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An acetylcholinesterase (AChE) biosensor was successfully fabricated with a stable structure and high detection accuracy. Graphene (Gra) nanofragments modified with chitosan (CS) and AChE were successively drip coated on the surface of a glassy carbon electrode via a layer-by-layer assembly method. The concentration range of the sensor to detect dichlorvos was 0.1-100,000 nM, and the limit of detection was 54 pM. CS was used to modify Gra for the first time, which enhanced the mechanical flexibility of these Gra nanostructures, significantly improving the stability and detection accuracy of this sensor.

Currently, torque ripple is a crucial factor hindering the application of the switched reluctance motor (SRM). Hence, it is of crucial importance to suppress this undesirable torque ripple. This paper proposes a new torque ripple suppression method of SRM based on the improved torque distribution function. Firstly, the electromagnetic characteristic model of a 8/6-pole four-phase SRM is established, and the cerebellar model articulation controller (CMAC) is used to complete the learning of each model. Then, the improved torque distribution function is planned based on the torque model to give the reference torque of each phase, and the inverse torque model is used to realize the mapping of the reference torque to the reference flux linkage. Finally, the duty of each phase voltage PWM wave modulation is output based on the PID control theory. The proposed accurate model-based planning scheme is implemented on the simulation platform, and the results shows that the maximum torque fluctuation of the output results is reduced to within 3%, and the average error is reduced to within 1%, which is much lower than the error of 15% under the traditional direct torque control method.

In this paper, the environmental stability of silicon nitride (SiNx) films deposited at 80 °C by plasma-enhanced chemical vapor deposition was studied systematically. X-ray photoelectron spectroscopy and Fourier transform infrared reflection were used to analyze the element content and atomic bond structure of the amorphous SiNx films. Variation of mechanical and optical properties were also evaluated. It is found that SiNx deposited at low temperature is easily oxidized, especially at elevated temperature and moisture. The hardness and elastic modulus did not change significantly with the increase of oxidation. The changes of the surface morphology, transmittance, and fracture extensibility are negligible. Finally, it is determined that SiNx films deposited at low-temperature with proper processing parameters are suitable for thin-film encapsulation of flexible devices.

Silver nanowires (AgNWs) and new-type two-dimensional material MXene have shown great prospects for applications in new-generation flexible electronic devices due to their brilliant properties. In this work, high-performance silver nanowire/MXene hybrid transparent conductive electrodes (TCEs) were prepared by a solution method. It studied the influence of AgNW concentration on the photoelectric properties of TCEs. Then, several-layer or even single-layer MXene were obtained by a delamination process and were added to the AgNW networks to prepared flexible hybrid electrodes on polyethylene naphtholate (PEN) substrates. The hybrid TCEs showed a low sheet resistance of 24.4 Ω/sq with the transmittance of 90.6% at 550nm. Moreover, the MXene sheets covered on the surface could improve the conductivity of the film and block the oxidation of silver nanowires. The results mean that the novel AgNW/MXene TCEs have great potential for the practical applications of high-performance flexible electronic devices.

Room-temperature phosphorescent (RTP) carbon dots (CDs) have been fascinated by a lot of scholars because of their stable triplet excited states and environmental friendliness. Herein, novel fluore...

An electrochemical acetylcholinesterase biosensor based on silver nanowire, graphene, TiO2 sol–gel, chitosan and acetylcholinesterase has been fabricated successfully for the detection of organophosphate pesticides. The outstanding electrical properties of silver nanowires and graphene, and moreover the self-assembly of these two nanomaterials make the biosensor highly sensitive. Simultaneously, the immobilization efficiency of the enzyme is greatly improved by the action of the TiO2 fixed matrix. Under optimum conditions, the biosensor exhibited excellent performance for the detection of dichlorvos with a linearity in the range of 0.036 μM to 22.63 μM and the detection limit was found to be 7.4 nM. The biosensor was highly reproducible and stable during detection and storage.

Layered group IV monochalcogenides have garnered considerable attention as a new class of two-dimensional (2D) semiconducting materials owing to their unique crystal structure and novel physical properties. The present work describes the chemical vapor transport synthesis of single-crystalline GeS nanoribbons. The findings demonstrate that with incrementally applied voltage, electrostrictive deformation and highly vertical current occur more significantly. Additionally, using a 2D fast Fourier transform power spectra, we demonstrate that the horizontal distribution of topography and current is more inhomogeneous than the vertical distribution, and that their monolithic spatial correlation weakens with increasing applied voltage. Moreover, we discovered that electrostrictive deformation has a sizable effect on the monolithic vertical resistance. Furthermore, local hollow positions are more conductive than bulge positions, as demonstrated by the ‘resistor’ model and local current–voltage curve. These findings on layered GeS nanoribbons not only shed light on the topographic and electrical properties of the material but also expand the possibilities for other nanoscale electronic and electromechanical device applications.

Graphene/silver nanowire composite films have great potential as transparent conductive electrodes in the field of optoelectronic devices. So far, antioxidant and reducing the junction resistance are two major parameters in the silver nanowire electrode studies. In this paper, a pseudo-biological inspired structure for transparent electrodes was proposed by combining hybrid diameters silver nanowire network with the chemical vapor deposition-grown (CVD-grown) graphene as a passivation layer. An environmentally friendly liquid, deionized water, was selected for the generation of capillary forces at the liquid bridge, thereby improving the wire junction problem. Compared with the silver nanowire films, the value of the figure of merit (FoM) of the graphene/silver nanowire composite film increased by 69.6% with a sheet resistance of 26.4 ?/sq, and a transmittance was 91.4% at 550nm. Also, graphene was chosen as an encapsulation layer to protecting silver nanowires from oxidation while improving electrical properties, which makes the composite films promising as electrodes for underwater optoelectronic devices or a possible development in high humidity environments. Furthermore, the capillary-welded silver nanowire/graphene composite film was used as the transparent anode to prepare organic light-emitting diode (OLED) devices. The devices emitted green electroluminescence at 516 nm, and the turn-on voltage was about 3.8 V. The maximum brightness was 50810 cd/cm2, which is higher than the indium tin oxide-based (ITO-based) devices with the same configuration. Therefore, it means that this novel silver nanowires/graphene electrode has great potential in optoelectronic device applications.

The enzymatic electrochemical biosensor has the advantages of simple operation, speed, and integration in the detection of organophosphorus pesticide (OPs) residues. It has the potential to become the best alternative to the traditional OP detection technology. This article introduces the OP identification principle of different enzymes, the OP detection mechanism of several common sensors, and the enzyme assembly method. In addition, the article discusses application of nanomaterials in sensor preparation and sensor performance parameters in the past decade. The related content of early sensors is outside the scope of this article.

An acetylcholinesterase biosensor modified with graphene and transition metal carbides was prepared to detect organophosphorus pesticides. Cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy were used to characterize the electrochemical catalysis of the biosensor: acetylcholinesterase/chitosan-transition metal carbides/graphene/glassy carbon electrode. With the joint modification of graphene and transition metal carbides, the biosensor has a good performance in detecting dichlorvos with a linear relationship from 11.31 μM to 22.6 nM and the limit of detection was 14.45 nM. Under the premise of parameter optimization, the biosensor showed a good catalytic performance for acetylcholine. Compared to the biosensors without modification, it expressed a better catalytic performance due to the excellent electrical properties, biocompatibility and high specific surface area of graphene, transition metal carbides. Finally, the biosensor exhibits good stability, which can be stored at room temperature for one month without significant performance degradation, and has practical potential for sample testing.

In this paper, study on the heat transfer of GaN-based high electron mobility transistors (HEMTs) on SiC substrate was carried out utilizing transient thermal analysis method. Gate-source voltage ( ${V}_{{\text {gs}}}$ ) is used as the temperature sensitive parameter. Linear relationship ( ${K}$ factor) between ${V}_{{\text {gs}}}$ and temperature is obtained. It is found that the ${K}$ factor is proportional with drain-source current and does not change with the variation of drain-gate voltage in the linear region of HEMT. We proposed and proved that the ${K}$ factor should be calibrated with HEMT working at linear region to insure the accuracy of subsequent thermal evaluation. Moreover, the temperature rise and thermal resistance keeps as constant when ${K}$ factors were calibrated within the linear region. Transient thermal measurement turns out to be a non-destructive, fast, and precise method for the thermal performance analysis and quality control of the power HEMTs.

A new graphene/silver nanowire (AgNW) hybrid transparent conductive films (TCFs) was prepared by wet-transferring a chemical vapor deposition (CVD)-grown monolayer graphene onto pristine spin coati...

Ultraviolet (UV) treatment has been demonstrated to be an effective way to removing the poly-methyl methacrylate (PMMA) residual during the process of the wet-etching transfer of graphene. ...

In this paper, a novel and scalable method to fabricate graphene/carbon nanotube (CNT) hybrid transparent conductive films on Cu substrates, which combines electroplating and chemical vapor deposition (CVD) is proposed and demonstrated. The Cu substrate was electroplated with electrolyte containing conductive CNTs; then, a uniform graphene film was grown on Cu. After a commonly utilized polymethyl methacrylate assisted transfer process, a hybrid graphene/CNT transparent conductive film was obtained at the target substrate. Conventional graphene grown on electropolished Cu was used as the reference sample. The comprehensive characterization using scanning electron microscopy (SEM), Raman microscopy system, and transmission electron microscopy (TEM) selected area electron diffraction pattern show that the CNTs are uniformly covered by a monolayer graphene with comparable quality to graphene grown on electropolished Cu. The hybrid thin films exhibit outstanding surface morphology (RMS of 1.26), obviously enhanced electrical properties (the square resistance decreases from 490 to 394 Ω sq−1), better surface wettability (7° decrease in contact angle), and a negligible transmittance loss (1.3% reduction at 550 nm) compared to CVD graphene that was grown on electropolished Cu. It is anticipated that the graphene/CNT hybrid films that are fabricated using the proposed approach can be a promising alternative to ITO to realize the emerging, particularly flexible optoelectronic devices.

A highly sensitive, stable electrochemical acetylcholinesterase biosensor with silver-graphene modified was fabricated to detect organophosphorus pesticides, dichlorvos. Complete production and test process were characterized by scanning electron microscopy, cyclic voltammetry, differential pulse voltammetry. The electrochemical signal of biosensor got to be well amplified because of the modification of silver-graphene. The better detection performance was got, compared with the sensors that lack or no modified. Moreover, TiO2 sol-gel and high viscous chitosan were used as a highly efficient immobilization matrix for immobilizing acetylcholinesterase. The detection linear range of the biosensor to dichlorvos, a model organophosphorus pesticides compound, was from 0.036 μM (7.9 ppb) to 22.6 μM, with a limit of detection of 7.4 nM (1.6 ppb). The biosensor was highly reproducible and stable in detection and storage.

Both graphene and silver nanowire electrodes are thermoelectric materials that replace ITO as electrodes for optoelectronic devices. In this paper, we studied the effect of silver nanowire concentrations on electrode performance. By taking into account factors such as the sheet resistance and the transmittance of the films, the concentration of silver nanowires solution was finally chosen to be 3 mg/ml. Compared with Joule heat welding, capillary welding has self-limiting, which can prevent silver nanowires from being blown because of excessive heat. Therefore, capillary welding was chosen to improve the junction problem. In addition, graphene was chosen as an encapsulation layer to protecting silver nanowires from oxidation while improving electrical properties. Finally, the graphene/silver nanowire composite electrode had the sheet resistance of 59.7 Ω/sq with the transmission of 91.8 % at 550 nm.

An electrochemical acetylcholinesterase biosensor based on silver nanowire, graphene, TiO

The transparent conductive films (TCFs) based on silver nanowires are expected to be a next-generation electrode for flexible electronics. However, their defects such as easy oxidation and high junction resistance limit its wide application in practical situations. Herein, a method of coating Ti3C2Tx with different sizes was proposed to prepare silver nanowire/MXene composite films. The solution-processed silver nanowire (AgNW) networks were patched and welded by capillary force effect through the double-coatings of small and large MXene nanosheets. The sheet resistance of the optimized AgNW/MXene TCFs was 15.1 Ω/sq, the optical transmittance at 550 nm was 89.3%, and the figure of merit value was 214.4. Moreover, the AgNW/MXene TCF showed higher stability at 1600 mechanical bending, annealing at 100 °C for 50 h, and exposure to ambient air for 40 days. These results indicate that the novel AgNW/MXene TCFs have a great potential for high-performance flexible optoelectronic devices.

We have investigated the characteristics of Cs 2 CO 3 doped 4, 7-Diphenyl-1, 10-phenanthroline (Bphen) film and the effects of its thickness on the optoelectrical performance of microcavity organic light-emitting diodes. By doping Cs 2 CO 3 into Bphen, higher current density and lower turn-on voltage could be achieved with barely changing the external quantum efficiency. Three types of resonators were fabricated by varying the thickness of every a quarter of electron transport layers, i.e. 152.5 nm (0.25 λ), 305 nm (0.5 λ), and 610 nm (1.0 λ). The 0.25λ-based microcavity organic light-emitting diode was found to exhibit the lowest waveguided loss coefficient of 118.0 cm −1 , while the relative emission intensity of edge mode to surface reached about 2118.8 times, indicating that this mode underwent a very low attenuation during its propagation.

We have proposed an approach to improve the photovoltaic performance of inverted polymer solar cells (i-PSC) using lithium ion doped ZnO (Li ZnO) as cathode buffer layer (CBL). The Li ZnO CBL was prepared using the diffusion technique, performed by inducing the Li ion of 8-hydroxyquinolatolithium (Liq) to diffuse into ZnO film through annealing the bi-layer ZnO/Liq film. Doping concentration of Li ion was controlled by using various thickness of Liq film and annealing temperature. Based on Li ZnO CBL, the poly (3-hexylthiophene) [6,6]:-phenyl C61-butyric acid methyl ester (P3HT:PCBM) i-PSC device possessed a optimal power conversion efficiency (PCE) of 4.07%, which was 30% improved than that of the device with neat ZnO as CBL. The enhancement of the device performance could be attributed to the enhanced electron mobility and better band matching of the Li ZnO CBL. Our finding indicates that the Li ZnO film fabricated with relatively low temperature treatment has great potential for high-performance i-PSCs.

We report herein the synthesis and characterization of an efficient versatile binaphthyl derivative, 4,4′-bis(4″-triphenylsilylphenyl)-1,1′-binaphthalene (SiBN) with a high glass transition temperature of 159.3 °C. Its role as a lasing dye was demonstrated by the amplified spontaneous emission, which exhibited an ultraviolet peak of 397 nm with a low threshold of 20 μJ/pulse. Meanwhile, the lasing film and blue electroluminescent device employing this novel binaphthyl derivative as the host showed superior performances to the reference based on the conventional host 4,4′-bis(carbazol-9-yl)biphenyl. Finally, extremely stable colour-tunable deep-blue (436 nm/472 nm) organic electroluminescent devices based on SiBN were demonstrated with very high brightness (>7521.2 cd/m2) and high external quantum efficiencies (1.90% for 436 nm and 4.66% for 472 nm emission). The development of such functional versatility materials will be an effective method for reducing the cost of organic photoelectric devices.

Highly accelerated decay testing ( HADT ) based on the subsystem isolation method for light-emitting diode ( LED ) lamps is proposed. The proposed test identifies the desired thermal stress limit ( TSL ) in a given humidity stress for LED light engine subsystems. In HADT , a monitoring procedure for the in situ pseudo-junction temperature ( T pj ) of LED lamps and a step-fitting analysis procedure for the monitored parameters are suggested to detect abnormal thermal stress levels. The obtained TSL is applied to step-stress accelerated degradation testing ( SSADT ) to verify the uniform decay mechanisms at various stress levels. Results show that the goodness-of-step fit analysis of the monitored parameters effectively identifies the abnormal thermal stress levels. Consequently, the TSL of LED light engines can be clearly observed through the in situ T pj and optical parameters monitored in HADT . The application study shows that SSADT rapidly and effectively accelerates the decay process of LED lamps, and the tested samples undergo uniform decay mechanisms at three reasonable stresses, which are selected based on the obtained TSL . The proposed HADT has significant potential use in the qualification specification of LED lamps.

We reported high-performance organic light-emitting diodes (OLEDs) with a maximum efficiency of 42.3 lm W−1 and color rendering index of 92 by facilitating charge carrier transportation and controlling the recombination zone. In addition, we demonstrated that inserting a thin interlayer in emitting layer prevented energy transfer between blue and green emission layers, providing a relative chromatic-stability OLEDs. Furthermore, the device also exhibit a rather low correlated color temperature (2202 K), corresponding to an excellent warm WOLED, which could meet the special requirement of energy saving and environment protection.

Inverted organic light-emitting diodes (IOLEDs) have great potential application in flat-panel displays. High energy consumption, efficiency roll-off, and poor electron injection are key issues limiting the use of IOLEDs. Here, we present IOLEDs with extremely low driving voltage, high efficiency and efficiency roll-up by employing double electron injection layers (D-EILs) composed of metal sulfide and cesium carbonate (Cs2CO3)-doped 4,7-diphenyl-1,10-phenanthroline (Bphen). We demonstrate that the use of D-EILs with metal sulfides can significantly improve the performance of IOLEDs. For a blue florescent device based on (2 nm-zinc sulfide)/Bphen: Cs2CO3, we achieve a power efficiency of 10.9 lm W−1 at a luminance of 1000 cd m−2, giving a turn-on voltage of 2.8 V. Notably, the external quantum efficiency increases from 6.9 to 7.5% and the current efficiency increases from 14.3 to 15.4 cd A−1 with the rise in luminance from 1000 to 10 000 cd m−2. Also, the copper sulfide-based device exhibits very-low operating voltages of 4.0 V and 5.3 V at the luminance of 1000 and 10 000 cd m−2, respectively. For a green phosphorescent device, approximately 1.2-fold improvement in external quantum efficiency was obtained compared to the conventional structure. We attributed the improved performance to dipole–dipole interactions at the sulfide-organic interface.

We have demonstrated a new composite thin film for encapsulation consisting of polydimethylsiloxane (PDMS) and graphene. Graphene is a kind of hydrophobic material, causing that moisture could hardly penetrate into the surface of graphene. Graphene is usually fabricated by the method of Chemical Vapor Deposition (CVD). The atomic distance of graphene is very small; thus the water vapor channel from one layer to the nearby layer has been cut off, achieving the barrier of oxygen, water vapor and particle. Graphene and PDMS have high transmittance, making this method be widely used in various photoelectric devices. Meanwhile, graphene owes excellent heat dissipation which can reduce the temperature gradient and the junction temperature, thus improves the comprehensive performance and reliability of the device. Furthermore, PDMS has a good cohesiveness property, by which fit graphene very well and meet the requirement of encapsulation at the same time. Using Ca film test, we have found that the composite thin film has the greatest water oxygen barrier effect among them.

The technique of organic light-emitting diodes developed rapidly over the last decade and small size OLED panels has also been commercially applied. However, organic semiconductors exhibit relatively poor electrical and thermal behavior and the device stability is always a tough issue since the high sensitivity to the environment, such as the water, oxygen, temperature, and so on. Therefore, the organic-inorganic hybrid light-emitting devices become research focus for combining both the high electron mobility of inorganic semiconductors and the extraordinary optoelectronic properties of polymers. However, according to previous research results, there are still problems such as low brightness, poor color rendering, low efficiency and low current density etc. Here, we present a novel inverted organic-inorganic hybrid blue fluorescent device with the hybrid p-n junction structure using n-type GaN (n-GaN) and organic semiconductor layers, which was proved to have extremely high luminance and low driving voltage. Single layer n-GaN with thickness of 2.5 um on patterned sapphire substrate (PSS) was used in this paper because it has excellent electrical conductivity and outstanding thermal conductivity. Based on this organic-inorganic hybrid system, we have proved a high-brightness and low turn-on voltage light-emitting devices.

In this paper, the regularization method and the Fourier domain deconvolution method with filter applied are used respectively for the operations of differentiation and deconvolution in the network identification by deconvolution (NID) process. The signal-to-noise ratio of the time-constant function is estimated as an evaluation of the result errors. The choosing criterion and influences of the parameters of the numerical algorithms are analyzed using the signal-to-noise ratio (SNR) estimation by numerical results.

In this paper, the effects of sample size, forward current, and temperature on the performance of organic light-emitting diodes (OLEDs) are discussed. It is found that the efficiency is inversely proportional to the sample size and current density due to the current spreading effect and limited carrier mobility of the organic material. The results demonstrate that both forward voltage and luminescent efficiency of all samples are functions of temperature. The voltage of devices decreases monotonously with temperature, while the efficiency shows the reverse trend. Carrier mobility, current injection efficiency, and the trap-recovery effect are regarded to be responsible for the temperature dependency.

The development of silver nanowire electrodes is always limited due to some disadvantages, such as roughness, oxidative properties, and other disadvantages. In this research, a capillary-welded silver nanowire/graphene composite film was used as an electrode for organic light-emitting diode (OLED) devices. As an encapsulation layer, graphene reduced the surface roughness and the oxidation probability of silver nanowires. The composite electrode showed an excellent transmittance of 91.5% with low sheet resistant of 26.4 ohm/sq. The devices with the silver nanowire/graphene composite electrode emitted green electroluminescence at 516 nm, and the turn-on voltage was about 3.8 V. The maximum brightness was 50810 cd/cm2, which is higher than the indium tin oxide-based (ITO-based) devices with the same configuration. Finally, it was proved that the silver nanowire/graphene composite electrodes possessed better heat dissipation than the ITO-based ones under energization. In summary, it means that this novel silver nanowires/graphene electrode has great potential in OLED device applications.

In this paper, we simulate graphene on the surface of substrate, using interconnection metallization to connect merge pin schottky (MPS) power diode chip and substrate, then build a thermal model of the device, and perform the finite-element analysis with ANSYS software. In recent years, graphene has attracted a large amount of interest because of its outstanding electric conductivity, ultrahigh thermal conductivity and large specific surface area. Therefore, if graphene films are used as heat sink, heat will diffuse through graphene surface at breakneck speeds, solving the heat accumulation problem of the device. In this work, we simulate the heat dissipation effect of the device with graphene films or without graphene films. Then, we change the thickness of graphene films, contact area between graphene films and heat dissipation substrate, and contact area between interconnection metallization and graphene films.

In this paper, the analysis of transient thermal resistance is carried out on the basis of the regularization smoothing method and the Fourier series expansion method. These two methods are used to smooth cooling curve under the hypothetical model, and their result of smoothing and derivation are contrasted to the move average smoothing method under the actual model measured by T3ster. From theoretical model analysis and actual measured data analysis and application, it can be seen that the methods proposed have good effect on the analysis of transient thermal resistance.

In this paper, multi-chip LED modules with aluminum nitride (AlN), Al and aluminum oxide (Al2O3) based substrates were successfully designed, fabricated and investigated. Finite element method (FEM) and electrical test method were used to evaluate the thermal performance of LED modules. Both simulation and experimental results show that the module with AlN-based substrate exhibits better thermal performances than the two others. Moreover, AlN-based substrate LED module shows the best optical performances. The optical performances of the LED modules with different substrates not only verify that the optical output and degradation of LED has a direct relation with the input current, but also show that the degradation could begin earlier if the thermal dissipation is not managed well.

In this paper, enhanced photolithography with Al film insertion for large-scale patterning of chemical-vapor-deposited graphene is proposed and demonstrated. An Al film was evaporated onto the surface of graphene, and then traditional photolithography and wet etching were used to make an Al mask. After Ar plasma treatment and removal of the Al mask, patterned graphene was obtained on the target substrate. Graphene patterns fabricated by conventional photolithography were utilized as reference samples. Compared to conventional patterned graphene, the graphene patterns exhibited obviously less residual photoresist (PR), high resolution, and can be applied to various shapes and sizes. It is worth mentioning that the pitch can reach the limit of photomasking utilized in this study. In addition, the graphene also shows a relatively lower work function (~4.2 eV), better photoelectric properties (sheet resistance ~531 Ω/sq, T550 = 94.2%), and outstanding surface hydrophilicity (water contact angle 49°). It is anticipated that graphene patterns realized by the proposed approach could make the application of graphene more promising in emerging and future optoelectronic devices.

In this letter, a theoretical degradation model of LEDs at dynamic working conditions was proposed. The Arrhenius effective temperature (AET) was used to describe the effect of dynamic working temperature. Utilizing the concept of AET, the lifetime prediction of LEDs at dynamic working conditions can be carried out based on the data base at steady state. It was found that the activation energy has a great effect on the numerical value of AET. The extraction of activation energy from steady state measurement made the degradation model available to predict the lifetime of LEDs at various input. The AET was found to be always higher than the arithmetical average temperature. The lifetime of LEDs was found to increase with the frequency and decrease with the duty cycle for the typical dynamic working conditions–pulse input.

In this paper, we present the thermal, electrical, and optical analyses of light emitting diode (LED) packages with different chip sizes. The LED packages under investigation employed the same configuration of package components, except for the chip sizes. The forward current was found to increase with the chip size at the same forward voltage due to the area increase of current spreading. The luminous flux and optical power were found to increase with the chip size at the same current density. The thermal analysis was made by the transient thermal measurement and thermal simulation using the finite volume method. It was demonstrated that the thermal resistance decreased with the chip size under the same package conditions both by simulation and experiment. The bulk thermal resistance and spreading thermal resistance were combined together to give out a quantitative investigation of the partial thermal resistance variation. Moreover, the spreading thermal resistance was found to have a great effect on the total thermal resistance of LED packages.

In this letter, the thermal evaluation of high-power LED packages at pulse conditions was reported. A theoretical calculation model was proposed based on the analogy between the thermal and electrical RC circuits. The thermal performance of LED packages driven by pulse input was calculated using the RC network extracted from transient thermal measurement. The junction temperature fluctuation band decreases with the frequency at certain duty cycles. The saturated average junction temperature rise linearly increases with the duty cycle at certain frequencies. These predictions were verified by the real-time junction temperature measurement using the peak shift method at pulse conditions. The theoretical model was found to be effective and applicable to the evaluation of the thermal performance of LEDs working at pulse conditions.

In this paper, we present the thermal and mechanical analysis of high-power light-emitting diodes (LEDs) with ceramic packages. Transient thermal measurements and thermomechanical simulations were performed to study the thermal and mechanical characteristics of ceramic packages. Thermal resistances from the junction to the ambient were decreased from 79.6 to 46.7degC/W by replacing the plastic mold with a ceramic mold for LED packages. Thermomechanical stress induced in the heat-block test was simulated using a finite-element method. Higher level of thermomechanical stress in the chip was found for LEDs with ceramic packages, despite less mismatching coefficients of thermal expansion, compared with that with plastic packages. The thermomechanical-stress components in the direction of the thickness were found to be larger than that in other two directions. The results suggest that the thermal performance of LEDs can be improved by using ceramic packages, but the mounting process of the high-power LEDs with ceramic packages is critically important and should be the reason for causing delaminating interface layers in the packages.

Degrading characteristics of InGaN/GaN-based blue light-emitting diodes (LEDs) in the optical increase stage (stage I) were investigated. The LEDs were under an ageing with dc current of different levels. Different ageing phenomena were observed for both the electrical and the optical characteristics. The tunnelling components of the forward current were found to change with different trends for the samples aged under current stresses of 100 mA and 1000 mA, respectively. The former decreased monotonically, while the latter decreased in the early period, and then kept increasing. Different ageing rates but the same trends were observed for the optical degradation of the samples aged under different stresses. The ageing factors induced by the dopant activation and the change of defects competed with each other in the ageing process and were proposed to be responsible for the ageing characteristics.