Search

By adopting atomic layer deposition (ALD) sidewall passivation, the electroluminescence (EL) and communication performances of mini-light emitting diodes (mini-LEDs) of different sizes were improved. In particular, the most significant improvement in the electroluminescence properties of the external quantum efficiency (EQE) (a 7.2% increase), leakage current, and the communication properties of the modulation bandwidth (a 20% increase) transmission rate and bit error rate (BER) was found in the smallest mini-LEDs (80 $\mu \text{m}\,\,\times $ 120 $\mu \text{m}$). According to the results of time-resolved photoluminescence (TRPL) measurements, the carrier lifetime of the samples can be affected by both the size and ALD sidewall passivation. In addition, the effects of ALD sidewall passivation for visible-light communication (VLC) were demonstrated. [ABSTRACT FROM AUTHOR]

The optical efficiency of GaN-based multiple quantum well (MQW) and light emitting diode (LED) structures grown on Si(111) substrates by metal-organic vapor phase epitaxy was measured and compared with equivalent structures on sapphire. The crystalline quality of the LED structures was comprehensively characterized using x-ray diffraction, atomic force microscopy, and plan-view transmission electron microscopy. A room temperature photoluminescence (PL) internal quantum efficiency (IQE) as high as 58% has been achieved in an InGaN/GaN MQW on Si, emitting at 460 nm. This is the highest reported PL-IQE of a c-plane GaN-based MQW on Si, and the radiative efficiency of this sample compares well with similar structures grown on sapphire. Processed LED devices on Si also show good electroluminescence (EL) performance, including a forward bias voltage of ∼3.5 V at 20 mA and a light output power of 1 mW at 45 mA from a 500×500 μm2 planar device without the use of any additional techniques to enhance the output coupling. The extraction efficiency of the LED devices was calculated, and the EL-IQE was then estimated to have a maximum value of 33% at a current density of 4 A cm-2, dropping to 30% at a current density of 40 A cm-2 for a planar LED device on Si emitting at 455 nm. The EL-IQE was clearly observed to increase as the structural quality of the material increased for devices on both sapphire and Si substrates. [ABSTRACT FROM AUTHOR]

A blue light emitting diode (LED) is grown on top of a (Ga, In)N/GaN multiple quantum well (QW) acting as a light converter from blue to green-yellow wavelength. The blue light is produced by electrical injection, while the green-yellow emitting QWs are optically pumped by the blue photons. It is shown that the final color of the LED is strongly dependent on the blue pumping wavelength, the absorption and the internal quantum efficiency of the light converter. Depending on these parameters, blue to green LEDs or even white LEDs can be obtained. In addition, the injection current dependence of the LED electroluminescence is measured and analyzed. A very low blueshift is observed as a function of the injection current. It is explained by the fact that the carrier density per QW in the light converter stays relatively low compared to the case of classical current-injected green LEDs. [ABSTRACT FROM AUTHOR]

The effect of image forces in GaN pyramidal nanorod structures is investigated to develop dislocation-free light emitting diodes (LEDs). A model based on the eigenstrain method and nonlocal stress is developed to demonstrate that the pyramidal nanorod efficiently ejects dislocations out of the structure. Two possible regimes of filtering behavior are found: (1) cap-dominated and (2) base-dominated. The cap-dominated regime is shown to be the more effective filtering mechanism. Optimal ranges of fabrication parameters that favor a dislocation-free LED are predicted and corroborated by resorting to available experimental evidence. The filtering probability is summarized as a function of practical processing parameters: the nanorod radius and height. The results suggest an optimal nanorod geometry with a radius of ∼50b (26 nm) and a height of ∼125b (65 nm), in which b is the magnitude of the Burgers vector for the GaN system studied. A filtering probability of greater than 95% is predicted for the optimal geometry. [ABSTRACT FROM AUTHOR]

We demonstrate GaInN multiple quantum well (MQW) light-emitting diodes (LEDs) having ternary GaInN quantum barriers (QBs) instead of conventional binary GaN QBs for a reduced polarization mismatch between QWs and QBs and an additional separate confinement of carriers to the MQW active region. In comparison with GaInN LEDs with conventional GaN QBs, the GaInN/GaInN LEDs show a reduced blueshift of the peak wavelength with increasing injection current and a reduced forward voltage. In addition, we investigate the density of pits emerging on top of the MQW layer that are correlated with V-defects and act as a path for the reverse leakage current. The GaInN/GaInN MQW structure has a lower pit density than the GaInN/GaN MQW structure as well as a lower reverse leakage current. Finally, the GaInN/GaInN MQW LEDs show higher light output power and external quantum efficiency at high injection currents compared to the conventional GaInN/GaN MQW LEDs. We attribute these results to the reduced polarization mismatch and the reduced lattice mismatch in the GaInN/GaInN MQW active region. [ABSTRACT FROM AUTHOR]

We develop a self-consistent model to describe the internal heating of high power light emitting diodes (LEDs) and use this model to simulate the operation of GaAs–AlGaAs double heterostructure LEDs. We account for the heating by nonradiative recombination processes in the simulations and solve self-consistently the steady state junction temperature. Based on the simulation results, we discuss the plausibility of unity conversion efficiency in LEDs and also the mechanisms underlying the efficiency droop. We show that the rise in the junction temperature limits the light output available from LEDs and further degrades the efficiency of operation at high operating currents. In addition to high power applications we study the optimal operating point and discuss the methods to increase the efficiency of LEDs toward the thermodynamical limits. [ABSTRACT FROM AUTHOR]

The recently increased efficiency of organic light emitting devices (OLED) brings lighting applications within reach. If the area of the OLED is of the order a cm2, voltage and brightness losses related to the square resistance of the transparent electrode become important. The homogeneity of the voltage and brightness can be improved by contacting the transparent electrode from all edges and by adding a metallic grid to the transparent electrode. This grid should have narrow lines to minimize transmission losses and improve the total light emission from the OLED. The voltage losses in grids with different shapes (triangular, square, and hexagonal) are evaluated and the grid parameters are optimized to maximize the total light emission. It turns out that a hexagonal grid has lower loss than a square grid with the same line width. [ABSTRACT FROM AUTHOR]

This work discusses the temperature-dependent forward current-voltage characteristics of an AlGaInP light-emitting diode. From 300 to 470 K, all curves have the same ideality factor of n=1.58. The temperature-dependent saturation currents are in excellent agreement with the thermal activation behavior over ten decades of current and with an activation energy of Ea=1.405 eV. Based on the discussion of the barrier for forward current flow, nEa corresponds to the band gap of the active layer. Various mechanisms of current flow for pn junctions and Schottky diodes were examined and verified. Therefore, the nEa=2.22 eV of the sample corresponds to the band gap of the barriers in the active layer. This value is consistent with the band gap of (AlxGa1-x)0.5In0.5P for x>0.58. [ABSTRACT FROM AUTHOR]

In this paper we demonstrate that the improvement in the emission intensity afforded by the introduction of multiquantum barrier (MQB) structures in an InGaN/GaN multiple-quantum-well (MQW) light-emitting diode (LED) is attributable to increased excitation cross sections. Over the temperature range from 300 to 20 K, the excitation cross sections of the MQW emissions possessing MQB structures were between 9.6×10-12 and 5.3×10-15 cm2, while those possessing GaN barriers were between 8.1×10-12 and 4.5×10-15 cm2. We found, however, that the figure of merit for the LED light output was the capture fraction of the cross section; we observed that the dependence of the optical intensity on the temperature coincided with the evolution of the capture fraction. This analysis permitted us to assign the capture cross-section ratios at room temperature for the MQWs with MQBs and with GaN barriers as 0.46 and 0.35. Furthermore, the MQW system possessing well-designed MQB structures not only exhibited the thermally insensitive luminescence but also inhibited energetic carrier overflow. [ABSTRACT FROM AUTHOR]

The carrier dynamics of electroluminescent metal-oxide-semiconductor silicon tunneling diodes is rigorously modeled in this study. Various tunneling and recombination current densities are formulated without using the Maxwell-Boltzmann approximation for the carrier concentrations. This model satisfactorily explains the current-voltage characteristics in strong accumulation using a self-consistent formulation. It also relates light emission efficiency to the interface-state density and the bulk-trap density in a straightforward manner. The internal radiative recombination efficiency in the bulk Si substrate is estimated to be in tens of percents. The model also explains clearly the strongly influenced small-signal light-current response time with respect to the injection current density. It enables the small-signal method to be useful in extracting the parameters of the interface-state density and the bulk-trap density. [ABSTRACT FROM AUTHOR]

350-nm-band quaternary InAlGaN quantum-well (QW) structures have been fabricated on low-dislocation-density GaN substrates and sapphire substrates, and the impact of the In composition on ultraviolet emission efficiency has been investigated. Photoluminescence, cathodoluminescence, and atomic force microscopy measurements have revealed that with increasing In concentration, band-gap inhomogeneity in InAlGaN layers is promoted and quantum efficiency becomes less sensitive to dislocation densities. However, this does not necessarily result in the enhancement of QW emission efficiency, because both a decrease in growth temperature and an increase in Al concentration degrade the InAlGaN crystalline quality. It is found that improving the InAlGaN crystalline quality by increasing the growth temperature while maintaining an In fluctuation effect with minimal In composition is essential for highly efficient InAlGaN QWs. The utilization of the optimized InAlGaN growth condition significantly improves the output power of 350-nm-band light-emitting diodes on GaN and sapphire substrates to 7.4 mW at 400 mA and 2.9 mW at 200 mA, respectively. [ABSTRACT FROM AUTHOR]

We report on the advantage of interlayers using transition-metal oxides, such as iridium oxide (IrOx) and ruthenium oxide (RuOx), between indium tin oxide (ITO) anodes and 4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl (α-NPD) hole transport layers on the electrical and optical properties of organic light-emitting diodes (OLEDs). The operation voltage at a current density of 100 mA/cm2 decreased from 17 to 11 V for OLEDs with 3-nm-thick IrOx interlayers and from 17 to 14 V for OLEDs with 2-nm-thick RuOx ones. The maximum luminance value increased about 50% in OLED using IrOx and 108% in OLED using RuOx. Synchrotron radiation photoelectron spectroscopy results revealed that core levels of Ru 3d and Ir 4f shifted to high binding energies and that the valence band was splitting from metallic Fermi level as the surface of the transition metal was treated with O2 plasma. This provides evidence that the transition-metal surface transformed to a transition-metal oxide. The surface of the transition metal became smoother with the O2 plasma treatment. The thickness was calculated to be 0.4 nm for IrOx and 0.6 nm for RuOx using x-ray reflectivity measurements. Secondary electron emission spectra showed that the work function increased by 0.6 eV for IrOx and by 0.4 eV for RuOx. Thus, the transition-metal oxides lowered the potential barrier for hole injection from ITO to α-NPD, reducing the turn-on voltage of OLEDs and increasing the quantum efficiency. [ABSTRACT FROM AUTHOR]

This study is focused on the modulation response of resonant-cavity light-emitting diodes (RCLEDs). Platinum (Pt) atoms are diffused into the 660 nm RCLED epitaxial layers to increase the concentration of recombination centers and to improve the modulation speed. The RCLED has an AlInGaP multi-quantum-well active layer which was embedded into AlGaAs-distributed Bragg reflectors to form a one-wavelength (1-λ) optical resonator. Afterwards, the deep-level Pt impurity was diffused into the RCLED and an improved average rise time, from 18.07 to 12.21 ns, was obtained. The corresponding modulation frequency can be increased from 19.54 to 30.21 MHz. [ABSTRACT FROM AUTHOR]

In this study, transparent indium tin oxide (ITO) deposited by sputtering was applied to laser lift-off (LLO) GaN-based vertical light-emitting diodes (VLEDs) and the electrical and optical properties of ITO films were measured as a function of annealing conditions. The measured minimum resistivity of ITO film was about 3.78×10-4 Ω cm and the measured optical transmittance at 460 nm was 96.8% after the annealing process. In this condition, about 1×10-5 Ω cm2 of ITO contact resistance to LLO n-GaN could be obtained. By applying the transparent ITO layer to the LLO GaN-based VLEDs, a significant decrease of the forward operating voltage from 3.3 to 3.8 V at 20 mA could be obtained. [ABSTRACT FROM AUTHOR]

The performance and efficiency of molecularly doped organic light-emitting devices (OLEDs) using voltage pulses have been investigated. The maximum current density and peak luminance have been found to depend on the pulse duration and device size, which was attributed to the heat effect in device. The self-quenching of dye molecules, especially at a high current density of ∼A/cm2, can be effectively inhibited using a low concentration of acceptor in donor system as a light-emitting layer (EML). For an OLED using a 0.4-mol % rubrene in Alq3 as an EML, a maximum peak luminance of 5.66×106 cd/m2 at 52.8 V can be sustained under the voltage pulses of duration of 5 μs. The current efficiency has been observed to decrease rapidly with the current density in device, which was believed to result from the annihilation of excitons. We have also investigated the performance of a pulsed OLED with an emitter layer of 4.0-mol % 1,4-bis[2-[4-[N,N-di(p-tolyl)amino]phenyl]vinyl]benzene in 4,4′-bis(9-carbazolyl)biphenyl, which has been known to exhibit low-amplified spontaneous emission under a pulsed nitrogen laser. [ABSTRACT FROM AUTHOR]

We measured delayed electroluminescence in small-molecule-based organic light-emitting diodes based on N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine hole-transport molecule and tris(8-hydroxyquinoline) aluminum electron-transport and emitter molecule after the excitation currents are switched off and reverse bias applied to the sample. The experiments indicate that delayed light emission is a result of two distinct processes: emissive excited singlet-state generation by either triplet-triplet annihilation or recombination of trapped positive and negative charges in the device. Under reverse device bias these two mechanisms have distinctly different signatures. We also found that upon device aging, delayed light emission decreases faster (by about a factor of 4–5) than prompt electroluminescence, which is attributed to an increase of the triplet decay rate due to the presence of aging induced spin-1/2 trapped charges in the device. [ABSTRACT FROM AUTHOR]

The low-frequency noise sources are investigated in as-prepared and aged GaN light-emitting diodes (LEDs). Accelerated aging is performed by thermal (300 h at 240 °C) and electrical forward-bias stressing (20 and 50 mA for 2500 h). At low currents I

This work presents the effects of electron traps in organic light-emitting diodes using a model which includes charge injection, transport, and recombination. For electron-only devices, the electron current is reduced by the traps for several orders of magnitude at fixed voltage, and the traps strongly increase the transient time. For bipolar devices, due to negative trapped charges, traps enhance the hole current and the total current, opposite to the electron-only devices. The traps also make the recombination region close to the cathode. There is a voltage-dependent critical trap density beyond which the quantum efficiency decreases and transient time rises dramatically. The quantum efficiency is doubled if the hole traps are added to balance the electron and hole injections. Finally, the trap effect can be used in a bilayer light-emitting diode to make the emission color-tunable. [ABSTRACT FROM AUTHOR]

The coupling effect between the cavity mode and excitonic states in a GaAs/GaAlAs asymmetric microcavity structure was studied via angle and temperature dependent reflectance and piezoreflectance (PzR) measurements in the angular range 10°<θinc<60° and temperature range 20 K

Three primary colors, red, green, and blue have been obtained from a single-emission layer organic light-emitting diode (OLED) through optical design using a half-wavelength all-metal-cavity device. Fullerene is used as an electron transport layer to further enhance the electrical performance of the cavity device and the optical tuning of the cavity OLED. This fullerene layer results in a ∼2 V driving voltage reduction and a ∼20% increase in power efficiency, as compared with traditional cavity OLED with Alq as the electron transport layer. The emissive spectra for the cavity OLEDs are well predicted by the Fabry–Perot cavity theory. The spectral narrowing and intensity enhancement at the resonance wavelength have been observed and are explained by the redistribution of optical-mode density inside the microcavity. Schemes to tune the emissive color by varying the cavity length through variations of indium tin oxide thickness, hole transport layer thickness, and electron transport thickness, individually or collectively, have been proposed and demonstrated. [ABSTRACT FROM AUTHOR]

Using numerical modeling techniques we predict the emission into air, substrate, polymer/indium tin oxide (ITO), and surface-plasmon modes of a polymer light-emitting diode. The results give good insight into the possible efficiency increases that can be expected for various light extraction techniques. In addition, the effects of various optical properties and layer thicknesses on the optical performance of the device are reported. We show how modification of the substrate can be used to concentrate light into mode types from which light can be easily extracted. We then report experimental results for two very different light extraction techniques. First, we demonstrate how Bragg gratings can be used to extract light from waveguide modes in the polymer/ITO layers. With an optimized Bragg grating, we have increased the external power efficiency by 25% at high brightness levels. In addition, we have used substrate-shaping techniques to extract light from waveguide modes in the glass substrate. In this case, we were able to increase the efficiency by over 60%. Finally, we compare our numerical and experimental results to summarize the possible benefits of various light extraction strategies. [ABSTRACT FROM AUTHOR]

Trichromatic white-light sources based on light-emitting diodes (LEDs) offer a high luminous efficacy of radiation, a broad range of color temperatures and excellent color-rendering properties with color-rendering indices (CRIs) exceeding 85. An analysis of the luminous efficacy and CRI of a trichromatic light source is performed for a very broad range of wavelength combinations. The peak emission wavelength, spectral width, and the output power of LEDs strongly depend on temperature and the dependencies for red, green, and blue LEDs are established. A detailed analysis of the temperature dependence of trichromatic white LED sources reveals that the luminous efficacy decreases, the color temperature increases, the CRI decreases and the chromaticity point shifts towards the blue as the junction temperature increases. A high CRI>80 can be maintained, by adjusting the LED power ratio so that the chromaticity point is conserved. [ABSTRACT FROM AUTHOR]

Presents information on a study that investigated the transport processes and degradation of GaP:N green light-emitting diodes using electrically detected magnetic response. Experimental procedure; Results and discussion on the study; Conclusions.

This article discusses the use of light-emitting diodes to generate white light – a research forefront in Physics and Ergonomics. We first present various technological approaches to white-light generation. After a general introduction to the human vision system, we discuss two key aspects of the quality of white light: the color of the light itself, and the color rendering of illuminated objects. We present the tools underlying modern color science, and review key color rendering metrics, from the well-known color rendering index to the latest improvements in the field. [ABSTRACT FROM AUTHOR]

We have demonstrated the improvement of the light-output power of GaN-based light-emitting diodes (LEDs) using hole-patterned indium tin oxide (ITO) p-type electrodes. Hole patterns were defined by a laser holographic lithography combined with a postlithography deposition process. It is shown that near-UV LEDs made with the patterned ITO with a hole period of 710 nm and a size of 320 nm give 23% and 67% higher light-output power (at 20 mA) than those of LEDs with unpatterned ITO and Ni/Au contacts, respectively. It is further shown that the reduction of the hole period results in an additional improvement of light-output power. [ABSTRACT FROM AUTHOR]

The characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminum with an ultrathin tetrahedral amorphous carbon (ta-C) film on indium tin oxide have been investigated. The device with a 1.0-nm ta-C layer has the highest current and power efficiency. The current efficiency of a device with a ta-C layer thickness of 1.0 nm is 3.7 cd/A at 20 mA/cm2, however, the current efficiency of a standard device without a ta-C layer is 2.56 cd/A at the same current density. The current efficiency is improved by 46% compared to the standard device. Although there is a 1.2-V increase in driving voltage for 100-cd/m2 luminance, the power efficiency is still improved by 22% compared to that of the standard device. The improvement of the efficiency is due to smoothing indium tin oxide surface, blocking hole injection from anode and balancing hole and electron currents. The optimal thickness of ta-C layer for hole injection mechanics can be understood by tunneling. [ABSTRACT FROM AUTHOR]

The current-voltage characteristics of poly [2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV)-based hole-only light-emitting diodes are measured as a function of temperature. The hole current is found to be space-charge limited, providing a direct measure of the mobility as a function of temperature and electric field. A thermal activation energy of 0.2 eV is obtained for the zero-field mobility, with a room-temperature low-field mobility value for holes of 3.3×10-7 cm2/V s. The hole mobility exhibits field dependence in accordance with the Poole-Frenkel effect. The combination of space-charge effects and field-dependent mobility thus provides a consistent description of hole transport as a function of temperature and bias voltage in MEH-PPV films. [ABSTRACT FROM AUTHOR]

The growth, fabrication, and device characterization of InP quantum-dot light-emitting diodes based on GaP are described and discussed. The diode structures are grown on gallium phosphide substrates using gas-source molecular-beam epitaxy and the active region of the diode consists of self-assembled InP quantum dots embedded in a GaP matrix. Red electroluminescence originating from direct band-gap emission from the InP quantum dots is observed at low temperatures.With increasing temperature, however, the emission line shifts to the longer wavelength. The emission light is measured to above room temperature. [ABSTRACT FROM AUTHOR]

Originating from a non-radiative recombination in the active region of a light-emitting diode (LED), the self-heating effect was investigated under continuous wave ( cw ) operation with a GaInN blue LED. Current–voltage (I–V) characteristics under pulse operation that lacked self-heating were measured and compared with the corresponding I–V measurement under the cw operation. Accordingly, a comparison of the measured I–V curves enabled estimation of the junction temperature of the LED during the cw operation. The junction temperature of the device increased as high as 323 K at an input current of 480 mA. The operating current showed a weaker temperature dependence in accordance with the bias increase, which is explained by the expansion of the Shockley diode equation. [ABSTRACT FROM AUTHOR]

Optimization of light-emitting diode (LED) design is highly dependent on the relationship between photometric, electric, and thermal parameters of the device. The photoelectrothermal (PET) theory connects all the features of LED device. In this paper, wall-plug efficiency equations of PET theory were extended to estimate heat-dissipation factor and optical power of low-power surface-mounted device (SMD) LED. The wall-plug efficiency as a function of ambient temperature and injection current is used to predict the heat-dissipation factor and the optical power of the SMD LED. The TeraLED and T3ster systems were utilized to provide optical and thermal test reports of the LED parameters at a certain range of injection current and ambient temperature. The presented equations were verified by testing on low-power SMD LED. Results demonstrate that the estimated and measured values were in good agreement for all the considered parameters. This validates the extension of PET theory equations for low-power SMD LED. [ABSTRACT FROM AUTHOR]

We demonstrate electrically driven single photon emission from a CdSe/ZnSSe/MgS single quantum dot embedded in a resonant cavity light emitting diode. Patterned Pd/Au top contacts are used to inject the charge carriers locally. Single quantum dot electroluminescence is obtained at T = 4 K from nanoapertures lithographically defined in the top contacts. At low current densities, antibunching with a value of g(2)(0) = 0.16 is achieved. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Highlights: [•] A very simplified HWOLED with P–F structure has been successfully developed. [•] The forward-viewing CE and PE are as high as 59.4cd/A and PE of 58.4lm/W, respectively. [•] A simple but effective way towards high-efficiency HWOLEDs has been demonstrated. [•] The origin of this high-efficiency device is explored comprehensively. [Copyright &y& Elsevier]

GaN-based light-emitting diodes (LEDs) rapidly degraded by a current-crowding-induced self-accelerating thermal process were investigated. The LEDs stressed at a dc current of 200 mA and a temperature of 100^\circC exhibited a rapid degradation rate of 2.7\times 10^-3~h^-1, while those stressed at 100 mA showed a relatively slow degradation rate of 3.0\times 10^-4~h^-1, in which the degradation rate corresponds to a slope of optical output power versus time plots. The corresponding degradation kinetics were also different depending on the stress current, i.e., the generation of nonradiative recombination defects was mainly responsible for the degradation of the 100-mA stressed LEDs, while multiple degradation kinetics including a drastic increase of defects, contact failure, and chip detachment from the lead frame were responsible for the rapid optical degradation of the 200-mA stressed LEDs. The origin of the multiple degradation kinetics could be the current-crowding effect, which causes local heating of the device and hence a self-accelerating thermal process. The rapidly degraded LEDs consistently showed a very large junction temperature from 36^\circC (pristine) to 74^\circC (failed). These results show that the LEDs should be avoided when current crowding occurs under harsh conditions in reliable lifetime testing and/or for practical use. [ABSTRACT FROM AUTHOR]

The articles in this special issue focus on the history and development of light emitting diode (LED) technology and industry applications over the last fifty years. [ABSTRACT FROM PUBLISHER]

Schichtbetrieb: Ein vernetzbares Wirt ‐ Gast ‐ System für moderne mehrschichtige Bauteile wurde entwickelt. Die Spirobifluoren ‐ basierten Wirtmaterialien wurden mit grün emittierenden phosphoreszierenden (fac) ‐ IrIII ‐ Komplexen vernetzt. So konnte eine fünfschichtige organische Leuchtdiode (OLED) produziert werden, in der jede organische Schicht aus der Lösung abgeschieden wurde. [ABSTRACT FROM AUTHOR]

Divalent europium doped barium orthosilicate is a very important phosphor for the production of light emitting diodes (LEDs), generally associated to the green emission color of micron-sized crystals synthesized by means of solid-state reactions. This work presents the combustion synthesis as an energy and time-saving preparation method for very small nano-sized Ba2SiO4 particles, flexibly doped to acquire different emission energies. The size of the resulting spherical nanoparticles (NPs) of the green emitting Ba2SiO4:Eu2+ was estimated to about 35 nm applying the Scherrer equation and further characterized with aid of atomic force microscopy (AFM) as well as scanning electron microscopy (SEM). This phosphor is able to build homogeneous luminescent suspensions and was successfully down-sized without changing the optical properties in comparison to the bulk phosphors. Besides the X-ray diffraction (XRD) analysis and the different types of microscopy, the samples were characterized by luminescence spectroscopy. Undoped Ba2SiO4 NPs are not luminescent, but show characteristic red emission of the 5D0 → 7FJ (J = 0-4) electronic transitions when doped with Eu3+ ions. Moreover, these orthosilicate nanoparticles generate blue light at low temperatures due to impurity-trapped excitons, introduced by the partial substitution of the Ba2+ with Sr2+ ions in the Ba2SiO4 lattice causing a substantial distortion. A model for the temperature behavior of the defect luminescence as well as for their nature is provided, based on temperature-dependent luminescence spectra and lifetime measurements. [ABSTRACT FROM AUTHOR]

The contributing factors that determine the reliability characteristics of GaN-based white light-emitting diodes (WLEDs) with dual degradation kinetics were investigated. For this study, an aging test of WLEDs was performed at temperatures of 60 ^\circ\C, 80 ^\circ\C, and 100 ^\circ\C under a constant dc of 300 mA. Evidence for dual degradation kinetics could be observed by the optical output degradation occurring with the modification of chromatic properties (due to the deterioration of packaging materials) and the evolution of leakage currents (due to the generation of nonradiative defects). By comparing temperature-dependent output degradation with the modification of chromatic properties or the evolution of leakage currents, the contributing factor that predominantly determines the reliability characteristics was estimated, i.e., the contributing factor for generated nonradiative defects versus the degraded packaging materials was 75:25 at 60 ^\circ\C and 32:68 at 100 ^\circ\C. This indicates that the predominant degradation kinetics change as the aging temperature varies. The activation energy of WLEDs was shown to be as low as 0.33 eV using Arrhenius plots of half-lifetime, which is attributed to the dual degradation kinetics. [ABSTRACT FROM AUTHOR]

In this paper, we present simulation results on the optical and transport properties of InGaN/GaN core-shell nanorod light-emitting diodes. The influence of contact position, surface recombination, and doping configuration on internal quantum efficiency is examined. The qualitative behavior when adding an electron blocking layer and the dependence on In content have been studied. [ABSTRACT FROM AUTHOR]

Multi-channel light-emitting diode (LED) light sources have been developed for various applications in lighting, but especially for spectrum-tunable lighting. However, how to select the optimal combination of LEDs for these light sources is rarely discussed. Unlike the negative-pruning process which operates by removing LEDs from a set of available LEDs based on an unrealizable negative-coefficient criterion alone, the proposed pruning process removes LEDs so as to minimize the synthesis error. This method not only produces the same optimal set of LEDs as those identified by the full-search method but also suggests when using phosphor-converted LEDs is beneficial. It does this by investigating the trade-off between the full width half maximum of the LEDs and the number of LEDs. [ABSTRACT FROM AUTHOR]

The exact relationship between voltage and frequency when negative capacitance (NC) appears in light-emitting diodes containing InGaN was determined both experimentally and by solving the physical equations for the complete equivalent circuit of a symmetrical p-n junction combined with its boundary condition. Results from the calculation revealed that NC depends on the recombination lifetime of carriers; therefore, we can accurately determine recombination lifetime from the appearance of NC. [ABSTRACT FROM AUTHOR]

The synthesis and characterisation of 2,5-bis(5'-hexyl-[2,2'-bithiophen]-5-yl)pyridine (Th4PY) and its use as a blue emitter in organic light emitting diodes (OLEDs) is reported. Th4PY was synthesised in high yield using a straightforward Suzuki coupling route with commercially available starting materials. As Th4PY is both soluble and has low molecular weight, blue OLEDs were fabricated using both spin-coating and vacuum deposition thin film processing techniques to study the effect of processing on device performance. OLED devices using a spin-coated layer consisting of 4',4"-tris(N-carbazolyl)triphenylamine (TCTA) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) as a host matrix together with Th4PY as emitter exhibited highly efficient sky-blue emission with a low turn-on voltage of 3 V, a maximum brightness close to 15000 cd m[sup -2] at 8 V, and a maximum luminous efficiency of 7.4 cd A[sup -1] (6.3 lm W[sup -1]) with CIE coordinates of x = 0.212, y = 0.320. The device performance characteristics are compared using various matrices and processing techniques. The promising sky-blue OLED performance, solution processability, and ambient stability make Th4PY a promising blue emitter for application in OLEDs. The synthesis, characterisation, and blue organic light emitting diodes (OLEDs) device characteristics using 2,5-bis(5'-hexyl-[2,2'-bithiophen]-5-yl)pyridine (Th4PY) as a blue emitter are reported. The OLED using 4',4"-tris(N-carbazolyl)triphenylamine (TCTA) matrix with Th4PY emitter exhibited highly efficient sky-blue emission with a low turn-on voltage of 3 V and a maximum brightness close to 15000 cd m[sup -2] at 8 V, and a maximum luminous efficiency of 7.4 cd A[sup -1] (6.3 lm W[sup -1]) with CIE coordinates of x = 0.21, y = 0.32. [ABSTRACT FROM AUTHOR]

We proposed an optical wireless position sensing system using light emitting diodes (LEDs), demonstrating it via experiment.During experiment, we used four LED lamps composed of 16 LED chips with a view angle of 65° at half power. Further, we assigned a unique address for each LED lamp, being transmitted using LED properties such as lighting and switching. Afterward, we used correlation to locate the target. The proposed positioning system displayed 4.38 cm of mean distance error among 36 points with dimensions of 1 × 1 × 1.2 m3. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1622-1626, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26880 [ABSTRACT FROM AUTHOR]

Abstract: Many manufacturers and distributors of LED tubes claim energy savings of 50% and more when replacing T8 fluorescent tubes with linear LED replacement lamps. Several distributors even pretend that the same visual comfort will be maintained after such a replacement. Optical and electrical parameters of twelve commercially available linear LED tubes have been determined and the evolution in time of these parameters has been monitored. Additionally, a case study is presented in which the fluorescent lamps in a small office room were replaced by LED linear replacement lamps in order to compare the illuminance distribution on the work plane, the glare perception and the overall visual appreciation. According to this study, it is clear that a one-to-one replacement of a classical fluorescent tube by a currently available linear LED lamp might have severe consequences on the lighting quality. [Copyright &y& Elsevier]

In InGaN quantum wells (QWs), effective active volume can be greatly reduced due to carrier localization in In-rich region and inhomogeneous carrier distribution. The authors investigate the efficiency droop of InGaN-based light-emitting diodes (LEDs) based on the carrier rate equation including the influence of the reduced effective active volume. It is found that efficiency droop characteristics can be modeled well without employing a large Auger recombination coefficient by assuming that only a small portion of the QWs is effectively used as active region. The presented model is expected to provide insight into the realization of droop-free operation in nitride LEDs. [ABSTRACT FROM AUTHOR]

Abstract: For the purposes of reducing the non-uniformity of images in LED display panel further, propose an algorithm. First, analyze the causes of the problem which images in LED display panel have shortcomings of the non-uniformity of images. Second, introduce the development course of the correction techniques which are used to reduce the nonuniformity of images. Simultaneously, point out the ground that location correction must be carried out. Third, for the example of a LED display panel which is spliced by modules, construct the algorithm model. Forth, describe the realization steps of the algorithm in accordance with this constructed model. Finally, this algorithm is implemented in a LED display panel, whose resolution is 640×480. Experimental results show that this algorithm is able to reduce the non-uniformity of images obviously. [Copyright &y& Elsevier]

A variety of light sources and reagents are available for the detection and identification of latent prints. This study was undertaken to explore the optimum light and filter combinations of laser and light-emitting diode (LED) light for use with indanedione and two new reagents, genipin and lawsone. The light sources utilized were Coherent TracER lasers operating at 460 nm, 532 nm, and 577 nm and the Rofin Polilight Flare Plus LED operating at 505 nm. Deliberate and randomly created latent prints were first examined utilizing the light sources alone and then again following treatment with the chemical reagents. Results indicated that treatment with indanedione-zinc chloride was the most effective at the excitation of latent prints. With the exception of the 577 nm laser and genipin, the two new reagents, genipin and lawsone, did not provide useful results under test conditions. Although the LED light source revealed a significant number of untreated impressions, the laser light source proved to be more sensitive at detecting untreated impressions, and the ridge clarity was frequently higher on the samples examined. Monochromatic sources (lasers) and broadband sources such as LEDs each exhibited the potential to detect evidence missed by the other. [ABSTRACT FROM AUTHOR]