High-power Ultraviolet Light-emitting Diodes Research Articles

Photocatalytic Oxidative Cleavage of Alkenes by Molecular Oxygen: Reaction Scope, Mechanistic Insights, and Flow Application.

The oxidative cleavage of C═C bonds with molecular oxygen was promoted effectively by a catalytic amount of a decatungstate photocatalyst using black light irradiation (365 nm). Not only aromatic ketones but also aliphatic ketones were obtained by the photocatalytic protocol. The continuous flow reaction of α-methylstyrene using a high-power ultraviolet light-emitting diode (365 nm) dramatically decreased the reaction time.

Photo-Sintered Silver Thin Films by a High-Power UV-LED Module for Flexible Electronic Applications.

In recent printed electronics technology, a photo-sintering technique using intense pulsed light (IPL) source has attracted attention, instead of conventional a thermal sintering process with long time and high temperature. The key principle of the photo-sintering process is the selective heating of a thin film with large light absorption coefficients, while a transparent substrate does not heat by the IPL source. Most research on photo-sintering has used a xenon flash lamp as a light source. However, the xenon flash lamp requires instantaneous high power and is unsuitable for large area applications. In this work, we developed a new photo-sintering system using a high-power ultraviolet light emitting diode (UV-LED) module. A LED light source has many merits such as low power consumption and potential large-scale application. The silver nanoparticles ink was inkjet-printed on a polyethylene terephthalate (PET) and photo-sintered by the UV-LED module with the wavelength of 365 and 385 nm. The electrical resistivity as low as 5.44 × 10−6 Ω·cm (just about three times compared to value of bulk silver) was achieved at optimized photo-sintering conditions (wavelength of 365 nm and light intensity of 300 mW/cm2).

Electrospun Semi-Alicyclic Polyimide Nanofibrous Membrane: High-Reflectance and High-Whiteness with Superior Thermal and Ultraviolet Radiation Stability for Potential Applications in High-Power UV-LEDs.

Polymeric nanofibrous membranes (NFMs) with both high whiteness and high thermal and ultraviolet (UV) stability are highly desired as reflectors for ultraviolet light-emitting diodes (UV-LEDs) devices. In the current work, a semi-alicyclic and fluoro-containing polyimide (PI) NFM with potential application in such kinds of circumstances was successfully fabricated from the organo-soluble PI resin solution via a one-step electrospinning procedure. In order to achieve the target, a semi-alicyclic PI resin was first designed and synthesized from an alicyclic dianhydride, 3,4-dicarboxy-1,2,3,4,5,6,7,8-decahydro-1-naphthalenesuccinic dianhydride (or hydrogenated tetralin dianhydride, HTDA), and a fluoro-containing diamine, 2,2-bis[4-(4-amino-phenoxy)phenyl]hexafluoropropane (BDAF), via an imidization procedure. The derived PI (HTDA-BDAF) resin possessed a number-average molecular weight (Mn) higher than 33,000 g/mol and was highly soluble in polar aprotic solvents, such as N,N-dimethylacetamide (DMAc). The electrospinning solution was prepared by dissolving the PI resin in DMAc at a solid content of 25–35 wt%. For comparison, the conventional high-whiteness polystyrene (PS) NFM was prepared according to a similar electrospinning procedure. The thermal and UV stability of the derived PI and PS NFMs were investigated by exposure under the UV-LED (wavelength: 365 nm) irradiation. Various thermal evaluation results indicated that the developed PI (HTDA-BDAF) NFM could maintain both the high reflectance and high whiteness at elevated temperatures. For example, after thermal treatment at 200 °C for 1 h in air, the PI (HTDA-BDAF) NFM exhibited a reflectance at a wavelength of 457 nm (R457) of 89.0%, which was comparable to that of the pristine PI NMF (R457 = 90.2%). The PI (HTDA-BDAF) NFM exhibited a whiteness index (WI) of 90.88, which was also close to that of the pristine sample (WI = 91.22). However, for the PS NFM counterpart, the R457 value decreased from the pristine 88.4% to 18.1% after thermal treatment at 150 °C for 1 h, and the sample became transparent. The PI NFM maintained good optical and mechanical properties during the high dose (2670 J/cm2) of UV exposure, while the properties of the PS NFM apparently deteriorated under the same UV aging.

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

Structural optimization and numerical thermal analysis of ultraviolet light-emitting diodes with high-power multi-chip arrays

In this paper, the finite element method and experiment were used to systematically study the influence of different factors on the heat dissipation of high-power ultraviolet light-emitting diode (UV LED) module, including chip spacing and structure, sub-mount, adhesive materials, the packaging area and void ratio of adhesive. The package structure was optimized by introducing Al barrier plate and changing the spacing between adjacent chips. Considering the balance of junction temperature and irradiance, the appropriate chip spacing (∼ 2.5 mm) should be chose. The simulations analysis demonstrated that both sub-mount and adhesive materials play an important role in the thermal performance of the UV LED module. The area ratio (∼0.9) between the adhesive area and the chip area can not only realize the excellent heat dissipation but also minimize the use of adhesive materials. During reflow soldering, the lager coalesced voids should be decreased as soon as possible, which have worse heat dissipation performance than the distributed voids. Furthermore, the reliability and validity of the numerical simulation were verified by the experiment data. The present conclusions provide significant insight on designing and optimizing high-power UV LED devices.

Thermally stable conformal encapsulation material for high-power ultraviolet light-emitting diodes

A conformal encapsulation material for use in high-power, thermally stable ultraviolet (UV) light-emitting diodes was successfully developed. For silicone, thermal degradation started at ∼200°C, and the transmittance was 85.5% at 365 nm. The transmittance decreased by 55% after thermal aging at 250°C for 72 h and it decreased further by 2.5%, even at room temperature, under continuous exposure to UV light at 365 nm for 72 h. By contrast, for the sol–gel material, thermal degradation started at ∼300°C, and the transmittance was 90% at 365 nm. The transmittance decreased negligibly after thermal aging at 250°C for 72 h and it did not decrease further even at 75°C under continuous exposure to UV light at 365 nm for 72 h.

Photoinduced grafting polymerization onto the surface with the use of radiation of high-power ultraviolet light-emitting diodes

The photoinduced grafting polymerization of acrylic acid onto the surface of a polypropylene film under the action of radiation with a wavelength of 365 nm and an intensity of 8–193 mW/cm2 emitted by high-power UV light-emitting diodes is studied. Grafting is performed from a thin layer of the aqueous solution of the monomer onto the surface containing a photoinitiator (benzophenone) deposited from a solution in a volatile solvent. The amount of graft polymer is determined with the use of ATR FTIR spectroscopy. At an intensity of UV radiation of 193 mW/cm2, the time of attainment of grafting-polymerization saturation is decreased by a factor of 6 relative to this time at an intensity of 8 mW/cm2, typical for emitters based on mercury lamps. The rate of grafting polymerization in the studied UV-radiation-intensity range grows in proportion to the square root of intensity.

Functional Analysis in Long-Term Operation of High Power UV-LEDs in Continuous Fluoro-Sensing Systems for Hydrocarbon Pollution.

This work analyzes the long-term functionality of HP (High-power) UV-LEDs (Ultraviolet Light Emitting Diodes) as the exciting light source in non-contact, continuous 24/7 real-time fluoro-sensing pollutant identification in inland water. Fluorescence is an effective alternative in the detection and identification of hydrocarbons. The HP UV-LEDs are more advantageous than classical light sources (xenon and mercury lamps) and helps in the development of a low cost, non-contact, and compact system for continuous real-time fieldwork. This work analyzes the wavelength, output optical power, and the effects of viscosity, temperature of the water pollutants, and the functional consistency for long-term HP UV-LED working operation. To accomplish the latter, an analysis of the influence of two types 365 nm HP UV-LEDs degradation under two continuous real-system working mode conditions was done, by temperature Accelerated Life Tests (ALTs). These tests estimate the mean life under continuous working conditions of 6200 h and for cycled working conditions (30 s ON & 30 s OFF) of 66,000 h, over 7 years of 24/7 operating life of hydrocarbon pollution monitoring. In addition, the durability in the face of the internal and external parameter system variations is evaluated.

High-Throughput Contact Flow Lithography.

High-throughput fabrication of graphically encoded hydrogel microparticles is achieved by combining flow contact lithography in a multichannel microfluidic device and a high capacity 25 mm LED UV source. Production rates of chemically homogeneous particles are improved by two orders of magnitude. Additionally, the custom-built contact lithography instrument provides an affordable solution for patterning complex microstructures on surfaces.

A compact multi-channel fluorescence sensor with ambient light suppression

A multi-channel fluorescence sensor has been developed for process monitoring and fluorescence diagnostics. It comprises a fiber-optic set-up with an immersion probe and an intensity-modulated high power ultraviolet light-emitting diode as a light source for fluorescence excitation. By applying an electronic lock-in procedure, fluorescence signals are selectively detectable at ambient light levels of 1000 000 times higher intensity. The sensor was designed to be compact, low cost and easily adaptable to a wide field of application. The set-up was used to simultaneously monitor three important metabolic fluorophores: NAD(P)H, flavins and porphyrins during the cultivation of a baker's yeast. Moreover, the accumulation and degradation kinetics of protoporphyrin IX induced by 5-aminolevulinic acid on the skin could be recorded by the sensor. The detection limit for protoporphyrin IX was determined to be 4 × 10−11 mol L−1. The linear signal amplification of the sensor and time courses of fluorescence signals monitored during yeast fermentations were validated using a commercial CCD spectrometer. The robust and flexible set-up of the fiber-optic measurement system promises easy implementation of this non-invasive analytical tool to fluorescence monitoring and diagnostics in R&D and production.

Optical Properties of a NiO/Al-based Reflector for High-power Ultraviolet Light-emitting Diodes

Optical Properties of a NiO/Al-based Reflector for High-power Ultraviolet Light-emitting Diodes

Electrically Detected Magnetic Resonance (EDMR) Measurements of Bulk Silicon Carbide (SiC) Crystals

Abstract Electrically detected magnetic resonance (EDMR) measurements of bulk silicon carbide (SiC) crystals, which have wide band-gaps, were conducted. A high-power ultraviolet light-emitting diode was used as a low-noise light source to excite excessive carriers. Under light irradiation, EDMR spectra of a 4H–SiC wafer (8-mm wide, 8-mm long) at an ESR frequency of 900 MHz was obtained with a g-value of 2.000 ± 0.003, indicating that the major recombination center was a vacancy. EDMR spectra were observed at all detection phases of magnetic field modulation. The spectrum observed at in-phase (0 degrees) was a single line; at out-of-phase (90 degrees), the different spectrum was observed.

White light generation using 280 nm light emitting diode pumps

High power 250–280 nm deep ultraviolet (UV) light emitting diode (LED) structures were grown by migration-enhanced metal organic chemical vapour deposition (MEMOCVD) approach allowing for very high material quality for high aluminum composition AlxGa1–xN layers (x up to 70%). Standard square geometry devices with CW power up to 0.5 mW at 80 mA and pulse power more than 3 mW at 200 mA were fabricated. These high power UV LEDs were mounted into TO type package with phosphor-coated windows. This approach yielded cold white (x = 0.26, y = 0.26) LED with corrected colour temperature of about 15000 K and averaged color rendering index as high as 90. Due to using of deep UV LED as a pump source excellent stability of CIE 1931 chromaticity coordinate and average colour rendering is achieved. The power conversion efficiency of about 20 % and luminous efficacy of about 210 lm/W was also measured. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-power UV InGaN/AlGaN double-heterostructure LEDs

Ultraviolet (UV) InGaN/AlGaN double-heterostructure (DH) light-emitting diodes (LEDs) with an external quantum efficiency of 7.5%, an output power of 5mW and an emission wavelength of 371nm were developed. High-power UV LEDs are obtained using an InGaN active layer with a thickness of 400Å instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based LEDs.