Efficiency droop in light‐emitting diodes: Challenges and countermeasures

Performance degradation resulting from efficiency droop during high-power operation is a critical problem in the development of high-efficiency light-emitting diodes (LEDs). In order to resolve the efficiency droop and increase the external quantum efficiency of LEDs, the droop's origin should be identified first. To experimentally investigate the cause of efficiency droop, we used null-point scanning thermal microscopy to quantitatively profile the temperature distribution on the cross section of the epi-layers of an operating GaN-based vertical LED with nanoscale spatial resolution at four different current densities. The movement of temperature peak towards the p-GaN side as the current density increases suggests that more heat is generated by leakage current than by Auger recombination. We therefore suspect that at higher current densities, current leakage becomes the dominant cause of the droop problem.

At room temperature, AlGaInP pn-junction light-emitting diodes (LEDs) emitting at 630 nm do not exhibit an efficiency droop. However, upon cooling the AlGaInP LEDs to cryogenic temperatures, they show a pronounced efficiency droop. We attribute the efficiency droop in AlGaInP LEDs to electron-drift-induced reduction in injection efficiency (i.e., carrier leakage out of the active region) mediated by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, with the concentration disparity exacerbated at low temperatures.

Nitride-based light-emitting diodes (LEDs) suffer from a reduction (droop) of the internal quantum efficiency with increasing injection current. This droop phenomenon is currently the subject of intense research worldwide, as it delays general lighting applications of GaN-based LEDs. Several explanations of the efficiency droop have been proposed in recent years, but none is widely accepted. In his Feature Article on pp. 2217–2225, Joachim Piprek provides a snapshot of the present state of droop research, reviews currently discussed droop mechanisms, contextualizes them, and proposes a simple yet unified model for the LED efficiency droop. This simple model considers both Auger recombination and carrier leakage as potential explanations of the efficiency droop. As both of these explanations exhibit some weaknesses, the search for additional and improved models of the LED efficiency droop continues.

White light sources based on III-Nitride light-emitting diodes (LEDs) hold great promise for developing energy-efficient solid-state lighting (SSL) technologies. However, the optical output power of InGaN-based green and blue LEDs at high driving currents is limited by quantum efficiency (QE) droop and low light outcoupling efficiencies. The former is a phenomenon that LEDs suffer a decline in quantum efficiency as the driving current increases, and prominently occurs in LEDs with high In contents. Poor light outcoupling is a challenge facing LEDs of all colors. Due to a large contrast of refraction index between nitrides and air, the majority of photons generated in the active region are trapped inside LED chips and reabsorbed. To develop powerful LED lighting sources capable of high-current operation, new strategies must be developed to overcome the droop obstacle and improve the efficiency of light extraction.;The objective of the current work is to improve the external quantum efficiencies of InGaN-based green and blue LEDs from these two aspects. The first part of this dissertation presents studies of the underlying mechanisms of efficiency droop in InGaN-based LEDs and our effort to develop LEDs with reduced droop through structure optimization. We investigated the optical characteristics of InGaN-based multiple-quantum-well (MQW) LEDs on sapphire with peak emission ranging from green to ultraviolet over a wide injection range. The current dependence of both the QE and peak shift appears to be a strong function of the In content in the active region. The QE of the green LED peaks at a current density as low as 1.4A/cm2, and decreases dramatically as current is increased, whereas the In-free deep-UV LEDs have a nearly constant QE at currents up 1 kA/cm2. To understand the role of threading dislocations in the droop behaviors, green LEDs were grown and fabricated on free-standing GaN. The density of microstructural defects in the LED structure was substantially reduced, leading to a significant reduction in defect-assisted tunneling currents a ∼65% peak internal QE. However, it suffered from even more dramatic efficiency droop which occurs at a current density as low as 0.3 A/cm2. These results offer a strong support for the argument that carrier overflow from localized states and loss at interfacial misfit defects is the nonthermal mechanism of the efficiency droop. Reduction of misfit defects in green LEDs by using an strain-compensated InGaN/InGaN MQW structure led to reduced efficiency droop at high currents, suggesting that strain engineering provides a

9 - Efficiency droop in GaInN/GaN LEDs

WITHIN FIVE years, you won't be buy-ing incandescent light bulbs. Instead of inefficient incandescent bulbs that get hot -- thus wasting a lot of electricity -- you will buy cool-to-the-touch LEDs (light emitting diodes). If you pay attention when you're driving, chances are you will see LED traffic lights. They are the ones that look like an array of smaller light bulbs -- which, in fact, they are. You might also notice that most school buses now have a pair of LED stoplights. Why LEDs? First, since LEDs produce almost no heat, they are very energy efficient -- using less than 10% of the electricity of an incandescent bulb. Second, LEDs have a lifetime of several hundred thousand hours, which prompts many manufacturers to warrant them for a lifetime. A flashlight I recently purchased illustrates the difference between incandescent and LED bulbs. The three-C-cell flashlight has a halogen bulb surrounded by an array of three LEDs. The halogen bulb will run one to three hours on a set of batteries; the LEDs will run 300 hours! I bought the flashlight for my car's glove box because it is so battery friendly. Admittedly, $50 or $60 is a lot for a flashlight, but imagine how many sets of batteries I won't have to buy. LED lighting is especially well suited for use in outdoor and marine environments. I boat in salt water, and over the years I have had a constant problem with my navigation lights. Salt is a natural enemy of the steel used in incandescent bulbs and their receptacles. I now have plastic-encased area lights and running lights that are guaranteed for a lifetime. Problem solved. Until recently LEDs came in colors, but not in white. What were sold as white LEDs had a distinct blue tint. Things are improving, and nearly pure white LEDs are now available. Of course, many applications like stoplights require colors anyway. Today, replacing a 30- to 60-watt incandescent bulb with an LED bulb is quite expensive -- in the range of $40 to $200. Night lights and accent lights are more reasonably priced at about $20 to $30. Experts are predicting a rapid decline in these prices. If every household switched from incandescent to LED lighting, the estimated energy saving would be between 10% and 20%. If I were a stock market investor, I would quickly put some funds in a manufacturer of LED home lighting products. The second technology that changes everything is Apple's new Airport Express wireless Ethernet base station. Airport Express weighs less than seven ounces; operates at 54 mbps (megabits per second), which is the fastest wireless Ethernet standard; and has an Ethernet networking port, a USB 2 port for printer sharing, and an analog/digital audio port. The feature that makes the Airport Express so ground breaking, however, is the audio port that lets you beam stereo to the Airport Express wirelessly. …

The effect of spacing among cell LEDs in high voltage (HV) LEDs performance was investigated in this study. Four kinds of HV-LEDs with different spacing are presented. From the results of LEDs output power and luminance intensity distribution, the spacing certainly influences the light extraction of LEDs. The 110% enhanced output power and 10% increased Wall-plug efficiency HV-LED was demonstrated with using 80 μm spacing in compared with 20 μm due to the suppression of light absorbed by neighbor cell LEDs.

Compact Fluorescent Lamps (CFLs) and Light Emitting Diode (LEDs) are fast replacing incandescent bulbs in most residential buildings because they are designed to fit into most lamp holders originally designed for incandescent bulbs. The aim of this work is to look at to what extent the usage of Compact Fluorescent Lamps (CFLs) or Light Emitting Diode (LEDs) can help reduce energy demand and how energy wastage in residential buildings due to lighting can be addressed by using energy efficient lamps. Energy meter was used to measure the actual energy in kWh (kilowatt-hour) of each bulb. Lux meter was used to measure the luminance (brightness) between CFLs, LEDs and Incandescent bulbs. Lighting point calculation for a three bedroom flat was carried out and the number of lighting points required to light up the building was calculated. An estimate of energy consumed for one year by the different bulbs using the readings from the energy meter was done and the prices of the bulbs was compared to ascertain if the cost outweighs the energy consumption. The prices of incandescent bulbs when compare to CFLs and LEDs in local stores is far cheaper than CFLs and LEDs with a burden of huge cost in lighting the incandescent bulbs due to energy wastage. Lighting design calculation for a three bedroom flat was carried out using IES (Illuminating Engineering Society) recommendation for different area. It was observed that LEDs bulbs can save almost 86.4% energy consumed by Incandescent bulbs while using CFLs bulbs can save 78.8% energy consumed by incandescent bulbs. Hence, LEDs is the most energy efficient bulb, by using LEDs in every home there will be a reduction of over 80% energy consumption due to lighting when compared to incandescent. DOI: 10.7176/JETP/10-5-04 Publication date: September 30 th 2020

A new structure of p-GaN/InGaN heterojunction has been proposed to enhance hole injection for blue GaN-based light-emitting diodes (LEDs). It is demonstrated by the simulation results that a p-GaN (50 nm)/In0.05Ga0.95N (150 nm) heterojunction can make a 25% and 10% increment of hole and electron concentration in the active region, respectively, finally resulting in a 55% improvement on the LED’s radiative recombination intensity. The simulation also reveals that the efficiency droop is alleviated from 32.9% to 21.7% at the current density of 100 A cm−2. The enhanced hole injection is mainly attributed to the increased average background hole concentration of the area between the p-AlGaN electron blocking layer (EBL) to the p-GaN/InGaN heterojunction. The increasing potential barrier of the conduction band, resulting from the introduction of p-GaN/InGaN heterojunction, would also weaken electron leakage and is favorable to the LED’s luminous performance. The experimental results show that the wall-plug efficiency (WPE) of the p-GaN/InGaN LED increases by 26.0% at the injection current of 75 mA, in spite of the increasing electric resistance, which impairs the improvement of the LED’s performance from the enhanced hole injection. The structure of the p-GaN/InGaN heterojunction is novel in the field of p-type region design, and is a simple but effective way to promote the LED’s performance, which is very promising for application in further high-performance LED fabrication.

Multiphysics processes such as recombination dynamics in the active region, carrier injection and transport, and internal heating may contribute to thermal and efficiency droop in InGaN/GaN light-emitting diodes (LEDs). However, an unambiguous methodology and characterization technique to decouple these processes under electrical injection and determine their individual roles in droop phenomena is lacking. In this work, we investigate thermal and efficiency droop in electrically injected single-quantum-well InGaN/GaN LEDs by decoupling the inherent radiative efficiency, injection efficiency, carrier transport, and thermal effects using a comprehensive rate equation approach and a temperature-dependent pulsed-RF measurement technique. Determination of the inherent recombination rates in the quantum well confirms efficiency droop at high current densities is caused by a combination of strong non-radiative recombination (with temperature dependence consistent with indirect Auger) and saturation of the radiative rate. The overall reduction of efficiency at elevated temperatures (thermal droop) results from carriers shifting from the radiative process to the non-radiative processes. The rate equation approach and temperature-dependent pulsed-RF measurement technique unambiguously gives access to the true recombination dynamics in the QW and is a useful methodology to study efficiency issues in III-nitride LEDs.

In conventional hexagonal InGaAlN multiple-quantum-well (MQW) (h-) light-emitting diodes (LEDs), carrier leakage from QWs is the main source of internal quantum efficiency (IQE) degradation without contributing to the LED efficiency droop. Our analysis based on the newly developed Open Boundary Quantum LED Simulator indicates that radiative recombination is hampered by the poor electron–hole wavefunction overlap induced by strong internal polarization for which QW carriers mostly recombine via Auger scattering rather than by radiative processes. By contrast, in non-polar h-LEDs, the IQE peak doubles its value compared to conventional h-LEDs while quenching the efficiency droop by 70% at current density of 100 A/cm2. Those effects are further enhanced in cubic InGaAlN MQW (c-) LEDs for which the IQE peak increases by an additional 30%, and the efficiency droop is further reduced by 80% compared to non-polar h-LEDs, thanks to the larger optical transition matrix element and the strong electron–hole wavefunction overlap in c-LEDs. Overall, a c-LED with a low efficiency droop of 3% at 100 A/cm2 is anticipated, paving a clear pathway toward ultimate solid-state lighting.

Blue InGaN LED suffers from a severe efficiency droop at high current density and electron leakage is believed to be one of the primary causes of it. In this study, InGaN LED was simulated using Sentaurus TCAD. The effects of thickness of the quantum wells on the device performances were examined through simulation. Results of the simulations suggested that to achieve a low efficiency droop, the wells have to be thick. Keywords: blue InGaN, quantum well, radiative-recombinations, electron leakage 1. INTRODUCTION Over the years, extensive developments of III-Nitride semiconductors had introduced many important lighting applications for light emitting diodes (LEDs) such as solid st ate lighting, full-colour displa y, liquid-crystal display back-lighting and automotives lighting. This LED is favoured by wide range of industries for its long life span, smaller size, energy saving and fast-response. Typical InGaN LED exhibits peak efficiency at current densities as low as 1-2 A/cm2 but with a gradual decrease subsequently which appeared to be a major drawback of the LED. This problem is termed as “efficiency droop” and the primary source of it is yet to be determined. However, researches have suggested carrier spillage [1-2], Auger recombination [3-4], carrier deloca lization [5] are some of the causes of the defect. Numerous researches about improving th e recombination efficiency and suppre ssing the electron escape have been pursued by novel quantum well design in nitride-based LEDs. Wang at al. [6] designed a graded-thickness multiple quantum well (GQW) and grew it via metal-organic chemical vapor deposition. In their work, the well thickness increased along [0001] direction and it was found that there was an extra emission from the narrower wells within GQWs which eventually improved the droop behavior as well as the output power of the LED. Lee, Chen and Lee [7] also proposed a graded quantum well but instead of the thickness, they focused on the compositions of Indium in the well. In their numerical investigation, they observed an increment in internal quantum efficiency (IQE) and subsequently suppressed efficiency droop. Increased in IQ E was due to improvement in the transpor t efficiency of injected holes. This method was supported by Zhu et al. [8]. Their results showed that efficiency droop occurred at a much higher current density than the conventional LEDs due to reduction in polarization field in the active region. Study on the effect of various thicknesses on multiple quantum wells was carried out by Li, Huang and Lai [9]. They reported dislocation recombination, Auger recombination and carrier overflow out of active region were among the causes of efficiency droop and this droop can be alleviated by employment of a thicker well structure. Optimization of InGaN LED done by Ghosh, Gomes and Biswas [10] showed that thicker quantum wells resulted in lower luminous intensity. Apart from well thickness, they optimized the indium concentration of the active region too. They observed luminous intensity increased with increasing Indium mole fr action up to 10% but decreased af ter this. Recently Chen at al. [11] suggested that insertion of 5-nm-thick p-In 0.01Ga0.99N layer managed to decrease 20 mA forward voltage by 0.05 V and the droop by 12.9%. In this paper, analysis on the electrical and optical performance of blue InGaN LEDs with multiple well thicknesses was studied using Sentaurus TCAD simulation program.

Efficiency droop is currently one of the most popular research problems for GaN-based light-emitting diodes (LEDs). In this work, a differential carrier lifetime measurement system is optimized to accurately determine carrier lifetimes (τ) of blue and green LEDs under different injection current (I). By fitting the τ-I curves and the efficiency droop curves of the LEDs according to the ABC carrier rate equation model, the impact of Auger recombination and carrier leakage on efficiency droop can be characterized simultaneously. For the samples used in this work, it is found that the experimental τ-I curves cannot be described by Auger recombination alone. Instead, satisfactory fitting results are obtained by taking both carrier leakage and carriers delocalization into account, which implies carrier leakage plays a more significant role in efficiency droop at high injection level.

Due to the superior advantages of high energy conversion efficiency, high brightness, high reliability, controllable color properties, long lifetime, and ease of miniature and digitalization, GaN-based light-emitting diodes (LEDs) are regarded as the next generation solid state lighting sources to replace the conventional lighting devices of fluorescent and incandescent lamps. Tremendous effort has been devoted to LED research, development and commercialization, thus drastic advances have been made in the past decades. However, despite these advantages and achievements, GaN-based LEDs still suffer from several technical issues, including insufficient heat dissipation, current crowding effect, low light extraction efficiency, and efficiency droop. All these problems, especially efficiency droop delay LED's further expansion in the general lighting market. Hence, a large amount of research work focusing on improving the efficiency droop has been carried out to achieve better LED performance. In this dissertation, the InGaN/GaN multiple quantum well (MQW) LEDs have been studied from multiple aspects, including material quality, light extraction efficiency, p-type doping influence, current crowding effect, electron overflow, and carrier transport. The LED epitaxial wafers studied are grown on c-plane patterned sapphire substrate using metal-organic chemical vapor deposition (MOCVD) system. Standard fabrication processes for flip-chip LEDs are developed and electrical and optical characterizations are conducted to evaluate the performance. Difficulty in p-type doping and low hole injection efficiency lead to nonuniform carrier distribution within MQWs. Consequently, the quantum well (QW)

This report covers the third part of a larger U.S. Department of Energy (DOE) project to assess the life-cycle environmental and resource impacts in the manufacturing, transport, use, and disposal of light-emitting diode (LED) lighting products in relation to incumbent lighting technologies. All three reports are available on the DOE website (www.ssl.energy.gov/tech_reports.html). • Part 1: Review of the Life-Cycle Energy Consumption of Incandescent, Compact Fluorescent and LED Lamps; • Part 2: LED Manufacturing and Performance; • Part 3: LED Environmental Testing. Parts 1 and 2 were published in February and June 2012, respectively. The Part 1 report included a summary of the life-cycle assessment (LCA) process and methodology, provided a literature review of more than 25 existing LCA studies of various lamp types, and performed a meta-analysis comparing LED lamps with incandescent and compact fluorescent lamps (CFLs). Drawing from the Part 1 findings, Part 2 performed a more detailed assessment of the LED manufacturing process and used these findings to provide a comparative LCA taking into consideration a wider range of environmental impacts. Both reports concluded that the life-cycle environmental impact of a given lamp is dominated by the energy used during lamp operation—the upstream generation of electricity drives the total environmental footprint of the product. However, a more detailed understanding of end-of-life disposal considerations for LED products has become increasingly important as their installation base has grown. The Part 3 study (reported herein) was undertaken to augment the LCA findings with chemical analysis of a variety of LED, CFL, and incandescent lamps using standard testing procedures. A total of 22 samples, representing 11 different models, were tested to determine whether any of 17 elements were present at levels exceeding California or Federal regulatory thresholds for hazardous waste. Key findings include: • The selected models were generally found to be below thresholds for Federally regulated elements; • All CFLs and LED lamps and most incandescent lamps exceeded California thresholds for Copper; • Most CFL samples exceeded California thresholds for Antimony and Nickel, and half of the LED samples exceeded California thresholds for Zinc; • The greatest contributors were the screw bases, drivers, ballasts, and wires or filaments; • Overall concentrations in LED lamps were comparable to cell phones and other types of electronic devices, and were generally attributable to components other than the internal LED light sources; • Although the life-cycle environmental impact of the LED lamps is favorable when compared to CFLs and incandescent lamps, recycling will likely gain importance as consumer adoption increases. This study was exploratory in nature and was not intended to provide a definitive indication of regulatory compliance for any specific lamp model or technology. Further study would be needed to more broadly characterize the various light source technologies; to more accurately and precisely characterize a specific model; or to determine whether product redesign would be appropriate.