Search

In this paper, a facile two-step method was adopted to in-situ develop a flexible electrode with NiCo-LDH nanowires@NiCo-LDH nanosheets core-shell structure on carbon fiber cloth (CFC). The electrochemical properties of the composite materials with different Ni/Co molar ratios nanosheets were systematically studied. It is found that the NiCo-LDH nanowires@Ni2Co1-LDH nanosheets (NiCo@Ni2Co1) composite material achieves a high areal capacitance of 9.67 F/cm2 at 5 mA/cm2 and a good cycling stability (75% capacitance retention after 2000 cycles). The excellent performance benefits from the distinct core-shell structure and the synergistic effect of NiCo-LDH nanosheets and NiCo-LDH nanowires. Moreover, a solid-state flexible asymmetric supercapacitor (ASC) assembled with NiCo@Ni2Co1/CFC as positive electrode and activated carbon on CFC as negative electrode can work stably at a voltage of 1.6 V. This ASC displays a high areal energy density of 298.6 uWh/cm2 at a current density of 1 mA/cm2 and with a corresponding areal power density of 0.80 mW/cm2.

Junction temperature is a critical metric for reliability evaluation of light-emitting diodes (LEDs) as light sources for automotive lamps. While numerous researchers have quantified the junction temperature accurately, uncertainty factors including electrical–thermal–mechanical boundary conditions and the die-attach layer (DAL) microstructure in automotive LEDs have not been addressed; the propagation of these variables degrades the precision of the junction temperature quantification. In this paper, a junction temperature uncertainty quantification framework involving finite element modeling, the evolution separation spectrum, and Monte Carlo simulation is introduced to build 3-D stochastic finite element models for automotive LEDs with stochastic DAL microstructures. Evolution of the statistical junction temperature probability for three types of automotive LEDs subjected to electrical–thermal–mechanical loadings is discussed and the failure risk is predicted. The results show that the temperature-mechanical displacement coupling of the DAL plays a major role in the junction temperature with regard to automotive boundary loadings, and the thermal power load is more likely to cause junction temperature deterioration of LED with DALs having voids. The automotive LED failure rate is dependent upon the boundary conditions, the DAL microstructures, and the LED numbers. The failure rate for LED modules with DALs having voids is higher than that for modules with uniform DALs; the failure rate is higher in aged LED modules; the failure risk at the given junction temperature threshold increases with increasing LED numbers in the module; and when operating at lower junction temperatures, the voids in LED DALs may not increase the module failure risk.

In order to reduce the Schottky barrier height and sheet resistance between graphene (Gr) and the p-GaN layers in GaN-based light-emitting diodes (LEDs), conductive transparent thin films with large work function are required to be inserted between Gr and p-GaN layers. In the present work, three kinds of transparent conductive oxide (TCO) zinc oxide (ZnO) films, Al-, Ga-, and In-doped ZnO (AZO, GZO, and IZO), are introduced as a bridge layer between Gr and p-GaN, respectively. The influence of different combinations of Gr/ZnO hybrid transparent conducting layers (TCLs) on the optical and thermal characteristics of the GaN-LED was investigated by the finite element method through COMSOL software. It is found that both the TCL transmittance and the surface temperature of the LED chip reduce with the increase in Gr and ZnO thickness. In order to get the transmittance of the Gr/ZnO hybrid TCL higher than 80%, the appropriate combination of Gr/ZnO compound electrode should be a single layer of Gr with ZnO no thicker than 400 nm (1L Gr/400-nm ZnO), 2L Gr/300-nm ZnO, 3L Gr/200-nm ZnO, or 4L Gr/100-nm ZnO. The LEDs with hybrid TCLs consisting of 1L Gr/300-nm AZO, 2L Gr/300-nm GZO, and 2L Gr/300-nm IZO have good performance, among which the one with 1L Gr/300-nm GZO has the best thermal property. Typically, the temperature of LEDs with 1L Gr/300-nm GZO hybrid TCLs will drop by about 7 K compared with that of the LEDs with a TCL without ZnO film.

In order to decrease the Schottky barrier height and sheet resistance between graphene (Gr) and the p-GaN layers in GaN-based light-emitting diodes (LEDs), some transparent thin films with good conductivity and large work function are essential to insert into Gr and p-GaN layers. In this work, the ultra-thin films of four metals (silver (Ag), golden (Au), nickel (Ni), platinum (Pt)) are explored to introduce as a bridge layer into Gr and p-GaN, respectively. The effect of a different combination of Gr/metal transparent conductive layers (TCLs) on the electrical, optical, and thermal characteristics of LED was investigated by the finite element methods. It is found that both the TCLs transmittance and the surface temperature of the LED chip reduces with the increase of the metal thickness, and the transmittance decreases to about 80% with the metal thickness increasing to 2 nm. The surface temperature distribution, operation voltage, and optical output power of the LED chips with different metal/Gr combination were calculated and analyzed. Based on the electrical, optical, and thermal performance of LEDs, it is found that 1.5-nm Ag or Ni or Pt, but 1-nm Au combined with 3 layered (L) Gr is the optimal Gr/metal hybrid transparent and current spreading electrode for ultra-violet (UV) or near-UV LEDs.

The thermal-electrical characteristic of a GaN light-emitting diode (LED) with the hybrid transparent conductive layers (TCLs) of graphene (Gr) and NiOx is investigated by a finite element method. It is indicated that the LED with the compound TCL of 3-layer Gr and 1 nm NiOx has the best thermal-electrical performance from the view point of the maximum temperature and the current density deviation of multiple quantum wells, and the maximum temperature occurs near the n-electrode rather than p-electrode. Furthermore, to depress the current crowding on the LED, the electrode pattern parameters including p- and n-electrode length, p-electrode buried depth and the distance of n-electrode to active area are optimized. It is found that either increasing p- or n-electrode length and buried depth or decreasing the distance of n-electrode from the active area will decrease the temperature of the LED, while the increase of the n-electrode length has more prominent effect. Typically, when the n-electrode length increases to 0.8 times of the chip size, the temperature of the GaN LED with the 1 nm NiOx/3-layer-Gr hybrid TCLs could drop about 7 K and the current density uniformity could increase by 23.8%, compared to 0.4 times of the chip size. This new finding will be beneficial for improvement of the thermal-electrical performance of LEDs with various conductive TCLs such as NiOx/Gr or ITO/Gr as current spreading layers.

Finite elements methods are used to investigate the thermal-electrical characteristics of gallium-nitride (GaN) light-emitting diodes (LEDs) with different transparent conductive layers (TCLs) and buried depths of electrodes, where the transparent conductive layers include indium tin oxide (ITO), graphene (Gr) and the combination of them (ITO/Gr). The optimal material parameters and the precision and accuracy of the simulation model are validated. Moreover, the parameters' sensitivity analysis is carried out as well. The results indicate that the LED with the TCL of a 100-nm ITO or 4-layer Gr has a good thermal-electrical performance from the viewpoint of the maximum temperature and the current density deviation of multiple quantum well (MQW), where the maximum temperature occurs at the n-Pad rather than p-Pad. The compound TCL with a 20-nm ITO and 3-layer Gr reaches a thermal-electrical performance better than that of a 100-nm ITO or 4-layer Gr. Moreover, their maximum temperatures decrease about −0.43% and 1.21%, and the current density uniformities increase up to −6.09% and 17.41%, respectively. Furthermore, when the electrode buried depth is 0.51 μm, the thermal-electrical performance of the GaN LEDs can be further improved.

A simple and efficient packaging technique with aluminium-core printed circuit board component embedded copper reflectors is presented for fabricating high-power white light emitting diode (LED) arrays. Three-dimensional finite element simulation and dynamic infrared imaging for analysis of the packaged nine-chips LED array are carried out to investigate the thermal and optical properties. The results show the method, which obtained a luminous efficiency of 126 lm/W and thermal resistance of 3.4 K/W, is suitable for high efficiency and low-cost LED based solid-state lighting.