High-efficiency Light-emitting Diodes Research Articles

A high PF, high efficiency LED driver without off-chip inductor and capacitor

A high power factor (PF), high efficiency light-emitting diode (LED) driver without off-chip inductor and capacitor is presented in this paper. The proposed driver consists of one resistor, one diode bridge and one controller integrated circuits (IC). The controller IC employs four LED strings and sequences their conduction to shape the output current in proportion to the input voltage. The controller IC was designed and implemented by using HHNEC 1 μm 700 V process. The experimental results show that high PF of 97.93 % and efficiency of 90.41 % under the input voltage of 220 V|rms utility voltage were obtained.

High Efficiency LED Power Supply

A new high efficiency light-emitting diode (LED) and an organic LED power supply are presented. An LED string is split into two sections. A first LED string is driven by a conventional dc-to-dc converter, while a second LED string is placed in series with the incoming dc supply current. Power is delivered to the second LED string without passing through the dc–dc converter, thus improving the system efficiency of the circuit. The equations presented show that the efficiency of the whole circuit will always be higher than the efficiency of the dc–dc converter alone. The performance of the new high efficiency circuit was tested and it was found that the new high efficiency circuit offered an improvement in efficiency of the LED driver of 3% at an automotive supply voltage of 14 V.

Theoretical analysis of quasi-random roughened surface on light extraction enhancement and optical field properties of GaN LED

Light extraction has been a limiting factor for the efficiency of light emitting diode (LED). In this study, a roughened surface increasing the light output of LED was demonstrated in numeric simulation. The roughening pattern was generated by imposing definite number of random imperfections on the surface of LED. Numeric calculations approved a remarkable enhancement of light extraction at targeting wavelengths compared with that of LED with unprocessed surface. In addition, optical field properties were also altered in the presence of roughened surface. The study suggested an easier and more flexible solution towards high efficiency LED by reducing complexity in design and allowing a broader margin for tolerance in production.

Modification of internal quantum efficiency and efficiency droop in GaN-based flip-chip light-emitting diodes via the Purcell effect.

The Purcell effect in GaN-based flip-chip (FC) light-emitting diode (LED) structures is investigated numerically using finite-difference time-domain simulations. Depending on the thickness of the p-GaN layer, the variation of the Purcell factor of FC LEDs is obtained to be as high as 20%, which results in the relative modification of the internal quantum efficiency (IQE) as large as 8% and 2.5% for the unmodified IQE of 0.4 and 0.8, respectively. Since the influence of the Purcell effect becomes more conspicuous as the IQE decreases, the Purcell enhancement can be advantageously used to mitigate the efficiency droop problem to some extent. When the Purcell effect is positively applied to the blue LED with the peak IQE of 0.8 and the droop ratio of 29.1%, the peak IQE and the droop ratio are found to be improved to 0.82 and 26.3%. This small but non-negligible effect on IQE is expected to be importantly adopted for industry development of high efficiency LEDs.

Spectral Effects of Artificial Light on Plant Physiology and Secondary Metabolism: A Review

With the expeditious development of optoelectronics, the light-emitting diode (LED) technology as supplementary light has shown great advancement in protected cultivation. One of the greatest challenges for the LED as alternative light source for greenhouses and closed environments is the diversity of the way experiments are conducted that often makes results difficult to compare. In this review, we aim to give an overview of the impacts of light spectra on plant physiology and on secondary metabolism in relation to greenhouse production. We indicate the possibility of a targeted use of LEDs to shape plants morphologically, increase the amount of protective metabolites to enhance food quality and taste, and potentially trigger defense mechanisms of plants. The outcome shows a direct transfer of knowledge obtained in controlled environments to greenhouses to be difficult, as the natural light will reduce the effects of specific spectra with species or cultivar-specific differences. To use the existing high-efficiency LED units in greenhouses might be both energy saving and beneficial to plants as they contain higher blue light portion than traditional high-pressure sodium (HPS) lamps, but the design of light modules for closed environment might need to be developed in terms of dynamic light level and spectral composition during the day to secure plants with desired quality with respect to growth, postharvest performance, and specific metabolites.

Improvement of the light extraction efficiency of light-emitting diodes based on ZnO nanotubes

Although ZnO nanotubes are emerging as a very promising approach for getting high-brightness light-emitting diodes (LEDs), the light extraction behavior of ZnO nanotubes LEDs has not been reported in detail previously. In this paper, numerical simulations based on finite-difference time-domain (FDTD) method were carried out to investigate the mechanisms through which ZnO nanotube arrays could effectively improve the light extraction efficiency (LEE). The effects of several parameters, including the inner radius, wall thickness, height and period of ZnO nanotubes on improving the LEE were studied. Furthermore, the LEE over the whole visible spectrum has also been investigated, two strong peaks and broadband visible spectrum has been demonstrated. An optimization of ZnO nanotubes geometries with respect to the optical properties has been presented. With the optimized structure, more than four times enhancement of LEE has been achieved. This enhancement is related to the wave-guiding and optical properties of ZnO nanotubes. The optimized structure presented here can be expected as a promising candidate for high efficiency LEDs.

Graphene oxide hole injection layer for high-efficiency polymer light-emitting diodes by using electrophoretic deposition and electrical reduction

Graphene oxide (GO) is a fascinating nanomaterial with tremendous potential for electronic devices which show high performance when the properties of GO are properly tuned like reduced graphene oxide (RGO). Here, we report a simple, cost effective and precisely controllable methods to fabricate high-quality RGO films as a hole injection layer (HIL) for high efficiency polymer light-emitting diodes (PLEDs). First, GO is electrophoretically deposited and electrically reduced to produce RGO films. The deposition and reduction of films are electrical methods; hence the thickness and the degree of reduction for GO films can be easily manipulated by controlling electrical parameters and time. The performance of PLEDs with RGO film as HIL is measured and compared with GO and (poly(3,4-ethylenedioxythiophene)polystyrene sulfonate) (AI4083), and shows better results for luminance and current density. The optimized maximum luminance and current efficiency of the PLEDs with RGO as HIL are found to be 12830cd/m2 and 3.35cd/A, respectively.

Design of near-unity transmittance dielectric/Ag/ITO electrodes for GaN-based light-emitting diodes

We designed a near-unity transmittance dielectric/Ag/ITO electrode for high-efficiency GaN-based light-emitting diodes by using the scattering matrix method. The transmittance of an ultrathin metal layer, sandwiched between a dielectric layer and an ITO layer, was investigated as a function of the thickness and the optical constant of each constituent layer. Three different metals (Ag, Au, and Al) were examined as the metal layer. The analytical simulation indicated that the transmittance of a dielectric/metal/ITO multilayer film is maximized with an approximately 10-nm-thick Ag layer. Additionally, the transmittance also tends to increase as the refractive index of the upper dielectric layer increases. By tailoring the thickness of the dielectric layer and the ITO layer, the dielectric/Ag/ITO structure yielded a transmittance of 0.97, which surpasses the maximum transmittance (0.91) of a single ITO film. Furthermore, this extraordinary transmittance was present for other visible wavelengths of light, including violet and green colors. A complex phasor diagram model confirmed that the transmittance of the dielectric/metal/ITO multilayer film is influenced by the interference of reflected partial waves. These numerical findings underpin a rational design principle for metal-based multilayer films that are utilized as transparent electrodes for the development of efficient light-emitting diodes and solar cell devices.

Design analysis of phosphor-free monolithic white light-emitting-diodes with InGaN/ InGaN multiple quantum wells on ternary InGaN substrates

Phosphor-free monolithic white light emitting diodes (LEDs) based on InGaN/ InGaN multiple quantum wells (MQWs) on ternary InGaN substrates are proposed and analyzed in this study. Simulation studies show that LED devices composed of multi-color-emitting InGaN/ InGaN quantum wells (QWs) employing ternary InGaN substrate with engineered active region exhibit stable white color illumination with large output power (∼ 170 mW) and high external quantum efficiency (EQE) (∼ 50%). The chromaticity coordinate for the investigated monolithic white LED devices are located at (0.30, 0.28) with correlated color temperature (CCT) of ∼ 8200 K at J = 50 A/cm2. A reference LED device without any nanostructure engineering exhibits green color emission shows that proper engineered structure is essential to achieve white color illumination. This proof-of-concept study demonstrates that high-efficiency and cost-effective phosphor-free monolithic white LED is feasible by the use of InGaN/ InGaN MQWs on ternary InGaN substrate combined with nanostructure engineering, which would be of great impact for solid state lighting.

Improvement of Optical and Electrical Properties of Indium Tin Oxide Layer of GaN-Based Light-Emitting Diode By Surface Plasmon in Silver Nanoparticles

Recently, GaN-based light-emitting diodes (LEDs) have found many applications, including automobile headlights, full-color displays, traffic light lamps, and solid-state lightings. However, the light extraction efficiency of GaN-based LEDs is limited by the total internal reflection of light at the interface between the LED structure and air caused by the large refractive index difference between the GaN film (n=2.5) and air (n=1). Photons with an incident angle lower than the critical angle of 23° are reflected from the interface, leading to low light extraction efficiency. Although the light extraction efficiency has been increased by using indium tin oxide (ITO) as a transparent conducting layer (TCL), further improvements in the TCL performance and the optical output power are still required to realize high efficiency LEDs. In this study, we demonstrate that the ITO layer with Ag nanoparticles can be used as an optically and electrically high performing TCL for LEDs. The transmittance of the ITO layer was improved by the resonance between incident light in the ITO layer and localized surface plasmons (LSPs) at the ITO-Ag interface. The optical output power and electrical characteristics of LEDs were improved by the Ag nanoparticles deposited on the ITO layer of the LEDs. Figure 1(a) shows a schematic diagram of blue LED with Ag nanoparticles deposited on ITO TCL. The LEDs with a blue emission at 465 nm were grown on a c-plane (0001) patterned sapphire substrate by metalorganic chemical vapor deposition. After growth of a GaN nucleation layer, a 2 μm-thick undoped GaN layer and a 2 μm-thick n-GaN layer were grown at 1025 °C. Then, multiple quantum wells, consisting of five periods of undoped InGaN wells and GaN barriers, were grown at 750 °C, followed by growth of a 200 nm-thick p-GaN layer at 980 °C. To fabricate the LEDs, the p-GaN layer was etched by an inductively coupled plasma etching process to expose the n-GaN layer for an n-type ohmic contact. Then, a 150 nm-thick ITO layer was deposited on p-GaN layer as a TCL by electron-beam evaporation. To investigate the effect of the Ag nanoparticles on the ITO layer, LEDs with Ag nanoparticles deposited on ITO layer were also fabricated. To form Ag nanoparticles, an Ag layer with different film thickness was deposited on the ITO layer by electron-beam evaporation, and then annealed at 500 °C for 1 min in a rapid thermal annealing (RTA) system. This was followed by deposition of Cr/Au on the n-GaN layer to form an n-pad electrode and on the ITO layer to form a p-pad electrode. Figure 1(a) also shows atomic force microscopy (AFM) image of the ITO layer with Ag nanoparticles after RTA processing. The surface of an as-deposited Ag layer on the ITO layer is smooth and featureless. However, the thin Ag layer is transformed into nanoparticles, and the particle size is increased by thermal annealing via the Ostwald ripening process. Figure 1(b) shows the optical output power of the blue LEDs with different density and size of Ag nanoparticle deposited on the ITO TCLs as a function of injection current. As shown in Fig. 1(b), the optical output powers of the LEDs with Ag nanoparticles on top of the ITO TCL are higher than that of the LED with the bare ITO TCL. In particular, the optical output power of an LED with Ag nanoparticles (1 nm-thick Ag layer) is increased by 16% at an injection current of 20 mA compared to that of the LED without Ag nanoparticles and the enhancement is increased with increasing the density of Ag nanoparticles. This improvement is mainly attributed to the increased transmittance of ITO by the resonance of the light with the LSPs in the randomly distributed Ag nanoparticles. The electrical properties of the LEDs are also improved by the enhanced conductivity of the ITO TCL caused by the Ag nanoparticles. These results indicate that the Ag nanoparticles/ITO with high transmittance and low resistivity can be used as a high efficiency TCL for LEDs. Figure 1

Enhanced light-extraction efficiency of GaN-based light-emitting diodes with hybrid Photonic Crystals

The light-extraction efficiency (LEE) of GaN-based light-emitting diodes (LEDs) is strictly limited by total internal reflection (TIR) and Fresnel reflection. In this paper, we sought to utilize hybrid Photonic Crystals (PhCs) with different etched depth to increase the LEE of GaN-based LEDs. The results showed that the structure proposed here could decrease not only the TIR but also the Fresnel reflection. Three-dimensional rigorous coupled waves approach was employed to calculate the total transmission efficiency at different incident angles, and numerical simulations based on finite-difference time-domain method were carried out to explore mechanisms by which hybrid PhCs can effectively improve the LEE. This improvement is related to the grade-refractive-index effect and diffraction properties of the hybrid PhCs. And the structure proposed here provides a train of thought for the design of high efficiency LEDs.

SU-8 Planarized InGaN Light-Emitting Diodes With Multipixel Emission Geometry for Visible Light Communications

The potential for visible light communications with SU-8 planarized InGaN light-emitting diodes (LEDs) is investigated experimentally. For large-size LEDs, current crowding occurring near the n-contact is addressed by shrinking the dimensions of the emitters/pixels, along with the use of parallel-connected schemes to achieve multipixel emissions. Through improved heat dissipation, current uniformity, and light extraction efficiency, the resulting LED matrices fabricated with 6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">$\times$</tex-math></inline-formula> 6 pixels outperform conventional LEDs in terms of light output power and current-induced spectral shift. It was also found that good control of the SU-8 planarization process and optimizing the number of pixels facilitates the fabrication of high-efficiency LED matrices. In addition, the presence of large junction capacitance caused by the parallel connection of the individual pixels prevents these LED matrices with 6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">$\times$</tex-math></inline-formula> 6 pixels from operating at high speed. After eliminating the slow-responding phosphorescent components emitting from the phosphor-converted white LEDs, an open eye diagram at 80 Mb/s is demonstrated over a distance of 100 cm in directed line-of-sight optical links.

Investigating the origin of efficiency droop by profiling the temperature across the multi-quantum well of an operating light-emitting diode

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.

Solution‐Based High‐Density Arrays of Dielectric Microsphere Structures for Improved Crystal Quality of III‐Nitride Layers on Si Substrates

The recent development of dielectric microsphere lithography has been able to open up new means of performing simple and easy patterning on the semiconductor surfaces. Here, we report uniform and high‐density arrays of microspheres using a solution‐based spin‐coating method. The arrays of microspheres were used for etching mask to form the arrays of III‐nitride microrods. By regrowing GaN layer on the microrod structures, high‐quality GaN layer was achieved in terms of surface morphology as well as XRD characterization. To apply the advantages such as improved crystal quality and light extraction enhancement, light‐emitting diodes (LEDs) were grown and then fabricated. The regrown LEDs with microspheres showed much improved optical output power and forward voltage characteristics in the same current injection. Therefore, we believe that this approach is quite useful for the development of high efficiency LEDs for future lighting.

Highly reflective Ti/Ag/Pt contacts to p-GaN for high-efficiency GaN-based light-emitting diodes

The authors report highly efficient GaN-based light-emitting diodes (LEDs) fabricated with Ti/Ag/Pt reflective p-type contact. Compared with the reference Ni/Ag/Pt contact, Ti/Ag/Pt contact annealed at optimized thermal annealing condition produced low specific contact resistances of 5.7 × 10−4 Ω cm2, and good optical reflectivity of 88.4% at 450 nm. This is due to the generated interfacial Ti–O layer upon thermal annealing in N2 ambient, which suppress the excessive inter-diffusion between Ag and GaN layers. Consequently, the LEDs fabricated with Ti/Ag/Pt contacts showed 32.6% brighter light output power and nearly the same forward voltages as compared to the reference LEDs fabricated with Ni/Ag/Pt contacts.

Strong modification of spontaneous emission rate in nanorod light-emitting diode structures.

This study investigates the characteristics of modifications in spontaneous emission (SE) from GaN-based nanorod light-emitting diode (LED) structures using the three-dimensional finite-difference time-domain method. The simulated nanorod LED structure is assumed to be enclosed by perfect conductors and includes InGaN multiple-quantum-well active layers emitting at 500 nm. In the simulation, the modification of the SE rate is calculated as the structural parameters of nanorods are varied. The SE rate is found to depend strongly on the radius of nanorods. Large enhancement of the SE rate is observed when the resonant modes are formed inside the nanorod cavity. For both the transverse-electric and transverse-magnetic modes, the SE rate of nanorod LED structures can be enhanced by more than six times when the radius or height of the nanorod is optimally chosen. This large increase in the SE rate can lead to considerable increase in the internal quantum efficiency of LEDs, making nanorod structures prime candidates for future high-efficiency LEDs than can overcome the efficiency limit of current LED structures.

Thermal effects in (oxy)nitride phosphors

Technologies that control the chemical composition of white lighting-emitting diodes are promising means to enhance thermal properties and renew spectra generation. Although (oxy)nitride red phosphors have been available for more than a decade, the drawbacks of these devices still evidently remain with respect to the local environments of activators in a variety of nitridosilicates. Thermal effects, such as, thermal quenching, thermal ionization, and thermal degradation, are technologically important parameters that determine product reliability. In recent years, red phosphors, which can alter novel complexes with particular wavelengths, have been easily synthesized and used to minimize losses during energy conversion process. Silicon nitride ceramics contain a more highly condensed network compared with silicate because of the higher degree of cross-linking, edge-sharing SiN4 tetrahedron, and more covalent and stronger crystal field splitting. To provide a reasonable explanation for the relationship between photoluminescence and structure, an empirical model has been proposed, in which the changes in the chemical environment of the activators are attributed to strains resulting from atom displacements. In addition, the development of high-efficiency and cost-effective light-emitting diodes based on these luminescent materials has difficult challenges.

Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light-emitting diodes

In this paper, recent advances in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) are demonstrated. 220–350-nm-band DUV LEDs have been realized by developing crystal growth techniques for wide-bandgap AlN and AlGaN semiconductors. Significant increases in internal quantum efficiency (IQE) have been achieved for AlGaN DUV emissions by developing low-threading-dislocation-density (TDD) AlN buffer layers grown on sapphire substrates. The electron injection efficiency (EIE) of the LEDs was also significantly increased by introducing a multiquantum barrier (MQB). We also discuss light extraction efficiency (LEE), which is the most important parameter for achieving high-efficiency DUV LEDs. We succeeded in improving LEE by developing a transparent p-AlGaN contact layer. The maximum external quantum efficiency (EQE) obtained was 7% for a 279 nm DUV LED. EQE could be increased by up to several tens of percent through the improvement of LEE by utilizing transparent contact layers and photonic nanostructures in the near future.

Integrating Surface Textures on ZnO Substrate for High Light Extraction Efficiency Light-Emitting Diode

Due to the large refractive index of ZnO material, light extraction efficiency (LEE) of ZnO based light-emitting diode (LED) is limited by the total reflection. We present a combined experimental and computational approach to study the effects of integrated surface textures on the efficiency of light extraction from the ZnO material. Hexagonal pyramids and pits textures were obtained by selective and anisotropic etching of O-polarity ZnO single crystal in HCl and KOH solutions, respectively. Dense hexagonal pyramids were integrated into the hexagonal pits via a two-step wet etching process. Ray-tracing simulations indicate that the substrate texturing can improve the performance of the ZnO based LEDs by enhancing the LEE and improving the light field uniformity. The integrated photoluminescence extraction was improved by a factor of 117% with respect to that of a planar ZnO single crystal through integrating the pyramids textures into the hexagonal pits.

High-Efficiency and Crack-Free InGaN-Based LEDs on a 6-inch Si (111) Substrate With a Composite Buffer Layer Structure and Quaternary Superlattices Electron-Blocking Layers

In this paper, a composite buffer layer structure (CBLS) with multiple AlGaN layers and grading of Al composition/u-GaN1/(AlN/GaN) superlattices/u-GaN2 and InAlGaN/AlGaN quaternary superlattices electron-blocking layers (QSLs-EBLs) are introduced into the epitaxial growth of InGaN-based light-emitting diodes (LEDs) on 6-inch Si (111) substrates to suppress cracking and improve the crystalline quality and emission efficiency. The effect of CBLS and QSLs-EBL on the crystalline quality and emission efficiency of InGaN-based LEDs on Si substrates was studied in detail. Optical microscopic images revealed the absence of cracks and Ga melt-back etching. The atomic force microscopy images exhibited that the root-mean-square value of the surface morphology was only 0.82 nm. The full widths at half maxima of the (0002) and (1012) reflections in the double crystal X-ray rocking curve were ~330 and 450 respectively. The total threading dislocation density, revealed by transmission electron microscopy, was <; 6× 108 cm-2. From the material characterizations, described above, blue and white LEDs emitters were fabricated using the epiwafers of InGaN-based LEDs on 6-inch Si substrates. The blue LEDs emitter that comprised blue LEDs chip and clear lenses had an emission power of 490 mW at 350 mA, a wall-plug efficiency of 45% at 350 mA, and an efficiency droop of 80%. The white LEDs emitter that comprised blue LEDs chip and yellow phosphor had an emission efficiency of ~110 lm/W at 350 mA and an efficiency droop of 78%. These results imply that the use of a CBLS and QSLs-EBL was found to be very simple and effective in fabricating high-efficiency InGaN-based LEDs on Si for solid-state lighting applications.