Lungang Feng

Laser patterning of Y 3 Al 5 O 12 :Ce 3+ ceramic phosphor platelets for enhanced forward light extraction and angular color uniformity of white LEDs.

We present a facile fabrication process to directly fabricate cone-shaped microwells arrays on single crystal Y3Al5O12:Ce3+ (YAG:Ce) ceramic phosphor platelets (CPPs) by short-pulse laser direct patterning. Compared to unpatterned YAG:Ce CPP with smooth surface, the forward-to-total ratio of emission photons of patterned YAG:Ce CPPs was enhanced from 53.2% up to 78.2%, and the total emission within 4-π degree is 6% higher. The fabricated patterns are also beneficial in increasing the color conversion efficiency of YAG:Ce CPPs by 7.6%. The patterned YAG:Ce CPPs display much better correlated color temperature (CCT) uniformity under varied currents. The angular correlated color temperature uniformity (ACU) of patterned YAG:Ce CPPs reaches as high as 0.933 compared to 0.730 of the unpatterned one. These results suggest that laser patterning of YAG:Ce CPP could effectively manipulate its luminance, chromaticity and illumination pattern, which may lead to further technological advancements for diversified applications of film-type CPPs in highly efficient white LEDs.

Mechanism of hole injection enhancement in light-emitting diodes by inserting multiple hole-reservoir layers in electron blocking layer

In this study, gallium nitride (GaN) based light-emitting diodes(LEDs) with single and multiple hole-reservoir layers (HRLs) inserted in the electron-blocking layer (EBL) have been investigated numerically and experimentally. According to simulation results, a better electron confinement and a higher hole injection level can be achieved by the multiple HRLs inserted in the EBL region. To further reveal the underlying mechanism of hole injection enhancement experimentally, the active regions were intentionally designed to emit photons with three different wavelengths of 440 nm, 460 nm, and 480 nm, respectively. Based on the experimental results of photoluminescence(PL) and time-resolved PL(TRPL) measurements conducted at 298 K, the remarkable enhancement (148%) of PL intensities and significant increase in the decay times of the quantum wells close to p-GaN can be obtained. Therefore, the mechanism is proposed that carriers are able to reserve in the EBL region with multiple HRLs for a much longer time. Meanwhile, carriers could diffuse into the active region by tunnelling and/or thermo-electronic effect and then recombine efficiently, leading to the better carrier reservoir effect and higher hole injection in LEDs. As a result, by inserting multiple HRLs in the EBL region instead of single HRL, the experimental external quantum efficiency is enhanced by 19.8%, while the serious droop ratio is markedly suppressed from 37.0% to 27.6% at the high current injection of 100 A/cm2.

Efficiency roll-off suppression in organic light-emitting diodes using size-tunable bimetallic bowtie nanoantennas at high current densities

Size-tunable bimetallic bowtie nanoantennas have been utilized to suppress the efficiency roll-off characteristics in organic light-emitting diodes (OLEDs) using both the numerical and experimental approaches. The resonant range can be widened by the strong dual-atomic couplings in bimetallic bowtie nanoantennas. Compared with the green OLED with conventional bowtie nanoantennas at a high current density of 800 mA/cm2, the measured efficiency roll-off ratio of the OLED with size-modulated bowtie nanoantennas is decreased from 53.2% to 41.8%, and the measured current efficiency is enhanced by 29.9%. When the size-modulated bowtie nanoantennas are utilized in blue phosphorescent OLEDs, the experimental roll-off ratio is suppressed from 43.6% to 25.9% at 250 mA/cm2, and the measured current efficiency is also enhanced significantly. It is proposed that the efficiency roll-off suppression is mainly related to the enhanced localized surface plasmon effect, which leads to a shorter radiative lifetime.

Efficiency roll-off suppression in organic light-emitting diodes at high current densities using gold bowtie nanoantennas

In this study, large-scale gold (Au) bowtie nanoantennas have been utilized to suppress the efficiency roll-off in organic light-emitting diodes (OLEDs) numerically and experimentally. Compared with the OLED without nanoantennas, the experimental roll-off ratio of the OLED with Au bowtie nanoantennas significantly decreases from 59.4 to 51.3% at a high current density of 1000 mA/cm2. We attribute the roll-off suppression to the localized surface plasmon (LSP) effect, which leads to a shorter radiative lifetime. The insufficient coupling between radiated light and LSP resonance could also be improved by a strong resonance coupling between the tips of bowtie nanoantennas.

Air-void embedded GaN-based light-emitting diodes grown on laser drilling patterned sapphire substrates

Air-void structure was introduced in GaN-based blue light-emitting diodes (LED) with one-step growth on periodic laser drilling patterned sapphire substrate, which free of any photolithography or wet/dry etching process. The influence of filling factors (FF) of air-void on crystal quality and optical performance were investigate. Transmission electron microscopy images and micro-Raman spectroscopy indicated that the dislocation was bended and the partially compressed strain was released. When FF was 55.43%, compared with the LED structure grown on flat sapphire substrate, the incorporation of air-void was observed to reduce the compressed stress of ∼20% and the luminance intensity has improved by 128%. Together with the simulated reflection intensity enhancement by finite difference time-domain (FDTD) method, we attribute the enhanced optical performance to the combined contribution of strong back-side light reflection of air-void and better GaN epitaxial quality. This approach provides a simple replacement to the conventional air-void embedded LED process.

Whispering gallery mode lasing from InGaN/GaN quantum well microtube

In this work, we have successfully fabricated microtubes by strain-induced self-rolling of a InGaN/GaN quantum wells nanomembrane. Freestanding quantum wells microtubes, with a diameter of 6 µm and wall thickness of 50 nm, are formed when the coherently strained InGaN/GaN quantum wells heterostructure is selectively released from the hosting substrate. Periodic oscillations due to whispering-gallery modes resonance were found superimposed on photoluminescence spectra even at low optical excitation power. With increasing pumping power density, the microtube is characterized by a stimulated emission with a threshold as low as 415 kW/cm2. Such emission shows predominant TM polarization parallel to the microtube axis.

Improving Peak-Wavelength Method to Measure Junction Temperature by Dual-Wavelength LEDs

In this paper, the improvement of the method measuring the junction temperature of light-emitting diodes (LEDs) has been studied experimentally. A practical method is proposed with only three measurement procedures. With the consideration of indium (In) composition and blue shift, the method has a high applicability, which is practical for the LED chips vary from blue to green chips under different currents, including the packaged chips. On the other hand, according to the experimental and derived results, the junction-temperature difference and peak-wavelength shift in both blue-shift and red-shift fields show similar parabolic-like relations. To simplify the experimental processes, dual-wavelength LEDs were fabricated and measured instead of conventional single-wavelength LEDs.

Growth, Characterization, and Application of Well-defined Separated Gan-based Pyramid Array on Micropatterned Sapphire Substrate

We tried to obtain microstructures on a three-dimensional (3D) micropatterned substrate by laser drilling. The influences of the dimensions of the drilling holes on the morphology and the material quality of the grown structures were studied. Uniform micropyramid arrays with relatively low dislocation density can be achieved by adjusting the laser drilling parameters. The internal quantum efficiency was estimated to be improved by a factor of 3 for a pyramid structure compared with that of planar LEDs. We fabricated 5 × 7 mm2 flexible LEDs employing the pyramid structure and the devices exhibited good flexibility without performance reduction after bending.

Freestanding GaN-based light-emitting diode membranes on Y3Al5O12:Ce3+ crystal phosphor plate for efficient white light emission

GaN-based light-emitting diode (LED) membranes were peeled from the substrate using electrochemical etching of the bottom sacrificial layer. The freestanding membranes were transferred onto a Y3Al5O12:Ce3+ (YAG:Ce3+) crystal phosphor plate to realize a compact white light source. Verified by the Raman test, the initial strain within the original GaN layers was greatly released after the exfoliation process, which induced alleviation of the quantum confined stark effect. The electroluminescence measurement of a blue LED membranes-on-YAG:Ce3+ plate-structured device was conducted exhibiting color coordinates and a correlated color temperature of and 5450 K at 10 mA, respectively.

Super flexible GaN light emitting diodes using microscale pyramid arrays through laser lift-off and dual transfer

We demonstrated a method to obtain super flexible LEDs, based on high quality pyramid arrays grown directly on sapphire substrates. Laser lift-off (LLO) and dual transfer processes were applied to transfer pyramid arrays face up onto the flexible substrates, which is more efficient than back light emission. Ag grid and Ag nanowires were employed as the electrical connection. No significant performance reduction appeared until the device reached a curvature radius of 0.5 mm. The performance reduction results from cracks appearing at the junction of the Ag grid, which can be improved by replacing the Ag grid with a strip electrode.