High-brightness Light-emitting Diodes Research Articles

Flexible High-Stability Self-Variable-Voltage Monolithic Integrated System Achieved by High-Brightness LED for Information Transmission.

The emergence of visible light information transmission systems is profoundly affecting the future of the Internet of Things (IoT) technology. The complex sensing and driving circuits of the IoT have become the key factor to hinder signal conversion and processing. Herein, a high-performance self-variable-voltage light information transmission integrated system (SVV-LTS) is reported and its application potential in low-power, self-powered optical communication transmission systems is demonstrated. Diffusion-adsorption regulation growth method and laser induction technology are innovatively used to realize high-brightness light-emitting diode (LED) and flexible micro-supercapacitor (MSC) on graphene. Meanwhile, MSC realizes the dual functions of supplying power to the system, realizing pressure signal response, and converting pressure signals into electrical signals. Finally, the MSC as power, sensor and LED as signal transmitter are integrated into an SVV-LTS. The response time of SVV-LTS is 80ms and the luminous wavelength fluctuation of the LED is stable at 1.2nm. This study will provide a new approach to realize low-power optical communication transmission systems affecting the IoT technology.

Dimming Techniques Focusing on the Improvement in Luminous Efficiency for High-Brightness LEDs

The pulse width modulation (PWM) dimming mode features good dimming linearity and has been widely used for driving high-brightness light-emitting diodes (HBLEDs), in which the brightness change is reached by modulating the duty cycle of the dimming signal to regulate the average current flowing through LEDs. However, the current-illuminance characteristic curve of most LEDs is nonlinear in nature. Namely, under the same lighting power fed, the conventional PWM dimming cannot make the LED exert its best luminous efficiency (LE) specified in datasheets. This paper focuses on the study of further improving LED luminous efficacy via dimming manipulation. Thereby, two multilevel current dimming techniques with varied dimming signal voltage and varied current sensing resistance are presented. With limited dimming capability, the proposed dimming strategies can efficiently raise the luminous flux ratio without increasing the power consumption. A prototype constructed for a 115 W HBLED driver is developed and the devised dimming schemes are realized by a digital signal controller (DSC). Experimental results exhibited with illuminance-power curves and CIE1931 and CIE1976 chromaticity diagrams are given to validate the theoretical derivation and effectiveness. Compared with conventional PWM dimming, under the same illuminance, the driver average output power is respectively reduced by 17.08% and 13.17%; the improvement in average illuminance under the same output power is 13.66% and 11.17%, respectively. In addition, the entire average LE boost has respectively increased by 21.36% and 16.37%.

DCM Boost PFC for High Brightness LED Driver Applications

This paper is concerned with the derivation of a discontinuous conduction mode boost PFC rectifier as a driver for high-power LED lighting applications. The proposed driver is operated in the current mode regime while emulating a resistance towards the line, thus attaining a near unity power factor and low total harmonic distortion of the line current. Theoretical analysis is reported and conditions for the low LED flicker are derived. A method of design for minimum THD is also suggested. Simulation and experimental results are reported.

Al2O3–Ce:YAG composite ceramics for high brightness lighting: Cerium doping effect

Efficient Al2O3–Ce:YAG composite ceramic phosphors were obtained via reactive vacuum sintering using commercial α-Al2O3, Y2O3, and CeO2 powders as starting materials, and SiO2, MgO as a complex sintering additive. The effect of cerium content (0.05–0.3 at%) on the structural-phase state, optical and photoluminescent properties of ceramic phosphors were investigated comprehensively by X-ray diffraction, high-resolution scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction, confocal laser scanning microscopy, ultraviolet-visible spectrophotometry, and photoluminescence examinations. It is shown that an increase in the cerium doping level of the composites initiates their partial recharging into the non-luminescent Ce4+ state and the parasitic introduction of magnesium ions into the aluminum oxide phase due to the stabilization of the crystal structure of Mg,Si,Ce:YAG solid solutions. The highest value of the photoluminescence intensity is observed for the 0.1 at% Ce3+ sample, which may be caused by the optimal ratio of its absorbance and radiative recombination rate compared to other samples. The obtained ceramic phosphors show excellent thermal quenching behavior – reduction in the photoluminescence emission intensities did not exceed 5% at heating temperatures to 150 °C, which can be regarded as almost unchanged. The correlated color temperature values of 1.0 and 0.4-mm-thick Al2O3–Ce:YAG samples with 0.05–0.3 at% Ce3+ doping varied between 4583 and 3890 K and 10,308–4146 K at color rendering index values of 61–35 and 74–55, respectively. It is concluded that 0.05–0.1 at% Ce3+-doped ceramic phosphors have an optimal balance between a high color rendering index, luminous efficiency, and appropriate correlated color temperature values for high-power and high-brightness natural white light-emitting diodes.

Broadband, Enhanced, and Antithermally Quenched Near-Infrared Phosphors via a Cosubstitution Approach

Wearable biosensing and food safety inspection devices with high thermal stability, high brightness, and broad near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) could accelerate the next-generation NIR light applications. In this work, NIR La3-xGdxGa5GeO14:Cr3+ (x = 0 to 1.5) phosphors were successfully fabricated by a high-temperature solid-state method. Here, by doping Gd3+ ions into the La3+ sites in the La3Ga5GeO14 matrix, a 7.9-fold increase in the photoluminescence (PL) intensity of the Cr3+ ions, as well as a remarkably broadened full width at half-maximum (FWHM) of the corresponding PL spectra, is achieved. The enhancements in the PL, PLE intensity, and FWHM are attributed to the suppression of the nonradiative transition process of Cr3+ when Gd3+ ions are doped into the host, which can be demonstrated by the decay curves. Moreover, the La1.5Gd1.5Ga5GeO14:Cr3+ phosphor displays an abnormally negative thermal phenomenon that the integral PL intensity reaches 131% of the initial intensity when the ambient temperature increases to 160 °C. Finally, the broadband NIR pc-LED was fabricated based on the as-explored La1.5Gd1.5Ga5GeO14:Cr3+ phosphors combined with a 460 nm chip, and the potential applications for the broadband NIR pc-LEDs were discussed in detail.

High-Brightness Perovskite Light-Emitting Diodes Based on FAPbBr3 Nanocrystals with Rationally Designed Aromatic Ligands

Despite rapid developments of light-emitting diodes (LEDs) based on emerging perovskite nanocrystals (PeNCs), it remains challenging to achieve devices with integrated high efficiencies and high brightness because of the insulating long-chain ligands used for the PeNCs. Herein, we develop highly luminescent and stable formamidinium lead bromide PeNCs capped with rationally designed short aromatic ligands of 2-naphthalenesulfonic acid (NSA) for LEDs. Compared with commonly used oleic acid ligands, the NSA molecules not only preserve the surface properties of the PeNCs during the purification but also notably improve the electrical properties of the assembled emissive layers, ensuring efficient charge injection/transport in the devices. The resulting champion LED with electroluminescence approaching the Rec. 2020 green primary color demonstrates a high brightness of 67 115 cd cm–2 and a peak external quantum efficiency of 19.2%. More impressively, the device shows negligibly decreased efficiency at an elevated brightness of 20 000 cd cm–2 and a well-retained efficiency of over 10% at around 65 000 cd cm–2, presenting a breakthrough in LEDs based on PeNCs.

Chip-level Ce:GdYAG ceramic phosphors with excellent chromaticity parameters for high-brightness white LED device.

Development of ceramic phosphors (CPs) featuring small volume and high efficacy is crucial for miniaturization of white LEDs and their integration in solid state lighting. In this study, the chip-level 2.5×2.5 mm Ce:GdYAG CPs with different thicknesses were packaged to the blue chips, and their luminous characteristics were analyzed under the different radiant flux. Notably, when thickness of the CPs was 1.4 mm, a luminous flux of 2000 lm, a correlated color temperature (CCT) of 6266 K and a color rendering index (CRI) of 70 were obtained under 11.0 W blue power (1.76 W/mm2) excitation. Phenomenon of colorimetric drift was explained simultaneously. These results indicate that Ce:GdYAG CPs is a promising candidate for automotive lighting and high-speed rail lighting.

Optimisation of a Polarisation Nephelometer

An analysis of the influence caused by polarization nephelometer parameters on the scattering matrix measurement accuracy in a non-isotropic medium is presented. The approximation errors in the actual scattering volume and radiation beam by an elementary scattering volume and an elementary radiation beam are considered. A formula for calculating the nephelometer base is proposed. It is shown that requirements to an irradiation source of a polarizing nephelometer, i.e. mono-chromaticity and high radiation intensity and directivity in a wide spectral range can be satisfied by a set of high brightness LEDs with a radiating (self-luminous) small size body. A 5-wavelength monochromatic irradiation source, with an emission flux of (0.15–0.6) W required for a polarization nephelometer, is described. The design of small-sized polarizing phase control units is shown. An electronic circuit of a radiator control unit based on an AVR-Atmega 8-bit microcontroller with feedback and drive control realized by means of an incremental angular motion sensor and a software PID controller is presented. Precise and smooth motion of the radiator is ensured by standard servo-driven numerical control mathematics and the use of precision gears. The system allows both autonomous adjustment of the radiator’s reference positions and adjustment by means of commands from a personal computer. Both the computer and microcontroller programs were developed with the use of free software, making it possible to transfer the programs to Windows‑7(10), Linux and embedded Linux operating systems. Communication between the radiator’s position control system and the personal computer is realised by means of a standard noise immune USB-RS485 interface.

Sandwich-like triboelectric nanogenerators integrated self-powered buoy for navigation safety

Powering marine distributed buoys sustainably and cost-efficiently has always been challenging in the ocean environment. Triboelectric nanogenerators (TENG), thanks to their robustness, high efficiency and low cost in energy harvesting, has provided a novel solution to the long-term electrical power supply for a series of distributed systems (including buoys). In this study, a wave energy harvesting sandwich-like TENG (S-TENG) has been proposed. In the on-land forced motion experiments, the S-TENG yields much superior performances compared to previous tower-like TENG in terms of the power density. The studies also reveal that the electrical output increases linearly even with larger number of parallel connected S-TENG layers. Consequently, a buoy integrated with 7 parallel connected S-TENGs is studied through systematic wave basin experiments and the relevance of most hydrodynamic parameters (including wave height, frequency, direction as well as mooring system) have been revealed. The presented wave basin experiments are original in wave energy harvesting TENG research practice and prove to be very helpful in that they simulate the most close-to-real ocean environments in the lab. The studies are concluded with a 12 W high-brightness LED lit up on a navigation supporting buoy. This study has set a benchmark of achieving self-powered buoy with TENG, which can be extended to other marine distributed systems such as sensors and small robotics. • Sandwich structure boosting output power density of triboelectric nanogenerators. • Accessible power promotion through parallel nanogenerators in layers. • First time triboelectric nanogenerator tested in an advanced wave basin. • Self-powered navigation buoy achieved by triboelectric nanogenerator integration.

Enhanced light extraction from AlGaInP-based red light-emitting diodes with photonic crystals.

The photonic crystal (PC) has been demonstrated to be very effective in improving the extraction efficiency of light-emitting diodes (LEDs). In this paper, high-brightness AlGaInP-based vertical LEDs (VLEDs) with surface PC (SPCLED) and embedded PC (EPCLED) were successfully fabricated. Compared with normal LED (NLED), photoluminescence intensities of SPCLED and EPCLED have been improved up to 30% and 60%, respectively. And the reflection patterns of SPCLED and EPCLED were periodic bright points array, showing the ability to control light in PC. Electroluminescent measurements show that three kinds of LEDs have similar threshold voltages. Simultaneously, the light output power (LOP) of SPCLED and EPCLED has been improved up to 24% and 11% at 200 mA, respectively, in comparison to NLEDs. But the LOP decays earlier for EPCLED due to the excessive heat production. Furthermore, it is demonstrated that the SPCLED and EPCLED luminous uniformity is better. This kind of high brightness PCLED is promising in improving the properties of all kinds of LEDs, especially mini LEDs and micro LEDs.

Interface and bulk controlled perovskite nanocrystal growth for high brightness light-emitting diodes [Invited

Halide perovskites have attracted great attention due to their high color purity, high luminance yield, low non-radiative recombination rate, and solution processability. Although the external quantum efficiency of perovskite light-emitting diodes (PeLEDs) is comparable with that of the organic light-emitting diodes (OLEDs) and quantum-dots light-emitting diodes (QLEDs), the brightness is still low compared with the traditional OLEDs and QLEDs. Herein, we demonstrate high brightness and high-efficiency CsPbBr3-based PeLEDs using interface and bulk controlled nanocrystal growth of the perovskite emission layer. The interface engineering by ethanolamine and bulk engineering by polyethylene glycol led to highly crystallized and cubic-shaped perovskite nanocrystals with smooth and compact morphology. As a result, PeLEDs with a high brightness of 64756 cd/m2 and an external quantum efficiency of 13.4% have been achieved.

Suppressing Auger Recombination in Cesium Lead Bromide Perovskite Nanocrystal Film for Bright Light-Emitting Diodes.

All-inorganic cesium lead halide perovskite colloidal nanocrystals are attractive for next-generation light-emitting diodes because of their high color purity, but the nonradiative Auger recombination in perovskite nanocrystal film limits the efficiency and brightness of the fabricated devices. Here, we introduce a surface-engineering process to exchange the original long-chain oleic acid/oleylamine ligands by the cerium-tributylphosphine oxide hybrid ligands to suppress nonradiative Auger recombination in CsPbBr3 NC film for bright and low-efficiency roll-off light-emitting diodes. Using ultrafast transient absorption and time-resolved photoluminescence spectroscopy, we demonstrate that the hybrid ligand passivation can efficiently remove surface trap states to enhance radiative recombination and homogenize the exciton concentration to suppress nonradiative Auger recombination in the CsPbBr3 nanocrystal thin film. Consequently, we fabricate a light-emitting diode with efficient charge injection into the CsPbBr3 nanocrystal emitting layer, achieving a pronounced improvement of electroluminescence with an external quantum efficiency from 5.5% to 9.1%. More importantly, the efficiency roll-off characteristics of high-brightness light-emitting diodes is effectively mitigated. Our reported hybrid ligand passivation suppressed Auger recombination strategy shows a great potential for fabricating high-brightness cesium lead halide perovskite light-emitting diodes.

Optimization study on magnetorheological fluid components and process parameters of cluster magnetorheological finishing with dynamic magnetic field for sapphire substrates

Sapphire is used as the base material of high-brightness LED devices, high-speed and high-frequency wireless communication devices, and solar photovoltaic conversion chips. Its surface quality determines the performance of the device. Magnetorheological (MR) finishing can avoid scratches and surface/subsurface damage caused by uneven abrasive particles due to the viscoelasticity of the polishing pad. When MR polishing was used to polish sapphire substrates, the adaptability of the MR fluid components and the rationality of polishing process was the key to quickly obtain high-quality workpiece surface. In this study, sapphire substrates were polished using cluster MR finishing with dynamic magnetic fields formed by multiple synchronous rotation magnetic poles. The components of the MR fluid were optimized by single-factor experiments, and the polishing process parameters were optimized through orthogonal experiment. The results showed that an ultra-smooth surface of Ra 0.27 nm could be obtained when sapphire substrate was polished 8 h by using the optimized MR fluid, which contained 120 nm silica sol with a mass fraction of 8 wt% and W3 carbonyl iron powders with a mass fraction of 16 wt%, also with the optimized process parameters as the machining gap, the workpiece speed, the magnetic pole speed and the rotation speed of the polishing disc were 1 mm, 350 r min−1, 45 r min−1 and 40 r min−1, respectively.

Compact illumination system with variable beam direction and beam divergence

Countless approaches to optimise lighting conditions for indoor and outdoor applications are described in the literature by adaptive dimming schemes or control algorithms. Those methods rely on adapting the emitted flux of individual luminaires in order to adjust lighting conditions to varying circumstances. Complex illumination requirements are tackled by controlling the output of every independent luminaire. In this case, the innovation is rather in software than in hardware, and such approaches are limited by how the LED fixtures can be manipulated. Lighting fixtures with a tuneable radiation pattern are rare, or these fixtures are rather large. This paper introduces a compact lighting system with a collimated light beam of which the beam direction and beam divergence can be adjusted electronically. To realise this optical functionality, a focus tuneable lens and rotatable mirror are combined with two custom-made aplanatic lenses in front of a compact, high-brightness LED. This paper elaborates the optical design of this novel illumination system and discusses the experimental performance of the realised demonstrator. The system functionality is validated by comparing the experimental lighting performance to optical simulations conducted with ray-tracing software. With the proposed system, a fast and flexible adaptation of the generated illumination distribution is possible.

Enhancement of light extraction efficiency of AlGaInP-based light emitting diodes by silicon oxide hemisphere array

Surface texturing has been demonstrated to be very effective in improving the extraction efficiency of light emitting diodes (LEDs). In this paper, high-brightness AlGaInP-based vertical LEDs (VLEDs) with nanoscale silicon oxide (SiO2) hemisphere array were successfully fabricated by photoresist thermal reflux technique and laser interference exposure. It is the first time to combine interferential lithography and thermal reflux technique to make a regular nanoscale hemisphere array to enhance LEE for LEDs Compared to LEDs with microscale SiO2 hemisphere array, with planar SiO2 and without SiO2, electroluminescent measurements show that four LEDs have a similar threshold voltage. Simultaneously, the light output power (LOP) of LEDs with nanoscale and microscale SiO2 hemisphere array has been improved up to 40% and 20% at 200 mA, respectively, in comparison to LEDs without SiO2. It fits the simulated results well, which were obtained by the finite-difference time-domain (FDTD) method. Furthermore, it is demonstrated that more light emits out in the direction perpendicular to the epitaxial wafer by angle-resolved EL spectra for LEDs with nanoscale SiO2 hemisphere array, which accords to the simulated optical field distribution images. This kind of high-brightness LEDs with nanoscale SiO2 hemisphere array will have promising applications in meeting different requirements in special situations, such as headlights, flashlights. The new method is capable of fabricating large-scale nanostructured materials with good controllability and low cost, which will be widely used in improving properties of all kinds of LEDs, especially Mini LEDs and Micro LEDs.

Morphology-Related Functionality in Nanoarchitectured GaN

Integrating silicon and III-nitride technologies for high-speed and large bandwidth communication demands optically interconnected active components that detect, process, and emit photons and electrons. It is imperative that multifunctional materials can enhance the performance and simplify fabrication of such devices. Spontaneously grown GaN in the nanowall network (NwN) architecture simultaneously displays unprecedented optical and electrical properties. A two-order increase in band-edge emission makes it suitable for high-brightness light-emitting diodes and laser applications. Decorating this NwN with silver nanoparticles further enhances emission through plasmonic interactions and renders it an excellent surface-enhanced Raman spectroscopy substrate for biomolecular detection. The observation of very high electron mobility (approximately 104 cm2/Vs) and large phase-coherence length (60 μm) is a consequence of two-dimensional (2D) electron gas formation applicable for high electron mobility transistors. Detecting ballistic transport in the nanowalls confirms proximity-induced superconductivity (<5 K and 8 T). Charge separation properties render it a device material for UV photodetectors, photoanodes for water splitting, and thermionic field emitters.

An Improved Dimmable LED Driving Scheme with Low Flicker Metrics for Low Voltage Application

In the field of dimming of high brightness LED, variable duty ratio pulse-width-modulation (PWM) operated LED dimmable drivers are commonly used which cause high flicker metrics resulting visual discomfort and health risks of the viewer as pointed in recently reported IEEE Std. 1789-2015. In IEEE Std. 1789-2015, a low flicker metrics such as flicker index and modulation depth are recommended to avoid those health hazards. Such flicker metrics are to be calculated from modulating LED current and have to be sufficiently low and also modulation depth should lie within a prescribed operating region as recommended in IEEE Std. 1789-2015. In this paper a variable-frequency constant duty ratio PWM operated novel driving scheme has been proposed for DC-DC wide range dimmable LED driver which performs dimming with a low flicker metrics. The reported driving scheme does not require any electrolytic capacitor in the driver unit and hence low life span of electrolytic capacitor does not deteriorate life span of LED driver unit. Both simulation and experimentation have been carried out and reported in this paper with adequate responses. Also comparative analyses of flicker metrics of the proposed driving scheme with existing conventional PWM operated dimmable LED drivers have been reported.

Characterization of the Interdependence Between the Light Output and Self-Heating of Gallium Nitride Light-Emitting Diodes

Abstract With the advent of gallium nitride (GaN) as an enabling material system for the solid-state lighting industry, high-power and high-brightness light-emitting diodes (LEDs) with wavelengths ranging from near ultraviolet to blue are being manufactured as part of a tremendously large and ever-increasing market. However, device self-heating and the environment temperature significantly deteriorate the LED's optical performance. Hence, it is important to accurately quantify the LED's temperature and correlate its impact on optical performance. In this work, three different characterization methods and thermal simulation were used to measure and calculate the temperature rise of an InGaN/GaN LED, as a result of self-heating. Nanoparticle-assisted Raman thermometry was used to measure the LED mesa surface temperature. A transient Raman thermometry technique was utilized to investigate the transient thermal response of the LED. It was found that under a 300 mW input power condition, self-heating is negligible for an input current pulse width of 1 ms or less. The temperature measured using nanoparticle-assisted Raman thermometry was compared with data obtained by using the forward voltage method (FVM) and infrared (IR) thermal microscopy. The IR and Raman measurement results were in close agreement whereas the data obtained from the widely accepted FVM underestimated the LED temperature by 5–10%. It was also observed that an increase in environment temperature from 25 °C to 100 °C would degrade the LED optical power output by 12%.

Suppressing Efficiency Roll-Off at High Current Densities for Ultra-Bright Green Perovskite Light-Emitting Diodes.

Perovskite light-emitting diodes (PeLEDs) have undergone rapid development in the last several years with external quantum efficiencies (EQEs) reaching over 21%. However, most PeLEDs still suffer from severe efficiency roll-off (droop) at high injection current densities, thus limiting their achievable brightness and presenting a challenge to their use in laser diode applications. In this work, we show that the roll-off characteristics of PeLEDs are affected by a combination of charge injection imbalance, nonradiative Auger recombination, and Joule heating. To realize ultrabright and efficient PeLEDs, several strategies have been applied. First, we designed an energy ladder to balance the electron and hole transport. Second, we optimized perovskite materials to possess reduced Auger recombination rates and improved carrier mobility. Third, we replaced glass substrates with sapphire substrates to better dissipate joule heat. Finally, by applying a current-focusing architecture, we achieved PeLEDs with a record luminance of 7.6 Mcd/m2. The devices can be operated at very high current densities (J) up to ∼1 kA/cm2. Our work suggests a broad application prospect of perovskite materials for high-brightness LEDs and ultimately a potential for solution-processed electrically pumped laser diodes.

An Isolated PFC Zeta-forward Single-stage Single-switch for LED Driver Without Electrolytic Capacitor

The high-brightness light-emitting diodes (LEDs) require an AC/DC converter with power factor correction (PFC). The large output electrolytic capacitor, which is used to minimize the low frequency LED current ripple, degrades the operating lifetime of the LED driver. In order to increase the lifetime of an AC–DC LED driver, the electrolytic capacitor should be eliminated without significantly increasing the output current ripple. In this article, an isolated single-stage single-switch AC/DC high power factor LED driver without electrolytic capacitor is proposed in which a zeta power factor (PF) corrector is integrated with a forward converter. The detailed theoretical analysis and design procedure of the proposed single-stage PFC converter is presented. The experimental results of a 110 Vrms, 21 W prototype verify the theoretical analysis. The input PF is 0.99 in the proposed converter that complies with lighting equipment standards such as IEC-1000-3-2 for class C equipment.