High-power phosphor-converted light-emitting diodes based onIII-Nitrides

Phosphor conversion of light-emitting diode light for white light sources and some monochrome applications requires particular phosphor properties and has to take into account specific issues if aimed at high-power output. Limitations and solutions will be discussed, giving special considerations to drive and temperature dependencies. Efficiencies of 32 lm/W for white with good color rendering at 4600 K and 35 lm/W for green (535 nm) have been demonstrated     

Compact ultrabroadband light-emitting diodes based on lanthanide-doped lead-free double perovskites

Impurity doping is an effective approach to tuning the optoelectronic performance of host materials by imparting extrinsic electronic channels. Herein, a family...     

High-efficiency organic light-emitting diodes

Progress in understanding the major loss mechanisms have led to rapid improvements in efficiency that have meant that OLEDs will soon be the material of choice for many display applications. In this review, we will discuss in details the main factors that need to be addressed in order to optimise the overall device performance     

Introduction to the issue on high-efficiency light-emitting diodes

Since their inception in the 1960s, light-emitting diodes (LEDs) have enjoyed sustained rapid development in terms of efficiency, available emission spectrum, higher power, higher speed, and application methods. These improvements, coupled with the inherent properties of solid-state devices-ruggedness, low-voltage operation, high reliability, and low cost-have catapulted LEDs into a wide range of applications, from telecommunications, to solid-state lighting, to displays. In each case, LEDs are increasingly replacing the conventional technologies. Even more exciting is the fact that progress in LED technology in each of these areas is continuing at very rapid rate, ensuring that LEDs will continue to see accelerated growth in new and emerging markets, and will find use in applications that would have been considered impossible just a few years ago. Indeed, in this special issue, Watt-class visible-spectrum LEDs are presented for the first time. Here we have 9 invited and 13 contributed papers that give a comprehensive cross-sectional view of the current state-of-the-art in LED technology. The papers are grouped into four main areas: communications, solid-state lighting, ultraviolet emitters, and organic LEDs     

Electrical characterization of polymer light-emitting diodes

This paper presents a device model for the current and light generation of polymer light-emitting diodes (PLEDs). The model is based on experiments carried out on poly(dialkoxy-p-phenylene vinylene) (PPV) devices. It is demonstrated that PLED's are fundamentally different as compared to conventional inorganic LEDs. The hole conduction in PPV is space-charge limited with a low-field mobility of only 5×10^-11 m²/Vs, which originates from the localized nature of the charge carriers. Furthermore, the hole mobility is highly dependent on the electric field and the temperature. The electron conduction in PPV is strongly reduced by the presence of traps. Combining the results of the electron- and hole transport a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. It is calculated that the unbalanced electron and hole transport gives rise to a bias dependent efficiency. By comparison with experiment it is found that the bimolecular recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes toward each other. This is in contrast to conventional semiconductors, in which the bimolecular recombination is governed by the joint density-of-states of electrons and holes and is not limited by charge transport. The occurrence of Langevin recombination explains why the conversion efficiency of current into light of a PLED is temperature independent. The understanding of the device operation of PLED's indicates directions for further improvement of the performance     

GaN-based light-emitting diodes using tunnel junctions

We demonstrate GaN-based light-emitting diodes (LEDs) with tunnel junction (TJ) structure and surface-emitting light-emitting diodes with TJ current aperture for lateral current confinement. The p/sup +//n/sup +/ GaN TJs are located in the upper cladding layers of conventional devices, allowing n-type GaN instead of p-type GaN as a top contact layer. The reverse-biased tunnel contact junction provides lateral current spreading without a semitransparent electrode and spatially uniform luminescence exhibiting an improved radiative efficiency. Also, the current confinement aperture for the lateral injection current in the LEDs was defined by mesa etching of a TJ structure and regrowth of the current blocking layer surrounding the TJ mesa. The very uniform light emission just through a buried TJ aperture confirms that the buried TJ structure acts very effectively as a confinement aperture of lateral current injection, particularly in GaN-based vertical-cavity surface-emitting lasers.     

Research progress of full electroluminescent white light-emitting diodes based on a single emissive layer

Carbon neutrality, energy savings, and lighting costs and quality have always led to urgent demand for lighting technology innovation. White light-emitting diod...     

Recent progress in group-III nitride light-emitting diodes

We review the recent progress of nitride-based light-emitting diodes (LEDs) and discuss the dislocation issue and luminous efficiency. First, candela-class blue LEDs have been developed. InGaN layer was used for nitride LEDs instead of GaN active layer. The quantum-well-structure InGaN active layer dramatically improved the external quantum efficiency. There are a number of threading dislocations in epitaxial layer of nitride-based LEDs. InGaN-LEDs, however, have quite high external quantum efficiency. With regard to this, it is thought that the fluctuation of indium mole fraction is strongly related to the high external quantum efficiency. We also discuss the method to improve the external quantum efficiency of nitride-based LEDs     

InGaN-GaN multiquantum-well blue and green light-emitting diodes

InGaN-GaN multiquantum-well (MQW) blue and green light-emitting diodes (LEDs) were prepared by organometallic vapor phase epitaxy, and the properties of these LEDs were evaluated by photoluminescence (PL), double crystal X-ray diffraction, and electroluminescence (EL) measurements. It was found that there were only small shifts observed in PL and EL peak positions of the blue MQW LEDs when the number of quantum well (QW) increased. However, significant shifts in PL and EL peak positions were observed in green MQW LEDs when the number of QW increased. It was also found that there was a large blue shift in EL peak position under high current injection in blue MQW LEDs. However, the blue shift in green MQW LEDs was negligibly small when the injection current was large. These observations could all be attributed to the rapid relaxation in green MQW LEDs since the In composition ratio in the InGaN well was high for the green MQW LEDs. The forward voltage V_f of green MQW LEDs was also found to be larger than that of blue MQW LEDs due to the same reason     

Towards efficient near-infrared fluorescent organic light-emitting diodes

The energy gap law(E_G-law)and aggregation quenching are the main limitations to overcome in the design of near-infrared(NIR)organic emitters.Here,we achieve unprecedented results by synergistically addressing both of these limitations.First,we propose porphyrin oligomers with increasing length to attenuate the effects of the E_G-law by suppressing the non-radiative rate growth,and to increase the radiative rate via enhancement of the oscillator strength.Second,we design side chains to suppress aggregation quenching.We find that the logarithmic rate of variation in the non-radiative rate vs.E_Gis suppressed by an order of magnitude with respect to previous studies,and we complement this breakthrough by demonstrating organic light-emitting diodes with an average external quantum efficiency of-1.1%,which is very promising for a heavy-metal-free 850 nm emitter.We also present a novel quantitative model of the internal quantum efficiency for active layers supporting triplet-to-singlet conversion.These results provide a general strategy for designing high-luminance NIR emitters.     

Multiple fields manipulation on nitride material structures in ultraviolet light-emitting diodes

As demonstrated during the COVID-19 pandemic, advanced deep ultraviolet (DUV) light sources (200–280 nm), such as AlGaN-based light-emitting diodes (LEDs) show ...     

AlGaN-GaN UV light-emitting diodes grown on SiC by metal-organicchemical vapor deposition

We report the study of the electrical and optical characteristics of AlGaN-GaN quantum-well (QW) ultraviolet light-emitting diodes grown on SiC by metal-organic chemical vapor deposition. These devices exhibit room-temperature electroluminescence emission peaked at λ = 363 nm with a narrow linewidth of Δλ = 9 nm under high-current-density dc injection. We have also applied a Mg-doped AlGaN-GaN superlattice structure as a p-cladding layer and vertical-geometry hole conduction improvement has been verified. A comparative study of the performance of light-emitting devices with single-QW and multiple-QW structures indicates that the single-QW structure is preferred     

Deep ultraviolet light-emitting diodes using quaternary AlInGaNmultiple quantum wells

We report on the growth, fabrication, and characterization of deep ultraviolet (UV) light-emitting diodes (LEDs) with quaternary AlInGaN-AlInGaN multiple quantum wells (MQWs) in the active region. These high quality quaternary MQWs were deposited over sapphire and n-SiC substrates using a novel pulsed atomic layer epitaxy (PALE) technique. LEDs with peak emission wavelengths from 305-340 nm were fabricated and characterized. Using square geometry devices over sapphire and n-SiC substrates we studied the role of current crowding. Numerical simulation results are also provided to explain the observed current-voltage and light-emission characteristics     

High-efficiency red and infrared light-emitting diodes using radialoutcoupling taper

In this paper, we give an overview of light-emitting diodes (LEDs) with radial tapers. Light is generated in the very center of a circularly symmetrical structure and is outcoupled at a tapered ring. Encapsulated devices with an emission wavelength of 980 nm achieve wallplug efficiencies of 48%. Non-encapsulated InGaAlP-based red-emitting LEDs show quantum efficiencies of 13%. A new device design combines the taper with a wafer-scale soldering technique promising a feasible fabrication method     

The role of surface plasmons in organic light-emitting diodes

Organic light-emitting diodes typically take the form of an optical microcavity in which one layer is a metallic cathode. Coupling between emissive species in the light emitting layer and surface plasmon (SP) modes associated with the metallic cathode result in a loss of efficiency; an aspect often discussed but not so far fully quantified. Here we numerically model the extent of this problem, both for organic light-emitting diodes based on small molecules (Alq₃) and those based on conjugated polymers (MEH-PPV). We show that SP modes can significantly detract from device efficiency, particularly those based on small molecules. We then report measurements of photo- and electroluminescence from organic light-emitting diodes incorporating wavelength scale periodic structure. These data demonstrate the existence of the SP modes in organic light-emitting diodes. Finally we consider ways in which the problems associated with SPs might be overcome, and may even be turned to advantage     

Emission zone in organic light-emitting diodes under AC operation

Organic light-emitting diodes consisting of a diamine/Al-quinolinol junction, a LiF/Al cathode, and an ITO anode were operated under application of rectangular voltage pulses alternated between 0 V (off-period) and 8 V (on-period) with frequencies in a range from 1 to 100 kHz. The emission intensity in the on-period was found to be lowered, if the off-period is longer than or equal to 100 /spl mu/s. From the analyses of the response of emission and the light-emitting zones in the devices under different operating conditions, it was concluded that, just after the turn-on, the light-emitting zone distributes in the Al-quinolinol layer deep from the diamine/Al-quinolinol interface. The region expands deeper as the off-period becomes longer. The expanded zone approaches the interface in the on-period of about 10 /spl mu/s. Such a change in the light-emitting zone with the operational conditions was attributed to the change in the energy barrier for the injection of electrons from the cathode into the Al-quinolinol layer, which is caused by the electric field applied to the organic layers.     

High-efficiency InGaN-GaN MQW green light-emitting diodes with CARTand DBR structures

Distributed Bragg reflector (DBR) and charge asymmetric resonance tunneling (CART) structures were applied to nitride-based green light-emitting diodes (LEDs) to enhance their output efficiency It was found that we can reduce the forward voltage at 20 mA from 3.7 to 3.2 V with the inclusion of CART structure. It was also found that the electroluminescence peak wavelength of the CART LED is less sensitive to the amount of injection current. The output power and external quantum efficiency of the CART LED with DBR structure measured at 20 mA can reach 7.2 mW and 11.25%, respectively     

Compact semiconductor light-emitting diodes for dynamic imaging of neuronal circuitry

We describe the use of compact green light-emitting diodes (LEDs) for fluorescence optical imaging and demonstrate this for dynamic recording from cultured neurons. The efficient gallium-nitride-based LEDs have individual element sizes comparable to typical biological cells and are operated in proximity illumination mode for individual neurons. As periodic arrays, and with direct electrical control of each LED, a spatially periodic multicellular target can be induced to fluoresce in a predesigned spatiotemporal sequence, thereby providing a new approach to dynamic recording of small neural circuits.     

Oxynitride/nitride phosphors for white light-emitting diodes (LEDs)

Oxynitride/nitride phosphors have attracted significant attention recently because they have promising luminescence properties and superior thermal and chemical stabilities which predestinates them for use in white LEDs to generate white light. This paper reports on luminescence spectra of Eu²⁺ or Ce³⁺-activated α-SiAlON, β-SiAlON, and alkaline earth silicon nitride (M₂Si₅N₈, M = Ca, Sr, Ba). A single broad emission band is observed for all samples, and the emission color depends on the type of activators and host lattice: α-SiAlON:Ce³⁺ (blue), α-SiAlON:Eu²⁺ (yellow), β-SiAlON:Eu²⁺ (green), and M₂Si₅N₈:Eu²⁺ (red). The excitation spectrum of these oxynitride/nitride phosphors covers a wide range from ultraviolet to visible light, enabling them to be used for white LEDs when an ultraviolet or blue LED chip is combined.