Lianqiao Yang

Study on the reliability of high-power LEDs under temperature cycle

High-power LEDs have been widely used and have a big potential market due to the many advantages compared to conventional light sources. Reliability is one of the most important issues for the application of LEDs. In order to study the degradation mechanism and the stability of LED withstanding cyclical exposures to temperature variations, life test was carried out under temperature cycle testing. The experiment was designed basing on the EIAJ ED-4701/100. Four groups of 1 W high power LEDs adopting different material and structures were utilized. The temperature cycle accelerated life test under the condition of −25°C/125°C. After the accelerated test, the optical performance of the four kinds of devices were compared and analyzed, and the thermal performance of the LEDs was also tested, analyzed and discussed. At last, the reliability differences of high-power LEDs were analyzed and degradation mechanism of high power LEDs under temperature cycling was discussed.

Transport Phenomena in a Novel Large MOCVD Reactor for Epitaxial Growth of GaN Thin Films

In this paper, a novel super large metal organic chemical vapor deposition reactor used for epitaxial growth of gallium nitride (GaN) thin film is designed and characterized. In this novel design, three layers of circular slits located on the periphery of reactor are used as the inlets. The outlet is located at the center of the reactor. In this design, the converging effects of gas flow in the radial direction could counterbalance the depletion of metal organic source during the GaN deposition. Therefore, uniform film thickness could be obtained. Besides, the spray board with circular slits located between top and middle inlets is used to help obtain the uniform flow distribution and sharp temperature gradient. The numerical simulation results show that even if in so large reactor (with the diameter of bigger than 1 m) all the field distributions are in an acceptable range in low vacuum degree. Controllable trimethylgallium distribution could be obtained by changing of the flow velocity and mass fractions of various species.

Effects of die-attach materials on the optical durability and thermal performances of HP-LED

Die interconnection layer is one of the main factors to improve the HP-LED thermal issue as it is the main thermal transfer media from the HP-LED chip to the substrate or heat sink. Silver epoxy is now widely used for LED interconnection, but it becomes more and more difficult for efficient heat dissipation as the HP-LED electric power density becomes higher and higher. Nowadays more and more attentions have been moved to metallic eutectic interconnection, especially for solder paste and AuSn20 eutectic interconnection which have big potential to be used for HP-LED. In order to further understand the thermal characteristics and reliability of HP-LED interconnected by different die-attach materials, the AuSn20 eutectic, solder paste and silver epoxy were applied to HP-LED respectively. The transient thermal resistance, steady state junction temperature and optical durability of the HP-LEDs are tested and studied. For HP-LED interconnected by AuSn20 eutectic, solder paste and silver epoxy, when applied 450 mA working current, the transient thermal resistance of die attach layer are 2.5 K/W, 3.2 K/W and 4 K/W respectively, and the junction temperature variation are 0.4%, 9.6% and 6.6% respectively after about 1180 hours 85 °C/85 H aging. The results show the AuSn20 eutectic has better thermal performances and optical durability than the solder paste and silver epoxy.

A simple and efficient approach to fabricate graphene/CNT hybrid transparent conductive films

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.

Highly transparent conductive films fabricated by combining CVD-grown graphene and silver nanowire

ABSTRACT 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 coating AgNW network. It has been demostrated that concentration, diameter and spin speed of AgNW are three effective approaches to adjust sheet resistance, optical transmittance and surface mophology. The graphene/AgNW hybrid TCFs exhibited excellent optical and electrical properties as well as good surface mophology because of the uniform AgNW network coated on the substrate. We anticipate that the highly conductive transparent graphene/AgNW hybrid TCFs have huge potential in high-performance emerging optoelectronic devices such as OLEDs, LCDs, and OSCs.

Study of light emitting diodes for the application of plant growth in green house

In this paper, the reliability issue of the LED lamp in high temperature and moisture condition was investigated. Thermal stress and hygroscopic stress in the LED package were obtained by coupled-field analysis using Finite element analysis (FEA). The results demonstrate that the temperature of the module can reach saturation in tens of seconds while the moisture field needs longer to reach balance. 85°C/85RH can reach the same distribution of relative humidity with 65°C/65RH, but the time spent is half. So temperature is the important factor to affect the humidity distribution. The moisture concentration is better than the relative humidity response to the material moisture level. So we made further research on hygroscopic stress. Both the thermal stress and hygroscopic stress contributed to the delaminations in LED packages.

Study of GaN-based light-emitting diodes with double roughened surfaces

In this paper, GaN-based blue light emitting diodes with double roughened surfaces were fabricated and compared with planar LEDs and these with single roughened layer. The double roughened surfaces were fulfilled by patterned sapphire substrates (PSS) and roughened p-GaN surface. Magnesium treatment during the growth process was used to produce p-GaN surface roughening. After the epitaxial growth, the four group samples were fabricated using the standard process (four mask steps) with a mesa area of 10×23 mil2. At last, 25 LED devices were packaged for each group using the same packaging material and by the same method. It was found that import of PSS and roughened p-GaN surface may lead to a little higher voltage by simply copying the epitaxial process of planar LED. However, even fabricated by the non-optimized epitaxial process, comparing with planar LEDs, the optical power of the package can be increased by 40.1% by using the patterned sapphire substrate, 10.2% by using p-GaN surface roughing, and 44.9% by using double roughing technologies.

Oxygen and water barrier performance of the composite thin film of graphene and polydimethylsiloxane (PDMS)

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.

Study on organic/inorganic hybrid light emitting diode

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.

Improved performance of graphene by effectively removing surface poly-methyl methacrylate residual during the process of wet-etching transfer

ABSTRACT 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. The UV/Ozone irradiation with appropriate time can effectively remove the PMMA residual, resulting in a performance improvement of graphene, including the surface morphology, carrier concentration, and sheet resistance. The work function of graphene could be tuned according to treatment time without loss to the transmittance of graphene. Furthermore, we have fabricated graphene-based OPVs with a power conversion efficiency of 3.33%, which increased by 18.7% compared to the OPV without graphene modification.