Light Output Of Light-emitting Diodes Research Articles

Energy cost control of LEDs considering the price of electricity and the desired lighting levels to meet the requirements of smart grids, smart cities, and unregulated energy markets

In order to manage the energy cost of LEDs (light-emitting diodes), this study presents an innovative solution considering the price of electricity and the desired lighting levels as the input variables of the control system. When electricity prices decline, customers tend to consume more electrical energy. Conversely, with the increase in prices, there is a tendency to use less electricity. Also, lighting levels are another important factor in determining the energy consumption and associated costs of light-emitting diodes. By using the proposed system, lighting levels are adjusted as a percentage of the nominal light output of LEDs. The advantage of this method is controlling the energy cost based upon the simultaneous effects of electricity prices and the desired lighting levels. Therefore, unlike conventional lighting control systems that focus only on controlling the lighting levels, the introduced method provides a more effective control solution. The accuracy of this technique is supported by the results of system simulation and implementation. The proposed method is recommended as a novel strategy for meeting the requirements of smart grids, smart cities, and unregulated energy markets.

Characterization of dynamic distortion in LED light output for optical wireless communications

Light-emitting diodes (LEDs) are widely used for data transmission in emerging optical wireless communications (OWC) systems. This paper analyzes the physical processes that limit the bandwidth and cause nonlinearities in the light output of modern, high-efficiency LEDs. The processes of carrier transport, as well as carrier storage, recombination, and leakage in the active region appear to affect the communications performance, but such purely physics-based models are not yet commonly considered in the algorithms to optimize OWC systems. Using a dynamic modeling of these phenomena, we compile a (invertable) signal processing model that describes the signal distortion and a parameter estimation procedure that is feasible in an operational communications link. We combine multiple approaches for steady-state and dynamic characterization to estimate such LED parameters. We verify that, for a high-efficiency blue GaN LED, the models become sufficiently accurate to allow digital compensation. We compare the simulation results using the model against optical measurements of harmonic distortion and against measurements of the LED response to a deep rectangular current modulation. We show how the topology of the model can be simplified, address the self-calibration techniques, and discuss the limits of the presented approach. The model is suitable for the creation of improved nonlinear equalizers to enhance the achievable bit rate in LED-based OWC systems and we believe it is significantly more realistic than LED models commonly used in communications systems.

Observation of Acoustic Charge Transport in GaN-Based Quantum Well Structures Under Surface Acoustic Wave Excitations.

Though the acoustoelectric and acoustooptic interactions have been widely studied on III-V semiconductors, most studies were conducted at low temperature to avoid the influence of lattice scattering. Here, we demonstrate larger-than-1-GHz optical oscillation and acoustic charge transport at room temperature from a nitride-based LED (light-emitting diode) by acoustic wave excitation. The optical oscillation is generated by the harmonics of acoustic waves, while the acoustic transport of carriers was observed from the LED light output at different acoustic excitation frequencies. Furthermore, with the change of bias voltage, the frequency response of light output varies correspondingly due to the thermal disturbance that affects the lattice vibration and acoustic wave propagation. The results also suggest that we are able to achieve a tunable optical resonant frequency by varying bias voltages without changing the LED structure.

Evaluation of Light-curing Intensity Output and Students’ Knowledge among Dental Schools in Riyadh City

AIM: The aim of this study is interpreting the dental student’s knowledge of light-curing units from different universities and colleges in Riyadh city and to evaluate the intensity of the light output of light-emitting diode (LED)-curing devices in dental school clinics in Riyadh, Saudi Arabia.
 METHODS: One hundred eighty-two visible light-curing (VLC) units were randomly selected to have their light intensity output evaluated using Demetron® L.E.D. Radiometer – Kerr among dental school clinics in Riyadh city. The university hospitals we visited are King Khalid University Hospital, King Saud bin Abdulaziz University Hospital, Dar Al Uloom University Hospital, Alfarabi colleges’ dental clinics, and Princess Nourah Bint Abdulrahman University Hospital. To evaluate the knowledge of dental students about light intensity output, a questionnaire was given to the students.
 RESULTS: Out of the VLC units, 22 units (12%) measured inadequate intensity, 91 units (50%) measured marginal intensity, and 69 (37%) measured adequate intensity. The statistical significance was up to p < 0.05, p = 0. Most students and interns did not think that light-curing unit intensity influenced the tooth pulp (55.8%) and also reported not know the minimum wavelength of light cure intensity (62%).
 CONCLUSION: A significant difference was found between the light cure intensities in universities. As for the students’ knowledge, the research revealed poor insight toward basic concepts of VLC units and its maintenance.

Imaging System Based on Silicon Photomultipliers and Light Emitting Diodes for Functional Near-Infrared Spectroscopy

We built a fiber-less prototype of an optical system with 156 channels each one consisting of an optode made of a silicon photomultiplier (SiPM) and a pair of light emitting diodes (LEDs) operating at 700 nm and 830 nm. The system uses functional near-infrared spectroscopy (fNIRS) and diffuse optical tomography (DOT) imaging of the cortical activity of the human brain at frequencies above 1 Hz. In this paper, we discuss testing and system optimization performed through measurements on a multi-layered optical phantom with mechanically movable parts that simulate near-infrared light scattering inhomogeneities. The baseline optical characteristics of the phantom are carefully characterized and compared to those of human tissues. Here we discuss several technical aspects of the system development, such as LED light output drift and its possible compensation, SiPM linearity, corrections of channel signal differences, and signal-to-noise ratio (SNR). We implement an imaging algorithm that investigates large phantom regions. Thanks to the use of SiPMs, very large source-to-detector distances are acquired with a high SNR and 2 Hz time resolution. The overall results demonstrate the high potentialities of a system based on SiPMs for fNIRS/DOT human brain imaging applications.

Full-Color III-Nitride Nanowire Light-Emitting Diodes

III-nitride nanowire-based light-emitting diodes (LEDs) have been intensively studied as promising candidates for future lighting technologies. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization fields, and quantum-conned Stark effect due to the effective lateral stress relaxation, promising high-efficiency full-color LEDs. Beside these advantages, however, several issues have been identified as the limiting factors for further enhancing the nanowire LED quantum efficiency and light output power. Some of the most probable causes have been identified as due to the lack of carrier confinement in the active region, non-uniform carrier distribution, electron overflow, and the nonradiative recombination along the nanowire lateral surfaces. Moreover, the presence of large surface states and defects contribute significantly to the carrier loss in nanowire LEDs. Consequently, reported nanowire LEDs show relatively low output power. Recently, III-nitride core-shell nanowire LED structures have been reported as the most efficient nanowire white LEDs with a record-high output power which is more than 500 times stronger than that of nanowire white LEDs without using core-shell structure. In this context, we will review the current status, challenges, and approaches for the high-performance IIInitride nanowire LEDs. More specifically, we will describe the current methods for the fabrication of nanowire structures including top-down and bottom-up approaches, followed by characteristics of III-nitride nanowire LEDs. We will then discuss the carrier dynamics and loss mechanism in nanowire LEDs. The typical designs for the enhanced performance of III-nitride nanowire LEDs will be presented next. The color-tunable nanowire LEDs with emission wavelengths in the visible spectrum and phosphor-free nanowire white LEDs will be finally discussed.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited.

Degradation of optical materials in solid-state lighting systems

ABSTRACTIn this paper, degradation mechanisms of optical materials, used in the light emitting diode (LED)-based products, are reviewed. The LED lighting is one of the fastest technology shifts in human history. Lighting accounts for almost 20% of the global electrical energy use, inferring that replacement of traditional lighting sources with LEDs with higher efficiencies will have major positive implications for the global energy consumption. Organic optical materials are key components in LEDs in the sense that they control the functionality of the device and they have decisive effects on the durability and reliability of LEDs. This paper aims at describing the influences of chemical structure and service conditions on the degradation mechanisms of organic optical materials in LEDs which lead to the lumen depreciation, discolouration, and colour shift of the LED light output. The contributions of different degradation mechanisms of optical and package materials in LED-based products to the lumen depreciation and colour shift are methodically reviewed.

Wide-range dimmable LED lighting based on QL-SEPIC converter

ABSTRACTThis paper proposes quadratic-like single-ended primary inductance converter (SEPIC) converter, a DC/DC converter suitable for wide-input voltage range. This circuit can be gated at higher duty ratio compared with conventional SEPIC converter, while the voltage conversion ratio is low. This feature is beneficial for high-input voltage because it prevents the controller from falling into extremely low duty ratio limitation. A prototype circuit is implemented with Pulse-Width-Modulation (PWM) control to drive the Light emitting diode (LED) strings for demonstration of the circuit application. Recent studies have advocated LEDs as efficient and durable technology for lighting applications. A stabilized output converter is required to regulate the current flowing through the LED lamp. The dimming control of the LED light output is implemented by the controller. This converter can be powered from either a 24-Vdc battery or from the universal AC input voltage (up to 360 Vdc). Such a wide-input voltage application can be found in emergency lighting in which the light is possibly powered either from battery or AC input voltages. Experimental results prove the feasibility of the circuit topology and theoretical analysis.

Enhanced Light Output of Near-Ultraviolet LEDs With Ta2O5/SiO2 Hybrid DBR Reflector

We designed and fabricated AlGaN-based near-ultraviolet light-emitting diodes (LEDs) operating at 368 nm with high output power using Ta2O5/SiO2 hybrid distributed Bragg reflectors (DBRs). The output powers of LEDs with eight-pair DBR and hybrid-DBR configurations were increased by 18% and 25% when compared with that of an LED without any DBR. The reflectance of the eight-pair DBR and hybrid-DBR were as high as 99%. In addition, the reflectance of the fabricated hybrid-DBR showed a slight dependence on the incident light wavelength and the angle of incidence. In comparison to the eight-pair DBR, the bandwidth of the hybrid-DBR’s reflectance band was broadened by up to 74 nm. Under current injection at 350 mA, output powers of 88, 104, and 110 mW were measured for LEDs without any DBR reflector, with the eight-pair DBR, and with the hybrid-DBR, respectively.

A statistical study to identify the effects of packaging structures on lumen reliability of LEDs

The development of high-power light-emitting diode (LED) devices has been bedeviled by the reliability problems. And most reliability issues are caused by the packaging materials rather than the chips. However, which packaging material is the most influential remains unrevealed. To answer this question, a statistical method was introduced in this paper. Optical simulations were conducted to calculate the optical output power of LED package according to the orthogonal experimental design. Range and variance analyses were carried out to determine the significance of the relevant factors on the LED's light output. The results showed that the dome lens among the non-luminescent packaging materials had the most significance in affecting the light output. It is concluded that this method is useful in detecting the most significant part of LED packaging materials during the development of new packaging structures and is beneficial for enhancing the whole reliability of LED package effectively.

Traceability validation of a high speed short-pulse testing method used in LED production

Industrial processes of LED (light-emitting diode) production include LED light output performance testing. Most of them are monitored and controlled by optically, electrically and thermally measuring LEDs by high speed short-pulse measurement methods. However, these are not standardized and a lot of information is proprietary that it is impossible for third parties, such as NMIs, to trace and validate. It is known, that these techniques have traceability issue and metrological inadequacies. Often due to these, the claimed performance specifications of LEDs are overstated, which consequently results to manufacturers experiencing customers' dissatisfaction and a large percentage of failures in daily use of LEDs. In this research a traceable setup is developed to validate one of the high speed testing techniques, investigate inadequacies and work out the traceability issues. A well-characterised short square pulse of 25 ms is applied to chip-on-board (CoB) LED modules to investigate the light output and colour content. We conclude that the short-pulse method is very efficient in case a well-defined electrical current pulse is applied and the stabilization time of the device is “a priori” accurately determined. No colour shift is observed. The largest contributors to the measurement uncertainty include badly-defined current pulse and inaccurate calibration factor.

Epitaxy and characterization of GaInP/AlInP light-emitting diodes on As-doped Ge/Si substrates.

We investigate the impact of threading dislocation density (TDD) and thermal conductivity of substrates on the performance of GaInP/AlInP light-emitting diodes (LEDs) for the integration of III-V optoelectronics on Si. We utilized an arsenic (As) doped Ge/Si substrate that showed a reduced TDD compared to undoped Ge/Si. Compared to LEDs on undoped Ge/Si, the leakage current density for LEDs on As-doped Ge/Si substrate is reduced by four orders of magnitude and the light output is increased six-fold. An increased junction temperature causes light output saturation for LEDs on bulk Ge at high injection current densities. The light output of LEDs on As-doped Ge/Si shows good linearity with injection current density and its junction temperature is ~25 ± 5 °C lower than that of LEDs on bulk Ge at high injection current densities due to better thermal conductivity of the Ge/Si substrate.

Performance improvement of VLC system by using GaN-based LEDs with strain relief layers

The GaN-based blue light-emitting diodes (LEDs) with superlatticelike strain relief layer (SRL) and without SRL were fabricated by metal organic vapor phase epitaxy (MOVPE). The luminescence characteristics of the different LEDs were measured experimentally and calculated theoretically. It is demonstrated that the efficiency droop of GaN-based LEDs is suppressed using the SRL due to the reduction of polarization electric field in quantum wells. This is beneficial to the mitigation of LED light output power-injection current (L-I) nonlinearity. Furthermore, using the different LEDs as light source, the performance of visible-light communication (VLC) system was also investigated. The results reveal that the VLC system performance is improved when the LEDs with SRL are applied. The reason is that the transmitted signal error decreases as the LED L-I nonlinearity mitigates. As a result, the GaN-based LEDs with SRL can be applied in the VLC to improve the system performance.

Enhanced Light Output of UVA GaN Vertical LEDs With Novel DBR Mirrors

The emission efficiency of GaN-based light-emitting diodes (LEDs) decreases significantly as the emission wavelength is shorter than 370 nm because of the absorption of emission light in the GaN layers. The vertical LED structure could be applied to avoid the absorption problem, because the undoped GaN layer would be removed and the n-GaN layer could be thinned as much as possible in the fabrication process. In this report, we propose a novel ultraviolet (UV) LED structure with a thin indium–tin–oxide (ITO) layer as the ohmic contact and a dielectric distributed Bragg reflector (DBR) as the high reflectivity mirror. Comparing referenced vertical UV LED structures with ITO/Ag as the metal mirror, the vertical UV LED with the ITO/DBR mirror shows the output power of approximately 190 mW at 350 mA with 7.9% enhancement.

<italic>In-Situ</italic> Measurement of Junction Temperature and Light Intensity of Light Emitting Diodes With an Internal Sensor Unit

This study designed and tested an innovative light-emitting diode (LED) chip with a built-in sensor. Two electrically isolated units, the LED (for light emission) and the sensor (for monitoring junction temperature and light intensity), were integrated on a single chip. The sensor unit determines the junction temperature by measuring the forward voltage; the light output power of the LED unit can be precisely extrapolated with a polynomial function based on the photocurrent and junction temperature. This novel structure enables the in-situ real-time monitoring of the LED junction temperature and light output power, which allows a highly detailed and/or in-field LED reliability analysis and provides valuable feedback information for smart LED lighting systems.

(Invited) Phosphor-Free III-Nitride Nanowire White Light Emitting Diodes: Challenges and Prospects

Group III-nitride nanowire heterostructures have been intensively studied as an emerging platform for future solid-state lighting and full-color display. Compared to the conventional GaN based planar light emitting diodes (LEDs), due to the effective lateral stress relaxation, III-nitride nanowires offer several distinct advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect [1-3], promising high efficiency full-color LEDs [4, 5]. However, several factors have been identified as the limiting process for further improving the nanowire LED quantum efficiency and light output power, which include the lack of carrier confinement in the active region, poor hole transport [6], electron overflow [7], and the nonradiative recombination along the wire lateral surfaces [8]. The quantum efficiency of nanowire LEDs generally exhibits a very slow rise with injection current and the peak quantum efficiency occurs at very high injection current (>100 A/cm2), which is much higher than the commonly reported values of conventional InGaN/GaN quantum well blue LEDs (in range of ~ 10-20 A/cm2) [4, 9]. Due to the highly effective lateral stress relaxation associated with the large surface areas, it is expected that both the polarization fields and dislocation densities can be significantly reduced in nanowire structures. However, the presence of large surface states and defects can contribute significantly to the carrier loss in nanowire LEDs. As a consequence, currently reported nanowire LEDs show relatively low output power, which is often in the range of μW, or lower [4]. Several approaches have been reported for the enhanced performance of III-nitride nanowire LED devices including usage of dots/disks in-a-wire to enhance carrier confinement. The poor hole transport and the electron overflow can be significantly improved by employing the p-type modulation doping technique and the usage of electron blocking layer, respectively [4, 7]. Such devices can exhibit relatively high internal quantum efficiency of ~57% and stable emission characteristics with increasing current. Most importantly, by reducing nonradiative surface recombination, the device performance including light output power and quantum efficiency can be significantly improved. Several surface passivation techniques have been employed including dielectric material and large bandgap energy semiconductors. However, passivating the InGaN/GaN nanowire structure with an in-situ grown AlGaN shell shows a record high output power which is more than 100 times stronger than that of nanowire white LEDs without using an AlGaN shell [10]. Moreover, a new self-organized InGaN/AlGaN dot-in-a-wire core-shell white LED heterostructure will also be presented. Multiple AlGaN shell layers are spontaneously formed during the growth of the InGaN/AlGaN quantum dot active region. Due to the drastically reduced nonradiative surface recombination, such core-shell nanowire structures exhibit significantly increased carrier lifetime (from ~ 0.3 ns to ~ 4.5 ns) and massively enhanced photoluminescence intensity, compared to that of nanowire LED without using core-shell structures. Strong white-light emission was recorded for those core-shell nanowire LEDs with an output power of >5 mW (at ~ 60 A/cm2), and a color rendering index of ~ 95 with stable white light emission for injection current from 50 mA to 500 mA, with the x and yvalues in the ranges of ~ 0.35 ‒ 0.36 and 0.37 ‒ 0.38, respectively. The significantly enhanced carrier lifetime is attributed to the greatly reduced nonradiative surface recombination and the effective lateral confinement offered by the large bandgap AlGaN shell. Such unique core-shell nanowire heterostructures, with controllable carrier dynamics, will significantly advance the achievement of a high power nanowire LEDs and also the development of broad range of nanowire photonic devices, including lasers, solar cells, and photodetectors. This presentation will cover current status of nanowire LED research, challenges and approaches to enhance nanowire LED performance. The progress being made to develop high efficiency phosphor-free nanowire LEDs for future solid-state lighting and full color display will also be presented.

Design of a homogeneous radiation field in an LED photo-reactor

In this paper, a radiation field model for a light-emitting diode-based photocatalytic reactor has been developed. The model was tested using experimental data, and good agreement between the model and experimental results was noted. The model was used to investigate the impact of irradiated distance on the homogeneity of radiation field because it is shown that a homogeneous radiation field is the most efficient. The model can be used to design an efficient photoreactor by selecting the optimal gap between adjacent light-emitting diodes, the irradiated distance and the light output of light-emitting diodes.

Spatial Correlation Between Efficiency and Crystal Structure in GaN-Based Light-Emitting Diodes Prepared on High-Aspect Ratio Patterned Sapphire Substrate With Sputtered AlN Nucleation Layer

In this paper, we investigate sputtered physical vapor deposition (PVD) AlN nucleation layer effects on crystal quality and efficiency of GaN-based light-emitting diodes (LEDs) prepared on high-aspect ratio patterned sapphire substrate (HARPSS). The crystal quality of the GaN epitaxial layer prepared on HARPSS with a PVD AlN nucleation layer was significantly better than that with a conventional metal organic chemical vapor deposition (MOCVD)-grown AlN nucleation layer. In addition, the pure wurzite structure GaN prepared on HARPSS could be obtained using PVD AlN nucleation layer. With the PVD AlN nucleation layer, the 20-mA LED light output power of the LEDs can be enhanced by 47.7% compared with the LEDs with the conventional MOCVD-grown AlN nucleation layer.

Strong Enhancement in Light Output of GaN-Based LEDs With Graded-Refractive-Index ITO Deposited on Textured V-Shaped Pits

An interesting structure of GaN-based light-emitting diodes (LEDs) with different indium-tin-oxide (ITO) films deposited on naturally textured V-shaped pits (V-pits) is fabricated and studied. The sputtered ITO assists the textured surface to get a better contact and thus reduces the forward voltage. Moreover, the ITO films prepared by different methods can serve as graded-refractive-index antireflective coatings, which can enhance the performance of the V-pits LED further. The results show that the V-pits LED with two layers ITO films have a stronger light output power than the reference LED by 52.1% at 20 mA, and the forward voltage is only a little higher than the standard LED.

Enhancing Light Output of GaN-Based LEDs With Graded-Thickness Quantum Wells and Barriers

GaN-based light-emitting diodes (LEDs) with graded-thickness quantum wells and barriers (GMQW-LEDs) are fabricated and researched in this letter. The light power and carrier distribution of GMQW-LEDs are compared with those of LEDs with original uniform MQW (OR-LEDs), graded-thickness quantum wells (GQW-LEDs), and graded-thickness quantum barriers (GQB-LEDs) through numerical simulation, respectively. The experimental results show that light power of GMQW-LEDs is enhanced significantly compared with that of OR-LEDs. The simulation results reveal that GMQW-LEDs show light output power enhancements of 25.7%, 14.3%, and 9.2% compared with OR-LEDs, GQW-LEDs, and GQB-LEDs at current density of 100 A/cm2, respectively. This is due to the superior hole distribution in quantum wells, which inhibits the electron leakage and enhances the radiative recombination.