Inhibition of Photoconversion Activity in Self-Assembled ZnO-Graphene Quantum Dots Aggregated by 4-Aminophenol Used as a Linker.

The aggregation of zinc oxide nanoparticles leads to an increased absorbance in the ultraviolet-visible region by an induced light scattering effect. Herein, we demonstrate the inhibition of photoconversion activity in ZnO-graphene core-shell quantum dots (QD) (ZGQDs) agglomerated by 4-aminophenol (4-AP) used as a linker. The ZnO-graphene quantum dots (QD) aggregates (ZGAs) were synthesized using a facile solvothermal process. The ZGAs revealed an increased absorbance in the wavelengths between 350 and 750 nm as compared with the ZGQDs. Against expectation, the calculated average photoluminescence lifetime of ZGAs was 7.37 ns, which was 4.65 ns longer than that of ZGQDs and was mainly due to the high contribution of a slow (τ2, τ3) component by trapped carriers in the functional groups of graphene shells and 4-AP. The photoelectrochemical (PEC) cells and photodetectors (PDs) were fabricated to investigate the influence of ZGAs on the photoconversion activity. The photocurrent density of PEC cells with ZGAs was obtained as 0.04 mA/cm2 at 0.6 V, which was approximately 3.25 times lower than that of the ZGQDs. The rate constant value of the photodegradation value of rhodamine B was also decreased by around 1.4 times. Furthermore, the photoresponsivity of the PDs with ZGAs (1.54 µA·mW-1) was about 2.5 times as low as that of the PDs with ZGQDs (3.85 µA·mW-1). Consequently, it suggests that the device performances could be degraded by the inhibition phenomenon of the photoconversion activity in the ZGAs due to an increase of trap sites.

Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern.

The future of solid-state lighting relies on how the performance parameters will be improved further for developing high-brightness light-emitting diodes. Eventually, heat removal is becoming a crucial issue because the requirement of high brightness necessitates high-operating current densities that would trigger more joule heating. Here we demonstrate that the embedded graphene oxide in a gallium nitride light-emitting diode alleviates the self-heating issues by virtue of its heat-spreading ability and reducing the thermal boundary resistance. The fabrication process involves the generation of scalable graphene oxide microscale patterns on a sapphire substrate, followed by its thermal reduction and epitaxial lateral overgrowth of gallium nitride in a metal-organic chemical vapour deposition system under one-step process. The device with embedded graphene oxide outperforms its conventional counterpart by emitting bright light with relatively low-junction temperature and thermal resistance. This facile strategy may enable integration of large-scale graphene into practical devices for effective heat removal.

High performance of InGaN light-emitting diodes by air-gap/GaN distributed Bragg reflectors.

The effect of air-gap/GaN DBR structure, fabricated by selective lateral wet-etching, on InGaN light-emitting diodes (LEDs) is investigated. The air-gap/GaN DBR structures in LED acts as a light reflector, and thereby improve the light output power due to the redirection of light into escape cones on both front and back sides of the LED. At an injection current of 20 mA, the enhancement in the radiometric power as high as 1.91 times as compared to a conventional LED having no DBR structure and a far-field angle as low as 128.2° are realized with air-gap/GaN DBR structures.

Improving the optical performance of InGaN light-emitting diodes by altering light reflection and refraction with triangular air prism arrays.

The effect of triangular air prism (TAP) arrays with different distance-to-width (d/w) ratios on the enhancement of light extraction efficiency (LEE) of InGaN light-emitting diodes (LEDs) is investigated. The TAP arrays embedded at the sapphire/GaN interface act as light reflectors and refractors, and thereby improve the light output power due to the redirection of light into escape cones on both the front and back sides of the LED. Enhancement in radiometric power as high as 117% and far-field angle as low as 129° are realized with a compact arrangement of TAP arrays compared with that of a conventional LED made without TAP arrays under an injection current of 20 mA.

Enhanced light emission in blue light-emitting diodes by multiple Mie scattering from embedded silica nanosphere stacking layers.

We demonstrate enhanced light emission in blue light-emitting diodes (LEDs) by multiple Mie scattering from embedded silica nanosphere stacking layers (SNSL). A honeycomb cone structure is introduced in the GaN epilayer to confine a maximum number of silica nanospheres (SNs). We found that the light is predominantly directed vertically by scattering and geometrical effect in SNSL embedded LEDs. Consequently, the light output power is enhanced by 2.7 times, which we attribute to the improvement in light extraction efficiency due to the multiple Mie scattering of light from the embedded SNSL. The experimental results are verified by simulation using finite difference time domain method (FDTD).

Effect of embedded silica nanospheres on improving the performance of InGaN/GaN light-emitting diodes.

We report on the effect of embedded silica nanospheres on improving the performance of InGaN/GaN light-emitting diodes (LEDs). The silica nanospehres were coated on the selectively etched GaN using a spin-coating method. With the embedded silica nanospheres structures, we achieved a smaller reverse leakage current due to the selective defect blocking-induced crystal quality improvement. Moreover, the reflectance spectra show strong reflectance modulations due to the different refractive indices between the GaN and silica nanospheres. By using confocal scanning electroluminescence microscopy, a strong light emission from silica nanospheres demonstrates that the silica nanospheres acted as a reflector. We found that the optimized embedded silica nanospheres structure, whose the average size of the etched pits was about 3.5 µm and EPD was 3 x 10(7) cm(-2), could enhance light output power by a factor of 2.23 due to enhanced the probability of light scattering at silica nanospheres.

Comparison of various surface textured layer in InGaN LEDs for high light extraction efficiency.

The various surface texturing effects of InGaN light emitting diodes (LEDs) have been investigated by comparison of experimented data and simulated data. The single-layer and double-layer texturing were performed with the help of ITO nanospheres using wet etching, where the ITO ohmic contact layer and the p-GaN layer are textured using ITO nanospheres as an etch mask. In case of single-layer texturing, p-type GaN layer texturing was more effective than ITO ohmic contact layer texturing. The maximum enhancement of wall-plug efficiency of double-layered textured LEDs is 40% more than conventional LEDs, after packaging at an injected current of 20 mA. The increase of light scattering at the textured GaN surfaces is a major reason for increasing the light extraction efficiency of LEDs.

Active packing method for blue light-emitting diodes with photosensitive polymerization: formation of self-focusing encapsulates.

A novel light-emitting diode (LED) packaging method, named the active packaging (AP) method, is presented in this paper. In this method, during the LED packaging process, the light emitted from a GaN LED chip itself is employed to package the LED encapsulant, thereby eliminating the need to utilize a mold. Current injection into a bare LED chip, triggers a photosensitive epoxy to polymerize, leading to the formation of mushroom lamp cap on the LED chip. The emission properties of LEDs fabricated by this method, including their emission beam profiles and light outputs, were characterized. The results showed that a self-focusing effect happened with the addition of an epoxy on the chip. The simulation demonstrated that the geometry the encapsulant controlled the beam pattern of emission. Further, the self-focusing effect was believed to be caused by the combination of the threshold energy of epoxy polymerization, the beam pattern and the power output of the LED chip.

White light emitting diodes realized by using an active packaging method with CdSe/ZnS quantum dots dispersed in photosensitive epoxy resins.

White light emitting diodes (LEDs) have been realized using the active packaging (AP) method. The starting materials were bare InGaN LED chips and CdSe/ZnS core-shell quantum dots (QDs) dispersed in photosensitive epoxy resins. Such hybrid LED devices were fabricated using QD mixtures with one ('single'), two ('dual') or four ('multi') emission wavelengths. The AP method allows for convenient adjustment of multiple parameters such as the CIE-1931 coordinate (x, y), color temperature, and color rending index (CRI). All samples show good white balance, and under a 20 mA working current the luminous efficacies of the single, dual, and multi hybrid devices were 8.1 lm W(-1), 5.1 lm W(-1), and 6.4 lm W(-1), respectively. The corresponding quantum efficiencies were 4.1%, 3.1%, and 3.1%; the CRIs were 21.46, 43.76, and 66.20; and the color temperatures were 12 000, 8190, and 7740 K. This shows that the CRI of the samples can be enhanced by broadening the QD emission band, as is exemplified by the 21.46 CRI of the single hybrid LED compared to the 66.20 value for the multi hybrid LED. In addition, we were able to increase the CRI of the single hybrid LED from 15.31 to 32.50 by increasing the working currents from 1 to 50 mA.