for the first time to the authors’ knowledge by bulk phase-matched second-harmonic generation from a stretched main chain polyurea. The polyurea was synthesized from 4,4’-diphenylmethane diisocyanate and 4,4’-methylene bis(cyclohexylamine). The spin-coated film has an initial positive birefringence. Drawing further increased the birefringence, and the film can be used for phase matching just as in biaxial single crystals. The drawn and poled polymer films belong to the 2mm point group. Three independent nonlinear-optic coefficients were determined. The type I phase-matching characteristics were calculated and confirmed by experiments. We demonstrate that a highly effective second-harmonic-generation device with a long optical path length can be obtained by use of bulk phase-matchable poled polymer.

A polymeric five-layer nonlinear optical waveguide with a large dimension tolerance and large overlap integral for mode conversion phase-matching

In order to produce a highly efficient second harmonic generation (SHG) device, phase-matching must be fulfilled. In this paper, we propose a novel polymeric five-layer nonlinear optical waveguide to improve both the fabrication tolerance and the overlap for mode conversion phase-matching. In the case of poled polymers, the introduction of the inversion of the nonlinear susceptibility structure which induces the high overlap integral value is very difficult. We propose a relatively simple technique to fabricate the inversion of the nonlinear susceptibility structure across the poled polymer waveguides based on a five-layer structure. The five-layer waveguide can be prepared by just pasting two poled polymer films deposited onto a substrate onto each other with a small gap consisting of silicon oil or air. The calculation results show that the phase-matching thickness can be satisfied in the range of 0.44 up to 0.57 μm, if the propagation length is 1 mm. This large dimension tolerance for phase-matching is at least 150 times larger than the fabrication tolerance of three-layer waveguides. The conversion efficiency can be improved also more than 6 orders of magnitude above that of three-layer waveguides due to their high overlap integral.

Novel phase-matching concept for polymer waveguides

The electric field induced dynamic phase-matching of SHG waveguide was first demonstrated using a main chain type NLO polymer. The optimum phase-matching thickness was controlled by applied an electric field to polymer waveguide. The effective phase-matching thickness variation induced by poling is about 0.025 micrometers which is 6 times larger than FWHM of phase-matching thickness in conventional slab waveguide. The efficient phase-matched SHG was observed from taperless slab waveguide. No decay of SH conversion efficiency is observed in the presence of electric field.

Luminescence of dye-doped polymer films induced by corona discharge

A type of luminescence was observed from fluorescent dye-doped polymer films by corona discharge excitation. The polymer film was deposited on a transparent electrode, and direct current corona discharge was caused in argon atmosphere using this electrode as the anode. Emission of three elemental colors, i.e., red, green, and blue, were obtained from polyvinilcarbazole films doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran, 3-(2′-Benzothiazolyl)7-diethylaminocoumarin, and 1,1,4,4-tetraphenyl-1,3-butadiene, respectively. The drive current required for the luminance of 10 cd/m2 was as low as 100 μA/cm2 and the maximum quantum efficiency was 9.95 cd/A, which is nearly four orders of magnitude higher than that of the common injection type electroluminescent cells with the same material. The origin of this high efficiency can be attributed to the efficient hole blocking by the gas phase and also by concurrent effects of different luminescence processes including electroluminescence b...