Light Emitting Diodes (light + emitting_diode)

Distribution by Scientific Domains

Kinds of Light Emitting Diodes

  • blue light emitting diode
  • organic light emitting diode
  • ultraviolet light emitting diode
  • white light emitting diode


  • Selected Abstracts


    Novel Light Emitting Diode Using Organic Electroluminescence Microcapsules

    MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 16 2003
    Jae-Seok Heo
    Abstract Novel light emitting diodes (LEDs) were prepared using electroluminescence (EL) material/polymer microcapsules (ELC). N-vinylcarbazole as a hole-transporting component and methyl methacrylate (MMA) were copolymerized for producing the seed particles using dispersion polymerization. An oxadiazole derivative, synthesized as a electron-transporting component, and tris(8-hydroxyquinolinato) aluminium(III) (Alq3) were incorporated into the polymer particles by using the solute co-diffusion method (SCM). The LEDs for the EL characterization were fabricated in a thin sandwich configuration: Al anode/ELC/ITO cathode. The surface imaging of the LED prepared using ELC was performed by atomic force microscopy (AFM). The EL characteristics of the ELC were investigated by UV, photoelectron and luminescence spectroscopy, and the current-voltage and the light-voltage characteristics for the LED were determined. Encapsulation procedure using solute co-diffusion method (SCM). [source]


    III-V concentrator solar cell reliability prediction based on quantitative LED reliability data,

    PROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 6 2007
    Manuel Vzquez
    Abstract III-V Multi Junction (MJ) solar cells based on Light Emitting Diode (LED) technology have been proposed and developed in recent years as a way of producing cost-competitive photovoltaic electricity. As LEDs are similar to solar cells in terms of material, size and power, it is possible to take advantage of the huge technological experience accumulated in the former and apply it to the latter. This paper analyses the most important parameters that affect the operational lifetime of the device (crystalline quality, temperature, current density, humidity and photodegradation), taking into account experience on the reliability of LEDs. Most of these parameters are less stressed for a III-V MJ solar cell working at 1000 suns than for a high-power LED. From this analysis, some recommendations are extracted for improving the long-term reliability of the solar cells. Compared to high-power LEDs based on compound semiconductors, it is possible to achieve operational lifetimes higher than 105,hours (34 years of real-time operation) for III-V high-concentration solar cells. Copyright 2007 John Wiley & Sons, Ltd. [source]


    Thermally Activated Delayed Fluorescence from Sn4+,Porphyrin Complexes and Their Application to Organic Light Emitting Diodes , A Novel Mechanism for Electroluminescence

    ADVANCED MATERIALS, Issue 47 2009
    Ayataka Endo
    Electroluminescence based on TADF, that is, thermally activated delayed fluorescence, is demonstrated in Sn4+,porphyrin complexes. On excitation by a short electrical pulse, prompt and delayed electroluminescence components were clearly observed. The delayed component was composed of both TADF and phosphorescence (see figure), and the TADF component significantly increased with increasing temperature. [source]


    A Novel Narrow Band Red-Emitting Phosphor for White Light Emitting Diodes

    INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 4 2009
    Sivakumar Vaidyanathan
    Research on down conversion phosphor materials is the key for the development of solid-state lighting (SSL). Especially finding alternative red phosphor for white light emitting diodes (LEDs) based on blue or near ultraviolet (NUV) LEDs is important research task. In this view, we have synthesized a series of Eu3+ -substituted La2W2,xMoxO9 (x=0,2, in step of 0.3) red phosphor and characterized by X-ray diffraction (XRD) and photoluminescence. XRD results reveal a phase transition from triclinic to cubic structure for x>0.2. All the compositions show broad charge transfer (CT) band due to CT from oxygen to tungsten/molybdenum and red emission due to Eu3+ ions. Select compositions show high red emission intensity compared with the commercial red phosphor under NUV/blue ray excitation. Hence, this candidate can be a possible red phosphor for white LEDs. [source]


    A Wide-Bandgap Semiconducting Polymer for Ultraviolet and Blue Light Emitting Diodes,

    MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 18 2003
    Ping Lu
    Abstract 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. The structure of the copolymer PDHFSCHD. [source]


    Electroluminescence uniformity in green LEDs and dual color blue/green stacking LEDs

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2007
    Grigory Onushkin
    Abstract The uniformity of Electroluminescence (EL) of green and blue/green stacking InGaN Light Emitting Diodes (LED) has been studied and analyzed by various techniques. Comparative measurements of Photo-Luminescence (PL), Cathode-Luminescence (CL) and EL allow us to conclude that non-uniform distribution of EL in studied LEDs is caused by non-uniformity of acceptors distribution over the p-GaN area. It was found that there are some defective areas in p-GaN having locally low level of acceptor concentration. These areas correspond to the surface hexagonal shaped pyramids on p-GaN surface. The locally low doping level changes the field distribution, injection properties and the EL emission properties for these defective points. Optimization of p-GaN growth conditions will improve the EL uniformity and increase the efficiency of green and blue/green LEDs. ( 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Proceedings of the 5th International Symposium on Blue Laser and Light Emitting Diodes (ISBLLED-2004)

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2004
    Eun-Kyung Suh
    The 5th International Symposium on Blue Laser and Light Emitting Diodes (ISBLLED-2004) was held in Gyeongju, Korea, 15,19 March 2004. The purpose of the symposium was to provide a forum for scientists and engineers to discuss recent progress and future trends in the rapidly advancing wide band gap semiconductor science and technologies and their applications in blue laser and light emitting diodes. [source]


    Preface: phys. stat. sol. (c) 1/10

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2004
    Eun-Kyung Suh
    The Fifth International Symposium on Blue Laser and Light Emitting Diodes (ISBLLED-2004) was held in Gyeongju, Korea from 15,19 March 2004. Gyeongju, the ancient capital of the thousand-year Silla kingdom (57 B.C. to 935 A.D.) provided additional pleasure to the participants as an exceptional open-air museum with antique treasures scattered all around the city. During the last decade we have witnessed remarkable developments in wide-gap semiconductors and light emitting devices in the spectral range from the visible to deep UV. The purpose of the Symposium was to provide a forum for intensive discussion on the issues and main progress especially in optoelectronic devices, material growth and characterization, and quantum structures of wide bandgap semiconductors. A total of 243 papers including 220 contributed and 23 invited ones were presented and discussed by 487 participants from 17 countries world-wide. Among them, 154 manuscripts were submitted and reviewed by the usual evaluation process of physica status solidi. Some were rejected or withdrawn, and finally 139 papers are published in the special issues of physica status solidi (a), (b), and (c). We gratefully acknowledge the referees for their careful review. The papers are grouped into 7 categories. The subheadings and the number of papers in each are as follows: Optoelectronic devices, 43; Growth and characterization, 45; Nano and quantum structures, 21; Contacts, 8; Zinc oxide, 9; Indium nitride and indium rich InGaN, 6; Others, 7. The special session of the Symposium, "The LED Highlight", designed partially to meet the challenging targets of the technology, i.e., energy savings and clean environment preservation, drew much attention and is edited as a special coloured section in this issue. The next symposium is scheduled for Montpellier, France, in 2006. We wish the organizers of that symposium the best of luck and hope to see all of the ISBLLED-2004 participants again at ISBLLED-2006. ISBLLED-2004 was sponsored by The Research Society for the Wide-gap Semiconductors, Korean Physical Society, Office of Naval Research, Korea Science and Engineering Foundation, Korea Research Foundation, Korea Association for Photonics Industry Development, Asian Office of Aerospace Research and Development, and Korea Photonics Technology Institute. We would like to thank Ms. E. S. Hwang for her devotion to the preparation and the Proceedings of the symposium including the manuscript handling for publication. [source]


    Oxidation of PCE with a UV LED Photocatalytic Reactor

    CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 1 2005
    D. H. Chen
    Abstract This paper is the first to investigate photocatalysis using a cutting-edge and energy-efficient solid-state light source: Ultraviolet (UV) Light Emitting Diodes (LED's). UV LED's do not involve mercury vapor, can be driven with direct current (DC), and have a long lifetime of 100,000 hours. UV LED's with a peak wavelength of 375,nm were tested for perchloroethylene (PCE) photocatalytic oxidation over Degussa P,25 TiO2. At a UV light output of only 49,,W/cm2, the designed reactor delivers a PCE conversion of up to 43,%. If the UV LED price continues to drop, it is very likely that UV LED's will replace UV lamps as the favored light source in photocatalysis applications. [source]


    Light emitting diodes and an infrared bulb as light sources of a fixed-film tubular photobioreactor for conversion of hydrogen sulfide to elemental sulfur

    JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2 2005
    Murtuza A Syed
    Abstract The effect of light quality on the performance a fixed-film continuous-flow photobioreactor for removal of hydrogen sulfide from synthetic industrial wastewater and conversion of it to elemental sulfur was investigated. Sixteen 150 mm long and 1.6 mm internal diameter (id) Tygon tubes formed the active part of the reactor. At the same light intensity, reactor performance in terms of optimal sulfide loading rates was compared between an infrared bulb and light emitting diodes (LEDs). The LEDs provided light within the peak absorption wavelength range of green sulfur bacteria (GSB) and were used as a light source for the GSB with the goal of reducing the cost of the required light. Though the reactor sustained higher sulfide loading rates using LEDs than when using an infrared bulb at equal light intensities, the infrared bulb has the potential to be more efficient overall. Copyright 2004 Society of Chemical Industry [source]


    Light emitting diodes on silicon substrates: preliminary results

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2009
    Alexandre Bondi
    Abstract III-V quantum wells (QW) superlattices have been grown by molecular beam epitaxy on GaP substrates for photonics applications on silicon. We first present room temperature photoluminescence (PL) results for GaAsP/GaP QWs. A detailed analysis of low temperature PL experiments is then performed. QW contribution is pointed out, and the structuration of the QW emission is attributed to LA phonon replica. A comparison with electronic bandstructure is performed, and a discussion is proposed on the nature of the observed transition (direct or indirect). Finally, it is shown that these QWs can be used as active zone in light emitters on silicon. Growth of good quality GaP epilayers on silicon is also presented. The crystalline quality of the deposited GaP near the GaP/Si interface is studied by Raman spectroscopy. ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Thick crack-free AlGaN films deposited by facet-controlled epitaxial lateral overgrowth

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2003
    R. Liu
    Abstract Thick crack-free AlGaN films have been grown on inclined-facet GaN templates. Light emitting diodes with , = 323 nm has been achieved on these epilayers. The GaN template was grown at a low temperature in order to obtain triangle-facet growth fronts. Subsequent growth of AlGaN on this template involving a lateral overgrowth process exhibits interesting properties. The microstructure and optical characterizations were done using transmission electron microscopy and cathodoluminescence. At the AlGaN/GaN interface, a high density of dislocations was created due to lattice mismatch strain. Another unexpected set of triangular boundaries was observed inside the AlGaN layer, which grew without any change of the growth parameters. These boundaries were found to arise from domains grown in different directions. Mono-chromatic cathodoluminescence images indicate that Al content is different between the vertically-grown and the laterally-grown domains, suggesting that lattice mismatch strain exists between them. Dislocations were created at these mismatched boundaries to relax the strain. ( 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Polythiophene-Fullerene Based Photodetectors: Tuning of Spectral Response and Application in Photoluminescence Based (Bio)Chemical Sensors

    ADVANCED MATERIALS, Issue 37 2010
    Kanwar S. Nalwa
    A photoluminescence (PL)-based oxygen and glucose sensor utilizing inorganic or organic light emitting diode as the light source, and polythiophene:fullerene type bulk-heterojunction devices as photodetectors, for both intensity and decay-time based monitoring of the sensing element's PL. The sensing element is based on the oxygen-sensitive dye Pt-octaethylporphyrin embedded in a polystyrene matrix. [source]


    Detection of Pretreated Fingerprint Fluorescence Using an LED-based Excitation System

    JOURNAL OF FORENSIC SCIENCES, Issue 4 2008
    Masahisa Takatsu Ph.D.
    Abstract:, Optimization of a light emitting diode (LED)-based excitation system for the detection of pretreated fingerprint fluorescence is described. Fluorescent ridges can usually be excited by irradiation with forensic light sources such as xenon arc lamps or quartz-halogen lamps with high-power output and suitable filters. However, they are too expensive for many crime laboratories in smaller organizations. We concentrated on LEDs which have advantages over conventional light sources in that they are simpler and of lower cost, but the power output and quality of each individual LED unit is not sufficient for the detection of weak fluorescent ridges. To resolve this subject, blue and green LED arrays composed of ninety LED units were adopted and suitable low pass filters for them were designed. An experimental system, consisting of blue and green LED arrays with the suitable low pass filters for illumination, high pass filters for viewing, a digital camera and a computer, was tested. The fluorescent images of cyanoacrylate ester fumed/rhodamine 6G stained fingerprint on white polyethylene sheet and weak fluorescent ridges of ninhydrin/indium chloride treated fingerprint on white paper were successfully detected and photographed. It was shown that the improvement of LED beam in intensity and quality can compensate the disadvantages, resulting in well-contrasted images. [source]


    Integrated fluorescence sensor based on ring-shaped organic photodiodes

    PHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 7 2010
    Bernhard Lamprecht
    Abstract We demonstrate a novel sensor type, which is based on the monolithic integration of luminescent optical sensor spots together with ring-shaped thin-film organic photodiodes on one substrate. The organic photodiodes serve as integrated fluorescence detectors, simplifying the detection system by minimizing the number of required optical components. The proposed concept enables filter-less discrimination between excitation light and generated fluorescence light. The functionality of the concept is demonstrated by an integrated oxygen sensor, exhibiting excellent performance. The sensor spots are excited by an assembled organic light emitting diode. The integrated optical sensor platform is suitable for the parallel detection of multiple parameters. Sensor schemes for the analytical parameters carbon dioxide, temperature and ammonia, are proposed. ( 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Preparation and luminescent properties of Eu2+ -activated glass ceramic phosphor precipitated with ,-Ca2SiO4 and Ca3Si2O7

    PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2009
    Takayuki Nakanishi
    Abstract Eu2+: calcium silicate glass ceramics (GC) containing ,-Ca2SiO4 and Ca3Si2O7 crystal phosphors were fabricated by cerammization process from a GC that were prepared by a novel GC preparation; frozen sorbet method. The GC obtained (as-made GC) consisted of glass and spherical ,-Ca2SiO4 crystal phases with size of about 20,40 ,m. After post-ceramization, Ca3Si2O7 crystals were also precipitated. Judging from the cathodoluminescence (CL) mapping images, two emission bands peaked at 515 nm and 600 nm were observed from different parts in the GC. The observed photoluminescence (PL) have two broad emission bands at 515 nm from Eu2+: ,-Ca2SiO4 and at 600 nm from Eu2+:Ca3Si2O7. With increasing heat-treatment temperature, the color coordinates shifted from green range to center region, which corresponds to pure white in the CIE chromaticity diagram. Eu2+ doped calcium silicate GC containing ,-Ca2SiO4 and Ca3Si2O7 are suitable phosphor as the use of phosphor converting white light emitting diode (pc-wLED) with high color rendering. ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Nanostructuring of an AlGaInP light emitting diode for surface plasmon enhanced emission of light output

    PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 8 2008
    R. M. Perks
    Abstract This work describes a successful post fabrication method of applying a plasmonic grating to the surface of a mounted AlGaInP LED device. A Carl Zeiss 1540 XB Focused Ion beam system was used to mill holes in a thin gold layer with a periodicity of around 540 nm. Measurements of the total light output of the device compared with a standard un-textured device show that the plasmonic grating was found to significantly affect the light output characteristics such that the overall light output was reduced. This is attributed to the fact that only p-polarised light can interact with the hole array. Furthermore, the onset of saturation in the textured device was not evident as compared with the un-textured device. This is attributed to improved current spreading in the device due to the top gold layer. ( 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    High efficiency AlGaInN-based light emitting diode in the 360,380 nm wavelength range

    PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003
    Hisao Sato
    Abstract High performance LEDs emitting in the wavelength range 360,380 nm, are fabricated on sapphire substrates by one-time metalorganic chemical vapor deposition (MOCVD) without using epitaxial lateral overgrowth (ELO) or similar techniques. By improving layer structures and growth conditions, the output power of the LEDs was much improved. The light output power of the LEDs at an injection current of 20 mA is 3.2 mW, 2.5 mW and 1 mW at wavelengths of 378 nm, 373 nm and 363 nm, which correspond to an external quantum efficiency of 4.8%, 3.8% and 1.4%, respectively. ( 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Characterization of InGaN/GaN and AlGaN/GaN superlattices by X-ray diffraction and X-ray reflectivity measurements

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010
    S. Sintonen
    Abstract The commercial significance of superlattice structures is increasing due to greater demand of optoelectronic devices, such as the light emitting diode (LED). In order to optimize these devices, an accurate and reliable characterization method is needed. This paper describes in detail the characterization of superlattices with X-ray scattering techniques. The thicknesses of the individual layers are determined by X-ray reflectivity (XRR) measurements and the state of strain, the lattice constants and the compositions of ternary compounds by X-ray diffraction (XRD) measurements. The method is non-destructive, and yields unique results, unlike characterizations based on simulation of symmetric XRD scans. These simulations were used for verification of results. The method was tested on InGaN and AlGaN superlattice structures. The measured and simulated parameter values agreed very well. ( 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Photon-assisted tunneling in GaN nanowire white light emitting diodes

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009
    P.-C. Yeh
    Abstract We reported intensive greenish white light electroluminescence, accompanied with 364 nm bandedge emission from GaN nanowires/p-GaN light emitting diode operated in the high-field forward bias regime. Strong bandedge emission, dominant over visible luminescence from bandgap state transitions, was also observed in the reverse bias regime. These observations were ascribed to a formation of n+ -p tunnel junction in the high-field regime and emission mechanism due to photon-assisted tunneling process. ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    CMP properties and fabrication of OLED using MEH-PPV

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2008
    Woo-Sun Lee
    Abstract The interface between the electrode and organic layer in organic light emitting diode (OLED) has been reported as an important factor to influence the electrical and luminescent properties. Indium tin oxide (ITO) is commonly used as the anode material. ITO thin film has been deposited on glass substrate by radio frequency (RF) magnetron sputtering. And it was used as an anode contact to fabricate OLED device. The OLED devices with ITO/MEH-PPV [2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene]/Al configuration were fabricated by thermal evaporation and spin coating, respectively. We investigated the electrical, structural and optical properties of ITO thin film, which was measured using the methods of atomic force microscope (AFM), field-emission scanning electron microscope (FESEM) and ultraviolet,visible (UV,VIS) spectrophotometer. The interface property between ITO thin film and MEH-PPV was improved as the surface of ITO thin film was smoothed and photoluminescence (PL) intensity was increased in the 680 nm range. ( 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    M -plane InGaN/GaN light emitting diodes fabricated by MOCVD regrowth on c -plane patterned templates

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2008
    Christopher A. Schaake
    Abstract In this work we demonstrate a light emitting diode (LED) with m -plane quantum wells fabricated on a (000) template. N-polar, n-type GaN was grown by MOCVD on vicinal sapphire substrates. Stripes, measuring 500 nm wide, 500 nm tall and spaced 2 ,m apart, were etched parallel to the ,110, direction leading to sidewalls that are approximately {100}. Sputtered AlN was used as a regrowth mask on the c -plane surfaces. An active region consisting of 5 InGaN quantum wells and GaN barriers followed by p-type was grown. The regrowth occurred mostly on the exposed m -plane sidewalls, leading to lateral growth in the ,100, direction. The LED was processed using conventional methods. A thick metal contact was used to connect the p-regions together. Current vs. voltage measurements showed good rectifying behavior with a turn on of about 6 volts. On-wafer electroluminescence measurements revealed a peak wavelength of 422 nm. ( 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Blue light emitting diode fabricated on a-plane GaN film over r-sapphire substrate and on a-plane bulk GaN substrate

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2007
    Y. Naoi
    Abstract We studied the growth technique for the dislocation reduction in a-plane GaN grown by metal organic chemical vapour deposition (MOCVD) using AlInN buffer layer, high temperature atomic layer epitaxy, and trenched r-sapphire technique. By using these techniques, the crystal quality was much improved. We also fabricated blue light emitting diodes (LEDs) on a-plane GaN film over r-sapphire substrate and on a-plane bulk GaN substrate. The electroluminescence (EL) characteristics of the LED samples were examined, and we found that the EL near field pattern from homo-epitaxially grown a-GaN based LED was spatially uniform, although the pattern from the LED on r-sapphire substrate was not uniform. The output power at the wavelength of 430nm was 0.72mW at the 20mA injection current for the sample on a-plane bulk GaN. ( 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Current crowding effects on blue LED operation

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2006
    I. Yu.
    Abstract A hybrid 1D/3D approach to modeling of the electric current spreading in a light emitting diode (LED) die is suggested and validated by comparing the theoretical predictions with available observations. Large-area LEDs with different contact electrode configurations suitable for high-power operation are analyzed in terms of simulations with the focus on the current non-uniformity and its impact on the device electrical and optical characteristics. The factors controlling the LED series resistance are identified by modeling, and ways for the resistance improvement are examined. ( 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Large enhancement of GaN-UV light emission using silver mirror resonator

    PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2006
    N. M. Ahmed
    Abstract In this paper we used new microcavity resonator design of extracting light with high efficiency from a high index material (GaN), n > 2 for use in light emitting diode (LED) and laser diode (LD). A GaN/sapphire structure as an active layer was sandwiched between two silver mirrors. For the study two types of microcavity were fabricated: (air/GaN/sapphire/silver) and (silver/GaN/sapphire/silver). Via photo-luminescence measurements we observed 2-fold intensity enhancement in the UV region (364 nm) at room temperature by using 400 nm silver back mirror compared with uncoated sample. The amplitude of the photoluminescence is enhanced 10-times when we used 400 nm back coated together with 50 nm silver as front mirror. Further, a tremendous enhancement was obtained when a small hole was made in the front mirror with 50 nm thickness. The large increment of GaN/sapphire light emission in this microcavity could be caused by few important factors: increasing absorption in the cavity, optical field enhancement due to the resonator and coupling of plasmon in the metallic layer with the UV light. ( 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


    Multifunctional Crosslinkable Iridium Complexes as Hole Transporting/Electron Blocking and Emitting Materials for Solution-Processed Multilayer Organic Light-Emitting Diodes

    ADVANCED FUNCTIONAL MATERIALS, Issue 7 2009
    Biwu Ma
    Abstract Here, a new series of crosslinkable heteroleptic iridium (III) complexes for use in solution processed phosphorescent organic light emitting diodes (OLEDs) is reported. These iridium compounds have the general formula of (PPZ-VB)2Ir(C,N), where PPZ-VB is phenylpyrazole (PPZ) vinyl benzyl (VB) ether; and the C,N ligands represent a family of four different cyclometallating ligands including 1-phenylpyrazolyl (PPZ) (1), 2-(4,6-difluorophenyl)pyridyl (DFPPY) (2), 2-(p-tolyl)pyridyl (TPY) (3), and 2-phenylquinolyl (PQ) (4). With the incorporation of two crosslinkable VB ether groups, these compounds can be fully crosslinked after heating at 180,C for 30,min. The crosslinked films exhibit excellent solvent resistance and film smoothness which enables fabrication of high-performance multilayer OLEDs by sequential solution processing of multiple layers. Furthermore, the photophysical properties of these compounds can be easily controlled by simply changing the cyclometallating C,N ligand in order to tune the triplet energy within the range of 3.0,2.2,eV. This diversity makes these materials not only suitable for use in hole transporting and electron blocking but also as emissive layers of several colors. Therefore, these compounds are applied as effective materials for all-solution processed OLEDs with (PPZ-VB)2IrPPZ (1) acting as hole transporting and electron blocking layer and host material, as well as three other compounds, (PPZ-VB)2IrDFPPY (2), (PPZ-VB)2IrTPY(3), and (PPZ-VB)2IrPQ(4), used as crosslinkable phosphorescent emitters. [source]


    Organic Light-Emitting Diodes Based on Poly(9,9-dioctylfluorene- co -bithiophene) (F8T2)

    ADVANCED FUNCTIONAL MATERIALS, Issue 6 2009
    Peter A. Levermore
    Abstract A study of the optical properties of poly(9,9-dioctylfluorene- co -bithiophene) (F8T2) is reported, identifying this polymer as one that possesses a desirable combination of charge transport and light emission properties. The optical and morphological properties of a series of polymer blends with F8T2 dispersed in poly(9,9-dioctylfluorene) (PFO) are described and almost pure-green emission from light emitting diodes (LEDs) based thereon is demonstrated. High luminance green electroluminescence from LEDs using only a thin film of F8T2 for emission is also reported. The latter demonstration for a polymer previously primarily of interest for effective charge transport constitutes an important step in the development of emissive materials for applications where a union of efficient light emission and effective charge transport is required. [source]


    Hole Injection in a Model Fluorene,Triarylamine Copolymer

    ADVANCED FUNCTIONAL MATERIALS, Issue 2 2009
    Hon Hang Fong
    Abstract Recent developments in synthesis and purification have yielded conjugated polymers with hole mobilities exceeding 0.01,cm2 V,1 s,1. Essential to harvesting the potential of these materials in organic light emitting diodes (OLEDs) is the identification of suitable ohmic contacts. Using a model fluorene copolymer that shows high-mobility, non-dispersive hole transport, it is demonstrated that electrodes commonly used as anodes in OLEDs are very poor hole injectors. Injection from Au and indium tin oxide anodes is limited by energy barriers of 0.75 and 0.65,eV, respectively, and the injected current is found to be temperature independent,a prediction that was not reproduced by the leading injection model for disordered organic semiconductors. Injection from a poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) anode, on the other hand, is found to become less efficient with electric field, a behavior which is currently not understood. In thinner poly[(9,9,-dioctylfluorenyl-2,7-diyl)- co -(4,4,-(N -(4- sec -butyl))diphenylamine)] films, which are of relevance to OLEDs, ohmic losses on the PEDOT:PSS layer are found to limit the flow of current. These results illustrate the opportunity to further improve the performance of OLEDs as well as the challenge posed by high mobility conjugated polymers for the design of hole injection layers. [source]


    Light Emission Characteristics and Mechanics of Foldable Inorganic Light-Emitting Diodes

    ADVANCED MATERIALS, Issue 28 2010
    Sang-Il Park
    Strategies are presented for forming inorganic, microscale light emitting diodes in interconnected arrays on plastic substrates, configured to accommodate extreme levels of bending, with negligible changes in the electrical properties or the emission wavelengths in the devices. The studies include quantitative analysis of the underlying mechanics and experimental measurements of bending induced shifts in the emission wavelength. [source]


    A Novel Narrow Band Red-Emitting Phosphor for White Light Emitting Diodes

    INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 4 2009
    Sivakumar Vaidyanathan
    Research on down conversion phosphor materials is the key for the development of solid-state lighting (SSL). Especially finding alternative red phosphor for white light emitting diodes (LEDs) based on blue or near ultraviolet (NUV) LEDs is important research task. In this view, we have synthesized a series of Eu3+ -substituted La2W2,xMoxO9 (x=0,2, in step of 0.3) red phosphor and characterized by X-ray diffraction (XRD) and photoluminescence. XRD results reveal a phase transition from triclinic to cubic structure for x>0.2. All the compositions show broad charge transfer (CT) band due to CT from oxygen to tungsten/molybdenum and red emission due to Eu3+ ions. Select compositions show high red emission intensity compared with the commercial red phosphor under NUV/blue ray excitation. Hence, this candidate can be a possible red phosphor for white LEDs. [source]