White electrolumineseence from a single-polymer system with simultaneous two-color emission: Polyfluorene as the blue host and a 2,1,3-benzothiadiazole derivative as the orange dopant on the main chain

New single-polymer electroluminescent systems containing two individual emission species - polyfluorenes as a blue host and 2,1,3-benzothiadiazole derivative units as an orange dopant on the main chain - have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue(lambda(max) = 421 nm/445 nm) and orange emission (lambda(max) = 564 nm)from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light-emitting diodes (PLEDs) based on the single-polymer systems has been investigated. The introduction of the highly efficient 4,7-bis(4-(N-phenyl-N-(4-methylphenyl)amino)phenyl)-2,1,3-benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single-layer device fabricated in air (indium tin oxide/poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure-white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m(-2), luminance efficiency of 7.30 cd A(-1), and power efficiency of 3.34 lm W-1 can be obtained.

Highly efficient pure-white-light-emitting diodes from a single polymer: Polyfluorene with naphthalimide moieties

Light-emitting diodes exhibiting efficient pure-white-light electroluminescence have been successfully developed by using a single polymer: polyfluorene derivatives with 1,8-naphthalimide chromophores chemically doped onto the polyfluorene backbones. By adjusting the emission wavelength of the 1,8-naphthalimide components and optimizing the relative content of 1,8-naphthalimide derivatives in the resulting polymers, white-light electroluminescence from a single polymer, as opposed to a polymer blend, has been obtained in a device with a configuration of indium tin oxide/poly(3,4-ethyleiledioxythiophene)(50 nm)/polymer(80 nm)/Ca(10 nm)/Al(100 nm). The device exhibits Commission Internationale de I'Eclairage coordinates of (0.32,0.36), a maximum brightness of 11900 cd m(-2), a current efficiency of 3.8 cd A(-1), a power efficiency of 2.0 lm W-1. an external quantum efficiency of 1.50 %, and quite stable color coordinates at different driving voltages, even at high luminances of over 5000 cd m(-2).

Negative differential resistance effect in organic devices based on an anthracene derivative

The authors observed a negative differential resistance (NDR) in organic devices consisting of 9,10-bis-(9,9-diphenyl-9H-fluoren-2-yl)-anthracene (DPFA) sandwiched between Ag and indium tin oxide electrodes. The large NDR shown in current-voltage characteristics is reproducible, resulting in that the organic devices can be electrically switched between a high conductance state (on state) and a low conductance state (off state). It can be found that the currents at both on to off states are space-charge limited and attributed to the electron traps at the Ag/DPFA interface. The large and reproducible NDR makes the devices of tremendous potential in low power memory and logic circuits.

Blue light-emitting polymer with polyfluorene as the host and highly fluorescent 4-dimethylamino-1,8-naphthalimide as the dopant in the sidechain

The dopant/host concept, which is an efficient approach to enhance the electroluminescence (EL) efficiency and stability for organic light-emitting diodes (OLEDs) devices, has been applied to design efficient and stable blue light-emitting polymers. By covalently attaching 0.2 mol % highly fluorescent 4-dimethylamino-1,8-naphthalimide (DMAN) unit (photoluminescence quantum efficiency: Phi(PL)=0.84) to the pendant chain of polyfluorene, an efficient and colorfast blue light-emitting polymer with a dopant/host system and a molecular dispersion feature was developed. The single-layer device (indium tin oxide/PEDOT/polymer/Ca/Al) exhibited the maximum luminance efficiency of 6.85 cd/A and maximum power efficiency of 5.38 lm/W with the CIE coordinates of (0.15, 0.19). Moreover, no undesired long-wavelength green emission was observed in the EL spectra when the device was thermal annealed in air at 180 degrees C for 1 h before cathode deposition. These significant improvements in both efficiency and color stability are due to the charge trapping and energy transfer from polyfluorene host to highly fluorescent DMAN dopant in the molecular level.

Modification of electrode surface through electrospinning followed by self-assembly multilayer film of polyoxometalate and its photochromic

Electrospun poly (vinyl alcohol) (PVA) nanofibers mat was collected on indium tin oxide (ITO) substrate. Heat crosslinked nanofibers mat became water-insoluble and firmly fixed on ITO substrate even in water. Oppositely charged poly (allylamine hydrochloride) (PAH) and Dawson-type polyoxometalate (POM), Na6P2Mo18O62 (P2Mo18), were alternately assembled on PVA nanofibers-coated ITO substrate to construct multilayer film through an electrostatic layer-by-layer (LBL) technique. The scanning electron microscope (SEM) images showed that P2Mo18 multilayer film was selectively deposited on PVA nanofibers while the unoccupied space by nanofibers on bare ITO was acted as substrate at the same time because the electrospun nanofibers have larger surface area and surface energy than the flat substrate. The cyclic voltammograms current responses of the P2Mo18 multilayer film on PVA/ITO electrode showed three well-defined redox couples of P2Mo18, but very small because P2Mo18 multilayer film was selectively deposited on PVA nanofibers with poor conductivity. In addition, the photochromic behavior of P2Mo18 multilayer film on PVA/ITO was investigated through UV-vis spectra and electron spin resonance (ESR). Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) proved that the charge-transfer complex was formed between PAH and P2Mo18 after UV irradiation.

Phenylene vinylene-based electroluminescent polymers with electron transport block in the main chain

We report a new route for the design of soluble phenylene vinylene (PV) based electroluminescent polymers bearing electron-deficient oxadizole (OXD) and triazole (TZ) moieties in the main chains with the aryloxy linkage. Both series of the PV-based polymers were prepared by Wittig reaction. By properly adjusting the OXD and/or TZ content through copolymerization, we can achieve an enhanced balance of hole- and electron injections, such that the device efficiency is significantly improved. Light-emitting diodes fabricated from P1, P2, P3, P4, P5, P6, and P7 with the configuration of Indium-Tin Oxide (ITO)/Poly (styrene sulfonic acid) doped poly (ethylenedioxythiophene) (PEDOT)/polymer/Ca/Al, emit bright green light with the maximum peak around 500 nm. For the device using the optimal polymer (P4) as emitting layer, a maximum brightness of 1300 cd/m(2) at 20 V and a maximum luminance efficiency of 0.325 cd/A can be obtained.

White electroluminescence from a poly(N-vinylcarbazole) layer doped with CdSe/CdS core-shell quantum dots

Hybrid organic/inorganic white light-emitting diodes (LEDs) were fabricated of semiconductor polymer poly(N-vinylcarbazole) (PVK) doped with CdSe/CdS core-shell semiconductor quantum dots (QDs). The device, with a structure of indium-tin-oxide (ITO)vertical bar 3,4-polyethylene-dioxythiophene- polystyrene sulfonate (PEDOT:PSS)vertical bar PVK:CdSe/CdS vertical bar Al, emitted a pure white light spanning the whole visible region from 400 to 800 nm. The Commission Internationale del'Eclairage coordinates (CIE) remained at x = 0.33, y = 0.34 at wide applied voltages. The maximum brightness and electroluminescence (EL) efficiency reached 180 cd m(-2) at 19 V and 0.21 cd A(-1) at current density of 2 mA cm(-2), respectively. The realization of the pure white light emission is attributed to the incomplete energy and charge transfer from PVK to CdSe/CdS core-shell QDs.

High efficiency red organic light-emitting diodes based on microcavity structure

We demonstrate high efficiency red organic light-emitting diodes (OLEDs) based on a planar microcavity comprised of a dielectric mirror and a metal Mirror. The microcavity devices emitted red light at a peak wavelength of 610 nm with a full width at half maximum (FWHM) of 25 nm in the forward direction, and an enhancement of about 1.3 factor in electroluminescent (EL) efficiency has been experimentally achieved with respect to the conventional noncavity devices. For microcavity devices with the structure of distributed Bragg reflectors (DBR)/indium-tin-oxide(ITO)/V2O5/N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine(NPB)/4-(dicy-anome-thylene)-2-t-butyl-6(1,1,7,7-tetrame-thyljulolidyl-9-enyl)-4H-pyran(DCJTB):tris(8-hydroxyquinoline) aluminium (Alq(3))/Alq(3)/LiF/Al, the maximum brightness arrived at 37000 cd/m(2) at a current density of 460.0 mA/cm(2), and the current efficiency and power efficiency reach 13.7 cd/A at a current density of 0.23 mA/cm(2) and 13.3 lm/W respectively.

NaCl/Ca/Al as an efficient cathode in organic light-emitting devices

An efficient cathode NaCl/Ca/Al used to improve the performance of organic light-emitting devices (OLEDs) was reported. Standard NM-bis(1-naphthyl)-NAP-diphenyl-1,1' biphenyl 4,4'-dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq(3)) devices with NaCl/Ca/Al cathode showed dramatically enhanced electroluminescent (EL) efficiency. A power efficiency of 4.6 lm/W was obtained for OLEDs with 2 nm of NaCl and 10 nm of Ca, which is much higher than 2.0 lm/W, 3.1 lm/W, 2.1 lm/ W and 3.6 lm/W in devices using, respectively, the LiF (1 nm)/Al, LiF (1 nm)/Ca (10 nm)/Al, Ca (10 nm)/Al and NaCl (2 nm)/ Al cathodes. The investigation of the electron injection in electron-only devices indicates that the utilization of the NaCl/Ca/Al cathode substantially enhances the electron injection current, which in case of OLEDs leads to the improvement of the brightness and efficiency.

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).

Green electroluminescent device with a terbium beta-diketonate complex as emissive center

In this study, a terbium complex, Tb(acac)(3)bath (acac: acetylacetone, bath: 4,7-diphenyl-1,10-phenanthroline), was synthesized and its luminescent properties were investigated compared with the reported terbium complex, Tb(acac)(3)phen (phen: phenanthroline). When it was used as an emitting material in organic electroluminescent (EL) device, the triple-layer-type device with a structure of glass substrate/ITO (indium-tin oxide)/TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine)/Tb(acac)(3)bath/Alq(3) (tris (8-hydroxyquinolinato) aluminum)/Al (aluminum) exhibited bright characteristic emission of terbium ion upon applying DC voltage. An apparent difference was observed between the photoluminescence spectrum and the EL spectrum. The EL device exhibited some characteristics of diode and the maximum luminance of 77 cd/m(2) was obtained at 17 V.

A novel terbium (III) beta-diketonate complex as thin film for optical device application

A novel terbium complex, Tb(acac)(3)AAP (acac: acetylacetone, AAP: 4-amino-antipyrine), was synthesized and its luminescent properties were studied. When it was used as an emitting center, triple-layer-type device with a structure of glass substrate/ITO (indium-tin oxide)/TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine)./Tb(acac)(3)AAP/PBD (2-(4-biphenyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole) or Alq(3) (tris(8-hydroxyquinolinato) aluminum)/Al (aluminum) exhibited bright characteristic emission of terbium ion upon applying d.c. voltage. The maximum luminance of the device is 56 cd/m(2) at 19 V and the maximum luminance efficiency is 0.357 lm/W.

Synthesis of novel nitrogen- and sulfur-containing conjugated polymers used as hole-transporting materials for organic light-emitting diodes

Two series of highly soluble novel nitrogen- and sulfur-containing conjugated polymers were synthesized via an acid-induced self-polycondensation of functional monomers with methyl sulfinyl and aromatic groups. The well-defined structures of synthesized polymers were confirmed by their NMR and IR spectra. The highest occupied molecular orbital energy values for these materials, estimated by cyclic voltammetry, showed a broad range of values from about 5.0 to 5.2 eV used as hole-transport layers (HTL) in two-layer light-emitting diodes ITO/HTL/Alq(3)/Mg:Ag [ITO = indium tin oxide, and Alq(3) = tris(8-quinolinato) aluminum]. The typical turn-on voltage of these diodes was about 4-5 V. The maximum brightness of the device was about 3440 cd/m(2) at 20 V. The maximum efficiency was estimated to be 0.15 1m/W at 10 V.

A novel way to enhance electroluminescence performance based on soluble binary and ternary europium 1,1,1-trifluoroacetylacetonate

Five rare earth complexes (Gd(acae)(3), Gd(TFacaC)(3), Eu(acaC)(3), Eu(TFacaC)(3) and Eu(TFacaC)(3)bipy; acac, acetylacetone; TFacac, 1,1,1-trifluoroacetylacetone; bipy, 2,2'-bipyridyl) were synthesized. By comparing the phosphorescence spectra of Gd(acac)(3) and Gd(TFacac)(3) the effect of the replacement of hydrogen by fluorine was examined. Organic light-emitting devices (OLEDs) based on the corresponding europium complexes as emissive layers were also fabricated by the spin-coating method. The triple-layer-type device with the structure glass substrate/ITO (indium-tin oxide)/PVK [poly(N-vinylcarbazole)]/(PVKEu)-Eu-.(TFacac)(3)bipy:PBD[2-(4-bibipyyl)-5-(4-t-butylbipyl-1,3,4-oxadiazole)]/PBD/Al (aluminum) exhibits a brighter red luminescence than those devices with Eu(acac)(3) and Eu(TFacac)(3) complexes as emissive centers upon applying a d.c. voltage.