New fluorescent dipolar pyrazine derivatives for non-doped red organic light-emitting diodes

Dipolar fluorescent compounds containing electron-accepting pyrazine-2,3-dicarbonitrile and electron-donating arylamine moiety have been designed and synthesized. The optical and electrochemical properties of these compounds can be adjusted by changing pi-bridge length and the donor (D) strength. Organic light-emitting devices based on these compounds are fabricated. Saturated red emission of (0.67, 0.33) and the external quantum efficiency as high as 1.41% have been demonstrated for one of these compounds.

Dependence of performance of organic light-emitting devices on sheet resistance of indium-tin-oxide anodes

The dependence of the performance of organic light-emitting devices(OLEDs) on the sheet resistance of indium-tin-oxide(ITO) anodes was investigated by measuring the steady state current density brightness voltage characteristics and the electroluminescent spectra. The device with a higher sheet resistance anode shows a lower current density, a lower brightness level, and a higher operation voltage. The electroluminescence(EL) efficiencies of the devices with the same structure but different ITO anodes show more complicated differences. Furthermore, the shift of the light-emitting zone toward the anode was found when an anode with a higher sheet resistance was used. These performance differences are discussed and attributed to the reduction of hole injection and the increase in voltage drop over ITO anode with the increase in sheet resistance.

A wide-bandgap semiconducting polymer for ultraviolet and blue light emitting diodes

A novel wide-bandgap conjugated polymer (PDHFSCHD) consisting of alternating dihexylfluorene and rigidly twisted biphenyl units has been synthesized. The new fluorene-based copolymer composed of rigid twisting segments in the main-chain exhibits an optical bandgap of as high as 3.26 eV, and a highly efficient ultraviolet emission with peaks at 368 nm and 386 nm. An electroluminescence device from PDHFSCHD neat film as an active layer shows UV emission which peaks at 395 nm with a turn on voltage below 8 V By optimizing the device conditions, a peak EL quantum efficiency of 0.054% and brightness of 10 cd.m(-2) was obtained. Furthermore, blending a poly(dihexylfluorene) in the PDHFSCHD host gave pure blue emission peaking at 417 nm, and 440 nm without long wavelength emission from aggregated species. Efficient energy transfer from PDHFSCHD to PDHF was demonstrated in these blended systems. Depressed chain-aggregation of PDHF in the PDHFSCHD host can correspond to pure blue emission behaviors.

Novel PPV-based light-emitting copolymers containing triazole moiety

Two new light-emitting PPV-based copolymers bearing electron-withdrawing triazole unit in the main chain have been synthesized by Wittig reaction between triazole diphosphonium salt and the corresponding dialdehyde monomers, respectively. Their optical and physical properties are characterized by UV-vis, photoluminescence (PL), TGA and DSC. The resulting copolymers are highly soluble in common organic solvents and have high Tg and Td values. They show blue-greenish fluorescence in solution (lambda(max) 502 and 508 nm) and green fluorescence in the solid state (lambda(max) 520 and 526 nm), respectively.

Novel PPV-based light-emitting polymers containing siloxane moiety

Two PPV-based copolymers consisting siloxane linkage have been synthesized by melt condensation of bisphenol and dianilinodimethylsilane. The rigid PPV segments act as chromosphere and allow fine turning of band gap for blue-light emission, while the flexible siloxane units lead to the effective interruption of conjugation and the enhancement of solubility. The UV-vis absorption, photoluminescent and eletroluminescent properties have been studied.

Zinc-based light-emitting materials containing oxadiazole moieties

Two oxadiazole-based zinc complexes containing naphthalene moiety with different coordination site are synthesized and characterized. Their thermal stability, photoluminescent and electroluminescent properties are investigated. The resulting complexes have good thermal stability and show bright blue fluorescence in the solid state. Their electroluminscent wavelengths are dependent on the coordination site of naphthalene moieties.

Enhanced efficiency of light-emitting polymers through intra-molecular energy transfer

To enhance the photoluminescence and electroluminescence efficiency, light-emitting polymers with energy transferring chromophores including N,N,N'N'-phenylene-diamine, naphthalene-imide, oxadiazole, meta-phenylene vinylene are designed and synthesized.

Efficient red organic light-emitting devices based on a europium complex

An efficient organic light-emitting device using a trivalent europium (Eu) complex Eu(Tmphen)(TTA)(3) (TTA=thenoyltrifluoroacetone, Tmphen=3,4,7,8-tetramethyl-1,10-phenanthroline) as the dopant emitter was fabricated. The devices were a multilayer structure of indium tin oxide/N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl-4,4-diamine (40 nm)/ Eu complex:4,4-N,N-dicarbazole-biphenyl (1%, 30 nm)/2,9-dimethyl,4,7-diphenyl-1,10phenanthroline (20 nm)/AlQ (30 nm)/LiF (1 nm)/Al (100 nm). A pure red light with a peak of 612 nm and a half bandwidth of 3 nm, which is the characteristic emission of trivalent europium ion, was observed. The devices show the maximum luminance up to 800 cd/m(2), an external quantum efficiency of 4.3%, current efficiency of 4.7 cd/A, and power efficiency of 1.6 lm/W. At the brightness of 100 cd/m(2), the quantum efficiency reaches 2.2% (2.3 cd/A).

A dinuclear aluminum 8-hydroxyquinoline complex with high electron mobility for organic light-emitting diodes

A dinuclear aluminum 8-hydroxyquinoline complex (DAlq(3)) with improved electron mobility was designed for organic light-emitting diodes. The electron mobility in DAlq(3) was determined via transient electroluminescence (EL) from bilayer devices with structure of indium tin oxide (ITO)/N,N-'-di(naphthalene-1-yl)-N,N-'-diphenyl-benzidine (NPB)/DAlq(3)/Mg:Ag. It was found that the electron mobility in DAlq(3) is between 3.7-8.4x10(-6) cm(2)/Vs at electric fields ranging between 1.2x10(6) and 4.0x10(6) V/cm, which is a factor of two higher than that in Alq(3). The DAlq(3) also shows a higher EL efficiency of 2.2 cd/A (1.2 Lm/W), as compared to Alq(3) with an EL efficiency of 2.0 cd/A (1.0 Lm/W), which is attributed to more balanced electron and hole recombination due to the improved electron mobility of DAlq(3).

Polymer light-emitting diodes based on a bipolar transporting luminescent polymer

A soluble electroluminescent polymer containing hole-deficient triphenylamine and electron-deficient oxadiazole units in the main chains has been designed and studied. The design is based on the consideration that the triphenylamine group possesses good hole-transporting property and the oxadiazole unit is known to be of electron-transporting character. Because of the good bipolar transporting performance, the brightness and electroluminescent efficiency are significantly improved and the turn-on voltage is reduced compared with a similar polymer without the electron-deficient oxadiazole units in the main chains. For a device with configuration ITO/PEDOT/polymer/CsF/Al, a maximum brightness of 3600 cd m(-2) and a maximum luminescent efficiency of 0.65 cd A(-1) (quantum efficiency of 0.3%) were obtained in the polymer with oxadiazole units, about 15 times brighter and 15 times more efficient than the corresponding polymer without oxadiazole units.

Blue organic light-emitting diodes based on an oxadiazole-containing organic molecule exhibiting excited state intramolecular proton transfer

A blue organic light-emitting device based on an emissive layer of 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole (HOXD), which exhibits excited state intramolecular proton transfer (ESIPT), was presented. The device had a luminance efficiency of 0.8 cd/A and a maximum brightness of 870 cd/m(2). Our studies indicate that some EL may originate from the triplet excitation state of the enol form of HOXD.

Determination of molecular weight and molecular sizes of polymers by high temperature gel permeation chromatography with a static and dynamic laser light scattering detector

Flow-mode static and dynamic laser light scattering (SLS/DLS) studies of polymers, including polystyrene, polyethylene, polypropylene and poly(dimethylsiloxane) (PDMS), in 1,2,4-trichlorobenzene (TCB) at 150 degreesC were performed on a high temperature gel permeation chromatography (GPC) coupled with a SLS/DLS detector. Both absolute molecular weight (M) and molecular sizes (radius of gyration, R-g and hydrodynamic radius, R-h) of polymers eluting from the GPC columns were obtained simultaneously. The conformation of different polymers in TCB at 150 degreesC were discussed according to the scaling relationships between R-g, R-h and M and the rho-ratio (p = R-g/R-h). Flow-mode DLS results of PDMS were verified by batch-mode DLS study of the same sample. The presented technique was proved to be a convenient and quick method to study the shape and conformation of polymers in solution at high temperature. However, the flow-mode DLS was only applicable for high molecular weight polymers with a higher refractive index increment such as PDMS.

Novel indigo light emitting long-lasting phosphors CdSiO3: RE3+ (RE = Y, La, Gd, Lu)

A series of novel indigo light emitting long-lasting phosphors CdSiO3: RE3+ (RE = Y, La, Gd, Lu) was prepared by the conventional high-temperature solid-state method. The XRD, photoluminescence (PL) spectra and afterglow intensity decay were used to characterize the synthesized phosphors. These phosphors emitted indigo light and showed long-lasting phosphorescence. The phosphorescence can be seen with the naked eye in the dark clearly even after the 254-nm UV irradiation have been removed for more than 30 min.

Liquid crystal alignment generated by linearly polarized UV light on photoactive low molecular mass compounds

Alignment films prepared from low molar mass photo-crosslinkable materials containing the cinnamate group can be used for aligning LCs after irradiating the films with linearly polarized UV light. The high contrast observed in the polarizing optical microscope between dark and bright images indicates that the alignment is quite uniform. As the photoreaction progresses. the average roughness of the films is increased. All the aggregate structures, 'lamellar crystals'. produced by the photo-crosslinking reaction are of a square shape.