Light Extraction Research Articles

Diffuser optimization for enhancing light extraction from light-emitting electrochemical cells

Light-emitting electrochemical cells (LECs) have enabled many novel solution-processable, low-cost, and flexible emission applications. Nevertheless, relatively inferior device efficiencies render LECs less competitive with other emission technologies. Light extraction would be a promising approach to enhance the device efficiencies of LECs. The diffuser film composed of a transparent photoresist layer doped with large and small TiO2 nanoparticles (NPs) embedded between the indium-tin-oxide layer and the glass substrate has shown superior light extraction performance (>2.5X). However, the physical insight of optimization, e.g., adjusting the concentration of small TiO2 NPs in diffuser film, has not been explored yet. In this work, increasing the concentration of small TiO2 NPs is found to increase the thickness, roughness, and refractive index of the diffuser film. The optical simulation indicates that the thickness of the diffuser film has little effect on the light extraction. However, a higher roughness but a lower refractive index is preferred for the diffuser film to achieve better light extraction. As such, it is rational to expect that a proper concentration of small TiO2 NPs to reach a high enough roughness but still keep a relatively low refractive index is required for diffuser optimization. This study clarifies the physical mechanism responsible for diffuser optimization and thus shows a high potential for lighting applications of LECs.

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.

Quasi-3D harnessing of visible light in emissive III-V on Si microstructures: Application to multiple-quantum-well color conversion layers

We report on the design, fabrication, and characterization of the first photonic crystal (PhC)-based red multiple-quantum-well (MQW) color converters fully optimized for augmented reality (AR) microdisplays through a quasi-3D light harnessing principle. This principle leverages an aluminum (Al) bottom reflector and a silicon dioxide (SiO2) gap to harness the bottom-emitted light, along with copper (Cu) lateral mirrors and a silicon nitride (SiN) phase-matcher for Bloch-mode replication. These structures were designed using 3D-FDTD simulations. As a proof-of-principle, we fabricated corresponding devices that exhibit promising characteristics, including record light extraction efficiencies over 40 % for 4 μm pixels and directional emission patterns. Time-resolved photoluminescence (TRPL) analyses, along with a four-wave intensity model developed in this work, indicate that there is still room for improvement. We believe that the guidelines established in this study could pave the way for the use of MQW color converters in the next generation of very bright, high-resolution RGB microdisplays for AR glasses and beyond.

Weld line structured light stripe center extraction approach based on data dimensionality reduction

In order to improve the accuracy and speed of extracting the center of the line structured light stripe when using the vision measurement technology to get the 3D information of the weld sample, this paper proposes a center extraction approach based on the idea of data dimensionality reduction. First, use the weighted gray centroid approach to get the initial center point of the light stripe. Then, build a 7×7 template centered on the pixel block where the initial point is located and performs singular value decomposition on the 49×3 matrix within the template to get the normal direction of the initial point. Finally, calculate the stripe's exact center points through second-order Taylor expansion and smooth repair by cubic spline interpolation of breakpoints. The comparison experimental results show that this approach can effectively reduce the influence of the noise outside the light stripe on the center extraction, reduce the extraction time while ensuring the measurement accuracy, which can lay a good foundation for the point cloud data processing and surface reconstruction.

Influence of the thickness of p-GaN ohmic contact layer on the performance of AlGaN-based deep-ultraviolet light-emitting diodes

276 nm AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) with varying p-GaN thicknesses were investigated. The simulation results demonstrate that DUV-LEDs with an ultra-thin 4 nm p-GaN layer exhibit a 96.9% increase in full-angle total radiant intensity compared to those with a 350 nm p-GaN layer. Meanwhile, the total radiant intensity of the DUV-LEDs with 4 nm p-GaN were all elevated during the degree of polarization (DOP) transition from 1 to −1. The experimental results demonstrate that the maximum external quantum efficiency (EQE) and wall-plug efficiency (WPE) values of 5.3% and 4.0%, respectively, at 4 A/cm2 for the DUV-LEDs with a 4 nm p-GaN layer. These figures represent remarkable improvements of 60.6% and 42.8% when compared to DUV-LEDs featuring a 350 nm p-GaN layer. Nevertheless, it's worth noting that the adoption of an ultra-thin p-GaN ohmic contact layer introduced some surface irregularities, potentially affecting the ohmic contact of the p-type layer and resulting in a slight voltage increase, thus tempering the WPE improvement. The light extraction efficiency (LEE) values of 10.0%, 7.4%, 6.7% and 6.3% were extracted for the four samples after fitting the experimental EQE curves by the ABC model.

Enhanced light extraction efficiency of GaN-based green micro-LED modulating by a thickness-tunable SiO2 passivation structure.

Green micro-light emitting diodes (micro-LEDs) is one of the three primary color light sources as full-color display, which serves as a key research object in the field of micro-LED display. As the micro-LED size decreases, the surface-area-to-volume ratio of the device increases, leading to more serious damage on the sidewall by inductively coupled plasma (ICP) etching. The passivation process of SiO2 provides an effective method to reduce sidewall damage caused by ICP etching. In this work, green rectangular micro-LEDs with passivation layer thickness of 0∼600 nm was designed using the finite-difference time-domain (FDTD) simulation. In order to verify the simulation results, the micro-LED array was fabricated by parallel laser micro-lens array (MLA) lithography in high speed and large area. The effect of the SiO2 passivation layer thickness on the performance of the green micro-LED was analyzed, which shows that the passivation layer thickness-light extraction efficiency curve fluctuates periodically. For the sample with 90 nm thickness of SiO2 passivation layer, there exists a small leakage current and higher operating current density, and the maximum external quantum efficiency (EQE) is 2.8 times higher than micro-LED without SiO2 passivation layer.

Modeling the co-assembly of binary nanoparticles

In this work, we present a binary assembly model that can predict the co-assembly structure and spatial frequency spectra of monodispersed nanoparticles with two different particle sizes. The approach relies on an iterative algorithm based on geometric constraints, which can simulate the assembly patterns of particles with two distinct diameters, size distributions, and at various mixture ratios on a planar surface. The two-dimensional spatial-frequency spectra of the modeled assembles can be analyzed using fast Fourier transform analysis to examine their frequency content. The simulated co-assembly structures and spectra are compared with assembled nanoparticles fabricated using transfer coating method are in qualitative agreement with the experimental results. The co-assembly model can also be used to predict the peak spatial frequency and the full-width at half-maximum bandwidth, which can lead to the design of the structure spectra by selection of different monodispersed particles. This work can find applications in fabrication of non-periodic nanostructures for functional surfaces, light extraction structures, and broadband nanophotonics.

Fabricating and investigating a beveled mesa with a specific inclination angle to improve electrical and optical performances for GaN-based micro-light-emitting diodes.

In this Letter, beveled mesas for 30 × 30 µm2 GaN-based micro-light-emitting diodes (µLEDs) with different inclination angles are designed, fabricated, and measured. We find that µLED with a mesa inclination angle of 28° has the lowest internal quantum efficiency (IQE) and the highest injection current density at which the peak IQE is obtained. This is due to the increased quantum confined Stark effect (QCSE) at the mesa edge. The increased QCSE results from the strong electric field coupling effect. Instead of radiative recombination, more nonradiative recombination and leakage current will be generated in the sidewall regions. Besides, the smallest angle (28°) also produces the lowest light extraction efficiency (LEE), which arises from the optical loss caused by the sidewall reflection at the beveled surface sides. Therefore, the inclination angle for the beveled mesa has to be increased to 52° and 61° by using Ni and SiO2 as hard masks, respectively. Experimental and numerical results show that the external quantum efficiency (EQE) and the optical power can be enhanced for the fabricated devices. Meanwhile, the reduced surface recombination rate also decreases the leakage current.

Refractive Index Engineering as a New Degree of Freedom for Designing High‐Performance AlGaN‐Based Ultraviolet C Light‐Emitting Diodes

This study delves into a profound exploration, using simple yet effective Monte Carlo ray‐tracing simulations, of the influence of the refractive indices of multiple quantum wells (MQWs) and p‐type electron blocking layer (p‐EBL) on the photon propagation and subsequent light extraction behavior in AlGaN‐based ultraviolet‐C (UVC) light‐emitting diodes (LEDs), covering both transverse‐electric (TE) and transverse‐magnetic (TM) polarized light. Remarkably, the refractive index contrasts at the heterointerfaces of p‐EBL/MQWs/n‐AlGaN have a relatively minor impact on the extraction efficiency of TE‐polarized light but exert a significant influence on TM‐polarized light. This discrepancy arises from the substantial effect of total internal reflection at these heterointerfaces on the propagation of photons emitted at large angles. Thus, this observation elucidates the oversight of photon propagation in the conventional design of MQWs and p‐EBL in AlGaN‐based UVA and UVB LEDs, where TE‐polarized emission predominates. Building upon simulation results, a highly effective strategy for extracting TM‐polarized light in AlGaN‐based UVC LEDs is further demonstrated by combining refractive index engineering with inclined sidewalls. The findings suggest refractive index engineering can serve as a novel design parameter for AlGaN‐based UVC LEDs dominated by TM‐polarized emission, expanding the range of techniques available for enhancing device performance.

Far-field pattern control and light-extraction enhancement of deep-ultraviolet light-emitting diodes with large-area Fresnel zone plate nano-structures

Conventional methods using high-purity quartz lenses to control deep-ultraviolet light-emitting diode (DUV-LED) far-field patterns have limitations, including small effective apertures and high cost. We apply phase-type Fresnel zone plates to control the beam angle and enhance light extraction efficiency (LEE) for DUV-LEDs on sapphire and AlN substrates. We demonstrate highly-collimated optics-free DUV-LED emissions with full width at half maximum far-field divergence angles of 40° and 10° on sapphire and AlN substrates at a peak emission wavelength of 279 nm and 273 nm, respectively. LEE enhancements of 1.4 and 1.5 times for DUV-LEDs on sapphire and AlN substrates, respectively, are also achieved.

Research on the center extraction algorithm of structured light fringe based on an improved gray gravity center method

Abstract In this study, we proposed a fast line-structured light stripe center extraction algorithm based on an improved barycenter algorithm to address the problem that the conventional strip center extraction algorithm cannot meet the requirements of a structured light 3D measurement system in terms of speed and accuracy. First, the algorithm performs pretreatment of the structured light image and obtains the approximate position of the stripe center through skeleton extraction. Next, the normal direction of each pixel on the skeleton is solved using the gray gradient method. Then, the weighted gray center of the gravity method is used to solve the stripe center coordinates along the normal direction. Finally, a smooth strip centerline is fitted using the least squares method. The experimental results show that the improved algorithm achieved significant improvement in speed, sub-pixel level accuracy, and a good structured light stripe center extraction effect, as well as the repeated measurement accuracy of the improved algorithm is within 0.01 mm, and the algorithm has good repeatability.

Patterned OLEDs: effect of substrate corrugation pitch and height

An ongoing OLED challenge is cost-effective enhancement of light extraction, i.e., increasing the external quantum efficiency (EQE ∼20% in conventional devices). OLEDs on corrugated substrates often show enhanced EQEs providing insight into light emission processes. In particular, patterned plastic substrates directly imprinted easily at room temperature and amenable to low-cost R2R production are ideal for studying/optimizing various structures, further elucidating the extraction process. We show new semi-quantitative data of the effect of the pitch (a) and height/depth (h) of plastic substrate patterns on the OLEDs’ stack and EQE, focusing on new designs, interestingly, some showing surprisingly enhanced EQEs that were neither reported nor discussed before. These include: (i) shallow (h < 200 nm) convex polycarbonate with a ∼ 750 versus∼400 nm, where the h gradually decreases as the OLED stack is built and (ii) concave PET/CAB with large a (∼2.8 and ∼7.8 μm), where the EQE enhancement of conformal OLEDs may be due largely to scattering. EQEs of green, blue, and white phosphorescent OLEDs were measured. OLEDs on substrates with narrow a ∼ 400 nm and low h < 200 nm showed no enhancement, resembling flat devices. In contrast, OLEDs on substrates with comparable or smaller h, but larger a ∼ 750 nm show significant EQE enhancement despite h reduction across the stack. Green OLEDs with a ∼ 750 nm and h ∼ 160 to ∼180 nm, showed EQEs ∼30%, reaching ∼58% with substrate mode extraction. Surprisingly, fully conformal OLEDs on a PET/CAB substrate with a ∼ 7.8 μm showed blue and white EQEs reaching ∼33%, without substrate mode extraction. The enhancing patterns increase the OLEDs’ EQE by reducing surface plasmon excitation and internal waveguiding.The experimental results for OLEDs on substrates with a < 2 μm are supported by scattering matrix simulations that assume conformal stacks, incorporating diffraction for internal losses reduction. EQE enhancement not predicted by simulations may be due additionally to scattering mostly for substrates with a significantly larger than the emitting wavelength.

Fabrications of twisted moiré photonic crystal and random moiré photonic crystal and their potential applications in light extraction

Twisted moiré photonic crystal is an optical analog of twisted graphene or twisted transition metal dichalcogenide bilayers. In this paper, we report the fabrication of twisted moiré photonic crystals and randomized moiré photonic crystals and their use in enhanced extraction of light in light-emitting diodes (LEDs). Fractional diffraction orders from randomized moiré photonic crystals are more uniform than those from moiré photonic crystals. Extraction efficiencies of 76.5%, 77.8% and 79.5% into glass substrate are predicted in simulations of LED patterned with twisted moiré photonic crystals, defect-containing photonic crystals and random moiré photonic crystals, respectively, at 584 nm. Extraction efficiencies of optically pumped LEDs with 2D perovskite (BA)2(MA) n−1PbnI3n+1 of n = 3 and (5-(2′-pyridyl)-tetrazolato)(3-CF3−5-(2′-pyridyl)pyrazolato) platinum(II) (PtD) have been measured.

Highly Efficient Light-Emitting Diodes based on Self-Assembled Colloidal Quantum Wells.

Nanocrystal-based light-emitting diodes (Nc-LEDs) have immense potential for next-generation high-definition displays and lighting applications. They offer numerous advantages, such as low cost, high luminous efficiency, narrow emission, and long lifetime. However, the external quantum efficiency (EQE) of Nc-LEDs, typically employing isotropic nanocrystals, has been limited by the out-coupling factor. Here we demonstrate efficient, bright, and long lifetime red Nc-LEDs based on anisotropic nanocrystals of colloidal quantum wells (CQWs). Through modification of the substrate's surface properties and control of the interactions among CQWs, we successfully spin-coated a self-assembled layer with an exceptionally high distribution of in-plane transitions dipole moment (TDM) of 95%, resulting in an out-coupling factor of 37%. Our devices exhibit a remarkable peak external quantum efficiency (EQE) of 26.9%, accompanied by a maximum brightness of 55,754cd m-2 and a long operational lifetime (T95 @100cd m-2 ) over 15,000h. These achievements represent a significant advancement compared to previous studies on Nc-LEDs incorporating anisotropic nanocrystals. We expect our work to provide a general self-assembly strategy for enhancing the light extraction efficiency of Nc-LEDs based on anisotropic nanocrystals. This article is protected by copyright. All rights reserved.

Enhancing light extraction efficiency and color stability for OLED by truncated micro-cone array films

A newly truncated micro-cone array film as light extraction layer for organic light emitting diodes (OLEDs) has been developed, which was fabricated by process combined with photolithography, polydimethylsiloxane (PDMS) molding, self-assembled anti-adhesion, and ultraviolet (UV) forming techniques. In this study, by varying the sidewall angle and fill factor of this film, their influences on the optical properties and light extraction efficiency for OLED are fully investigated. As a result, device performance of OLED employed truncated micro-cone array film, with a high fill factor and sidewall angle of 76.49⁰, was found to be significantly enhanced up to 52% compared with the reference OLED. Notably, the proposed truncated micro-cone array film also stabilized emissive color stability while the viewing angle changes from 0° to 70°, achieving an excellent angular shift with CIE-indices of (0.0025, 0.0066). Aforementioned findings contribute to the development in advanced OLEDs for display and lighting applications.

Laser stripe center extraction method base on Hessian matrix improved by stripe width precise calculation

The line laser measurement technology based on triangulation is widely used in the field of complex surface topography measurement. The key technology of line laser measurement is laser stripe center extraction. For high reflective surfaces, due to the low surface roughness, some areas of the laser stripe image are overexposed, resulting in uneven stripe width distribution, asymmetric light intensity distribution of the stripe cross section, and difficulty in extracting the center of the stripe. In this paper, Laser stripe center extraction method based on Hessian matrix method improved by stripe width is proposed. Firstly, the laser stripe image is preprocessed, the influence of noise in the image is alleviated by image filtering and adaptive threshold segmentation, and the region of interest of the image is extracted to reduce the amount of subsequent calculation. The laser stripe width is calculated based on BPNN and divided into different width intervals. The Gaussian kernel is calculated according to the light stripe width and the Hessian matrix is constructed. The Hessian matrix method is used to roughly extract the center coordinates of the light stripe. Based on the continuity of the light stripe, the center coordinates of the light stripe are weighted and corrected to realize the smooth optimization of the center coordinates of the light stripe. The experimental results show that the proposed light stripe center extraction method is superior to the traditional method under different surface roughness and exposure time conditions, which can effectively improve the measurement accuracy of the sensor.

Influence of Shape and Size on GaN/InGaN μLED Light Emission: A Competition between Sidewall Defects and Light Extraction Efficiency

Influence of Shape and Size on GaN/InGaN μLED Light Emission: A Competition between Sidewall Defects and Light Extraction Efficiency

Investigation on the optical properties of Al nanograting deep-ultraviolet LEDs with rough surface of sapphire

The optical properties of Al nanograting deep ultraviolet LEDs with a rough surface of sapphire are investigated by the finite-difference time-domain simulation. The rough surface of sapphire is characterized by rms amplitude and correlation length. The calculation results indicate that the rough sapphire surface is easier to extract s-polarized light than p-polarized light, which leads to an increase in the polarization degree. When the rms and correlation length are around 150 nm, the light extraction efficiency (LEE) of LED devices can reach a maximum. Compared to the smooth surface condition, the LEE of Al nanograting LEDs with a period of 300 nm is improved by 65.47% at rms = 150 nm and correlation length = 100 nm. This can be attributed to the critical angle of light extraction increasing from ∼23° on a smooth surface to ∼46° on a rough surface. In addition, due to surface plasmon coupling, when the period of Al nanograting is 100–800 nm, the peak intensity of the TE or TM polarized radiation recombination rate is basically 37%–50% higher than that of the control structure with an Al plane.

Multi-optimization of operational parameters for malt extract production by centrifugal block freeze concentration

This study aims to optimize light and dark malt extract production by centrifugal block freeze concentration (CBFC), considering simultaneous optimization of solids concentration (SC), concentration efficiency (CE), and concentrate percentage (CP) using a desirability function approach. Two cycles and one recycle of CBFC were evaluated for both malt extracts. A central composite rotational design was employed to predict the effects of freezing temperature (FT) and centrifugal time (t) on malt extract production. The desirability function approach was used for optimization. In scenarios with the highest optimization, operational parameters varied from −29.63 °C to −25.90 °C for FT and 7.00 min to 10.16 min for t. Optimized response values ranged from 2.99°Brix to 26.13°Brix for SC, 69.14% to 86.23% for CE, and 38.30% to 43.86% for CP. The results suggest that optimizing CBFC cycles hinges on liquid mixture properties, and diverse input parameters may be necessary for attaining optimal outcomes.

Early failure of high-power white LEDs for outdoor applications under extreme electrical stress: Role of silicone encapsulant

Solid state lighting systems have deeply penetrated the market and are widely available. White light emitting diodes (LEDs) are based on a gallium nitride blue-emitting chip and a phosphor-based layer is used to convert blue radiation to white light, in combination with an optical system to enhance light extraction. High temperatures can be detrimental for LED reliability, resulting in a gradual or catastrophic failure. Therefore, the design of such devices, especially in relation to use cases that require operation at high current and temperature levels, needs to be based on reliability considerations.In this work we analyze the robustness of high power white LEDs submitted to driving currents exceeding the nominal levels by means of electrical and optical characterization, as well as on failure analysis techniques. The results highlight a significant degradation of the main optical parameters (luminous flux, color coordinates, correlated color temperature, …). Device cross sectioning highlighted a heavy darkening of the silicone material in the overstressed devices, and a delamination of the phosphors. Raman spectroscopy analysis highlighted an enhancement in luminescence of degraded samples. Such degradation was ascribed to the high temperature reached by the LEDs during the stress, that caused the degradation of the silicone lens; the process can be enhanced by the low efficiency of the phosphors and LEDs at high temperatures, resulting in a stronger heating of the devices.