Visible light communication (VLC), which utilizes LEDs, promises superior privacy and security and reduced impact on surrounding electronics compared to traditional Wi-Fi. This paper explores the potential of InGaN-based micro-light-emitting diodes (micro-LEDs) in high-speed VLC applications, focusing on yellow-green micro-LEDs with nanoporous distributed Bragg reflector (NP-DBR) and red InGaN micro-LEDs. Yellow-green micro-LEDs achieved a maximum external quantum efficiency (EQE) of 8.7%, bandwidth of 442 MHz, and data rate of 800 Mbit/s, while red micro-LEDs demonstrated an EQE of 5.95%, maximum bandwidth of 424 MHz, and data rate of 800 Mbit/s. The application of four core technologies, including circular devices and electrodes, reduced contact electrode area, atomic layer deposition (ALD) for passivation protection, and multi-chip parallel arrays, enhanced optoelectronic characteristics. This paper also highlights the superior performance of InGaN-based red micro-LEDs with a single quantum well (SQW) structure over double quantum wells (DQWs) for VLC applications. The SQW structure yielded higher maximum EQE, modulation bandwidth, and faster transmission rates, paving the way for the potential of full-color micro-display and high-speed VLC applications.
In this study, we review our progress in micro/mini LED arrays and their numerical simulation in optical fields. The variation in substrate thickness will be included in the calculation and the results can provide detailed insight for the optimization of the micro/mini LED structures.
This article presents a chip-scale package (CSP) with conformal and uniform structures for white light-emitting diodes used in lighting and backlight unit (BLU) applications. The CSP structures produce higher light extraction efficiency and lower assembly-dependent packaging compared with conventional surface-mounted devices (SMDs). Simulation results show that compared with SMDs, the luminous efficiency of CSP structures is 8.81% higher in lighting applications and 9.43% higher in BLU applications. This is likely due to light loss in the light bowl of the SMDs. Moreover, CSPs with a conformal phosphor structure exhibit low assembly dependence and redundancy, and rb-CSPs with a conformal structure are a more effective light source in both lighting and BLU applications.
A flexible large area lighting devices have been demonstrated by PDMS films. The (polydimethylsiloxane) PDMS films doped with organic/inorganic materials. The PDMS film is favorable due to its heat stability, good transparency, and flexibility. This study aimed to combine both organic and inorganic materials for flexible large area lighting applications. The architecture consists of blue LEDs coupled to a leaky waveguide that is covered with the PDMS film. The white light was generated with the poly (9, 9-dioctylfluorene-co-benzothiadiazole)F8BT blended into the PDMS slurry. Organic wavelength conversion materials were chosen owing to their ability to decompose in nature. The more conventional inorganic phosphors such as YAG are difficult to decompose and may present environmental issues which can bring concerns in many lighting applications. These flexible PDMS films had thicknesses of 100μm, 440μm, and 980μm. The resulting white light devices had color temperatures of 8944K, 4863K, and 4429K, respectively. In this study, we have also compared the performance of the organic versus conventional YAG phosphor embedded films.
This paper presents a “hybrid” structure for the coating of yellow YAG∶Ce3+ phosphor on blue GaN-based light-emitting diodes (LEDs). The luminous efficiency of the hybrid phosphor structure improved by 5.9% and 11.7%, compared with the conventional remote and conformal phosphor structures, respectively, because of the increased intensity of the yellow component. The hybrid structure also has an advantage in the phosphor usage reduction for the LEDs. Furthermore, the electric intensity of the hybrid phosphor structure was calculated for various thicknesses by conducting TFCalc32 simulation, and the enhanced utilization of blue rays was verified. Finally, the experimental results were consistent with the simulation results performed using the Monte-Carlo method.
We use thinner-quantum well to improve the droop behavior of GaN-base light emitting diode in simulation. Taking the advantage of that the thin quantum well will saturate easily, this characteristic of thin well will improve carrier distribution. Furthermore, this structure has more wave-function overlap than that of the thick well. This simulation result showed that decreasing the well thickness in specific position will not only improve the holes transport but also increase the quantum efficiency at high current density in the active region, and the efficiency droop behavior can be effectively suppressed. In this research, we designed three thin well structures by inserting different numbers of thin wells in the active region. We have compared them to the conventional LEDs, for which, the well thickness of 2.5 nm is used. The thin well structures have better droop behavior than conventional LED.
In recent year, InGaN-based alloy was also considered for photovoltaic devices owing to the distinctive material properties which are benefit photovoltaic performance. However, the Indium tin oxide (ITO) layer on top, which plays a role of transparent conductive oxide (TCO), can absorb UV photons without generating photocurrent. Also, the thin absorber layer in the device, which is consequent result after compromising with limited crystal quality, has caused insufficient light absorption. In this report, we propose an approach for solving these problems. A hybrid design of InGaN/GaN multiple quantum wells (MQWs) solar cells combined with colloidal CdS quantum dots (QDs) and back side distributed Bragg reflectors (DBRs) has been demonstrated. CdS QDs provide down-conversion effect at UV regime to avoid absorption of ITO. Moreover, CdS QDs also exhibit anti-reflective feature. DBRs at the back side have effectively reflected the light back into the absorber layer. CdS QDs enhance the external quantum efficiency (EQE) for light with wavelength shorter than 400 nm, while DBRs provide a broad band enhancement in EQE, especially within the region of 400 nm ~ 430 nm in wavelength. CdS QDs effectively achieved a power conversion efficiency enhancement as high as 7.2% compared to the device without assistance of CdS QDs. With the participation of DBRs, the power conversion efficiency enhancement has been further boosted to 14%. We believe that the hybrid design of InGaN/GaN MQWs solar cells with QDs and DBRs can be a method for high efficiency InGaN/GaN MQWs solar cells.
We demonstrate the GaAs solar cells which utilize the high-transmittance textured polydimethylsiloxane (PDMS) film can outstanding increase the short circuit current density and power conversion efficiency of solar cells. The transmittance of PDMS film is exceeded 90%, which can pass through almost all the light of GaAs Solar cells can be absorbed. We used a special imprint technology to let the PDMS film possess a highly textured surface. Then we measured the characteristics of textured PDMS film and found out that it has a very excellent Haze performance. The effect of flexible textured PDMS film on the suppression of surface reflection in GaAs solar cells is also investigated. The presented technology provides an inexpensive surface anti-reflection process, which can potentially replace typically complex anti-reflection coating (ARC) layer. The GaAs solar cells with textured PDMS layer can effectively enhance the short-circuit current density from 22.91 to 26.54 mA/cm2 and the power conversion efficiency from 18.28 to 21.43 %, corresponding to a 17 % enhancement compared to the one without textured PDMS. The open-circuit voltage (Voc) and the fill-factor (FF) of GaAs solar cells exhibit negligible change, because the textured PDMS film was pasted up on the surface of GaAs solar cells and did not interfere with the diode operation. At the same time, we observed through the EQE measurement that the textured PDMS film not only proved wonderful light scattering effect but also generated more electron-hole pairs in all absorption spectrum range. Finally, through this simple PDMS process, we believe this technology shall be a great candidate for next generation of highly efficient and low-cost photovoltaic devices.
We fabricated the colloidal quantum-dot light-emitting diodes (QDLEDs) with the HfO2/SiO2-distributed Bragg reflector
(DBR) structure using a pulsed spray coating method. Moreover, pixelated RGB arrays, 2-in. wafer-scale white light
emission, and an integrated small footprint white light device were demonstrated. The experimental results showed that
the intensity of red, blue, and green (RGB) emissions exhibited considerable enhancement because of the high
reflectivity in the UV region by the DBR structure, which subsequently increased the use in the UV optical pumping of
RGB QDs. In this experiment, a pulsed spray coating method was crucial in providing uniform RGB layers, and the
polydimethylsiloxane (PDMS) film was used as the interface layer between each RGB color to avoid crosscontamination
and self-assembly of QDs. Furthermore, the chromaticity coordinates of QDLEDs with the DBR structure
remained constant under various pumping powers in the large area sample, whereas a larger shift toward high color
temperatures was observed in the integrated device. The resulting color gamut of the proposed QDLEDs covered an area
1.2 times larger than that of the NTSC standard, which is favorable for the next generation of high-quality display
technology.
This study explores the optical field distribution of 1.55μm InGaAsP distributed feedback Laser with an air gap in the
middle section. The optical field distribution was analyzed by different depth and width of an air gap. From the
calculation, we could observe how the gap affect the coupling of the optical field into the other cavity. The percentage of
the coupling is a crucial factor to the injection-locking operation. Both effective index model and commercial software
were used to predict this coupling.
KEYWORDS: Scattering, Air contamination, Mie scattering, Light scattering, Thin film solar cells, Thin films, Solar cells, Absorption, Particles, Glasses
Light trapping techniques such as textured interfaces and highly reflective back contacts are important to thin-film solar
cells. Scattering at rough interfaces inside a solar cell leads to enhanced absorption due to an increased optical path
length in the active layers, which is generally characterized by a haze ratio. In this work, we demonstrate the measured
haze characteristics of indium tin oxide nano-whiskers deposited on an ITO-coated glass substrate. A theoretical model
based on a modified Mie theory is also employed to analyze the scattering effects of nano-whiskers. Instead of spherical
model, a cylindrical condition is imposed to better fit the shapes of the whiskers. The calculated haze-ratio of an ITO
whisker layer matches the measurement closely.
Improvement of efficiency for crystalline silicon (c-Si) with nanopillar arrays (NPAs) solar cell was demonstrated by
deployment of CdS quantum dots (QDs). The NPAs was fabricated by colloidal lithography of self-assembled
polystyrene (PS) nanospheres with a 600 nm in size and reactive-ion etching techniques, and then a colloidal CdS QDs
with a concentration of 5 mg/mL was spun on the surface of c-Si with NPAs solar cell. Under a simulated one-sun
condition, the device with CdS QDs shows a 33% improvement of power conversion efficiency, compared with the one
without QDs. Additionally, we also found that the device with CdS QDs shows a 32% reduction in electrical resistance,
compared with the one without QDs solar cell, under an ultraviolet (UV) light of 355nm illumination. This reduced
electrical resistance can directly contribute to our fill-factor (FF) enhancement. For further investigation, the excitation
spectrum of photoluminescence (PL), absorbance spectrum, current-voltage (I-V) characteristics, reflectance and
external quantum efficiency (EQE) of the device were measured and analyzed. Based on the spectral response and
optical measurement, we believe that CdS QDs not only have the capability for photon down-conversion in ultraviolet
region, but also provide extra antireflection capability.
Highly efficient InGaN-base light emitting diodes are crucial for next generation solid state lighting. However,
drawbacks in substrate materials such as lattice and thermal expansion coefficient mismatches hold back the lamination
efficiency improvement. In the past, patterned sapphire sustrate (PSS) has been proven to be effect to enhance the LED's
performance. In this work, we reviewed several promising nano-scale technologies which successfully increase the
output of LED through better material quality and light extraction. First, we presented a study of high-performance blue
emission GaN LEDs using GaN nanopillars (NPs). It exhibits smaller blue shift in electroluminescent peak wavelength
and great enhancement of the light output (70% at 20 mA) compared with the conventional LEDs. Secondly, GaN based
LEDs with nano-hole patterned sapphire (NHPSS) by nano-imprint lithography are fabricated structure grown on
sapphire substrate. At an injection current of 20mA, the LED with NHPSS increased the light output power of LEDs by
1.33 times, and the wall-plug efficiency is 30% higher at 20mA indicating that it had larger light extraction efficiency
(LEE). Finally, we fabricated the high performance electrical pumping GaN-based semipolar {10-11} nano-pyramid
LEDs on c-plane sapphire substrate by selective area epitaxy (SAE). The emission wavelength only blue-shifted about
5nm as we increased the forward current from 40 to 200mA, and the quantum confine stark effect (QCSE) had been
remarkably suppressed on semipolar surface at long emission wavelength region. These results manifest the promising
role of novel nanotechnology in the future III-nitride light emitters.
We investigate the optical and electrical characteristics of the GaN-based light emitting diodes (LEDs)
grown on Micro and Nano-scale Patterned silicon substrate (MPLEDs and NPLEDs). The transmission
electron microscopy (TEM) images reveal the suppression of threading dislocation density in
InGaN/GaN structure on nano-pattern substrate due to nanoscale epitaxial lateral overgrowth (NELOG).
The plan-view and cross-section cathodoluminescence (CL) mappings show less defective and more
homogeneous active quantum well region growth on nano-porous substrates. From temperature
dependent photoluminescence (PL) and low temperature time-resolved photoluminescence (TRPL)
measurement, NPLEDs has better carrier confinement and higher radiative recombination rate than
MPLEDs. In terms of device performance, NPLEDs exhibits smaller electroluminescence (EL) peak
wavelength blue shift, lower reverse leakage current and decreases efficiency droop compared with the
MPLEDs. These results suggest the feasibility of using NPSi for the growth of high quality and power
LEDs on Si substrates.
Free-space smart-pixel optical interconnect architecture promises to relive the interconnect bottleneck in high-speed parallel interconnection and switching systems. Vertical cavity Fabry-Perot multiple quantum well modulators, which offers high on-off contrast, low insertion loss, low operating voltage swing, low chirp and easy to be integrated with silicon electronics ins one of the most promising devices. A low chirp vertical cavity Fabry-Perot modulator where the top and bottom mirrors are made of distributed Bragg reflector , the undoped regions between top mirrors and intracavity contact region contain 26 GaAs/Al0.35Ga0.65As quantum wells. By applying a voltage across the quantum well regions through intracavity contacts, the absorptive characteristics of the active region at the cavity's resonance wavelength can be modified through the quantum-confined Stark effect. The device under test is integrated with microwave probe for efficient wafer level GHZ probing without further high frequency packaging. To minimize the parasitic capacitance of the probe pads, the conducting region underneath probe pads is etched away and planarized with low dielectric material-electronics resins BCB. In this paper, we report the low switching voltage with only 3.5V, high contrast ratio of 10:1 and high bandwidth of 16GHz. The effects of incident laser power on the contrast ratio and modulation bandwidth were also discussed.
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