Low Light Extraction Efficiency Research Articles

Improved light extraction efficiency of AlGaN DUV light emitting diodes using Al/MgF2-based highly reflective film

AlGaN DUV light emitting diodes (DUV-LEDs) (275 nm emission) are safe, eco-friendly and smart alternatives for inactivating viruses and bacteria. However, DUV-LEDs suffer from the main bottleneck of low external quantum efficiencies, which are strongly associated with the low light extraction efficiency caused by the strong optical polarisation of Al-rich AlGaN. Optical simulation results show that the luminous intensity of DUV-LEDs was increased by 10% owing to the synergistic effect of the sidewalls and substrates. The optical power of DUV-LEDs was increased to 16.8%, the far-field pattern was expanded to 130 degrees and the emission intensity was more focused on the central region, proving that the highly reflective sidewalls and substrates could re-direct the sideways-travelling photons for extraction. Moreover, we also investigated the reflective mechanism of Al/MgF2 layers. Optimizing the refractive index distribution of Al/MgF2 layers could change the electric field intensity and improve the reflectivity. At the same time, the temperature of the sample after coating was significantly reduced by 6.83%. Thermal radiation benefits and the high stability of bonding interfaces are the main reasons to reduce the temperature of DUV-LEDs after Al/MgF2 coating. The present strategy is proposed from the point of view of chip fabrication, which is cost-effective and able to be manufactured at a large scale.

High performance and high yield sub-240 nm AlN:GaN short period superlattice LEDs grown by MBE on 6 in. sapphire substrates

We develop ultraviolet (UV) light emitting diodes (LEDs) that operate in the sub-240 nm (far UVC) regime. The traditional limitations of high Al content AlGaN—the low light extraction efficiency due to transverse magnetic polarization and the poor doping efficiency—were overcome by using AlN:GaN short period superlattices (SPSLs) of varying compositions throughout almost the entire device structure. The quality of our molecular beam epitaxy process for growing SPSL devices was verified via transmission electron microscopy and x-ray diffraction—together, these demonstrate our capacity for growing high quality interfaces with a great deal of control over the thicknesses of each individual layer. Resistivity measurements of our SPSL were three times smaller than bulk AlGaN of similar composition due to the improved doping efficiency. Devices fabricated on 6 in. sapphire substrates showed uniform performance, leading to over 10 000 functional devices per wafer. Packaged diodes exhibited a peak wall plug efficiency of 0.55% and an output power of 1.7 mW when operated at peak efficiency. The devices had an operating life (70% initial power) of 2800 h (at 20 mA continuous wave). Additionally, they could be driven as high as 1 A, which resulted in a record for radiant flux of a far UVC LED grown on sapphire (17.4 mW).

Two-step sintered phosphor ceramics for high-power LED applications: Fine microstructure and luminescent property

With the development of the high-power solid-state lighting (SSL) industry, there is an urgent demand for low-cost color converters with good luminescent performance and thermal robustness. Conventional phosphor ceramics face the problems of high sintering energy consumption and low light extraction efficiency. In the present work, a two-step sintering (TSS) method has been developed to obtain a series of YAG:Ce and YAG:Ce-Al2O3 samples at a lower temperature of 1600 °C, where the operating power is reduced to 67% of the traditional one-step process. The TSS samples have a fine microstructure, which leads to a better scattering enhancement effect. The impacts of grain size and composition on optical and thermal properties were investigated in detail. The luminous characteristics and stability were analyzed by packaging phosphor ceramics with blue LEDs, with a maximum LE of 188.72 lm/W, CRI of 58.7, and CCT in the range of 3628–5048 K, respectively. Such suitable properties of the TSS material make it a promising candidate for high-power white LEDs applications.

Unveiling light collection and pump enhancement from quantum wells with plasmonic metasurfaces using power dependent measurements

Low light extraction efficiency (LEE) is the greatest limiting factor for the brightness of reduced-size light-emitting diodes (LEDs) as it limits their emission intensity. In addition, LEDs have a Lambertian emission, which requires secondary optics to control the emission directionality. Plasmonic metasurfaces can introduce a way of manipulating the generated light from LEDs to enhance their LEE and steer the emitted light by reshaping the far-field emission. Here, we fabricate resonant plasmonic metasurfaces on top of a typical blue emitting wafer consisting of InGaN/gallium nitride quantum wells developed for commercial LED devices. The metasurface is separated from the InGaN quantum wells by p-GaN and indium-tin-oxide (ITO) layers with a cumulative thickness of 110 nm. Since this distance value is close to the emission wavelength in the corresponding medium, enhanced near-fields of localized plasmonic resonances do not reach the active region. Despite this, we observe a strong influence of the metasurfaces on the far-field photoluminescence emission from the quantum wells as demonstrated by Fourier imaging. Power-dependent excitation measurements of the samples allow us to retrieve the pump and light collection enhancement factors provided by the plasmonic metasurfaces. We demonstrate that the plasmonic metasurfaces can provide a pump enhancement factor of up to 4.1 and a collection enhancement factor of up to 3.2. We have also performed simulations based on the reciprocity principle and achieved a good qualitative agreement with the experimental results.

Ultra High Luminous Efficiency p-Type Surface Defect Structure GaN LED

In this paper, we propose a double-layer SiO2 photonic crystal LED with a linear defect structure in the [Formula: see text]-GaN layer, the purpose is to solve the problem of low light extraction efficiency caused by diffusion scattering effect. We used FDTD modeling to analyze the light-emitting characteristics of GaN LED, after optimizing the structural parameters of two-dimensional photonic crystals, the light extraction efficiency is improved from 19.4% to 35.2%, the light extraction efficiency is increased by 1.8 times, meanwhile the light output power is also increased by 1.86 times. The introduction of the defect structure does not affect the [Formula: see text]–[Formula: see text] characteristics of the LED, and the output power of the LED is increased by 11% under the current of 300[Formula: see text]mA.

Photonics design theory enhancing light extraction efficiency in quantum dot light emitting diodes

The external quantum efficiency (EQE) of quantum dot light emitting diodes (QLEDs) needs improvement for more power-efficient devices. One of the main limitations is the low light extraction efficiency (LEE). Generally, only 20% of the light that is generated inside the emissive layer makes its way out of the device into air, with the rest being lost to waveguide and substrate modes and surface plasmon polaritons. Different photonics structures have been previously tested to help extract the light that is trapped inside the device. Here we report a photonics design which is a combination of nanopillars and grating structures for improving the LEE of QLEDs. The effect of changing the nanopillar height, radius and material has been studied. It was found that ZnO nanopillars of 500 nm pitch, 200 nm height and 400 nm width alongside 150 nm width and pitch grating structure can increase the LEE at 460 nm by 50% and at 640 nm by 20%. It was also found that different materials can help extract light at different wavelengths. TiO2 nanopillars increased the extraction efficiency at ∼590 nm region which was not observed by the other materials. As around 19% of the world’s electricity consumption is due to lighting applications, increasing the LEE can significantly reduce the power consumption.

Enhancing opto-thermal performances of white laser lighting by high reflective phosphor converter

Phosphor-converted white laser lighting has been considered as a promising candidate in next-generation high-brightness lighting. However, the color converter confronts the inevitable roadblock of low light extraction efficiency and poor thermal stability in reflective white laser diodes (LDs). Herein, a high reflective converter of phosphor-in-glass film-BN layer-Al substrate (PiG-BN-Al) was prepared for high-brightness white laser lighting. The effects of laser power and excitation time on opto-thermal performances of PiG-BN-Al-based converter were investigated and compared with the traditional PiG-Al-based converter. Attributing to the high reflectivity of BN glass composite and the high thermal conductivity of Al substrate, the PiG-BN-Al-based converter achieves a luminous flux of 458.6 lm at a laser power of 3.15 W, which is about 70 % higher than the PiG-Al-based converter. Furthermore, the PiG-BN-Al-based converter still remains low surface temperature of 46.4 °C under the continuous laser excitation. The PiG-BN-Al-based converter provides an effective strategy to enhance the opto-thermal performances of white laser lighting. • A high reflective PiG-BN-Al-based converter was prepared for white laser lighting. • The PiG-BN-Al-based converter presents high luminous flux of 458.6 lm. • The PiG-BN-Al-based converter shows thermal stability and low surface temperature. • The converter enhances the opto-thermal performances of white laser lighting.

Role of surface microstructure and shape on light extraction efficiency enhancement of GaN micro-LEDs: A numerical simulation study

One of the bottlenecks for today’s micro-light-emitting-diodes (micro-LEDs) is the low light extraction efficiency. How to effectively boost this efficiency is especially important for micro-LEDs because their surface-to-volume ratio is much larger than normal LEDs. The trapezoidal chip structure, surface grating, and surface coarsening are simulated in this paper, with the aim to obtain their contributions to enhance the light extraction efficiency. Through the simulation, we have found that all of the three methods can increase the light extraction, but with different extents. When the trapezoidal angle of the mesa is 12°, the extraction efficiency reaches its maximum (by this method) of 64.5%. If the surface grating on n-GaN (the light output surface) has a period of 300 nm, a height of 147 nm and width of 243 nm, the light extraction efficiency can be as high as 72%, which is much higher than that of the micro-LED without any grating. For the random roughening of n-GaN, a maximum value of 56% can be obtained if the roughness extent is optimized. Our results are of reference value for scientists and engineers working in micro-LEDs and can serve as guidelines for GaN based high efficiency micro-LED design for future displays.

Application of patterned sapphire substrate for III-nitride light-emitting diodes.

Recent decades have witnessed flourishing prosperity of III-nitride emitters in solid-state lighting and high-resolution displays. As one of the widely used substrates, sapphire shows superiority for heteroepitaxial growth of III-nitride light-emitting diode (LED) structure, due to the advantages of stability, low cost, high mechanical strength, as well as mature fabrication technology. However, realization of efficient LEDs grown on sapphire substrate is impeded by high density of defects in epilayers and low light extraction efficiency. The emergence of patterned sapphire substrate (PSS) turns out to be a promising and effective technology to overcome these problems and enhance the LED performances. In this review, we first introduce the background and recent advances of PSS applied in III-nitride visible and ultraviolet LEDs are. Then, we summarize the fabrication methods of PSS, together with novel methods to define nanometre-scale patterned structures. We further demonstrate the effect of PSS that contributes to reduce the threading dislocation density (TDD) of epilayers in detail. Meanwhile, mechanism of light extraction efficiency enhancement by adopting PSS is presented based on numerical analysis. Next, we explore the influence of PSS structural parameters (e.g. pattern size, pattern shape and aspect ratio) on LED performances, spanning from visible to deep ultraviolet UV emission region. Finally, challenges and potential prospects in PSS for future LED development are proposed and forecasted as well.

Enhanced Light Extraction of DUV LEDs by Using a Quartz Lens With Three-Dimensional Structure

AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs) have wide applications in many fields, such as air and water purification, disinfection, and biochemical inspection. However, the wall plug efficiency (WPE) of DUV LEDs is relatively low for these applications, which is due to the low light extraction efficiency (LEE) yielding limited light output from LED package. In this work, we proposed a quartz lens with three-dimensional (3D) structure to enhance the light extraction of DUV LEDs. The optical models of DUV LEDs were established and the optical performances of DUV LEDs were investigated by using optical simulation. The DUV LEDs with the 3D quartz lens exhibit higher light efficiency than those with traditional quartz lens structure. Compared with the traditional structure, the light output power of the 3D lens structure is enhanced by 25.93% at the current of 140 mA and the packaged DUV LED can achieve a wide light angle of 153.2°. It is attributed to the significant light extraction enhancement of the sidewall emission from DUV LED chip.

Light extraction in tandem organic light emitting diodes

Since the invention of organic light emitting diodes (OLEDs), great research efforts have been dedicated to improving their efficiency and lifetime. For high-brightness applications, tandem OLED structures have advantages because of the lower current densities required to achieve high brightness. With the successful development of highly efficient charge generation layers, high brightness tandem OLEDs are used in displays and lighting. However, the major challenge for tandem OLEDs is the low light extraction efficiency, because about 50% of the light is trapped inside the device as waveguide modes. In this Perspective, we first review the recent works done on light extraction, analyze different waveguide mode extraction structures, and then identify the key factors determining the extraction efficiencies in tandem OLEDs.

Light extraction efficiency of AlGaN nanowire deep ultraviolet light-emitting diodes on Si with different photonic structures

The low light extraction efficiency (LEE) stemming from the transverse-magnetic dominant emission is one of the major factors limiting the performance of aluminum gallium nitride (AlGaN)-based deep ultraviolet (UV) light-emitting diodes (LEDs) when the light emission wavelengths are close to 200 nm. These wavelengths, nonetheless, are pivotal to applications including sensing and sterilization and are considered human safe. We investigate the LEE of AlGaN/AlN nanowire (NW) deep UV LEDs emitting at 225 nm, with a focus on the top-surface LEE by considering different NW arrangements, typical NW spacings and radii from AlGaN NWs grown by selective area epitaxy, the influence of Si on the LEE in comparison to an Al reflector, and graphene as the top electrode. Our results show that given the selected range of design parameters, the top-surface LEE for honeycomb, square, and hexagonal lattices can be up to around 42% for complete devices on Si with the graphene top electrode; and compared to using an Al reflector, the Si substrate does not reduce LEE considerably. In the end, the light extraction mechanism is discussed by simulating the two-dimensional photonic crystal band structures.

Enhanced Light Extraction Efficiency (LEE) of GaN-based LED Die through Substrate Side Surface Grooving

High refractive index difference between GaN-based light-emitting diode (LED) die material and surrounding medium causes low light extraction efficiency. Introduction of surface grooves to sapphire substrate side surfaces improves the light extraction efficiency of GaN-based LED die6 by altering the normal for the impinging light rays inside the substrate layer. Comparison of light output power given out by GaN-based LED die with grooved sapphire substrate and un-grooved sapphire substrate were studied using ray-tracing simulation. Three types of surface grooves at different aspect ratios were simulated which includes elliptical groove, triangular groove and rectangular groove. Based on the results obtained, GaN-based LED die with elliptically grooved sapphire substrate gives the best enhancement in light output power by 121.2426% compared to an un-grooved GaN-based LED die at an aspect ratio of 0.05. Surface grooves with curvature or angled plane gives a better enhancement in light extraction efficiency compared to a flat surface.

Optimized Aluminum Reflector for Enhancement of UVC Cathodoluminescence Based-AlGaN Materials with Carbon Nanotube Field Emitters.

The far ultraviolet C (UVC) light sources based on carbon nanotube (CNT) field emitters as excitation sources have become promising light sources for sterilization, disinfection, and water purification. However, the low light extraction efficiency of UVC–CNT light sources still hinders the practical application of these structures. Herein, we report an optimized aluminum (Al) reflector to enhance the light extraction efficiency of UVC–CNT light sources. Optical analysis of UVC-CNT light sources covered by the Al reflectors with various thicknesses ranging from 30 to 150 nm was performed to realize the optimized reflector. The UVC-CNT light sources exhibit the highest light extraction efficiency when the Al reflector layer has an optimized thickness of 100 nm. For comparison, the cathodoluminescence (CL) spectra were recorded for UVC–CNT light sources with and without the optimized Al reflector. The measured light output power and the estimated power efficiency of the UVC–CNT light-source-tube with Al reflector were enhanced by about 27 times over the reference. This enhancement is mainly attributed to the outstanding reflection effect of the Al reflector.

Effect of HfO2 Passivation Layer on Light Extraction Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes

Diverse applications of ultraviolet (UV) light in the field of purification, sterilization, detection, surface disinfection, sensing, and many others have been studied mostly on AlGaN light-emitting-diodes (LEDs). However, the external quantum efficiency (EQE) of AlGaN deep UV LEDs is limited due to strong induced polarization fields, high dislocation density, severe electron leakage, difficulty in p-type doping, low internal quantum efficiency (IQE), IQE droop, and low light extraction efficiency (LEE). Alternatively, AlInN semiconductor has been recently gained tremendous attention for high power UV LEDs [1-3]. Our group has successfully demonstrated the first AlInN core-shell nanowire UV LEDs with a relatively high IQE of ~52%[1]. However, the light extraction study of such nanowire LEDs is unexplored. In this context, we have studied the LEE of a single AlInN nanowire UV LED at 220 nm - 400 nm wavelength regime. Moreover, we report that the LEE of a single AlInN nanowire UV LED could be increased by ~60% using HfO2 passivation compared to conventional nanowire LED at 282 nm.The schematic structure of AlInN nanowire UV LED is shown in Figure 1(a). The nanowire is composed of 200 nm n-GaN, 100 nm n-AlxIn1-xN quantum barrier (QB)/40 nm i-AlyIn1-yN quantum well (QW)/100 nm p-AlxIn1-xN QB, and 20 nm p-GaN layer. We have estimated the LEE of nanowire LEDs using the finite difference time domain (FDTD) simulation method. Silicon (Si) is used as a substrate, and the n-GaN layer is tapered with a diameter from 70 nm (bottom) to 100 nm (top). The diameter is set to be 100 nm for the rest of the regions. We have placed a single TM-polarized dipole source at the QW center to calculate LEE for a nanowire LED in the simulations. The estimated LEE of 282 nm wavelength single nanowire AlInN UV LED with various HfO2 passivation layers thickness is provided in Figure 1(b). The LEE of such LED without any passivation is ~25.2%, whereas the maximum recorded LEE is ~40.6% with the utilization of 35nm thickness HfO2 passivation layer. This study provides a promising approach for enhancing the light extraction efficiency of nanowire UV LEDs. Reference R. T. Velpula, B. Jain, M. R. Philip, et al., Sci. Rep., 10(1) 2547-2556 (2020).R. T. Velpula, B. Jain, H.Q.T. Bui, et al., Opt. Mater. Express, 10(2), 472-483 (2020).B. Jain, R. T. Velpula, M. Tumuna, et al., Opt. Express, 28(15), 22908-22918 (2020). Figure 1

Transparent YAG:Ce ceramic with designed low light scattering for high-power blue LED and LD applications

Yellow-emitting YAG:Ce transparent ceramic is recognized as an ideal color converter in high-power blue LEDs and LDs, but the absence of scattering centers in its microstructure leads to a low light extraction efficiency and poor light uniformity. Here, taking advantage of the scattering effect and the transparency of YAG:Ce ceramics, Ce-free YAG phase was used as a second component to form a composite with YAG:Ce phosphor. The sintered YAG:Ce-YAG ceramic possessed a high transparency of ∼63 % and a thermal conductivity of 8.9 Wm−1 K−1. Due to its beneficial thermal properties and high external quantum efficiency of 70.2 %, the YAG:Ce-YAG ceramic could be excited under a high blue-laser flux density of up to 9.60 W/mm2 and showed a luminous emittance of 1220 lm/mm2. Due to light scattering arising from the slightly different refractive indices of the two phases, the designed YAG:Ce-YAG ceramic showed better lighting effects than a single-phase transparent YAG:Ce ceramic.

Improvement of Light Extraction in Deep Ultraviolet GaN Light Emitting Diodes with Mesh P-Contacts

One of the main reasons that the emission efficiency of GaN-based light-emitting diodes (LEDs) decreases significantly as the emission wavelength shorter than 300 nm is the low light extraction efficiency (LEE). Especially in deep ultra-violet (DUV) LEDs, light propagating outside the escape cone and being reflected back to the semiconductor or substrate layer is absorbed not only by active layers but also by p-type layers with narrower bandgaps and electrodes that are neither transparent nor reflective of the DUV wavelength. In this report, we propose a DUV LED structure with mesh p-GaN/indium-tin-oxide (ITO) contacts and a Ti/Al/Ni/Au layer as a reflective layer to improve LEE. The mesh p-GaN/ITO DUV LED showed an output power of 12% higher than that from the conventional DUV LED due to the lower light absorption at 280 nm.

Convenient method for improving the light output of scintillators by using buffer layers coated with photonic crystals

The low light-extraction efficiency of scintillators is due to total internal reflection and has led to the extensive use of photonic crystals to improve the light output. However, in some applications, photonic crystals cannot be fabricated directly on scintillators. Here, we demonstrate a promising method to improve the light output of scintillators by using a buffer layer coated with photonic crystals and then fixed to the scintillator. Through both numerical simulations and experiments, we investigate how the refractive indexes of the buffer layer and photonic crystal affect the light output from scintillators. The experimental results indicate that the light output of (Lu,Y)2SiO5:Ce scintillators is enhanced 1.9 times by using a sapphire buffer layer coated with an array of polystyrene nanospheres. This method can be used to improve the detection efficiency of radiation-detection systems when photonic crystals cannot be fabricated directly on the scintillator.

Research progress of enhancing perovskite light emitting diodes with light extraction

Perovskite light emitting diodes (PeLEDs) have developed rapidly in recent years due to their advantages of tenability of band gap and high color purity. At present, the external quantum efficiency of PeLED has rised up to 20%. Like the scenario of organic light emitting diode, there exist various internal losses in PeLED with low light extraction efficiency. It arises from the absorption of substrates, waveguide transmission and surface plasmon resonance of metal electrode. To improve the luminescence performance of PeLED, a well-matched optical admittance between the thin-films inside the devices is required. In this paper, the strategies of enhancing the light extraction efficiency are adopted as the materials and structures in PeLED are concerned. The applications of alternative electrode in PeLED are discussed, such as graphene, silver nanowires, metal transparent electrode and some new-types of electrodes. In addition, the plasma effect is also introduced into the PeLED to deflect the emitting light. What is more, the nano-structure grating is inserted into device to reduce the optical losses due to the large refractive index difference between the interfaces in device. Therefore, the external quantum efficiency of PeLED rises up to 28.2%, and the current efficiency can reach 88.7 cd/A.

Interplay between various active regions and the interband transition for AlGaN-based deep-ultraviolet light-emitting diodes to enable a reduced TM-polarized emission

Al-rich AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) have a low light extraction efficiency, especially when the emission wavelength is shorter than 280 nm, and this is partially because of the dominant transverse-magnetic polarized light. Our results show that the transverse-electric (TE) polarized light can be obtained even if the emission wavelength becomes even shorter by reducing the quantum well thickness. The ultrathin quantum well enables the enhanced TE-polarized emission that arises from the redistributed subbands for holes. On the contrary to the common belief, we observe a blueshift for the emission wavelength when the AlN composition in the quantum barrier increases. The internal quantum efficiency (IQE) for DUV LEDs with ultrathin quantum wells is no longer determined by the quantum-confined Stark effect, while quantum barrier with high AlN composition is vitally important to improve the electron injection efficiency and thus enhance the IQE.