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Now showing 1 - 9 of 9

The Photothermal Stability Study of Quantum Dots Embedded in Sodium Chlorides
(2020-01-01)
Huang, Yu-Ming
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Hsu, Shun-Chieh
;
Li, Ning
;
Yu, Chung-Ping
;
Ke, Li-Ann
An efficient and useful method for the incorporation of colloidal quantum dots (QDs) into ionic matrices is demonstrated. We prepared three different synthesis methods, which are traditional saturated-salt water, methanol-assisted, and ethanol-assisted methods. The continuous thermal and photonic stress tests indicate that the high temperature, instead of photonic excitation stress, is more detrimental to the illumination capability of the quantum dots. While the traditional saturated-salt water synthesis and methanol-assisted method are quite effective in low temperature and low photon excitation intensity, the quantum dots sealed by the ethanol-assisted method cannot hold under all conditions. An over-1000-h aging test can provide crucial information for the longevity of these quantum dots, and more than 10,000 h of lifetime can be expected.
Highly Efficient and Stable White Light-Emitting Diodes Using Perovskite Quantum Dot Paper
(1970-01-01)
Kong, Chieh-Yu
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Lin, Chun-Ho
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Lin, Chih-Hao
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Li, Ting-You
;
Chen, Sung-Wen Huang
Perovskite quantum dots (PQDs) are a competitive candidate for next-generation display technologies as a result of their superior photoluminescence, narrow emission, high quantum yield, and color tunability. However, due to poor thermal resistance and instability under high energy radiation, most PQD-based white light-emitting diodes (LEDs) show only modest luminous efficiency of approximate to 50 lm W-1 and a short lifetime of <100 h. In this study, by incorporating cellulose nanocrystals, a new type of QD film is fabricated: CH3NH3PbBr3 PQD paper that features 91% optical absorption, intense green light emission (518 nm), and excellent stability attributed to the complexation effect between the nanocellulose and PQDs. The PQD paper is combined with red K2SiF6:Mn4(+) phosphor and blue GaN LED chips to fabricate a high-performance white LED demonstrating ultrahigh luminous efficiency (124 lm W-1), wide color gamut (123% of National Television System Committee), and long operation lifetime (240 h), which paves the way for advanced lighting technology.
Luminous efficiency enhancement of white light-emitting diodes by using a hybrid phosphor structure
(2015-02-19)
Lin, Bing-Cheng
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Huang, Jhih-Kai
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Chen, Kuo-Ju
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Chiu, Sheng-Huan
;
Tu, Zong-Yi
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. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)
Efficiency droop behavior improvement through barrier thickness modification for GaN-on-silicon light-emitting diodes
(2015-03-05)
Tzou, An-Jye
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Lin, Bing-Cheng
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Lee, Chia-Yu
;
Lin, Da-Wei
;
Liao, Yu-Kuang
Crack-free GaN-based light-emitting diodes (LEDs) were grown on 150-mm-diameter Si substrates by using low-pressure metal-organic chemical vapor deposition. The relationship between the LED devices and the thickness of quantum barriers (QBs) was investigated. The crystal quality and surface cracking of GaN-on-Si were greatly improved by an AlxGa1-xN buffer layer composed of graded Al. The threading dislocation density of the GaN-on-Si LEDs was reduced to < 7 x 10(8) cm(-2), yielding LEDs with high internal quantum efficiency. Simulation results indicated that reducing the QB thickness improved the carrier injection rate and distribution, thereby improving the droop behavior of the LEDs. LEDs featuring 6-nm-thick QBs exhibited the lowest droop behavior. However, the experimental results showed an unanticipated phenomenon, namely that the peak external quantum efficiency (EQE) and light output power (LOP) gradually decreased with a decreasing QB thickness. In other words, the GaN-on-Si LEDs with 8-nm-thick QBs exhibited low droop behavior and yielded a good peak EQE and LOP, achieving a 22.9% efficiency droop and 54.6% EQE. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)
Multi-Azimuth Failure Mechanisms in Phosphor-Coated White LEDs by Current Aging Stresses
(2018-04-01)
Peng, Zhangbao
;
Guo, Ziquan
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Wu, Tingzhu
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Zhuang, Peng
;
Ye, Zhicheng
We have experimentally analyzed multi-azimuth degradation mechanisms that govern failures of commercially-available high-power (1 Watt) phosphor-coated white (hppc-W) light-emitting diodes (LEDs) covered with peanut-shaped lenses under three current-stress aging (CSA) conditions. Comprehensive analyses focus on photometric, chromatic, electrical, thermal and packaging characteristics. At the packaging level, (a) the decrease of the phosphor-conversion efficiency; (b) the yellow-browning of the optical lens; and (c) the darkening of the silver-coated reflective layer deposited with extraneous chemical elements (e. g., C, O, Si, Mg, and Cu, respectively) contribute collectively to the integral degradation of the optical power. By contrast, Ohmic contacts, thermal properties, and angles of maximum intensity remain unchanged after 3840 h aging in three cases. Particularly at the chip level, the formation of point defects increases the number of non-radiative recombination centers, and thus decreases the optical power during aging stages. Nevertheless, in view of the change of the ideality factor, the Mg dopant activation and the annealing effect facilitate the increase of the optical power in two specific aging stages (192 h similar to 384 h and 768 h similar to 1536 h). This work offers a systematic guidance for the development of reliable LED-based light sources in general-lighting areas.
Enhanced Luminous Efficiency of WLEDs Using a Dual-Layer Structure of the Remote Phosphor Package
(2013-06-15)
Chen, Kuo-Ju
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Chen, Hsin-Chu
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Shih, Min-Hsiung
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Wang, Chao-Hsun
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Tsai, Hsin-Han
This study demonstrates how the insertion of a thin silicone layer into a dual-layer remote phosphor structure enhances light extraction in white light-emitting diodes (WLEDs). In the experiment, a dual-layer phosphor structure yielded a higher intensity of blue and yellow components than a conventional structure. Moreover, the lumen flux was 5% higher than a conventional remote phosphor package at the same correlated color temperature (CCT). Using a TFCalc32 simulation, the electric field intensity was calculated for different thicknesses of the dual-layer remote phosphor structures, and the enhanced use of blue rays was verified. Additionally, the dual-layer structure reduces chromaticity deviations as the driving current increases.
Resonant-Enhanced Full-Color Emission of Quantum-Dot-Based Display Technology Using a Pulsed Spray Method
(2012-12-19)
Chen, Kuo-Ju
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Chen, Hsin-Chu
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Tsai, Kai-An
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Lin, Chien-Chung
;
Tsai, Hsin-Han
Colloidal quantum-dot light-emitting diodes (QDLEDs) with the HfO2/SiO2-distributed Bragg reflector (DBR) structure are fabricated using a pulsed spray coating method. Pixelated RGB arrays, 2-in. wafer-scale white light emission, and an integrated small footprint white light device are demonstrated. The experimental results show that the intensity of red, green, and blue (RGB) emission exhibited considerable enhancement because of the high reflectivity in the UV region by the DBR structure, which subsequently increases the use in the UV optical pumping of RGB QDs. A pulsed spray coating method is crucial in providing uniform RGB layers, and the polydimethylsiloxane (PDMS) film is used as the interface layer between each RGB color to avoid cross-contamination and self-assembly of QDs. Furthermore, the chromaticity coordinates of QDLEDs with the DBR structure remain constant under various pumping powers in the large area sample, whereas a larger shift toward high color temperatures is observed in the integrated device. The resulting color gamut of the proposed QDLEDs covers an area 1.2 times larger than that of the NTSC standard, which is favorable for the next generation of high-quality display technology.
Light Extraction Enhancement of GaN-Based Light-Emitting Diodes Using Crown-Shaped Patterned Sapphire Substrates
(2012-07-15)
Chiu, Ching-Hsueh
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Hsu, Lung-Hsing
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Lee, Chia-Yu
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Lin, Chien-Chung
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Lin, Bo-Wen
In this letter, we report the high performance GaN-based light-emitting diodes (LEDs) with embedded air void array grown by metal-organic chemical vapor deposition. The donut-shaped air void was formed at the interface between crown-shaped patterned sapphire substrates (CPSS) and the GaN epilayer by conventional photolithography. The transmission electron microscopy images demonstrate that the threading dislocations were significantly suppressed by epitaxial lateral overgrowth (ELOG). The Monte Carlo ray-tracing simulation reveals that the light extraction of the air-voids embedded LED was dramatically increased due to a strong light reflection and redirection by the air voids.
Photonic-crystal light-emitting diodes on p-type GaAs substrates for optical communications
(2008)
Yang, Hung-Pin D.
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Liu, Jui-Nung
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Lai, Fang-I.
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Kuo, Hao-Chung
;
Chi, Jim Y.
Oxide-confined photonic-crystal (PhC) light-emitting diodes (LEDs) on p-type GaAs substrate in the 830 nm range are reported. The device consists of a bottom distributed Bragg reflector (DBR), quantum wells (QWs), and a top DBR, with a photonic-crystal structure formed within the n-type ohmic contact ring for light extraction. The etching depth of the PhC holes is 17-pair out of the 22-pair top DBR being etched off. The internally reflected spontaneous light emission can be extracted out of PhC holes because of lower reflectance within those areas. High-resolution micrographic imaging studies indicate that the device emits light mainly through the photonic-crystal holes and it is suitable for optical communications.

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