We have synthesized four pyrene-derived blue emitting materials using Suzuki cross coupling reactions. All OLED devices using these materials as emitting materials showed efficient blue electroluminescence (EL). Particularly, a device using 1,1'-(9,9-dimethyl-9H-fluorene-2,7-diyl)bis-pyrene (1) showed best EL properties with the luminous efficiency of 4.32 cd/A, the power efficiency of 3.98 lm/W and the external quantum efficiency of 2.48% at 500 cd/m2.

Compared with other materials, zinc oxide (ZnO) exhibits stability in air, high-electron mobility, transparency and low light sensitivity. We investigated these properties in ZnO thin-film transistors (TFTs) containing a cross-linked poly(vinyl alcohol) (C-PVA) (1:3) buffer layer stacked between the semiconductor and gate dielectric. We measured the impact of this C-PVA layer on gate bias stress. We measured the transfer characteristics of the saturation region to determine the threshold voltage and the field-effect mobility of the transistors. We recorded a threshold voltage of 11.53 V in the ZnO TFTs with the C-PVA buffer layer, the field-effect mobility was 0.2 cm2/Vs. There was a positive shift in the threshold voltage of deltaV(TH) approximately 10 V in response to the application of a gate bias stress of 20 V. The positive shift in the threshold voltage was lower than that in pristine ZnO TFTs. This finding suggests that the shift in threshold voltage was due to reduced charge trapping at the semiconductor-gate dielectric interface. Our report indicates that the organic buffer layer enhanced the stability of ZnO TFTs.

The electrical and optical properties of blue organic light-emitting devices (OLEDs) containing a mixed host emitting layer (EML) consisting of a 1,3-bis(carbazole-9-yl)benzene (mCP) layer and a 3-tert-butyl-9,10-di(naphtha-2-yl)anthracene (TBADN) layer were investigated. The driving voltage of the OLEDs with a mixed host EML was smaller than that of the OLEDs with a single EML. The electroluminescence spectra for OLEDs containing a mixed host EML showed a dominant peak related to the mCP or the TBADN layer. The color coordinates of the OLEDs containing a 5% TBADN-doped mCP EML were (0.146, 0.091), indicative of the deep blue color coordinates.

We demonstrated that the efficiency of green organic light-emitting diodes (OLEDs) was improved by co-doping with hole transport material, N'-bis-(1-naphyl)-N,N'-dipheny1-1,1'-biphenyl-4,4 '-diamine (NPB) and electron transport material, bis(2-methy1-8-quninolinato)-4phenylphenolate aluminum (BAlq), into emitting layer. Co doping with NPB and BAlq can improve charge balance which makes it possible to enhance device efficiency. The maximum luminous and quantum efficiency of BDAT-P doped device was measured to be 5.56 cd/A and 1.99 %, respectively. Device co-doped with NPB, BAlq and BDAT-P resulted in the most efficient device, in which maximum luminous and quantum efficiency were 8.36 cd/A and 3.19 %, respectively.

The electrical and optical properties of organic light-emitting devices (OLEDs) with three periods of 4,7-diphenyl-1,10-phenanthroline (BPhen)/aluminum tris(8-hydroxyquinolate) (Alq(3)) multiple heterostructures acting as an electron transport layer (ETL) were investigated. While the leakage current of OLEDs with multiple heterostructures was smaller than that of OLEDs without multiple heterostructures, the luminance efficiency was larger than that of OLEDs without multiple heterostructures. The BPhen layers in the multiple heterostructures blocked holes from the emitting layer (EML) to the ETL, and they enhanced the electron injection from the cathode to the EML, resulting in an increase in the luminance efficiency.

White organic light-emitting diodes (WOLEDs) have drawn increasing attention due to their potential use in various applications such as solid-state lighting and backlight of liquid crystal displays and full-color OLEDs of red, green, and blue pixel. N,N'-dicabazolyl-3,5-benzene (mCP), the host material, was co-doped with Iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2']-picolinate (FIrpic), which functions not only as phosphorescent sensitizer but also blue emitter, and (2Z,2'Z)-3,3'-[4,4"-bis (dimethylamino)-1,1':4',1"-terphenyl-2',5'-diyl]bis (2-phenylacrylonitrile) (ABCV-P), which is a red fluorescent material. The fabricated device structures were as follows: (device A) Indium tin oxide (ITO)/N,N'-bis-(1-naphyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB)/(mCP)/mCP:ABCV-P (1%)/4,7-diphenyl-1,10-phenanthroline (Bphen)/lithium quinolate (Liq)/aluminum (Al), (device B) ITO/NPB/mCP/mCP:FIrpic (8%)/Bphen/Liq/Al and (device C) ITO/NPB/mCP/mCP:FIrpic:ABCV-P (8%, 1%)/Bphen/Liq/Al, respectively. Phosphorescent FIrpic harvesting both singlet and triplet excitions not only emitted blue light but also transferred energy to fluorescent ABCV-P. The maximum luminance efficiency, external quantum efficiency, and luminance of white light device were measured to be 5.95 cd/A, 2.45% and 2500 cd/m2, respectively. The white device gave practically white light with the Commision Internationale de l'Eclairage (CIE(xy)) coordinate of (0.44, 0.49) which was close to warm white color (CIE(xy) = 0.45, 0.45).

Electron injection mechanisms of the luminance efficiency of green organic light-emitting devices (OLEDs) fabricated utilizing a cesium carbonate (Cs2CO3)/fullerene (C60) heterostructure acting as an electron injection layer (EIL) were investigated. Current density–voltage and luminance–voltage measurements showed that the current densities and the luminances of the OLEDs with a Cs2CO3 or Cs2CO3/C60 EIL were higher than that of the OLEDs with a Liq EIL. The luminance efficiency of the OLEDs with a Cs2CO3 EIL was almost three times higher than that of the OLEDs with a Liq EIL. Because the electron injection efficiency of the Cs2CO3 layer in OLEDs was different from that of the C60 layer, the luminance efficiency of the OLEDs with a double EIL consisting of a Cs2CO3 layer and a C60 layer was smaller than that of the OLEDs with a Cs2CO3 EIL. The electron injection mechanisms of OLEDs with a Cs2CO3 and C60 double EIL are described on the basis of the experimental results.

The luminance efficiency of the blue organic light-emitting devices (OLEDs) fabricated utilizing a double emitting layer (DEML) with a 4,4'-Bis(2,2-diphenyl-ethen-1-yl)diphenyl (DPVBi) layer doped with 4,4'-Bis[4-(diphenylamino)styryl]biphenyl (BDAVBi) fluorescence dopant and a 4,4'-Bis(carbazol-9-yl)biphenyl (CBP) layer doped with a bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium III (FIrpic) phosphorescence dopant at 20mA/cm(2) was 6.2 cd/A, indicative of highly efficient OLEDs. Electroluminescence spectra for the OLEDs with a DEML showed that a dominant peak at 469nm corresponding to the BDAVBi doped DPVBi layer together with a shoulder at 491nm related to the combination of the BDAVBi doped DPVBi layer and the FIrpic doped CBP layer appeared.

Electrical and optical properties of blue organic light-emitting devices (OLEDs) with doped or undoped emitting layers (EMLs) consisting of a heterostructure layer or a single layer were investigated. The driving voltage of the OLEDs with an undoped heterostructure multiple EML was lower than those of both OLEDs with a single EML and a doped heterostructure multiple EML. The electroluminescence spectra of the OLEDs with an undoped and a doped heterostructure EMLs showed dominant peaks related to two heterostructure layers.

The luminance efficiency of the blue organic light-emitting devices (OLEDs) with a mixed host emitting layer (EML) consisting of a 2-t-butyl-9,10-di-2-naphthylanthracene (TBADN) and 4,4′-Bis[4-(diphenylamino)styryl]biphenyl (BDAVBi)-doped 1,3-bis(carbazol-9-yl)benzene (mCP) layer at 20 mA/cm2 was 5.78 cd/A, indicative of highly efficient OLEDs. Electroluminescence spectra for the OLEDs with a mixed host EML showed that a dominant peak at 467 nm corresponding to the BDAVBi doped mCP layer together with a shoulder at 491 nm related to the combination of the TBADN doped mCP layer and the BDAVBi doped mCP layer appeared.

While the hole transport decreased in the mixed layer in the organic light-emitting devices (OLEDs) with an Al:lithium quinolate (Liq) mixed layers in the electron transport layer (ETL), the electron injection increased the Al:Liq mixed layer. The enhancement of luminance efficiency in the OLEDs with an Al:Liq mixed layer in the ETL originated from the more balance between the electrons and the holes in the emitting layer due to the decrease in the hole injection and the increase in the electron injection resulting from the existence of the Al:Liq mixed layer.

While the current density of the organic light-emitting devices (OLEDs) with 5,6,11,12-tetraphenylnaphthacene (rubrene) layers in both the hole transport layer (HTL) and the emitting layer (EML) was lower than those of the OLEDs without a rubrene layer or with a rubrene layer in only a HTL or an EML, the luminance efficiency of the OLEDs with rubrene layers in both the HTL and the EML were significantly enhanced. While the rubrene layer in the HTL acting as a hole trap decreased the hole mobility of the HTL, the rubrene layer in the EML acting as an emitting site increased the luminance.

A red fluorescent material (2E,2'E)-3,3'-[4,4"-bis(dimethylamino)-1,1': 4',1 "-terphenyl-2',5'-diyl]bis[2-(2-thienyl)acrylonitrile] (ABCV-Th) was synthesized for use in organic light emitting diodes (OLEDs) as the host emissive material. It has been reported some green and blue host emissive materials used in OLEDs revealed high device performance but, owing to concentration quenching, comparable red light emitting materials are still rare in OLEDs application. Non-doped organic light emitting diodes, with the structure of ITO/NPB/ABCV-Th (30 nm and 50 nm)/BCP/Alq3/Liq/Al were fabricated using ABCV-Th as the host emitter. The peak wavelength and full width at half maximum (FWHM) of electroluminescence (EL) were 629.5 nm and 68.5 nm, respectively. The maximum brightness and turn on voltage of the device were measured to be 1330 cd/m2 at 14.6 V and 3.4 V, respectively. The device exhibited authentic red emission (Commission Internationale De L'Eclairage (CIE(xy)) = 0.65, 0.34) which is almost close to the standard red (CIE(xy) = 0.67, 0.33) demanded by the national television system committee (NTSC).

While the current densities of hole only devices with a 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F-4-TCNQ) doped N,N'-bis-(1-naphthyl)- N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) hole transport layer (HTL) slightly changed with increasing F-4-TCNQ doping concentration, those of hole only devices with a F-4-TCNQ doped 4,4',4 ''-tris(N-(2-naphthyl)-Nphenylamino)triphenylamine (2-TNATA) HTL significantly increased. The hole injection and hole transport of hole only devices were enhanced by inserting an ultra thin F-4-TCNQ layer between an indium-tin-oxide layer and a NPB HTL or a 2-TNATA HTL, regardless of the HTL materials. These results indicate that the hole injection and hole transport in OLEDs utilizing a F-4-TCNQ doped HTL or a F-4-TCNQ thin layer is enhanced.

Most organic light-emitting diodes (OLEDs) have a multilayer structure composed of organic layers such as a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL) and an electron injection layer (EIL) sandwiched between two electrodes. The organic layers are thin solid films with a thickness from a few nano meters to a few tenths nano meter, respectively. Surface morphology of an organic thin solid film in OLEDs depends on the molecular structure of the organic material and has an affect on device performance. To analyze the effect of surface morphology of an organic thin solid film on fluorescence and electroluminescence (EL) properties, thin solid films of 4-(dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4H-pyran (DCM2) and new red fluorophores, (2E,2′E)-3,3′-[4,4″-bis(dimethylamino)-1,1′:4′,1″-terphenyl-2′,5′-diyl]bis[2-(2-thienyl)acrylonitrile] (ABCV-Th) and (2Z,2′Z)-3,3′-[4,4″-bis(dimethylamino)-1,1′:4′,1″-terphenyl-2′,5′-diyl]bis(2-phenylacrylonitrile) (ABCV-P) were investigated by atomic force microscopy (AFM). The samples for EL and AFM measurement were fabricated by the high-vacuum thermal deposition (8×10−7Torr) of organic materials onto the surface of indium tin oxide (ITO)-coated glass substrate, in which the layer structures of samples for AFM measurement and those for EL measurement were ITO/NPB (40nm)/red emitters (80nm) and ITO/NPB (40nm)/red emitters (80nm)/BCP (30nm)/Liq (2nm)/Al (100nm), respectively. The analysis based on AFM measurements well supported that the photoluminescence properties and the device performance were very much dependent upon surface morphology of an organic thin layer.

The influence of the presence of a double emitting layer (DEL) consisting of 4,4'-Bis(carbazol-9-yl)bipheny1 (CBP) and 4,4'-Bis(2,2-diphenyl-ethen-1-yl)diphenyl (DPVBi) acting as electron and hole trapping layers in the organic light-emitting devices (OLEDs) was studied. While the luminance efficiency of the OLEDs with a DEL was very stable, regardless of variations in the applied voltage, that of the OLEDs with a CBP or a DPVBi emitting layer varied with the applied voltage. The rate of decrease is smaller than the single emitting layer, about 30%. The dominant peak corresponding to the CBP layer, in the electroluminescence spectrum for the OLEDs with a DEL appeared at 451 nm, which is in the deep-blue region. The Commission Internationale de l'Eclairage chromaticity coordinates of the OLEDs with a DEL at 11 V were (0.150, 0.137), indicative of a deep, stabilized blue color. These results indicate that luminance efficiency-stabilized blue OLEDs can be fabricated using a CBP/DPVBi DEL acting as electron and hole trapping layers. (C) 2007 Elsevier B.V. All rights reserved.

The light extraction efficiencies of organic light emitting diodes (OLEDs) utilizing various kinds of porous alumina films with different pore diameters were investigated. The OLEDs with the porous alumina film deposited on the glass surface were fabricated to improve their light extraction efficiency. The porous alumina film was fabricated by using a two step anodizing electrochemical procedure. The current densities as functions of the applied voltage do not significantly change, regardless of the existence and the magnitude of the pore diameter in the porous alumina film. The luminance efficiency of the OLEDs increased with increasing pore diameter. The luminance efficiency of the OLEDs utilizing the porous alumina film with a pore diameter of 70 nm was enhanced approximately 9% in comparison with that of the OLEDs without the porous alumina film. These results indicate that highly efficient OLEDs can be fabricated using a porous alumina film with an optimum pore diameter. (c) 2007 Elsevier B.V. All rights reserved.

The degradation behaviors of the electrical and the optical properties of organic light-emitting devices (OLEDs) fabricated with an emitting layer (EML) doped with or without a wide-bandgap-impurity were investigated. The OLEDs with a wide-bandgap-doped Alq3 EML were more stable than those with an undoped Alq3 EML. The existence of the doped wide-bandgap-impurity in the EML decreased the trap-charge density in the EML, resulting in an increase in the number of electrons in the Alq3 EML. That increases in the number of electron in the Alq3 EML for the OLEDs with a wide-bandgap-impurity decreased the staying time of the holes in the Alq3 EML, resulting in an enhanced lifetime for the OLEDs. These results indicate that OLEDs with a wide-bandgap-impurity-doped EML hold promise for potential applications in long-lifetime OLED displays.

The optical properties of organic light-emitting devices (OLEDs) with multiple heterostructures consisting of N, N'-bis-(1-naphthyl)-N, N'-diphenyl-1,1'-biphenyl-4,4'-diamine(NPB)15,6,11,12-tetraphenylnaphthacene (rubrene) emitting layers were investigated. The optical properties of the OLEDS were significantly affected by the number of heterostructures. The electroluminescence intensity corresponding to the rubrene peak for the OLEDs increased with increasing driving voltage and with increasing number of heterostructures. The Commission Internationale de I'Eclairage (CIE) chromaticity coordinates became stabilized with increasing the number of heterostructures, and the coordinates maintained almost constant, regardless of the driving voltage. The CIE chromaticity coordinate of the OLEDs with 5-periods of the heterostructures was (0.37, 0.54), indicative of a yellow color.

The electrical and the optical properties of organic light-emitting devices (OLEDs) with a mixed layer acting as a hole transport layer and as an emitting layer/electron transport layer were investigated. The OLEDs with a mixed layer showed the highest efficiency, and the emitting color of the OLEDs was pure yellow. The enhancement of the luminous efficiency in the OLEDs with a mixed layer was attributed to a decrease in hole mobility.