Maofeng Guo

Study of high CRI white light-emitting diode devices with multi-chromatic phosphor

In this paper, we investigated the connection between the employment of multi-chromatic phosphor and CRI value for WLED devices by simulation, using the excitation and emission spectra of phosphor and surface source property of LED chips acquired from experiments. CRI value was examined in the range of 56.8-76.0 due to the change of phosphor concentration, for traditional blue-pumped yellow phosphor. When red phosphor was added, it was found that as red contents increased, red shift occurred in CIE, and CRI value was enhanced only within a limited range. The highest enhancement in such case was 13.8% for blue-pumped yellow phosphor, and when more multi-chromatic phosphor such as red, yellow, green was mixed, the value of CRI enhancement was 19.2% higher than that of dichromatic LED. Ray tracing simulation revealed that multi-chromatic phosphor also had an impact on luminous efficacy and color temperature for high-power WLED devices. It was also showed in our simulation that CRI value increased with the increase of total phosphor concentration, up until the point of an optimum concentration where CRI value started to decrease. Other parameters such as quantum efficiency and molar absorbance index also contributed to white-LED devices performance. Such simulation results are useful to design the optimum phosphor mixture concentration and are helpful to fabricate high CRI blue-pumped or ultraviolet-pumped WLED devices with the best multi-chromatic phosphor proportion.

Defect-induced color-tunable monolithic GaN-based light-emitting diodes

We have demonstrated defect-induced color-tunable monolithic GaN-based vertical light-emitting diodes (VLEDs). With Ag nanorod arrays embedded in p-GaN, large numbers of Ga vacancies (VGa) were produced during the thermal bonding process in VLED fabrication. VGa-related donor–acceptor pair (DAP) transitions in p-GaN resulted in red emission in photoluminescence (PL) measurements as well as a broad electroluminescence (EL) emission spectrum extending from green to red. In combination with high-emission-efficiency blue InGaN/GaN multiple quantum wells (MQWs), the emission color of VLEDs can be changed from red to white by increasing the injection current.

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.

ACU Enhancement of WLEDs Realized by Multilayer Phosphor With Convex Shape

A new package structure with three convex phosphor layers is shown to enhance the angular color uniformity based on modeling and experimental verification. The color uniformity has been improved by tailoring the phosphor concentration distribution of the three layers. The experimental measurements indicate that the deviation of angular correlated color temperature (CCT) can be reduced from 152.9, 487.2, and 586.4 K to 51.3, 55.1, and 78.7 K compared with the conventional structure at the average CCT of 4500, 5500, and 6200 K, respectively. Luminous flux and optical output power of LED packages with optimized multilayer phosphor have been measured and are reduced no more than 5% compared with the single phosphor layer packages.

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.

Polystyrene-catalytic indium–tin–oxide nanorods grown on green light-emitting diodes for enhancing light extraction

A novel technique for fabricating indium–tin–oxide (ITO) nanorods (NRs) on hexagonal-pyramid-surface green vertical light-emitting diodes (VLEDs) is demonstrated using electron-beam deposition with polystyrene spheres catalysis. The ITO NRs have high optical transmittance (>90%) in green wavelength and good crystal quality with a cubic structure. The VLED with ITO NRs has a 31% enhancement of light output power at 200 mA, compared with those without ITO NRs. Finite-difference time-domain simulations suggest that the power enhancement is attributed to the gradient refractive indices of the ITO NRs, and that the light extraction enhancement is caused by changes in ITO NR heights.

Modulating dual-wavelength multiple quantum wells in white light emitting diodes to suppress efficiency droop and improve color rendering index

In this paper, gallium nitride (GaN) based white light-emitting diodes (WLEDs) with modulated quantities of blue (In0.15Ga0.85N) quantum wells (QWs) and cyan QWs (In0.18Ga0.82N) in multiple QW (MQW) structures have been investigated numerically and experimentally. It is demonstrated that the optical performance of LEDs is sensitive to the quantities of cyan QWs in dual-wavelength MQW structures. Compared to the LEDs with respective 0, 4, and 8 cyan QWs (12 QWs in total), the optical performance of the sample with 6 cyan QWs is the best. The deterioration of the optical performance in the sample with less (4 pairs) cyan QWs or more (8 pairs) cyan QWs than 6 cyan QWs may be ascribed to weakened reservoir effect or more defects induced. Compared to conventional blue LEDs (12 blue QWs), the sample with 6 cyan QWs could effectively suppress the efficiency droop (the experimental droop ratio decreases from 50.3% to 39.5% at 80 A/cm2) and significantly improve the color rendering index (CRI, increases from 66.4 to 77.0) simultaneously. We attribute the droop suppression to the strengthened reservoir effect and carrier confinement of deeper QWs (higher indium composition) incorporated in the dual-wavelength MQW structures, which lead to the better hole spreading and enhanced radiative recombination. Meanwhile, the remarkable experimental CRI improvement may result from the wider full-width at half-maximum of electroluminescence spectra and higher cyan intensity in WLED chips with dual-wavelength MQW structures.

Surface plasmon enhanced green light emitting diodes with silver nanorod arrays embedded in p-GaN

We demonstrated surface-plasmon (SP) enhanced green light-emitting diodes (LEDs). Three types of Ag nanorod arrays with a minimum distance between the quantum well (QW) and Ag of 20, 40, and 55 nm respectively were fabricated on p-GaN layer. Photoluminescence measurements showed ?175% emission enhancement for the 20 nm spacing while almost no enhancement for the 55 nm spacing. Simulation result showed that a localized surface plasmon resonance (LSPR) at a wavelength of ?500 nm generated by Ag nanorod arrays induced InGaN/GaN QW and SP coupling. However, the electrical field of the LSPR generated by Ag nanorods only spread ?40 nm in the vertical direction in GaN. This simulation result well explains the observation of SP-QW coupling emission enhancement for 20 nm spacing between Ag and QW, and the lack of enhancement for the 55 nm spacing samples.

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.