High Light Extraction Efficiency Research Articles

Enhanced forward emission by backside mirror design in micron-sized LEDs.

Tiny InGaN micro-LEDs (μ-LEDs) play a pivotal role in emerging display technologies, particularly augmented reality (AR) applications. Achieving both high internal quantum efficiency (IQE) and efficient light extraction efficiency (LEE) is essential. While wet chemical etching can recover the IQE after dry etching, it alters the pixel shape, impacting optical properties and reducing the LEE. In this study, we overcome this issue by fabricating 1 μm thin-film-based μ-LED emitter arrays with a metallic backside mirror deposited on a patterned dielectric material around the μ-LED mesa. This concave mirror can be straightforwardly integrated into a thin-film LED process chain, and it redirects photons within the μ-LED structure, enhancing the LEE in the forward direction. Electro-optical measurements show a 2.1-fold improvement in light output within the ±15∘ emission cone compared to μ-LEDs with vertical sidewalls. These findings hold significant implications for μ-LED projection displays, where maximizing the overall efficiency and directionality is critical.

Highly light extraction efficiency in multiple resonance OLED by PMMA-silica composite microlens arrays

This study reports a cost-effective method for fabricating densely packed micro-lens arrays (MLAs). These microstructures were created by using different particle sizes of 2, 6, and 10 μm modified silica, polymethyl methacrylate (PMMA), and ethyl acetate solvent. The proposed PMMA-silica composite MLAs were identified to feature highly transparent with a transmittance of 91.2 % and a high haze of 80.2 % in UV–vis regime. Consequently, external quantum efficiency (EQE) of OLED attached MLAs was found to be significantly improved, raising from 6.30 % to 11.67 %, corresponding to a remarkable enhancement of 85 % compared with reference OLED. Additionally, a substantially small CIE coordinate difference of (0.003, 0.0129) was obtained as viewing angles increased from 0° to 60°. The origin of high light extraction efficiency and angular-spectrum stability originated from the strong scattering induced by the high haze, leading to an emissive profile broader than Lambertian. It's worth noting that the fabrication for proposed MLAs is facile, and the substantial improvement suggests that this approach promise for next generation display and lighting applications.

A scattered volume emitter micropixel architecture for ultra efficient light extraction from DUV LEDs

Deep ultraviolet light-emitting diodes (DUV LEDs) typically suffer from strong parasitic absorption in the p-epitaxial layer and rear metal contact/mirror. This problem is exacerbated by a substantial portion of the multiple quantum well (MQW) emissions having a strong out-of-plane dipole component, contributing to emission in widely oblique directions outside the exit cone of the front semiconductor emitting surface. To address this, we propose an architecture that leverages such a heavily oblique angular emission profile by utilizing spaced-apart or scattered volume emitter micropixels that are embedded in a low-index dielectric buffer film with a patterned top surface. This approach achieves high light extraction efficiency at the expense of enlarging the effective emission area, however, it does not require a high-index (e.g., sapphire) substrate or a lens or a nanotextured epi for outcoupling purposes. Hybrid wave and ray optical simulations demonstrated a remarkable larger than three to sixfold increase in light extraction efficiency as compared to that of a conventional planar LED design with a sapphire substrate depending on the assumed epi layer absorption, pixel size, and ratio of light emission area to the MQW active area. An extraction efficiency three times greater than that of a recent nanotextured DUV LED design was also demonstrated. This architecture paves the way for DUV LEDs to have a plug efficiency comparable to that of mercury lamps while being significantly smaller.

P‐144: High efficiency MIP with external DBR package

In this work, we design a new type of MIP(Micro LED in Package) chips which have higher LEE(Light Extraction Efficiency) by adding an external DBR(Distributed Bragg Reflector) so that the EQE (External Quantum Efficiency) of the MIP is increased. Such MIP chips provide a new choice for Mini LED display with high brightness and low power consumption.

Enhancing external quantum efficiency of deep ultraviolet micro-leds through geometry design and multi-physics field coupling analysis

Abstract High-efficiency deep-ultraviolet (DUV) micro light-emitting diodes (LEDs) are explored for inspiring development in numerous fields, such as non-line-of-sight solar-blind communication, optical pumping, and maskless lithography. In this study, we performed FDTD and SimuLED calculations to investigate the optimized DUV micro-LED structure geometry for high light extraction efficiency (LEE) by designing different mesa structures, including square, hexagonal, and circular geometries of micro-LEDs emitted at a wavelength of 275 nm. The results showed that a circular mesa of 5 μm diameter achieved a LEE of 27% from the bottom and sidewall emissions of as-prepared DUV micro-LED. And both the near- and far-field transverse magnetic polarized light intensities were enhanced by a factor of 1.5 over the square and hexagonal mesas. Meanwhile, the transverse electric (TE) polarized light of the circular mesa structure was enhanced and concentrated along the normal direction. Moreover, the internal quantum efficiency (IQE) of circular mesas with varied sizes was comprehensively investigated in the interactions of the thermal and electric fields. An AlGaN-based DUV micro-LED with a diameter of 5 μm was found to obtain the highest IQE owing to a high current-density distribution and its self-heating properties, thereby achieving a sufficiently high external quantum efficiency of 26.75%. This study provides a comprehensive technical report, including electrical, thermal, and optical analyses, and a new perspective for developing high-efficiency, high-performance DUV micro-LEDs in practical applications.

Droop and light extraction of InGaN-based red micro-light-emitting diodes

In this letter, we investigate the impact of periphery, width, length and area on the external quantum efficiency (EQE) of stripe-type InGaN-based red micro-light-emitting diodes (µLEDs). A longer periphery resulted in a higher light extraction efficiency () via the sidewall regardless of the area of the µLEDs. However, as the injection current increased a somewhat larger efficiency droop was observed at the longer periphery due to current crowding. Additionally, larger µLEDs experienced more self-heating than smaller ones, resulting in a red shift of wavelengths and a larger efficiency droop. When the current density exceeded 100 A cm−2, the EQE ratio of smaller-area μLEDs to larger-area ones increased significantly due to the difference in efficiency droop. Besides, a short light propagation length and a long emission width yielded a higher . Hence, the periphery, width, length and area of the µLEDs determine EQE, which provides insight into the pixel design of µLED displays.

Design of photonic crystals for light‐emitting diodes

AbstractPhotonic crystals (PCs) can greatly enhance the optoelectronic performance of light‐emitting diodes (LEDs) due to their distinctive color, photonic band gap, etc. Therefore, many scholars have conducted extensive research based on the high light extraction efficiency, good monochromaticity, and other excellent optoelectronic properties of PC LEDs. This review discusses the main principles of photonic crystals to improve the optoelectronic performance of LEDs and summarizes 12 structural applications of photonic crystal LEDs, such as PC slabs, Bragg grating, backside reflectors, surface PC, embedded PC, dual PC, PC beads, CPC, PC thin films, LIPC, defective PC, and composite architectures with other materials that boost LED optoelectronic qualities. In summary, it is found that photonic crystals can not only greatly improve the light extraction efficiency of LEDs but also improve other optoelectronic properties such as luminescent color and directional radiation angle, and reduce the manufacturing cost of LEDs. Photonic crystal LEDs are expected to be a strong candidate for future lighting technology. Finally, the prospects and challenges of PC LEDs are summarized.

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.

Research on the High Light Out-Coupling Efficiency Deep-Blue Top-Emitting Organic Light-Emitting Diode through FDTD Optical Simulation.

We have studied high light out-coupling efficiency top-emitting organic light-emitting diodes (TOLEDs) under the guidance of the finite-difference time-domain (FDTD) simulation. TOLED achieves an extraordinarily high light extraction efficiency at 468 nm, in deep-blue regions, of 49.70%, which is approximately 3.5 times that of the bottom light-emitting diode (BOLED) by changing the thickness of the organic layer and the position of the light-emitting layer in the FDTD simulation. Based on the simulation results, the TOLED with ultrahigh efficiency and narrow full width at half maximum is successfully fabricated, and the maximum external quantum efficiency of TOLED is almost 3.3 times that of the BOLED, which is perfectly consistent with the FDTD simulation results. Meanwhile, the shift of the electroluminescence spectrum of the TOLED is restricted within 10° in the angular-dependence test (0° to 80°). The optimized performance of the OLED indicates a new method to develop a high-performance device under the guidance of simulation.

A Review on the Progress of AlGaN Tunnel Homojunction Deep-Ultraviolet Light-Emitting Diodes

Conventional deep-ultraviolet (UV) light-emitting diodes (LEDs) based on AlGaN crystals have extremely low light-emission efficiencies due to the absorption in p-type GaN anode contacts. UV-light-transparent anode structures are considered as one of the solutions to increase a light output power. To this end, the present study focuses on developing a transparent AlGaN homoepitaxial tunnel junction (TJ) as the anode of a deep-UV LED. Deep-UV LEDs composed of n+/p+-type AlGaN TJs were fabricated under the growth condition that reduced the carrier compensation in the n+-type AlGaN layers. The developed deep-UV LED achieved an operating voltage of 10.8 V under a direct current (DC) operation of 63 A cm−2, which is one of the lowest values among devices composed of AlGaN tunnel homojunctions. In addition, magnesium zinc oxide (MgZnO)/Al reflective electrodes were fabricated to enhance the output power of the AlGaN homoepitaxial TJ LED. The output power was increased to 57.3 mW under a 63 A cm−2 DC operation, which was 1.7 times higher than that achieved using the conventional Ti/Al electrodes. The combination of the AlGaN-based TJ and MgZnO/Al reflective contact allows further improvement of the light output power. This study confirms that the AlGaN TJ is a promising UV-transmittance structure that can achieve a high light-extraction efficiency.

Excitation-dependent spatially resolved photoluminescence in InGaN/GaN LEDs with air-cavity arrays grown on patterned sapphire substrates

We investigated the excitation-dependent spatially resolved luminescence properties of InGaN/GaN light-emitting diodes (LEDs) with air-cavity arrays using scanning near-field optical microscopy (SNOM), and quantified the effect of the air-cavity structure on the spatial distributions of light-extraction efficiency, internal quantum efficiency, and external quantum efficiency through experiments and numerical simulations. We found that higher light-extraction efficiency and less stress were always observed in the air-cavity areas. Compared to flat areas, the average light-extraction efficiency of the air-cavity areas was improved by 65%, and the residual stress of the air-cavity center was reduced by 89 MPa, which resulted in increased PL intensity, reduced wavelength shift, and narrowed full width at half maximum. The highest external quantum efficiency and aggravated efficiency droop were found in the air-cavity center. At 3.1 W/cm2, the external quantum efficiency and internal quantum efficiency of the air-cavity center were enhanced by 255% and 223%, respectively.

Microdisk-Type Multicolor Semipolar Nitride-Based Light-Emitting Diodes

Recently, III-nitride semiconductors have gained immense attention for application in high-performance optoelectronic devices. However, in the device fabrication process, it is essential to develop a wet-etching process for conventional polar nitride-based devices to reduce the dry-etching damage. In this study, semipolar microdisk (MD)-type light-emitting diodes (LEDs) are fabricated using only a wet-etching process. The wet-etching rate of the n-type GaN film is the highest, followed by those of the undoped and p-type GaN films. The n- and p-type GaN films exhibit upward and downward surface-band-bending heights, respectively, because of which they can easily attract and repel OH– to form Ga2O3. In addition, the fully wet-etched MD-LED shows high light extraction efficiency from the etched nanofacets because of the increase in the escape cone angle with low etching damage. The smaller diameters of the MD-LEDs result in shorter emission wavelengths, allowing multicolor emission ranging from the violet (∼420 nm) to amber (∼620 nm) wavelengths to be achieved on a single LED wafer. Thus, monolithic multicolor emissions and increased light output power are achieved by fabricating parallel-connected MD-LED arrays using only a wet-etching process.

Surface-emitting photonic crystal terahertz quantum cascade laser adopting uniform triangular prism photonic crystal with a double-metal waveguide

Herein, we investigate surface-emitting photonic crystal terahertz quantum cascade lasers adopting double metal waveguide for high light extraction efficiency by utilizing finite-difference time-domain and filter diagonalization method. For the ease of fabrication and scalability of beam power, only a typical uniform 2D unit cell with triangular lattice and connected metal electrodes are considered in this study. Cylinder or uniform triangular prism photonic crystals are adopted as resonators within the selected unit cell structure. Candidate -point modes for efficient surface-normal emission are searched based on the unit cell analysis as a first step. After that, far-field, field distribution, and light-extraction efficiency are calculated to verify the candidate modes’ suitability by deploying 15(16) × 15 unit cells. Extracted results show that the surface-emitting quantum cascade lasers with triangular prism photonic crystal slabs have the possibility to attain high light-extraction efficiency with Gaussian beam shape even with compact chip size and high active region occupancy.

Silk Fibroin‐Based Flexible Organic Light‐Emitting Diode with High Light Extraction Efficiency

AbstractIt is an essential issue for the efficiency of flexible organic light‐emitting diodes (OLEDs) to be improved in display and lighting applications. Herein, the use of silk fibroin (SF) as a flexible OLED substrate with enhanced light extraction efficiency (LEE) is reported. Regenerated SF is prepared via a casting method and then coated with conductive indium tin oxide (ITO) via magnetron sputtering. SF‐based flexible OLED outperforms its reference device in terms of turn‐on voltage, peak brightness, and LEE, where glass and polyethylene terephthalate are used as rigid and flexible substrates, respectively. The improved device performance can be ascribed to the low refractive index of SF and its lower surface roughness and more matched energy levels compared to those of other substrates, resulting in a high LEE. Notably, a molecular dipole layer can be formed between the interface of ITO and SF. The dipole moment pointing to the ITO surface effectively changes the ITO work function and reduces the potential barrier height for hole transfer. The work provides new ideas for constructing high‐performance OLEDs on biomass‐based substrates with excellent flexibilities and high LEEs.

Computational upper-limit of directional light emission in nano-LED via inverse design.

We demonstrate directional light emission in nano-LEDs using inverse design. Standard light-extraction strategies in LEDs have been limited to surface roughening or suppressing guided modes via LED structure modifications, which are insufficient for simultaneously achieving high-light extraction efficiency and directional emission. In this work, we use inverse design to discover high-efficiency directional emitting nano-LEDs. We first investigate the computational upper bounds of directional emission using free-form grayscale material, where material permittivity indicates an intermediate state between air and SiO2. For a narrow emission angle (<±30°), the optimized grayscale design offers 4.99 times enhancement from the planar LED surface. Then, we apply fabrication constraints to our inverse design for discovering a single material (SiO2) based design. The optimized SiO2 surface design shows 4.71 times light extraction (<±30°) improvement compared with the planar configuration. This is a first theoretical demonstration of high light-extraction efficiency and directional emitting nano-LED designs.

Analysis of the efficiency improvement of 273 nm AlGaN UV-C micro-LEDs

UV-C LEDs have great application prospects in the fields of sterilization, disinfection, and non-line-of-sight solar-blind communication. However, their efficiency is currently not high enough to satisfy these applications and needs to be improved. In this work, AlGaN based UV-C LEDs (273 nm) with size from 8 to 200 μm were fabricated to evaluate the size effect of external quantum efficiency (EQE). Higher EQE was detected in smaller-size UV-C micro-LEDs. Simulations on EQE were performed. The analysis on experimental data and the simulation results reveals that higher EQE originates from both higher internal quantum efficiency at high current densities due to better current spreading and higher light extraction efficiency (LEE) due to the larger proportion of sidewall. From the efficiency improvement mechanism, it is concluded that the most sever current density non-uniformity and LEE loss of UV-C devices compared with visible or near ultraviolet LEDs can be effectively restrained by preparing micro-LEDs. Our work helps to optimize the efficiency characteristics for UV-C applications.

Higher Light Extraction Efficiency in Organic Light-Emitting Devices by Employing 2D Periodic Corrugation.

Photons trapped in the form of waveguide (WG) modes associated with the organic–organic interface and in the form of surface plasmon polariton (SPP) modes associated with the metallic electrode–organic interface result in a large energy loss in organic light-emitting devices (OLEDs). Introducing gratings onto the metallic electrode is especially crucial for recovering the power lost to the associated SPP modes. In our research, we demonstrate the efficient outcoupling of SPP modes in TE mode by two-dimensional (2D) grating, which cannot excited in one-dimensional (1D) grating OLED. This causes a 62.5% increase in efficiency from 2D grating OLED than 1D grating OLED. The efficient outcoupling of the WG and SPP modes is verified by the numerical simulation of both the emission spectra and the field distribution.

Simulation Model of GaN-Based Micro-LED with High Light Extraction Efficiency

Simulation Model of GaN-Based Micro-LED with High Light Extraction Efficiency

Self‐Assembled Nanowrinkle‐Network‐Structured Transparent Conductive Zinc Oxide for High‐Efficiency Inorganic Light‐Emitting Diodes

AbstractA ZnO nanowrinkle network (NWN) film is applied as a transparent conductive electrode (TCE) to improve the light extraction efficiency of inorganic light‐emitting diodes (LEDs). The NWN structures in sol–gel‐derived ZnO films are spontaneously formed by the evaporation of the solvent during prebaking to achieve the gelation of the sol–gel ZnO film. Despite the NWN structure, the optical transmittance of the NWN‐ZnO film prebaked at 130 °C is maintained at ≈92% with respect to that of unmodified glass. However, the optical transmittance of the NWN‐ZnO film prebaked at 150 °C is significantly decreased because of the increase in nanowrinkle density. As a result, the electroluminescence intensity of LEDs having the NWN‐ZnO TCEs increases significantly for the samples prebaked at temperatures up to 130 °C but decreases slightly in those treated at 150 °C. In addition, LEDs having the NWN‐ZnO film show enhanced spectral distribution over wide angles, which is important for lighting applications. On the basis of these results, the spontaneously assembled NWN‐ZnO films formed by the sol–gel method show potential for the low‐cost production of inorganic LEDs having high light extraction efficiencies.

The Light Extraction Efficiency of GaN-based LED with Air-Hole Photonic Crystal Structures

In order to improve the light extraction efficiency (LEE) of GaN-based light-emitting diodes (LEDs), a layer of cylindrical air-hole photonic crystal (PC) structure inserted into P-GaN is proposed and investigated numerically. Finite difference time domain (FDTD) method is used to make a series of simulations in the LEE of GaN-based LED with air-hole photonic crystal structure. According to the variable-controlling approach, the PC structure is simulated and optimized. The results of the simulations show that the LEE depends on the PC’s position and relevant structural parameters. When PC is etched in the active layer, and its dielectric constant [Formula: see text][Formula: see text][Formula: see text]m, etching [Formula: see text][Formula: see text][Formula: see text]m and air-hole radius [Formula: see text][Formula: see text][Formula: see text]m, higher LEE is obtained as 44.5%, translated into a 13.6-fold enhancement for the case of a planar LED. The remarkable enhancement is of particular interest for improving LEE of LED and provides a theoretical reference for future LED structure design efforts.