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Abstract: We demonstrate a general method for tuning the color performance of white organic light-emitting diodes (WOLEDs) by inserting 0.5 nm thick red emitting layer in different location of blue phosphorescent emitting layer. The Commission Internationale de L''Eclairage (CIE) coordinates of WOLEDs were dependent on the position of red emitting layer and they were correlated with recombination zone of the blue phosphorescent emitting layer. Red shift of white CIE was observed as the location of red emitting layer get close to recombination zone of blue emitting layer. In addition, CIE of WOLEDs was kept stable between 100 cd/m2 and 10,000 cd/m2. [Copyright &y& Elsevier]

Abstract: Quantum efficiency, driving voltage, color stability and recombination zone of organic white-light-emitting diodes (OWLEDs) with blue/orange/blue stacked emitting structure were correlated with host structure of emitting layer. A mixed host structure of 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) was used in orange emitting layer and host composition was critical to device performances of OWLEDs. TPBI host structure was better than other host structures in terms of quantum efficiency and color stability, while TCTA shows poor quantum efficiency in spite of good color stability. TCTA:TPBI mixed host structure showed better driving voltage than other host structures. In addition, recombination zone in blue/orange/blue stacked OWLEDs could be controlled by changing host structure in orange light-emitting layer. OWLEDs with TPBI host in orange emitting layer showed high quantum efficiency of 10.3% at 1000 cd/m2 with little change of Commission international De L’Eclairage coordinates of (0.32, 0.34) from 100 cd/m2 to 10,000 cd/m2. [Copyright &y& Elsevier]

Abstract: A deep blue phosphorescent organic light-emitting diode was developed using a diphenyldi(4-(9-carbazoly)lphenyl)silane(SiCa) host material and a tris((3,5-difluoro-4-cyanophenyl)pyridine) iridium(FCNIr) dopant material. Triplet bandgap of the SiCa was 3.0 eV, while triplet bandgap of the FCNIr was 2.8 eV. Light-emitting performances of blue devices were investigated by changing doping concentration of the FCNIr and a pure blue emission with a color coordinate of (0.15, 0.19) could be obtained. [Copyright &y& Elsevier]

Abstract: Relationships between the work function of indium tin oxide (ITO) and device performances of C60 modified organic light-emitting diodes were investigated. The work function of ITO was controlled by changing the degree of oxidation of ITO using ultraviolet/O3 treatment. High efficiency could be obtained in the C60 modified device with short ITO treatment time, while driving voltage was lowered in the C60 modified device with long ITO treatment time. [Copyright &y& Elsevier]

Abstract: Stacked organic light-emitting diodes (SOLEDs) with 30-nm nanoparticle (NP) interfacial layers were investigated. Zinc oxide (ZnO) was used as an interfacial layer between two green polymer (GP) layers. SOLEDs with NP interfacial layers had higher device efficiency than did a single-unit device due to the high probability of exciton recombination that originated from the Auger electron-assisted energy up-conversion process. Although the current density and luminance of SOLEDs with ZnO NP interfacial layers were smaller than those of the reference device, the efficiency was doubled because of the big band alignment difference and the large band gap between GP and ZnO NP interfacial layers, which induced more radiative-exciton recombination. [Copyright &y& Elsevier]

Abstract: Improved efficiency in green phosphorescent organic light-emitting diodes using a composite hole transport layer (HTL) with hole transport and exciton blocking function was investigated. Mixed layer of (4,4′-N,N′-dicarbazole)biphenyl (CBP) and N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (NPB) was used as a HTL and quantum efficiency could be enhanced from 4.3% to 8.1% at 100cd/m2. Best performances could be obtained in the device with 50% CBP and 50% NPB in the HTL. [Copyright &y& Elsevier]

Abstract: Device performances of phosphorescent organic light-emitting diodes (PHOLEDs) with triplet mixed host structure were correlated with dopant energy levels and mixed host composition. A mixed host of 4,4′,4″-tris-(N-carbazolyl)triphenylamine (TCTA) and spirobifluorene type host (PH1) was used and three different dopants with different bandgap were doped in the mixed host. Optimum quantum efficiency was obtained in TCTA:PH1 (50:50) mixed host device in the case of green dopants, while the optimum host composition for red and orange dopants was TCTA:PH1 (25:75). [Copyright &y& Elsevier]

Origin of efficiency roll-off in phosphorescent organic light-emitting diodes was investigated with triplet mixed host devices and stable devices with little efficiency roll-off was developed. Efficiency roll-off was significant in the device with narrow recombination zone (RZ) and charge leakage out of emitting layer at high luminance was critical to efficiency roll-off. Efficiency roll-off could be reduced in triplet mixed host device with broad RZ and little charge leakage at high driving voltage. Triplet mixed host devices with an exciton blocking layer showed a quantum efficiency over 90% of maximum quantum efficiency at a luminance of 20 000 cd/m2. [ABSTRACT FROM AUTHOR]

Color stability of white organic light-emitting diodes was improved by using a charge confining device structure, which can confine charges at the center of the emitting layer. A red phosphorescent emitting layer was sandwiched between blue phosphorescent emitting layers and there was little change of color coordinate from 100 to 10 000 cd/m2. [ABSTRACT FROM AUTHOR]

Device performances of green devices with cathode structure of lithium quinolate (Liq)/Ca/Al were investigated and electron injection mechanism was studied using ultraviolet photoelectron spectroscopy. Power efficiency could be improved by 70% by using Liq/Ca/Al cathode structure due to efficient electron injection, and interfacial energy barrier lowering by Liq/Ca/Al metal cathode was observed. [ABSTRACT FROM AUTHOR]

Green phosphorescent organic light-emitting diodes (PHOLEDs) with a triplet mixed host emitting layer were developed and device performances were studied by changing host materials in light-emitting layer. Power efficiency of green PHOLEDs could be improved from 12.7 to 29.1 lm/W by using triplet mixed host emitting layer. Combination of hole-transport-type host with good hole injection properties and electron-transport-type host with good electron injection properties was effective to get high efficiency in triplet mixed host devices. [ABSTRACT FROM AUTHOR]

Device performances of green phosphorescent organic light-emitting diodes using (4,4′-N,N′-dicarbazole)biphenyl (CBP) and N,N′-dicarbazolyl-3,5-benzene (mCP) as an exciton blocking layer were investigated. CBP and mCP were introduced between hole transport layer and emitting layer to block triplet exciton quenching and efficient hole transport to emitting layer. The efficiency of green devices could be improved by more than three times by using mCP exciton blocking layer. [ABSTRACT FROM AUTHOR]

Lifetime improvement of green phosphorescent organic light-emitting diodes by charge confinement inside an emitting layer was investigated. Excitons were confined within the emitting layer by using a charge confining structure with a high doping concentration at the center of the emitting layer. The lifetime of green devices could be improved by more than five times by confining the excitons at the center of the emitting layer. [ABSTRACT FROM AUTHOR]

Abstract: Light emission efficiency of red phosphorescent organic light-emitting diodes was improved by using a double doping structure in light emitting layer. Red devices with tris(2-phenylpyridine) iridium (Ir(ppy)3) as a co-dopant for tris(1-phenylisoquinoline) iridium (Ir(piq)3) showed higher efficiency than Ir(piq)3 device. Ir(ppy)3 acted as a sensitizer for energy transfer from (4,4′-N,N′-dicarbazole)biphenyl to Ir(piq)3 and enhanced luminance efficiency of Ir(piq)3 by 30%. [Copyright &y& Elsevier]

The effect of indium tin oxide (ITO) surface treatment on hole injection in organic light-emitting diode with C60 as a buffer layer on ITO was studied. Double surface dipole layer was induced on oxygen plasma treated ITO surface, while no dipole formation was observed on ITO without surface treatment. Interfacial energy barrier between ITO and hole transport layer was reduced by 0.4 eV by C60 modification on oxygen plasma treated ITO surface, while there was no change of interfacial energy barrier by C60 on ITO without surface treatment. [ABSTRACT FROM AUTHOR]

Abstract: The surface of a non-polar a-plane GaN light-emitting diode (LED) was intentionally damaged with a KOH wet etch to enhance the extraction of light. This roughening technique has been commonly applied to c-plane polar GaN LEDs to extract photons that would otherwise suffer from total internal refraction. We show that wet etching of the non-polar LED does create a textured surface that increases the light extraction efficiency; however, the mechanism of the etch is quite dissimilar to the etch mechanism observed for c-plane LEDs. In fact, the etch proceeds perpendicular to the a-plane surface along unstable N-face (000−1) plane with the Ga-face plane resistant to the etch. The photoluminescence intensity from a-plane non-polar LED after KOH-based wet etching was increased by 83% in our experiments. Therefore, surface roughening by KOH-based wet etch was found to be very effective to extract photons from a-plane non-polar GaN-based LEDs. [Copyright &y& Elsevier]

Abstract: Alkali earth selenide phosphor, SrSe:Eu2+ and Sr1−x Zn x Se:Eu2+, was prepared using a solid-state chemical reaction method. The synthesized phosphor displayed a broad excitation band, which is suitable for blue or near UV LED applications and emits green light (563nm). Zn2+ substitution for Sr2+ greatly enhances the green emission of the phosphors. The samples were also excited with various wavelength emitting diodes (410, 425, and 455nm) to assess their potential use as phosphors for white LED applications. The LED fabricated with the synthesized phosphor produced white light using blue LED with a CIE-1931 coordinate, T c and R a of (0.3199, 0.2777) 6499K and 49.7, respectively. [Copyright &y& Elsevier]

Abstract: Phosphor conversion white LED was developed using a 460nm InGaN LED chip as an excitation source and yellow emitting Ce-activated YAG as the phosphor. YAG:Ce phosphor was obtained through solid state reaction (SSR) at 1500°C, and spray pyrolysis (SP) at 900°C. The SSR and SP prepared YAG:Ce had strong excitation peaks at 460nm, and broad band emissions from 540nm to 580nm due to d–f transition of the Ce ion. The fabricated white LED using SSR-YAG:Ce and SP-YAG:Ce showed a luminous efficiency of 65.8lmW−1 with CRI of 65.1, and 92.1lmW−1 with CRI of 67.7 at 20mA, respectively. For high CRI values, CdSe/ZnSe nanoparticles emitting at 622nm were prepared via the wet chemical method. The high CRI values of the white LED were achieved by efficient energy transfer from YAG:Ce to CdSe/ZnSe nanoparticles in hybrid phosphor system. The luminous efficiency of the hybrid phosphor white LED was 77.1lmW−1 and this device showed an excellent CRI of 92.3 at 20mA. The CIE of the white LED was varied from (0.34, 0.31) to (0.35, 0.31) when the applied currents increased from 20 to 60mA. [ABSTRACT FROM AUTHOR]

Abstract: White light emitting diodes (LEDs) were fabricated using an InGaN 460nm blue emission LED chip as the excitation source and CdSe quantum dots dispersed in PMMA as the phosphor. CdSe quantum dots were synthesized by the wet chemical method using CdO and Selenium powder as precursors. The three different size, 2.9, 3.4 and 4.3nm in diameter, of CdSe quantum dots obtained using this method exhibited emission peaks at 555, 580 and 625nm, respectively with a quantum yield of 10–30%. Mixed phosphors containing different weight ratio of CdSe and PMMA (1:0.1, 1:1, 1:5 and 1:10 wt%) were deposited on the LED chip to investigate the effects of different weight ratios of CdSe and PMMA on the performance of the white LEDs. The fabricated white LEDs that contained CdSe and PMMA weight ratio at 1:10 showed the best performance and the CIE color coordinates varied less with different applied currents. The luminous efficiency of single phosphor (580nm CdSe) white LEDs was 5.62lm/W with a CRI of 15.7, whereas the luminous efficiency of dual phosphors (555, 625nm CdSe) white LEDs was 3.79lm/W with a CRI of 61.4 at 20mA. The CIE coordinates of single and dual phosphors white LEDs varied from (0.33, 0.28) to (0.29, 0.26) and from (0.39, 0.33) to (0.39, 0.32), respectively, when the working current ranged from 5 to 80mA. [Copyright &y& Elsevier]

Abstract: In this work we fabricated white LEDs using a blue InGaN LED precoated conjugated copolymer/quantum dots (QDs) composite (green-emitting Poly {(9,9-dioctyl-2,7-divinylenefluorenylene)-alto-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)}/red-emitting CdSe quantum dots) as a hybrid phosphor. The emission peak wavelengths of the blue and red emissions in the spectra were 462 and 618nm, respectively. The green-emitting polymer had two emission peaks at 515 and 543nm. The subpeak of the copolymer (515nm) was decreased by the absorption of QDs as the ratio of QDs increased, while the emission of the QD (618nm) increased. Therefore, changes in the CIE-1931 coordinate, color temperature (Tc) and color-rendering index (Ra) were dependent on the QDs to copolymer ratios. The white LED of the hybrid phosphor containing 20wt.% QDs had a luminous efficiency of 44.2lm/W at 20mA with a CIE-1931 coordinate, Tc and Ra of (0.3297, 0.3332), 5620K and 75.3, respectively. [Copyright &y& Elsevier]

Abstract: The flip-chip configuration is employed for the production of high-brightness GaN-based light emitting diodes to improve the extraction of heat. A lithographic approach based on a sacrificial SiO2 nanosphere etch mask was developed to enhance the external extraction of light from the sapphire substrate. Closed-packed arrays of SiO2 nanospheres were prepared by a simple solution-based method on the sapphire substrate. Subsequent dry-etching via inductively coupled plasma using a gas mixture of BCl3 and Cl2 transferred a pattern into the sapphire substrate with the lowest etching at the center of the SiO2 nanosphere. This process created an array of circular cones in the surface of the sapphire that were found to be effective in enhancing the light extraction efficiency through multi-photon scatterings. Room temperature photoluminescence exhibited an increase of 22.5% in intensity after the surface of sapphire was textured. [Copyright &y& Elsevier]

Abstract: In this study, alkali earth thio-selenide phosphor, SrSxSe1−x:Eu2+ , was prepared using the conventional solid state reaction method. The synthesized phosphor displayed a broad excitation band that was suitable for blue or near UV LED applications and emitted light at various wavelengths (563–605nm) with different chemical compositions. Like the case of other alkali phosphors, Zn2+ substitution for Sr2+ greatly enhanced the emission of this thio-selenide phosphor. The LED fabricated with the synthesized phosphor produced white light using a blue LED with a CIE-1931 coordinate, T c and R a of (0.3199, 0.3107), 6577K and 51.6, respectively. [Copyright &y& Elsevier]

Abstract: Thio-selenide phosphor, SrGa2SeS3:Eu2+, was prepared using a solid-state chemical reaction method without the addition of toxic gases or other excess materials. The synthesized phosphor displayed a broad excitation band, which is suitable for blue or NUV LED applications and emits green light (528nm). The samples were also excited with various wavelength emitting diodes (410nm, 425nm, and 455nm) to assess their potential use as phosphors for white LED applications. The LED fabricated with the synthesized phosphor produced bluish green light using NUV LED with a CIE-1931 coordinate, Tc and Ra of (0.2309, 0.3436), 11732.3K and 18.2, respectively. [ABSTRACT FROM AUTHOR]