Orange Light-emitting Diodes Research Articles

Fe‐Doped p‐ZnO Nanostructures/n‐GaN Heterojunction for “Blue‐Free” Orange Light‐Emitting Diodes

In order to obtain the necessary band gap for light‐emitting diodes (LEDs) with zero emission at blue wavelengths (“blue‐free”), quasi‐3D nanostructures of p‐type Fe‐doped zinc oxide (ZnO:Fe) are fabricated on an n‐type GaN substrate. The ZnO:Fe nanostructure comprises an array of vertical nanowires attached at the nodes of a 2D network. Elemental analysis and field‐effect‐transistor (FET) and current–voltage (I–V) measurements indicate the successful iron‐doping of ZnO. After doping, the ZnO exhibits p‐type conductivity, a local‐charge‐reservoir layer, and an abundance of Fe‐related deep levels. The cathodoluminescence (CL), photoluminescence (PL) at various temperatures (down to 4.65 K), and electroluminescence (EL) confirm the creation of Fe‐related donor and acceptor levels. New donor levels may be attributed to the substitution of Fe for some Zn sites (“FeZn”), while new acceptor levels may be due to the bind of FeZn with Zn vacancies, (producing FeZn–VZn pairs). These doping‐induced energy levels are helpful in restricting the intrinsic ZnO UV and blue emission, and a stable blue‐free orange LED device is achieved. It has chromaticity coordinates of ∼(0.483, 0.447) and a color temperature of ∼2574 K under bias voltages of 6–16 V, making it potentially applicable to electronic display systems.

Highly Simplified Reddish Orange Phosphorescent Organic Light-Emitting Diodes Incorporating a Novel Carrier- and Exciton-Confining Spiro-Exciplex-Forming Host for Reduced Efficiency Roll-off.

A novel exciplex-forming host is applied so as to design highly simplified reddish orange light-emitting diodes (OLEDs) with low driving voltage, high efficiency, and an extraordinarily low efficiency roll-off, by combining N,N-10-triphenyl-10H-spiro [acridine-9,9'-fluoren]-3'-amine (SAFDPA) with 4,7-diphenyl-1,10-phenanthroline (Bphen) doped with trivalent iridium complex bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)iridium(III) (Ir(MDQ)2(acac)). The reddish orange OLEDs achieve a strikingly high power efficiency (PE) of 31.80 lm/W with an ultralow threshold voltage of 2.24 V which is almost equal to the triplet energy level of the phosphorescent reddish orange emitting dopant. The power efficiency of the device with the exciplex-forming host is enhanced, achieving 36.2% mainly owing to the lower operating voltage by the novel exciplex forming cohost, compared with the reference device (23.54 lm/W). Moreover, the OLEDs show extraordinarily low current efficiency (CE) roll-off to 1.41% at the brightness from 500 to 5000 cd/m2 with a maximal CE of 32.87 cd/A (EQEmax = 11.01%). The devices display a good reddish orange color (CIE of (0.628, 0.372) at 500 cd/m2) nearly without color shift with increasing brightness. Co-host architecture phosphorescent OLEDs show a simpler device structure, lower working voltage, and a better efficiency and stability than those of the reference devices without the cohost architecture, which helps to simplify the OLED structure, lower the cost, and popularize OLED technology.

Demonstration of InGaN-based orange LEDs with hybrid multiple-quantum-wells structure

We demonstrate the effectiveness of a hybrid multiple-quantum-wells (MQWs) structure in InGaN-based orange light-emitting diodes (LEDs) grown by metalorganic vapor phase epitaxy. The hybrid MQWs-LED is composed of orange InGaN double QWs and a blue-green InGaN single QW. Using the hybrid MQWs structure, the orange LEDs exhibited electroluminescence spectra with narrow full widths at half maximum of 51 nm at 20 mA. The light output power and external quantum efficiency of the InGaN-based orange LEDs were 0.23 mW and 0.6%, respectively, at 20 mA.

Implantable self-reset CMOS image sensor and its application to hemodynamic response detection in living mouse brain

A self-reset pixel of 15 × 15 µm2 with high signal-to-noise ratio (effective peak SNR ≃64 dB) for an implantable image sensor has been developed for intrinsic signal detection arising from hemodynamic responses in a living mouse brain. For detecting local conversion between oxyhemoglobin (HbO) and deoxyhemoglobin (HbR) in brain tissues, an implantable imaging device was fabricated with our newly designed self-reset image sensor and orange light-emitting diodes (LEDs; λ = 605 nm). We demonstrated imaging of hemodynamic responses in the sensory cortical area accompanied by forelimb stimulation of a living mouse. The implantable imaging device for intrinsic signal detection is expected to be a powerful tool to measure brain activities in living animals used in behavioral analysis.

Effects of spectral distribution and photosynthetic photon flux density for overnight LED light irradiation on tomato seedling growth and leaf injury

We investigated the effects of spectral distribution and photosynthetic photon flux density (PPFD) for overnight light irradiation supplied by light-emitting diodes (LEDs) on the growth and degree of continuous light (CL)-induced leaf injury of tomato seedlings in a growth-chamber experiment. Over a 14-day treatment period, all plants were irradiated daily for 12h with white LED light at 300μmolm−2s−1 PPFD. During each 12-h nighttime interval, all plants except for the controls were irradiated with white LED light at 150 or 300μmolm−2s−1, or alternatively with blue, orange, or red LED light at 150μmolm−2s−1. Plants irradiated overnight exhibited injuries, with their visually rated severity of injury increased as the PPFD of nighttime white light was increased. In plants irradiated overnight at 150μmolm−2s−1, a higher degree of injury was experienced under blue light than under orange and red light. Overnight white-light irradiation at 150μmolm−2s−1 PPFD promoted dry matter production compared with the no-irradiation control, but no further increase was observed at a PPFD of 300μmolm−2s−1. Among different spectra, blue light was less effective than orange light at promoting dry matter production when white light was supplied during the day. Thus, both spectral distribution and PPFD of overnight LED light irradiation were demonstrated to affect tomato seedling growth and degree of injury.

Condenser-free contrast methods for transmitted-light microscopy.

Phase contrast microscopy allows the study of highly transparent yet detail-rich specimens by producing intensity contrast from phase objects within the sample. Presented here is a generalized phase contrast illumination schema in which condenser optics are entirely abrogated, yielding a condenser-free yet highly effective method of obtaining phase contrast in transmitted-light microscopy. A ring of light emitting diodes (LEDs) is positioned within the light-path such that observation of the objective back focal plane places the illuminating ring in appropriate conjunction with the phase ring. It is demonstrated that true Zernike phase contrast is obtained, whose geometry can be flexibly manipulated to provide an arbitrary working distance between illuminator and sample. Condenser-free phase contrast is demonstrated across a range of magnifications (4–100×), numerical apertures (0.13–1.65NA) and conventional phase positions. Also demonstrated is condenser-free darkfield microscopy as well as combinatorial contrast including Rheinberg illumination and simultaneous, colour-contrasted, brightfield, darkfield and Zernike phase contrast. By providing enhanced and arbitrary working space above the preparation, a range of concurrent imaging and electrophysiological techniques will be technically facilitated. Condenser-free phase contrast is demonstrated in conjunction with scanning ion conductance microscopy (SICM), using a notched ring to admit the scanned probe. The compact, versatile LED illumination schema will further lend itself to novel next-generation transmitted-light microscopy designs. The condenser-free illumination method, using rings of independent or radially-scanned emitters, may be exploited in future in other electromagnetic wavebands, including X-rays or the infrared.

Rare-Earth-Free Direct-Emitting Light-Emitting Diodes for Solid-State Lighting

The advent of a wide-bandgap GaN p-n junction has enabled highly efficient blue light-emitting diodes (LEDs) in GaInN/GaN heteroepitaxy. The system also enables a much wider range of emission wavelengths. We summarize progress in epitaxial materials development of green, yellow, and orange direct-emitting LEDs that bypass the steps of external phosphor conversion to achieve higher stability, efficiency, and higher overall color rendering quality for wider adoption of one of the widest power savings resources so far identified.

Regulation of the growth and photosynthesis of cherry tomato seedlings by different light irradiations of light emitting diodes (LED)

The growth and photosynthetic characteristics of cherry tomato seedlings were investigated under seven light irradiations such as dysprosium lamps (white light; control, C), red light emitting diodes (LEDs) (R), blue LEDs (B), orange LEDs (O), green LEDs (G), red and blue LEDs (RB) and red, blue and green LEDs (RBG) with the same photosynthetic photon flux density (about 320 μmol m-2 s-1) for 30 days. Morphological appearances of seedlings were significantly different between light treatments, that is, the plants under RB and RBG were shorter and stronger than those under C, while those under O, G and R were higher and weaker. The higher carbohydrate contents were in plants containing blue treatment, B, RB and RBG. Photosynthetic pigments were shown to have significant difference under respective light irradiations of LEDs. The higher photosynthetic pigments were in leaves of seedlings containing blue light treatment, RBG, RB, B, C and G treatments, the lower the pigments were in those with R and O treatments. Net photosynthesis (Pn) was the highest in leaves of seedlings with RB and RBG and the lowest in those with G. Compared with C treatment, light compensation point and light saturation point of seedlings with R, RB and RBG increased, but those with O and G decreased. Electron transport rate (ETR), quantum efficiency of photosystem II photochemistry (ΦPSII), photochemical quenching (qP) and efficiency of excitation energy capture by open PSII reaction centres (Fv′/Fm′) in seedlings with B, RB and RBG treatments were significantly greater than those of the other treatments. Taken together, RB and RBG of LEDs were shown to be beneficial factors for the growth and photosynthesis in cherry tomato seedlings. Key words : Light-emitting diode (LED), light quality, cherry tomato, growth, photosynthetic characteristics.

Orange a-plane InGaN/GaN light-emitting diodes grown on r-plane sapphire substrates

We report on orange a-plane light-emitting diodes (LEDs) with InGaN single quantum well (SQW) grown on r-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). The peak wavelength and the full-width at half maximum (FWHM) at a drive current of 20mA were 612.2 nm and 72 nm, respectively. The device demonstrated a blue shift in emission wavelength from 614.6 nm at 10 mA to 607.5 nm at 100 mA, representing a net shift of 7.1 nm over a 90 mA range, which is the longest wavelength compared with reported values in nonpolar LEDs. The polarization ratio values obtained from the orange LED varied between 0.36 and 0.44 from 10 to 100mA and a weak dependence of the polarization ratio on the injection current was observed.

Motorist Understanding of Alternative Displays for Speed Limits in Work Zones

In Texas, speed limits for short-term work zones are reduced speed limits that are posted only when work activity is present. When work activity is not present, the reduced speed limit signs are to be removed or covered; the legal speed limit for that segment reverts to the normal zone speed limit. The major challenge of speed limits for short-term work zones is the daily process of installing and removing, or uncovering and covering, signs. In this study, researchers conducted a laptop-based laboratory survey to determine (a) motorists' understanding and opinions of alternative short-term signs for speed limits in work zones and the enforceability of limits, (b) the success of the orange or white light-emitting diode (LED) color used on electronic speed limit (ESL) signs and portable changeable message signs (PCMSs) in indicating to motorists that they were advisory or regulatory signs, and (c) motorists' preference for short-term speed limit displays. The four categories of signs included in the survey were static work zone signs, ESL signs, speed limits displayed on PCMSs, and “Your Speed” signs (distracters). On the basis of the results obtained from this survey, researchers recommend the use of ESL signs and flexible roll-up signs for speed limits in static work zones for displaying speed limits in short-term work zones. To ensure that the speed limit displayed on ESL signs is considered a regulatory speed limit, the changeable display should be composed of white LEDs.

Optical–thermal characterization of cutaneous transilluminators

In recent years, there has been an increase in the popularity of light-emitting diode (LED)-based, battery-powered transilluminators (BPTs) for facilitating transdermal vascular access in adults and neonates. BPTs are believed to have lower potential for inducing skin burns than prior devices based on high-power broadband lamps; however, the optical and thermal outputs of BPTs are not well documented and safety limits for these devices are not well established. In this study, we characterize and assess the optical and thermal outputs of six BPTs that incorporate red, orange and white LEDs. Optical measurements included spectral irradiance and peak local irradiance. Thermal measurements included transient temperature readings for an exposure time of 4 min in ambient air and ex vivo tissue pre-heated to physiological temperatures. The greatest mean temperature rise produced in tissue by a non-white-light diode BPT was 2.5 °C, whereas a mean temperature rise of 9.1 °C was measured in a BPT that incorporated white-light diodes with relatively high irradiance levels. The dominant cause of temperature rise was most likely heat generation within the devices. Thermal damage analyses based on temperature limits and the Arrhenius equation indicate that although some of the devices studied approach the threshold for damage, none appear to exceed it under normal operating conditions. The results demonstrated that ambient air measurements may be suitable for identifying worst-case BPT temperatures. This study highlights the potential risk of LED-based medical devices as well as the need for additional research on related issues such as neonatal thermal injury thresholds.

A novel orange phosphor of Eu 2+-activated calcium chlorosilicate for white light-emitting diodes

Novel orange phosphor of Eu 2+-activated calcium chlorosilicate was synthesized at 1273 K by conventional solid-state reactions under reductive atmosphere and investigated by means of photoluminescence excitation, diffuse reflectance and emission spectroscopies. These results show that this phosphor can be efficiently excited by the incident light of 300–450 nm, well matched with the emission band of 395 nm-emitting InGaN chip, and emits an intense orange light peaking at 585 nm. By combining this phosphor with a 395 nm-emitting InGaN chip, an intense orange light-emitting diode (LED) was fabricated. Under 20 mA forward-bias current, its CIE chromaticity coordinates are (0.486, 0.446). The dependence of as-fabricated orange LED on forward-bias current indicates that it shows excellent chromaticity stability and luminance saturation. These results show that this Eu 2+-activated calcium chlorosilicate is a promising orange-emitting phosphor for near-ultraviolet (UV) InGaN-based white LED.

Orange and red emitting OLEDs based on phenothiazine polymers

Two novel polymers PQP(poly(3, 7-N-octyl phenothiozinyl cyanoterephthalylidene)) and PQM (poly(3, 7-N-octyl phenothiozinyl cyanoisophthalylidene)) containing phenothiazine were synthesized for application in red and orange light-emitting diodes. The synthesized polymers were characterized using UV-visible, photoluminescence and electroluminescence (EL) spectroscopy. The single-layer EL devices of indium tin oxide(ITO)/PQP(or PQM)/Mg : Ag and multi-layer devices of ITO/PQP(or PQM): N, N′-diphenyl-N, N′-bis(3-methylphenyl) -[1, 1′-biphenyl]-4, 4′ -diamine(TPD) (44 nm)/2, 9-dimethyl-4, 7-diphenyl -1, 10-phenanthroline (BCP, 5 nm)/tris(8-hydroxyquinolinato) aluminium(AlQ3, 20 nm)/Mg : Ag were fabricated. The EL spectra from the devices based on PQP peaked at the wavelength of 664 nm and covered red and infrared regions. The maximal brightness was measured to be about 60 cd m−2 at the applied voltage of 17 V. The devices based on PQM gave bright orange light-emission peaking at 608 nm and the maximal luminance was 150 cd m−2 at the applied voltage of 14 V.

Budding and Bolting Responses of Horticultural Plants to Night-break Treatments with LEDs of Various Colors

We investigated the use of light-emitting diodes (LEDs) of particular wavelength emissions as light sources for night-break treatment of plants. We used the long-day plants godetia (Godetia cvs.), petunia (Petunia×hybrida Vilm.), and spinach (Spinacia oleracea L., two cultivars); and the short-day plants perilla (Perilla ocymoides L.), cosmos (Cosmos bipinnatus Cav.), yellow cosmos (Cosmos sulphureus Cav.), zinnia (Zinnia elegans Jacq.), and Japanese morning glory (Pharbitis nil Choisy). The plants were exposed daily to LED irradiation from 2300 to 0100h during the short-day season (late autumn to early spring). Red and orange LED lights were effective in controlling flowering in all plants used in this study. Yellow and green LEDs were almost as effective. Far-red LED was effective in some, but not all, cases. One spinach cultivar bolted in response to far-red night-break treatment, but the other cultivar did not. Blue LED was effective in only one trial with perilla, but not so effective in other cases.

ZnCdSeTe-based orange light-emitting diode

We report the fabrication of orange light-emitting diodes (LEDs) by introducing Te into the well layers of ZnCdSe-ZnSSe multiple quantum-well active region. It was found that a 15% Te can result in a 91 nm photoluminescence (PL) red-shift at 16 K and a 81 nm PL red-shift at room temperature. Such a ZnSe-based orange LED can be integrated easily with conventional ZnSe-based blue-green LED on the same GaAs substrate. Thus, such a ZnSe-based orange LED is potentially useful in realizing a new one-chip type white LED without the use of phosphor.

Band gap engineering of amorphous silicon quantum dots for light-emitting diodes

Amorphous silicon quantum dots (a-Si QDs), which show a quantum confinement effect were grown in a silicon nitride film by plasma-enhanced chemical vapor deposition. Red, green, blue, and white photoluminescence were observed from the a-Si QD structures by controlling the dot size. An orange light-emitting diode (LED) was fabricated using a-Si QDs with a mean size of 2.0 nm. The turn-on voltage was less than 5 V. An external quantum efficiency of 2×10−3% was also demonstrated. These results show that a LED using a-Si QDs embedded in the silicon nitride film is superior in terms of electrical and optical properties to other Si-based LEDs.

Internal photoluminescence in ZnSe homoepitaxy and application in blue–green–orange mixed-color light-emitting diodes

We discuss the controllable color-range in ZnSe-based light-emitting diodes (LEDs) realized by ZnSe homoepitaxy and internal photoluminescence. ZnSe-based LED structures were grown by molecular-beam epitaxy (MBE) on mostly conductive ZnSe substrates, which exhibit under short wavelength light excitation at room temperature strong orange emission around 600 nm. This fact is exploited to fabricate integrated mixed-color LED chips, where light from the active layer sandwiched in a p–n-junction acts as internal excitation source. We named this effect recently “Internal Photoluminescence” (Wenisch et al., J. Appl. Phys. 82 (1997) 4690). It leads to electroluminescence spectra with two distinct emission peaks originated from the active layer and from the ZnSe substrate, respectively. In view of color impression, just by varying the Cd x Zn 1− x Se quantum-well composition and the radiant recombination rate in the substrate by it's choice, as much as two thirds of the visible color space is covered. Under conditions, when only the substrate emission is present, Commission Internationale d'Eclairage (CIE) chromaticity coordinates for orange color LEDs of (0.54, 0.45, 0.01) for the red, green and blue color, respectively, were determined. 490-nm quantum-well-emitting LEDs were found to be best suited in reaching the technologically important balanced white emission (“White Point”) and a value of (0.31, 0.39, 0.30) for the color coordinates close to it was experimentally achieved.

Reddish orange light-emitting diodes made with N,N′-bis (1-naphthyl) -3,4,9,10-perylenbis(dicarboximide)

The electroluminescence (EL) of N,N′-bis(1-naphthyl)-3,4,9,10-perylenbis(dicarboximide) (BPD) is reported. BPD is an electron acceptor. Red emission from multilayered light-emitting diodes (LEDs) using BPD as light-emitting layer with a maximum brightness of 26 cd/m 2 was observed. The metal materials of the cathode have an effect on both the turn-on voltage and the peaks and shapes of the EL spectra of the devices. This implies that some kind of interaction exists between BPD and the cathode. In LEDs with both hole transport layer and electron transport layer, it was found that the peaks and shapes of the EL spectra of BPD change greatly with the applied bias. When the forward biased voltage is low, the EL emission colour is scarlet. As the voltage increases, the emission colour gradually blue-shifts to reddish orange. Fine structure of the EL spectrum was observed.

High reliability of AlGaInP LED's with efficient transparent contacts for spatially uniform light emission

The indium-tin oxide (ITO) films with low sheet resistance and conducting transparency have been used on the AlGaInP double-heterostructure orange light-emitting diodes (LED's) to improve the light extraction from the LED surface and device reliability. The orange LED's with ITO films exhibit a light output power of 1 mW at 50 mA, which corresponds to an external quantum efficiency of 1.1%. They can operate stably for more than 3000 h at 20 mA and 25/spl deg/C without any degradation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Highly reliable operation of indium tin oxide AlGaInP orange light-emitting diodes

Indium-tin oxide (ITO) transparent conducting films with a low resistivity of 2–3 × 10-4 Ω·cm have been introduced as a window layer in (Al0.7Ga0.7)0.5In0.5P/(Al0.1Ga0.9)0.5In0.5P double-heterostructure orange light-emitting diodes to obtain a uniform spatial distribution of the emission light, a good device performance and high reliability. An output power of 450 µW at 20 mA corresponding to an external quantum efficiency of 1.1% for the 620 nm emission can be achieved.