Light-emitting Diode Chip Research Articles

A review of high ideality factor in gallium nitride-based light-emitting diode

Theory concerning the high ideality factor of gallium nitride (GaN) based light- emitting diode (LED) has been reviewed. The presence of a high ideality factor indicates a large forward voltage that results in efficiency reduction. The paper suggests that tunneling is the main reason defining the exponential behaviour of current-voltage measurements, which leads to a high ideality factor. However, there is also a paper that suggests that the design of current geometry in the LED chip defines the value of ideality factor. An effective current spreading geometry in the LED chip will minimize the ideality factor and make it fall between the ideal range of 1 to 2. Besides, how the ideality factor is calculated will also play a major role in defining its value. By calculating the ideality factor based solely on the radiative recombination current formula, the value of ideality factor can result in an ideal ideality factor of 1.08.

Electrical model formulation for multicolor light‐emitting diode modules and its application to design dimmable driver

SummaryA multicolor light‐emitting diode (LED) module comprises LED chips of more than one type or color and is frequently used for spectrally tunable lighting applications. In this paper, electrical models of such multicolor LED modules are formulated taking two basic circuit configurations into account, namely, (i) conventional, where a parallel string is made of one particular LED type, and (ii) nonconventional, where a parallel string is composed of different LED types. For deriving the model equations, the basic exponential model of a single LED chip has been applied, which is experimentally found to be the suitable option out of the four existing models, namely, linear, parabolic, exponential, and modified Shockley models. The formulated models are experimentally validated, results of which show maximum deviations of 4.71% and 4.19% from the measured data in the nonlinear and the linear operating regions, respectively. The formulated models are utilized to design dimmable, isolated LED drivers for red‐green‐blue (R‐G‐B) LED module. Performance evaluation of the drivers is conducted in Matlab‐Simulink. The results reveal the satisfactory operation of the LED modules in terms of current and power regulation, branch current matching, and fast response and thus establish the effectiveness of the proposed algorithm.

Using multi-site substitution to design blue-exciting phosphor Mg2Y2Al2Si2O12:Mn2+ for full-spectrum plant growth LEDs

Full-spectrum plant growth light emitting diode (LED) prepared using a variety of phosphors has some problems, such as energy loss and spectral distortion. It is the solution that designing a kind of phosphor, which is the single activated ion doped phosphor with the multiple emission. To achieve this goal, the host Mg2Y2Al2Si2O12 (Abbreviated as: MYASO) was selected from a large number of garnet materials which has the multi-sites, and the Mn2+ was selected as the activated ion which can produce the different emission in the different crystal field. The results show that Mn2+ can produce the green light (536 nm), red light (635 nm) and deep red light (735 nm), which correspond to the Mn2+ substituting for the tetra-coordinated Al3+, the octa-coordinated Mg2+ and the hexa-coordinated Mg2+, respectively, importantly, this phosphor can be efficiently excited by the 456 nm blue light, which matches well with the blue LED chip. Therefore, it may be a great potential application value in the production of plant growth LEDs.

Highly Efficient Liquid-Quantum Dot/Melamine- Modified Urea-Formaldehyde Microcapsules for White Light-Emitting Diodes

A liquid matrix is beneficial for improving the luminescence performance of quantum dots (QDs), although it is difficult to be packaged with light-emitting diode (LED) chips. In this study, liquid-QDs with chlorobenzene solution and melamine-modified urea-formaldehyde (MUF) were used as the core and shell structures, respectively, successfully yielding liquid-QD/MUF microcapsules for white LED packaging. The concentration of the liquid-QDs in the microcapsules was optimized, demonstrating an increase of 15.9% in the luminous efficiency, as compared to the traditional solid-QDs at a specific QD energy proportion. The efficiency roll-off can be attributed to the increased reabsorption and back-scattering loss at higher liquid-QDs and microcapsules concentrations. It was also confirmed that the liquid-QD/MUF microcapsules have an excellent thermal stability below 200 °C, owing to the protection of the MUF shells. Finally, white LEDs compounding the red liquid-QD/MUF microcapsules and YAG phosphor particles were fabricated, achieving a high luminous efficiency of 149.2 lm/W at 20 mA, correlated color temperature of 3301 K, and color rendering index of 88.8. This study will open new research avenues for liquid-QD packaging in white LEDs.

Hypergravity-Induced Accumulation: A New, Efficient, and Simple Strategy to Improve the Thermal Conductivity of Boron Nitride Filled Polymer Composites.

Thermal conductive polymer composites (filled type) consisting of thermal conductive fillers and a polymer matrix have been widely used in a range of areas. More than 10 strategies have been developed to improve the thermal conductivity of polymer composites. Here we report a new “hypergravity accumulation” strategy. Raw material mixtures of boron nitride/silicone rubber composites were treated in hypergravity fields (800–20,000 g, relative gravity acceleration) before heat-curing. A series of comparison studies were made. It was found that hypergravity treatments could efficiently improve the microstructures and thermal conductivity of the composites. When the hypergravity was about 20,000 g (relative gravity acceleration), the obtained spherical boron nitride/silicone rubber composites had highly compacted microstructures and high and isotropic thermal conductivity. The highest thermal conductivity reached 4.0 W/mK. Thermal interface application study showed that the composites could help to decrease the temperature on a light-emitting diode (LED) chip by 5 °C. The mechanism of the improved microstructure increased thermal conductivity, and the high viscosity problem in the preparation of boron nitride/silicone rubber composites, and the advantages and disadvantages of the hypergravity accumulation strategy, were discussed. Overall, this work has provided a new, efficient, and simple strategy to improve the thermal conductivity of boron nitride/silicone rubber and other polymer composites (filled type).

Ultrashort Pulse Laser Lift-Off Processing of InGaN/GaN Light-Emitting Diode Chips

Gallium nitride (GaN) film delamination is an important process during the fabrication of GaN light-emitting diodes (LEDs) and laser diodes. Here, we utilize 520 nm femtosecond laser pulses, exploiting nonlinear absorption rather than single-photon absorption such as in conventional laser lift-off (LLO) employing excimer or Q-switched laser sources. The focus of this study is to investigate the influence of laser scanning speed and integrated fluence corresponding to laser energy per area during the LLO processing of GaN LED chips and their resulting structural properties. Because both the sapphire substrate and InGaN/GaN heterostructures are fully transparent to the emission of the laser system, a key question is related to the impact of laser pulses on the quality of a thin film structure. Therefore, several characterization methods (i.e., scanning electron microscopy, atomic force microscopy, X-ray diffraction, Raman spectroscopy, and electroluminescence spectroscopy) were employed to understand the material modifications made by femtosecond LLO (fs-LLO). We demonstrated that by adjusting the laser scanning speed, smooth GaN surfaces and good crystal quality could be obtained regardless of the existing delamination of metal contact, which then slightly downgraded the LED performance. Here, the integrated fluence level was set in the range of 2.6–4.4 J/cm2 to enable the fs-LLO process. Moreover, two mitigation strategies were developed and proven to improve the optoelectrical characteristics of the lifted-off LEDs (i.e., modification of the processing step related to the metal creation and reduction of laser energy).

Demonstration of GaN-based white LED grown on 4-inch patterned sapphire substrate by MOCVD

A blue light emitting diode (LED) chip has been facilely fabricated on a 4-inch patterned sapphire substrate based on an InGaN/GaN multiple quantum well structure, composed of a Ce3+ doped Yttrium Aluminum Garnet (YAG) yellow phosphor packaged into a white LED chip. The epitaxial growth, fabrication, and package process of the device have been briefly described; the structural properties and device performance have also been systematically studied. The surface morphology of the sample in a whole wafer has a good uniformity. And the curvature distribution with a maximum value of ~70 μm, it gradually becomes smaller from the middle to the periphery, which is mainly resulting from the strain state evolution during the film growth. When increasing the injection current, it is significant interestingly observed in the electroluminescence (EL) spectrum that the blue light peak is blue-shifted, it is due to the influence reducing of the polarization field and/or the energy band filling, while the blue light peak and the yellow-green light peak are red-shifted due to the thermal effect of the chip. In addition, the light output power exhibits a linearly incremental relationship with a little saturation when the injection current varies from 0 to 100 mA. Therefore, it is firmly believed that the corresponding chip fabrication and characterization technology will play a certain role in promoting the development of solid-state lighting field.

Cyan-emitting Ba0.45Ca2.5La6(SiO4)6: 0.05 Eu2+ and Ba1.45Ca1.5La6(SiO4)6:0.05 Eu2+ solid-solution phosphors for white light-emitting diodes

Two cyan-emitting phosphors with different bandwidths were successfully synthesized through the high-temperature solid-state method in a reducing atmosphere. The crystal structures, morphologies, and luminescence properties of the as-prepared phosphors were investigated. The Rietveld refinements of the powder X-ray diffraction (XRD) data demonstrated the single phase of the samples, and two crystallographic sites of La3+ were observed in the crystal structure. Under the excitation of UV light, both Ba0.45Ca2.5La6(SiO4)6: 0.05 Eu2+ and Ba1.45Ca1.5La6(SiO4)6: 0.05 Eu2+ phosphors emitted cyan light due to the 4f65 d1→4f7 transitions of the Eu2+ ion. The emission spectra could be well fitted by two component Gaussian peaks corresponding to two different coordination environments of the Eu2+ ions. The temperature-dependent photoluminescence spectra show a large difference on the thermal stability between the two phosphors. The two phosphors exhibit effective absorption of near-UV light and their internal quantum efficiencies (IQEs) were calculated as 31.5% and 42.4% under 295 nm UV-light excitation. The experimental results indicate that the novel cyan phosphors might have potential applications in white light-emitting diodes (LEDs) based on the near-UV LED chip.

Discretely-supported nanoimprint lithography for patterning the high-spatial-frequency stepped surface

Non-planar morphology is a common feature of devices applied in various physical fields, such as light or fluid, which pose a great challenge for surface nano-patterning to improve their performance. The present study proposes a discretely-supported nanoimprint lithography (NIL) technique to fabricate nanostructures on the extremely non-planar surface, namely high-spatial-frequency stepped surface. The designed discretely imprinting template implanted a discretely-supported intermediate buffer layer made of sparse pillars arrays. This allowed the simultaneous generation of air-cushion-like buffer and reliable support to the thin structured layer in the template. The resulting low bending stiffness and distributed concentrated load of the template jointly overcome the contact difficulty with a stepped surface, and enable the template to encase the stepped protrusion as tight as possible. Based on the proposed discretely-supported NIL, nanostructures were fabricated on the luminous interface of light emitting diodes chips that covered with micrometer step electrodes pad. About 96% of the utilized indium tin oxide transparent current spreading layer surface on top of the light emitting diode (LED) chips was coated with nanoholes array, with an increase by more than 40% in the optical output power. The excellent ability of nanopatterning a non-planar substrate could potentially lead innovate design and development of high performance device based on discretely-supported NIL.

CH3NH3Pb1–xCoxBr3–2xCl2x Perovskite Quantum Dots for Wide-Color Backlighting

In recent years, perovskite quantum dots (PQDs) with the incorporation of transition-metal ions for reducing toxicity and enhancing stability have been in a research hotspot. However, the PQDs containing transition-metal ions are quite unstable and display low photoluminescence quantum yields (PLQYs), owing to the existence of defects. Herein, we have endeavored to introduce simultaneous cation and anion exchange with cobalt and chloride ions into the host CH3NH3Pb1–xCoxBr3–2xCl2x PQDs followed by matrix encapsulation via poly(methyl methacrylate) (PMMA) as a favorable approach to stabilize PQDs with improved PLQY. We have utilized cobalt chloride as the precursor for doping ions. The emission wavelength can be regulated from green to cyan color with the increase in doping concentration without negotiating on color quality, which is believed to be the modification of band gap and corresponding electron density estimated from the density functional theory. A high PLQY of 95% is observed even after the incorporation of cobalt and chloride ions into host PQDs. We demonstrate white light-emitting diode (LED) prototypes for backlight applications using a PQDs/PMMA composite on a blue LED chip, which exhibits excellent stability toward an ambient and robust atmosphere and provides a wide-color gamut of 124% of the NTSC standard. This research paves a way for the research community to synthesize highly efficient, color-tunable PQDs for wide-color backlighting applications.

Vibratory Feeding Performance Improvement Through Discrete Element Analysis for High-Speed Optical Detection of LED Chips

A high-speed vibratory bowl feeder is a key equipment of the optical detection of light emitting diode (LED) chips. Addressing the stationarity-losing problem induced by transporting small-size and light-weight LED chips, this study designs a vibratory bowl feeder for the high-speed detection of small LED chips based on theoretical and numerical analyses. To understand the influence of the structural parameters of the feeder on its performance, we create a mathematical model of the part's behavior base on the Hertz-Mindlin contact theory. The numerical model indicates that the property of the part, the track angle and the friction coefficient of the track are the main factors that affect the behavior of the part in the feeding process. Through the discrete element method (DEM) simulation, we observe the part behavior under different track angles and friction coefficients of the feeder. The motion stationary of the LED chip is evaluated by its standard deviation (STD) of velocity and hopping height. Thus, we can appropriately determine these parameters of the vibratory feeder in order to get the optimal feeding performance. Finally, an experimental platform of the LED chip detection device is built to verify the performance of the vibratory bowl feeder. The experimental results show that the performance of the feeder was in good agreement with the simulation results. With the theoretical analysis and experimental verification, the vibratory bowl feeder was successfully implemented for LED transportation and reached a high feeding velocity of 1516.5 pieces/min at the working frequency of 223 Hz.

Insight into the crystal structure and photoluminescence properties of an extremely broadband yellow-emitting phosphor Sr8MgCe(PO4)7:Eu2.

The combination of binary complementary color phosphors with near-ultraviolet (NUV) light-emitting diode (LED) chips was put forward to lower the energy loss caused by the reabsorption of the combination of tricolor phosphors with NUV LED chips. Thus investigating broadband yellow-emitting phosphors with enriched red components in the spectral range has gained much attention. Here, an extremely broadband yellow-emitting phosphor Sr8MgCe(PO4)7:Eu2+ with sufficient red component in the spectral region was synthesized. Due to the introduction of Ce into the host, Sr8MgCe(PO4)7 shows an emission band with a peak at 375 nm when excited at 310 nm. Meanwhile, Eu2+ doped Sr8MgCe(PO4)7 exhibits an extremely broad yellow emission band with a full width at half-maximum of 175 nm and a peak located at about 598 nm, due to the 5d-4f transitions of Eu2+ ions substituting five Sr2+ sites. The excitation peak of the host at 310 nm was detected in the excitation spectrum of Sr8MgCe(PO4)7:Eu2+ monitored at 598 nm, indicating the energy transfer from the host to Eu2+, which was also proved by the decay curves. On the other hand, the excitation band at about 400 nm due to the 4f-5d transitions of Eu2+ was also detected in the excitation spectrum of Sr8MgCe(PO4)7:Eu2+, and this matches well with the NUV LED chips. Moreover, the energy transfer between Eu2+ ions at different crystallographic sites was demonstrated by time-resolved photoluminescence (TRPL) spectra. A white LED with a CRI (Ra) of 82.88 and a CCT of 4238 K was prepared through a 400 nm NUV LED chip, Sr8MgCe(PO4)7:Eu2+ and BaMgAl10O17:Eu2+ (BAM:Eu2+). These results pave the way for designing better Eu2+-doped phosphors for NUV WLEDs with binary complementary color phosphors.

A study of negative-thermal-quenching (Ba/Ca)AlSi5O2N7:Eu2+ phosphors.

Phosphor is an important part of the new generation of light-emitting diodes (LEDs), which requires high luminous intensity and high-temperature resistance. In this study, a series of excellent (Ba1-x-yCax)AlSi5O2N7:yEu2+ phosphors was developed, which were synthesized by a high-temperature solid-state reaction in a reducing atmosphere. In addition, the crystallinity and luminescence intensity of the samples could be enhanced by some amount of Ca2+ substitution. The luminescence intensity was the highest when the Eu2+ concentration reached 0.06. Furthermore, the thermal stability of the luminescence was studied in detail. The results were satisfactory, showing that the luminescence intensity of the (Ba1-xCax)AlSi5O2N7:Eu2+ phosphors exhibited unique negative-thermal-quenching characteristics both at high (273-473 K) and low (4-273 K) temperatures. And the phosphor combined with UV LED chip and red phosphor Sr2Si5N8:Eu2+ can achieve a CRI of 90.4 in white LED application which indicated the (Ba1-xCax)AlSi5O2N7:Eu2+ phosphor has potential in LED applications.

Deep machine learning of the spectral power distribution of the LED system with multiple degradation mechanisms

Abstract The performance and reliability of the light-emitting diode (LED) system significantly depend on the thermal–mechanical loading-enhanced multiple degradation mechanisms and their interactions. The complexity of the LED system restricts the theoretical understanding of the root causes of the luminous fluctuation or the establishment of the direct correlation between the thermal aging loading and the luminous outputs. This paper applies the deep machine learning techniques and develops a gated network with the two-step learning algorithm to build the empirical relationship between the design parameters and the thermal aging loading and the luminous output of LED products. The flexibility of the proposed method will be demonstrated by integrating it with different neural network architectures. The proposed gated network concept has been validated in both multiple LED chip packaging and LED luminaire under thermal aging loading. The validation of the luminous data of multiple LED chip packaging shows that the maximum differences of the correlated color temperature (CCT) and color coordinate are 2.6% and 1.0%, respectively. Moreover, the machine learning results of the LED luminaire exhibit that the differences of lumen depreciation, CCT and color coordinate are 1.6%, 1.9% and 1.1%, after 2160 h of thermal aging.

Toward 200 Lumens per Watt of Quantum-Dot White-Light-Emitting Diodes by Reducing Reabsorption Loss.

In this study, we analyze the influence of the pore structure of an SBA-15 particle on the light emission from its inner adsorbed quantum dots (QDs) and outer light-emitting diode (LED) chips. It is found that the particle features of a high refractive index, comparable feature size of pore structure, and lower amount of QD adsorption help with QD light extraction, demonstrating a mechanism to suppress QD light propagating through pores and thus reducing the reabsorption loss. We consequently developed highly efficient QD white LEDs with wet-mixing QD/SBA-15 nanocomposite particles (NPs) by further optimizing the packaging methods and the introduced NP mass ratio. The LEDs demonstrated a record luminous efficacy (the ratio of luminous flux to electrical power) of 206.8 (entrusted test efficiency of 205.8 lm W-1 certificated by China National Accreditation Service) and 137.6 lm W-1 at 20 mA for white LEDs integrating only green QDs and green-red QD color convertors, respectively, with improved operating stability. These results are comparable to conventional phosphor-based white LEDs, which can be a starting point for white LEDs only using QDs as convertors toward commercialization in the near future.

Design and application of a cooling device based on peltier effect coupled with electrohydrodynamics

Miniaturization has become the development trend of light-emitting diodes (LEDs), which results in a significant increase in heat flux production and operating temperature. The accumulated heat should be dissipated effectively to ensure that the chip can work properly. The conventional cooling techniques have now reached their limits in heat removal rates. A new cooling device coupled thermoelectric cooling (TEC) with corona wind cooling (CWC) was proposed and optimized experimentally based on the second law of thermodynamics and the principle of entropy generation minimization. The results indicate that a better cooling effect and a high coefficient of performance value is obtained by turning on the TEC near the heat source firstly and adjusting the CWC power at the hot side. Increasing the input power of TEC and keeping the total cooling power unchanged, another 4.7% improvement is obtained with the highest heating power of 14 W. The target temperature can be maintained at an acceptable level (＜80 °C) if the TEC and CWC are well coupled. However, the coupling effect between TEC and CWC will be worse when the input power is large. A higher output luminous flux will be obtained when the LED chip works in the constant current mode due to the good coupling between CWC and TEC. The luminescence intensity and chromaticity properties are improved significantly. The principal component analysis (PCA) also indicates excellent comprehensive performance. The results verify the feasibility of a novel cooling device coupled TEC with CWC in cooling high heat flux electronics.

Enhancement of luminescence and thermal stability in Sr2.92-xTmxSiO5:Eu2+ for white LEDs

Sr2.92-xTmxSiO5:Eu2+ (x = 0–0.09) orange emitting phosphors for white LED were prepared by solid-state method. The crystal structure and morphology of Sr2.92-xTmxSiO5 were determined by XRD and SEM. The phosphor showed strong absorption spectra and can be excited by near-ultraviolet (NUV) light in the range of 200−550 nm which matched well with the NUV light-emitting diode (LED) chips. The emission spectra showed asymmetric band around 585 nm under NUV and assigned to the 4f65d-4f7 transition of Eu2+ occupied Sr2+ sites in the host lattice. It is observed that 5d electron state of Eu2+ which easily influenced by the crystal field was not changed after Tm3+ co-doping in the host. Enhanced emission intensity, improved thermostability and fluorencence decay time have been made via crystal chemical unequivalence substitution of Tm3+ in Sr2+ site. The energy transfer took place between the Tm3+ and Eu2+ activator, the underlying mechanisms of enhanced luminescence and decay fluorescence properties by energy transfer were analyzed.

Sensitizing Full‐Spectrum Lanthanide Luminescence within a Semiconductor CaZnOS Host

Lanthanide ions are appealing for luminescence applications due to their sharp and tunable emission lines spanning the whole spectral range. However, the parity‐forbidden nature of the electronic transition in lanthanide ions typically results in ineffective excitation processes, which is a major obstacle to practical applications of lanthanide‐based luminescent materials. Herein, a general method to sensitize lanthanide luminescence within a semiconductor host of CaZnOS is developed. Efficient energy transfer from the host to a series of lanthanide ions with the assistance of Cu+ or Mn2+ codopants is demonstrated, which introduces intermediate energy states to mediate the energy transfer processes. Accordingly, a wide spectrum of emission is achieved by a single band excitation in the near‐ultraviolet region. Due to the efficient host sensitization, a phosphor‐converted light‐emitting diode (LED) is constructed by integrating the lanthanide‐doped CaZnOS with an ultraviolet LED chip, which is used for near‐infrared studies of biological tissues.

Rapid synthesis of Ce3+:YAG via CO2 laser irradiation combustion method: Influence of Ce doping and thickness of phosphor ceramic on the performance of a white LED device

A new, rapid method was reported to produce Y3Al5O12 phosphors doped with different concentrations of Ce3+ (YAG:Ce3+) within 1.5 ​min via continuous-wave CO2 laser irradiation, in which a CO2 laser is utilised as the combustion source. X-ray powder diffraction confirmed the formation of pure cubic phase in the as-synthesised powder. Thus, laser irradiation plays an important role in optimising reaction kinetics to obtain high-quality crystal with high luminescence intensity. Furthermore, the incorporation of Ce3+ ions into the YAG matrix can be enhanced through sintering powders at 1050 ​°C for 5 ​h. Optical properties were examined comparatively using photoluminescence emission. Additionally, the performance of white light-emitting diode (LED) can be regulated easily by altering the Ce3+ doping concentration and YAG:Ce3+ layer thickness on blue InGaN LED chip. Excellent white light, which has a CIE coordinate of (0.31, 0.36) that approached the standard WLED values, a colour rendering index of about 69.4, a correlated colour temperature of about 6299 ​K and a luminous radiation efficiency exceeding 169 lm/W, was obtained for YAG doped with 2 ​mol% Ce concentration with a coating layer thickness of 1 ​mm.

The effects of low-color-temperature dual-primary-color light-emitting diodes on three kinds of retinal cells

Long-term illumination of the retina with blue-light-excited phosphor-converted light-emitting diodes (LEDs) may result in decreased retinal function, even if the levels of blue light emitted are low. New low-color-temperature dual-primary-color LEDs have been developed that are composed of only two LED chips: a red chip and a yellow chip. These LEDs are expected to become a new type of healthy lighting source because they do not emit blue light, they lack phosphor, and they solve the problem of low efficiency encountered with phosphor-converted low-color-temperature LEDs. Many studies have indicated that these new low-color-temperature LEDs are likely to have therapeutic effects. However, the biological safety of these LEDs needs to be explored before the therapeutic effects are explored. Therefore, this experiment was conducted to investigate the effects of the new low-color-temperature LEDs and fluorescent white LEDs on three types of retinal cells. We observed that the viability and numbers of retinal cells decreased gradually with increasing LED color temperature. The new low-color-temperature LEDs caused less death and adverse effects on proliferation than the fluorescent white LEDs. After irradiation with high-color-temperature LEDs, the expression of Zonula Occludens-1 (ZO-1) was decreased and discontinuous in ARPE-19 cells; the stress protein hemeoxygenase-1 (HO-1) was upregulated in R28 cells; and glial fibrillary acidic protein (GFAP) and vimentin were upregulated in rMC-1 cells. We therefore conclude that the new white LEDs cause almost no damage to retinal cells and reduce the potential human health risks of chronic exposure to fluorescent white LEDs.