Yukun Zhao

Phonon boundary scattering effect on thermal conductivity of thin films

In the study, we introduced the local mean free path of phonons with boundary effects. The local thermal conductivity distribution from boundary to film bulk region was obtained, and the boundary scattering effect was examined by introducing a phonon Knudsen layer thickness. We calculated the ratio of effective thermal conductivity to the bulk one and the results are in agreement with available data.

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.

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.

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.

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.

Metamaterial study of quasi-three-dimensional bowtie nanoantennas at visible wavelengths

In this paper, a novel array of quasi-three-dimensional (quasi-3D) bowtie nanoantennas has been investigated numerically and experimentally. A low-cost and facile method has been designed and implemented to fabricate the quasi-3D bowtie nanoantennas. The fabrication processes containing laser patterning and wet etching have demonstrated the advantages of easily tuning the periodic and diameter of microhole arrays. According to the simulated results, the electric and magnetic resonances at visible wavelengths are obtained in the tips and contours of the metamaterials made of the quasi-3D bowtie nanoantennas, respectively. The effects of the size and gap of quasi-3D bowtie nanoantennas on the array performance have also been studied. The underlying mechanism suggests that different electric and magnetic resonant ranges of the metamaterials could contribute to the broad resonant range for the monolithic metamaterials.

Mechanism of suppressing efficiency droop by modulating the thickness of quantum barriers in light-emitting diodes

In this paper, modulating the thickness of quantum barriers (QBs) in gallium nitride (GaN) based light-emitting diodes (LEDs) has been investigated numerically and experimentally. To reveal the underlying mechanism of efficiency droop for LEDs with different QB thicknesses, the dual-wavelength active regions were specially designed. Compare to LEDs with 18-nm-thick QBs and 9-nm-thick QBs, the efficiency droop ratio of the LED with 12-nm-thick QBs is the lowest at the high current density of 80 A/cm2, which indicates that LEDs with medium QB thickness are the best for suppressing efficiency droop. The measured external quantum efficiency (EQE) of the LED with 18-nm-thick QBs is the highest at the low current density of 5 A/cm2, which may result from the lowest leakage current. However, due to the higher hole injection level, the measured EQE results of the LED with 12-nm-thick QBs are higher than the LED with 18-nm-thick QBs at 80 A/cm2.