General Lighting Applications Research Articles

Exploring single rare earth doped nanophosphor for efficient white emission through codoping of Na+ and Bi3+ ions into Dy3+ activated LaPO4

Using the simple hydrothermal technique, high-purity nanocrystalline Na+ and Bi3+ co-doped Dy3+ activated LaPO4 was effectively prepared. The structural characterisation of the prepared samples was carried out using XRD, EDS, SEM and FTIR analysis. The existence of Na+, Dy3+, La3+, P5+, O2– and Bi3+ was demonstrated using EDS analysis. The direct band gap value of 4.72 eV is obtained from UV visible absorption spectrum and infer that the doped samples are suitable for optical applications. The photoluminescence excitation spectrum monitored at 474 nm and 570 nm emission, the Na+ and Bi3+ co-doped Dy3+ activated LaPO4 exhibit narrow, and intense characteristics f-f peak of Dy3+ ion at 349 nm, 363 nm, 386 nm and 452 nm. Photoluminescence emission spectrum, monitored under 349 nm, 363 nm and 386 nm shows high intense sharp blue emission peaks of Dy3+ with stark splitting at 475 nm, 478 nm, and 486 nm, yellow emission at 571 nm and 580 nm and a very feeble peak at 658 nm in the red region. When the sample is excited at 452 nm only a yellow band is present. Since the intensity of blue and yellow emission in this host matrix is so high and nearly equal, it is appropriate for cool white emission under 350 nm, 363 nm and 386 nm (near UV) excitation and under 452 nm (blue LED) excitation the prepared phosphor may emerge as highly efficient warm white light emitter suitable for general solid-state lighting applications, which can be further confirmed by CIE chromaticity analysis of the prepared sample. In photoluminescence excitation and emission spectra intensities of peaks of the samples with Bi3+ are more intense, this confirms the positive impact of co-doping of Bi3+ into the sample. The Q value from the Dexter’s formula is calculated as an average of 6.4 and is very close to 6 indicates that the interaction is dipole − dipole, which causes the non-radiative energy migration among the Dy3+ ions in the sample. The lifetime of characteristic yellow emission of Dy3+ in the Na+ and Bi3+ co-doped Dy3+ activated LaPO4 host matrix monitored under 350 nm is calculated as 0.642 ms from decay analysis, fitted by a bi-exponential function. The results support the idea that co-doping of Bi3+ and Na+ into the LaPO4:Dy3+ matrix could emerge as a potential white light emitting phosphor for phosphor-converted white LED application.

Speckle-Free, Angle-Free, Cavity-Free White Laser with a High Color Rendering Index.

The freedom from efficiency droop motivates monochromatic lasers to progress in general lighting applications due to the demand for more efficient and sustainable light sources. Still, a white light based on monochromatic lasers with high lighting quality, such as a high color rendering ability, an angle-independent output, and a speckle-free illumination, has not yet been fabricated nor demonstrated. Random lasers, with the special mechanism caused by multiple scattering, the angle-free emission, and the uncomplicated fabrication processes, inspire us to investigate the feasibility of utilizing them in general lighting. In this work, a white random laser with a high color rendering index (CRI) value, regardless of pumping energy and observing direction, was performed and discussed. We also investigated the stability of white RL as its CIE chromaticity coordinates exhibit negligible differences with increasing pump energy density, retaining its high-CRI measurement. Also, it exhibits angle-independent emission while having a high color rendering ability. After passing through a scattering film, it generated no speckles compared to the conventional laser. We demonstrated the advances in white laser illumination, showing that a white random laser is promising to be applied for high-brightness illumination, biological-friendly lighting, accurate color selections, and medical sensing.

A Versatile Photochromic Dosimeter Enabling Detection of X‐Ray, Ultraviolet, and Visible Photons

AbstractAn accurate measurement of radiation doses is required to ensure efficient use of electromagnetic radiation in medical diagnostics, agriculture, or general lighting applications. Yet, existing dosimeters usually face the issues of cumbersome manipulation, time‐consuming analysis, or power‐supply requirement. Here, an all‐round dosimeter is reported based on BaMgSiO4, a photochromic material that exhibits a reversible white‐pink color change upon irradiation. Electron paramagnetic resonance measurements under in situ coloring and bleaching reveal that the charge carrier trapping and detrapping at oxygen vacancy‐related defects determine the photochromic behavior. This can be utilized for dosimetry of X‐rays, ultraviolet, and visible light, as the coloring and bleaching are dependent on the irradiation wavelength and the dose. The distinct color variation of BaMgSiO4 allows for on‐site measurement of irradiation doses by a colorimetric method, and the special wavelength‐responsive behavior in the ultraviolet region is suitable for personal solar ultraviolet light monitoring. By virtue of good stability and excellent cycling robustness, BaMgSiO4 can be an ideal integrating detector for X‐rays, ultraviolet, and visible light dosimetry. A prototype device is also developed for long‐duration daylight measurements. These findings enhance the understanding of the photochromic behavior in inorganic materials and can stimulate the exploration of new photochromic dosimeters.

Evaluation of different general V(λ) mismatch indices of photometers for LED-based light sources in general lighting applications

The general V(λ) mismatch index, , of a photometer quantifies the deviation between its determined relative spectral responsivity, , and its nominal responsivity, the spectral luminous efficiency function for photopic vision, V(λ). LED-based light sources now dominate in general lighting, displacing incandescent and fluorescent lamps. Furthermore, the calibration of photometers will very likely see the replacement of the reference CIE standard illuminant A with CIE reference spectrum L41. The article evaluates the consequences. Should we also change the definition of the general V(λ) mismatch index? Based on performance criteria, the individual indices are evaluated using various datasets with the help of statistical analyses. As a result, the authors can conclude that the current definition works very well even under changed calibration and application conditions and does not need to be changed. Other evaluated indices for a new index definition perform only slightly better in some cases, but do not generate generally better properties.

White electroluminescence from cross relaxation-free CaSiO3:Tb3+ film on silicon wafer and its concentration dependent white-color tunability

A full-color white light source is continuously required for general lighting applications. Herein, the white electroluminescence (EL) from Tb3+-singly doped CaSiO3 (CSO) thin film on a silicon substrate has been reported for the first time. The EL device has a metal-oxide-semiconductor (MOS) structure which consists of two CSO and SiOx oxide layers on silicon wafer. Above the threshold voltage in AC power, it shows a full-color white EL spectrum with two dominant emission groups of ultraviolet-blue and green-red peaks from the f-f intra-transition of Tb3+ ions without the cross-relaxation effect in the four blue peaks which suffers from their severe cross-relaxation quenching in its photoluminescence spectrum. It is attributed to the strong electric field splitting (Stark effect) which prevents the resonance energy transfer due to energy perturbation. With increasing Tb3+ concentrations, the blue bands get more dominant along with increasing the threshold voltage. Furthermore, we demonstrate the EL-voltage transient behavior with a large hysteresis in a positive voltage polarity.

Ligand Control of Structural Diversity in Luminescent Hybrid Copper(I) Iodides

Copper(I) iodide hybrids are of interest for next-generation lighting technologies because of their efficient luminescence in the absence of rare-earth elements. Here, we report 10 structurally diverse hybrid copper(I) iodides that emit in the green–red region with quantum yields reaching 67%. The compounds display a diversity of structures including ones with one-dimensional (1D) Cu–1 chains, Cu2I2 rhomboid dimers, and structures with two different arrangements of Cu4I4 tetramers. The compounds with Cu2I2 rhomboid dimers or Cu4I4 cubane tetramers have higher photoluminescence quantum yields than those with Cu–I 1D chains and octahedral Cu4I4 tetramers, owing to the optimal degree of condensation of the inorganic motifs, which suppresses nonradiative processes. Electronic structure calculations on these compounds point out the critical influence of the inorganic motif and organic ligand on the nature of the band gaps and thus the excitation characteristics. Temperature-dependent photoluminescence spectra are presented to better understand the nature of luminescence in compounds with different inorganic motifs. The emerging understanding of composition–structure–property correlations in this family provides inspiration for the rational design of hybrid phosphors for general lighting applications.

Electrothermal Tristability Causes Sudden Burn‐In Phenomena in Organic LEDs

AbstractOrganic light‐emitting diodes (OLEDs) have been established as a mature display pixel technology. While introducing the same technology in a large‐area form factor to general lighting and signage applications, some key questions remain unanswered. Under high‐brightness conditions, OLED panels were reported to exhibit nonlinear electrothermal behavior causing lateral brightness inhomogeneities and even regions of switched‐back luminance. Also, the physical understanding of sudden device failure and burn‐ins is still rudimentary. A safe and stable operation of lighting tiles, therefore, requires an in‐depth understanding of these physical phenomena. Here, it is shown that the electrothermal treatment of thin‐film devices allows grasping the underlying physics. Configurations of OLEDs with different lateral dimensions are studied as a role model and it is reported that devices exceeding a certain panel size develop three stable, self heating‐induced operating branches. Switching between them causes the sudden formation of dark spots in devices without any preexisting inhomogeneities. A current‐stabilized operation mode is commonly used in the lighting industry, as it ensures degradation‐induced voltage adjustments. Here, it is demonstrated that a tristable operation always leads to destructive switching, independent of applying constant currents or voltages. With this new understanding of the effects at high operation brightness, it will be possible to adjust driving schemes accordingly, design more resilient system integrations, and develop additional failure mitigation strategies.

Long-Term Reliability Characteristics of OLED Panel and Luminaires for General Lighting Applications

Organic light-emitting diode is one of the future-proof solid-state-based lighting sources. OLED shows great aesthetic advantages and good color quality without glare. Moreover OLED is a kind of surface light sources naturally. There are some studies on the long-term reliability characteristics of OLED light source. However, these studies focused on the light output degradation of OLED light sources mainly. In this paper, we have investigated the long-term reliability characteristics of OLED panel and luminaires in terms of lumen maintenance, correlated color temperature, color rendering index, and operating voltage. Total twelve OLED panels with four different kinds and six OLED luminaires with two different kinds were analyzed up to six thousand hours and analyzed for the general lighting applications.

Tunable Luminescence in Hybrid Cu(I) and Ag(I) Iodides.

Hybrid materials are increasingly demonstrating their utility across several optical, electrical, and magnetic applications. Cu(I) halide-based hybrids have attracted attention due to their strong luminescence in the absence of rare-earths. Here, we report three Cu(I) and Ag(I) hybrid iodides with 1,5-naphthyridine and additional triphenylphosphine (Ph3P) ligands. The compounds are built on (Cu/Ag)-I staircase chains or on a rhomboid Cu2I2 dimer and display intense and tunable luminescence. Replacing Cu with Ag, and adding the second kind of organic ligand (Ph3P) tunes the emission color from red to yellow and results in significantly enhanced quantum yield. Density functional theory-based electronic structure calculations reveal the separate effects of the inorganic module and organic ligand on the electronic structure, confirming that bandgap, optical absorption, and emission properties of these phosphors can be systemically and deliberately tuned by metal substitution and organic ligands cooperation. The emerging understanding of composition-structure-property relations in this family provides powerful design tools toward new compounds for general lighting applications.

A laser-driven phosphor converted system with enhanced optical efficiency by using light-recycling dichroic filters

Laser-driven lighting has been considered as a high potential general lighting application according to its high luminous and pure chroma. However, the large optical reflective loss of the excitation blue laser on the phosphor converter interface decreases the optical efficiency seriously. In this work, the dichroic filters were utilized to confine the blue laser inside the phosphor material and thereby improve the phosphor converted efficiency. The optical and thermal performances were experimentally investigated, respectively. The utilization of dichroic filters could greatly improve the optical efficiency of a laser-driven phosphor converter, approximately 1.8 times higher than the conventional converter.

Enhanced Visible Light Communication Modulator With Dual-Feedback Control

This article presents a modulator for optical wireless communication (OWC) via LEDs, controlled by a dual closed loop. While the potential communication speed and performance of OWC have received a lot of research attention, the design of appropriate modulator circuits has not been studied so widely. This article addresses the control aspects to minimize power losses and to ensure automatic adaptation to component spreads or aging, for “high-power” LED emitters. We initially address a “low-bandwidth– system, particularly suited for indoor camera-based detection, and then a “higher bandwidth” one for Li-Fi data communication. In contrast to maximizing data throughput as it applies to high-speed modulators, low-speed modulators aim at high power and high efficiency in general lighting applications. Specifically, for the high-power modulator, we propose a control strategy that allows a wide range of combinations of LED types and LED power supplies. The power consumption of the modulator is minimized by voltage margin setting that is automatically adapt to the modulation and the LED type using the nonlinear detector. The common industry practice is to use constant-current LED drivers for powering LEDs. After creating a theoretical framework of some generality to evaluate designs and control loops with rationals and verified models, we realize modulator designs that are supplied by constant current LED drivers and verify their performance by simulation and measurements.

Luminescence and luminescence quenching of K2Bi(PO4)(MoO4):Sm3+ phosphors for horticultural and general lighting applications

Novel and highly thermally stable K2Bi(PO4)(MoO4):Sm3+ phosphors were prepared and used to produce pcLEDs for horticulture and general lighting application.

Accelerated Degradation Test Investigation for Life-Time Performance Analysis of LED Luminaires

LED luminaires are a common lighting source used in general lighting application as they are energy-efficient and long-lasting. However, the reliability of an LED luminaire is not well established, and no standard method for estimating the performance of an LED luminaire is developed. This article presents an investigation to understand the performance of LED luminaires at the accelerated degradation test (ADT) condition and analyzes the reliability of LED luminaires. An LED luminaire has two subsystems, an LED light engine and the LED constant current driver that are subjected to ADT, and the light engine factors, lumen output, and Duv changes are analyzed. Similarly, the driver output stage electrolytic capacitor capacitance and equivalent series resistance (ESR) are estimated. The study of continuous operation in time and study when subjected to switching cycles to quantify reliability in terms of operating cycles is performed. All four key parameters-lumen maintenance, Duv, capacitance, and ESR-are estimated up to their specified threshold and reliability is analyzed. The LED light engine lumen output is found to degrade very quickly compared with other factors. LED luminaire failure at ADT is mainly due to lumen depreciation followed by color shift and, after significant intervals of time and cycles, the driver capacitor ESR follows as reason for the failure of the LED luminaire. A scanning electron microscope (SEM) energy dispersive spectroscopy (EDS) analysis is performed on the LED package, and results show that LED silver (Ag) mirror tarnish is the main reason for a significant reduction in light output.

Organic light-emitting diodes with split recombination zones: A concept for versatile color tuning

Abstract White organic light emitting diodes (WOLEDs) for general lighting applications are still mostly considered as a possibility rather than a definitive next step in solid-state lighting technology. Vacuum deposited WOLEDs usually run into troubles of complicated manufacturing processes, incompatible emitters, high-precision requirements for emitting layer structuring and monolithic integration. In this work, we present a simple method of achieving white emission from a single-stack OLED with three (red, green and blue) emitters utilizing a laterally structured shadow mask. Only the green emitter is deposited via this mask without any alignment requirements, allowing the decoupling of recombination zones for achieving simultaneous emission from all emitters, circumventing the need of additional charge generating layers or vacuum-breaking structuring steps. A highly tunable white emission is demonstrated, irrespective of charge transporting systems or green emitters used. The resulting WOLEDs show external quantum efficiencies and luminous efficacies up to 14% and 26 lm/W, respectively, with CIE coordinates of (0.40, 0.40) and CRI values of up to 80.

Hollow Light Guide Module Involving Mini Light-Emitting Diodes for Asymmetric Luminous Planar Illuminators

Light-emitting diodes (LEDs) have numerous advantages. However, LEDs only offer a point light source. Therefore, transforming LEDs into planar light sources is a new objective in general lighting applications. Solid light guides have strong uniformity but are marred by their material absorption characteristics and weight. Hollow light guides constitute a solution to the weight problem but exhibit poor uniformity and necessitate sacrificing efficiency to enhance uniformity. To resolve the uniformity, weight, and efficiency problems simultaneously, we propose a hollow light guide architecture involving mini-LEDs with asymmetric luminous intensity. To develop this guide module, we first optimized the aspect ratio of the cavity and then modulated the light path by using varied angles of the reflection surface on the end wall of the module. We then designed a beveled reflection surface near the mini-LEDs to further enhance uniformity. An archetype of the proposed architecture for planar light source modules had a width and depth of 51.5 and 9.95 mm, respectively. Experimental results revealed a total efficiency of 83.9% and uniformity of 92.3%. The module weight was determined to be 215 g, which was 40% lighter than that of similarly sized solid light guide modules.

Filler Particle-Induced Light Absorption in Underfill-Encapsulated Flip-Chip Light-Emitting Diodes

Light-emitting diode (LED) devices depend strongly on proper thermal management to mitigate negative effects caused by high operating temperature, especially in general lighting applications. Flip-chip packaging has been used as a solution to reduce thermal resistance, with some iterations involving an epoxy underfill material to mechanically support solder joints and assist in thermal management. The benefits of underfill encapsulation on the thermal resistance of flip-chip LED (FCLED) packages have been studied, but the effects on optical performance have not been addressed. This paper identifies that the radiant power of the FCLED die suffers as a result of the underfill fillet. The selected underfill materials were applied to FCLEDs, and radiant power was measured. Radiant power was found to decrease as filler loading increased.

Efficient Design Method of Segmented Lenses for Lighting Applications with Prescribed Intensity and Low Peak Luminance

ABSTRACT Light emitting diodes (LEDs) that are used today in general lighting applications provide high luminous flux from small packages, which can result in high peak luminance and glare. The use of diffusors allows reducing peak luminances, but these components do not allow accurate control over the resulting intensity pattern as is possible with nonimaging lenses for low-étendue sources such as LEDs. In this article, a new design method is presented to create a rotational or transverse symmetric segmented lens that results in a reduced peak luminance but still allows excellent control over the resulting intensity distribution. Each segment of the lens spreads out the light in a certain angular range, but the different contributions of each lens segment result together in the desired target intensity distribution. The design method is based on the iterative optimization of a linear transformation of a source intensity distribution into a target intensity distribution, starting from a transformation that yields maximal light beam spreading.

Novel orange-emitting Ba2LaNbO6:Eu3+ nanophosphors for NUV-based WLEDs and photocatalytic water purification

Energy conservation and environmental safety are the key requirements in the modern world. We report novel orange-emitting double perovskite Ba2LaNbO6:Eu3+ (BLN:Eu3+) nanophosphor fabricated using a citrate sol-gel method for use in general illumination and photocatalysis. After annealing at 800 ℃, the particles exhibited a nanorod-like morphology with monoclinic structure. The photoluminescence emission spectra exhibited an intense 5D0→7F1 transition at 594 nm and a moderate 5D0→7F2 transition at 615 nm, demonstrating that the Eu3+ ions occupied the La3+ sites with inversion symmetry. The optimal concentration of Eu3+ ions was found to be about 5 mol% for the BLN host lattice. Energy transfer from the NbO67- octahedrons to the Eu3+ ions was clearly witnessed when the BLN:Eu3+ nanophosphors were excited with both the characteristic excitation bands of Eu3+ (7F0→5L6) and NbO67- octahedrons at 392 and 380 nm, respectively. The thermal quenching temperature of 5 mol% Eu3+ ions doped BLN nanophosphors was found to be 183 ℃, indicating that these nanophosphors are very stable at high temperatures. In addition, the dye removal efficiency of the proposed BLN nanophosphors was verified using Rhodamine B (RhB) dye as a model pollutant under UV irradiation. Compared to a commercial nano-ZnO catalyst, our synthesized BLN nanophosphors showed superior RhB de-colorization efficiency. Therefore, the proposed BLN:Eu3+ nanophosphors are promising multifunctional materials for photocatalysis and general lighting applications.

Hybrid White Light-Emitting Diodes Utilizing Radiative or Nonradiative Energy Transfer for Wavelength Conversion

Hybrid light-emitting diodes (LEDs) were fabricated by spin coating poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) on blue InGaN/GaN multiple-quantum-well (MQW) LED chips as a downconverter. As the F8BT film thickness was in the range of 235–290 nm, the hybrid LEDs emitted cool white light with a downconversion efficiency of 28.8%. The F8BT film quality was markedly improved by postannealing at 200 °C, raising the downconversion efficiency to 82.6%. To obtain more efficient white light emission, we proposed to insert Ag nanoparticles (NPs) with a diameter of 13 nm at the inorganic–organic interface to form an MQWs/NPs/F8BT structure. Simulations showed that nonradiative energy transfer from the MQWs to F8BT can be mediated by localized surface plasmons, leading to more efficient downconversion and overall white light emission. The proof of concept of such a hybrid LED will pave the way toward high-performance phosphor-free white LED sources for microdisplay and general lighting applications.

Characteristics Study of Die-Bond Process of High-Power LED Package

High-power light-emitting diodes (LEDs) are essential in general lighting applications. Because of the characteristics of semiconductors, the optical and electrical properties of LEDs vary with the operating temperature. To increase the reliability of high-power LEDs, this study conducted a shear stress test and examined the thermal dissipation alongside electrical and optical properties. The simulated and experimental trends of the thermal distribution of the LED package overlapped effectively. The results revealed that changing the side glue height from 10% to 100% of die height, which consequently increases the shear stress of 1693 g/mm2, reduces the thermal resistance of 1 °C/W, and reduces the luminous flux of 0.4 lm, can optimize the LED reliability.