Browsing by Author "Chen, Kuo-Ju"

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A high quality liquid-type quantum dot white light-emitting diode
(2016-01-01)
Sher, Chin-Wei
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Lin, Chin-Hao
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Lin, Huang-Yu
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Lin, Chien-Chung
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Huang, Che-Hsuan
This study demonstrates a novel package design to store colloidal quantum dots in liquid format and integrate them with a standard LED. The high efficiency and high quality color performance at a neutral white correlated color temperature is demonstrated. The experimental results indicate that the liquid-type quantum dot white light-emitting diode (LQD WLED) is highly efficient and reliable. The luminous efficiency and color rendering index (CRI) of the LQD WLED can reach 271 lm W-op(-1) and 95, respectively. Moreover, a glass box is employed to prevent humidity and oxygen erosion. With this encapsulation design, our quantum dot box can survive over 1000 hours of storage time.
A high quality liquid-type quantum dot white light-emitting diode (vol 8, pg 1117, 2016)
(2018-04-07)
Sher, Chin-Wei
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Lin, Chih-Hao
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Lin, Huang-Yu
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Lin, Chien-Chung
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Huang, Che-Hsuan
A Highly Efficient Hybrid GaAs Solar Cell Based on Colloidal-Quantum-Dot-Sensitization
(2014-07-18)
Han, Hau-Vei
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Lin, Chien-Chung
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Tsai, Yu-Lin
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Chen, Hsin-Chu
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Chen, Kuo-Ju
This paper presents a hybrid design, featuring a traditional GaAs-based solar cell combined with various colloidal quantum dots. This hybrid design effectively boosts photon harvesting at long wavelengths while enhancing the collection of photogenerated carriers in the ultraviolet region. The merits of using highly efficient semiconductor solar cells and colloidal quantum dots were seamlessly combined to increase overall power conversion efficiency. Several photovoltaic parameters, including short-circuit current density, open circuit voltage, and external quantum efficiency, were measured and analyzed to investigate the performance of this hybrid device. Offering antireflective features at long wavelengths and luminescent downshifting for high-energy photons, the quantum dots effectively enhanced overall power conversion efficiency by as high as 24.65% compared with traditional GaAs-based devices. The evolution of weighted reflectance as a function of the dilution factor of QDs was investigated. Further analysis of the quantum efficiency response showed that the luminescent downshifting effect can be as much as 6.6% of the entire enhancement of photogenerated current.
A Novel Tunable Green- to Yellow-Emitting beta-YFS:Ce(3+) Phosphor for Solid-State Lighting
(2011-08-01)
Wu, Yun-Chen
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Wang, De-Yin
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Chen, Teng-Ming
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Lee, Chi-Shen
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Chen, Kuo-Ju
A Ce(3+)-activated fluorosulfide phosphor (beta-YFS: Ce(3+)) was synthesized by solid-state reaction in a sealed tube. The crystal structure has been refined from the XRD profiles and there are two different crystallographic rare earth sites, namely, Y(1) and Y(2), where the Ce(3+) ions occupied. The emission band with a maximum at 495 nm of beta-Y(0.99)Ce(0.01) FS phosphor was characterized by the 4f-5d transitions of Ce(3+) ion. With increasing Ce(3+) concentration, the emission variations were observed from 495 to 547 nm. When beta-YFS:Ce(3+) phosphors were utilized to incorporate with n-UV/blue chip, greenish-white light with color rendering index of 65-77 were obtained. The results indicate that the tunable green- to yellow-emitting beta-YFS:Ce(3+) can serve as a potential phosphor for incorporation in fabrication for solid-state lighting. The preparation, spectroscopic characterization, quantum efficiency, thermal-quenching behavior, and related LED device data are also presented.
A Novel Tunable Green- to Yellow-Emitting beta-YFS:Ce3+ Phosphor for Solid-State Lighting
(2011-08-01)
Wu, Yun-Chen
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Wang, De-Yin
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Chen, Teng-Ming
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Lee, Chi-Shen
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Chen, Kuo-Ju
A Ce3+-activated fluorosulfide phosphor (beta-YFS: Ce3+) was synthesized by solid-state reaction in a sealed tube. The crystal structure has been refined from the XRD profiles and there are two different crystallographic rare earth sites, namely, Y(1) and Y(2), where the Ce3+ ions occupied. The emission band with a maximum at 495 nm of beta-Y0.99Ce0.01 FS phosphor was characterized by the 4f-5d transitions of Ce3+ ion. With increasing Ce3+ concentration, the emission variations were observed from 495 to 547 nm. When beta-YFS:Ce3+ phosphors were utilized to incorporate with n-UV/blue chip, greenish-white light with color rendering index of 65-77 were obtained. The results indicate that the tunable green- to yellow-emitting beta-YFS:Ce3+ can serve as a potential phosphor for incorporation in fabrication for solid-state lighting. The preparation, spectroscopic characterization, quantum efficiency, thermal-quenching behavior, and related LED device data are also presented.
Advantages of Blue LEDs With Graded-Composition AlGaN/GaN Superlattice EBL
(2013-11-01)
Lin, Bing-Cheng
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Chen, Kuo-Ju
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Han, Hau-Vei
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Lan, Yu-Pin
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Chiu, Ching-Hsueh
InGaN/GaN light-emitting diodes (LEDs) with graded-composition AlGaN/GaN superlattice (SL) electron blocking layer (EBL) were designed and grown by metal-organic chemical vapor deposition. The simulation results demonstrated that the LED with a graded-composition AlGaN/GaN SL EBL have superior hole injection efficiency and lower electron leakage over the LED with a conventional AlGaN EBL or normal AlGaN/GaN SL EBL. Therefore, the efficiency droop can be alleviated to be similar to 20% from maximum at an injection current of 15-120 mA, which is smaller than that for conventional AlGaN EBL (30%). The corresponding experimental results also confirm that the use of a graded-composition AlGaN/GaN SL EBL can markedly enhance the light output power by 60%.
An Investigation of the Optical Analysis in White Light-Emitting Diodes With Conformal and Remote Phosphor Structure
(2013-11-01)
Chen, Kuo-Ju
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Chen, Hsin-Chu
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Lin, Chien-Chung
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Wang, Chao-Hsun
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Yeh, Chia-Chi
An effective emission model of phosphor film is proposed by using bidirectional scattering distribution function system (BSDF), and the model is verified by white light-emitting diodes (LEDs) with conformal and remote phosphor structure. The emission model is built to clarify the optical characteristics by analyzing the angular-dependent distribution of emission and excitation behaviors in phosphor film. The white LEDs with conformal and remote phosphor structure are also fabricated for experimental comparison. The uniformity of angular correlated color temperature (CCT) in white LEDs can be determined by the angular distribution of blue and yellow light, which is in turns decided by the refractive index variation between chip a (c) nd phosphor layers. Finally, the experimental results are found to have good agreement with the simulation results performing by the Monte Carlo method.
Crystal structure characterization, optical and photoluminescent properties of tunable yellow- to orange-emitting Y-2(Ca,Sr)F4S2:Ce3+ phosphors for solid-state lighting
(2012)
Wu, Yun-Chen
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Chen, Yi-Chin
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Chen, Teng-Ming
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Lee, Chi-Shen
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Chen, Kuo-Ju
In this study, a new efficient Ce3+-doped fluorosulfide phosphor, Y-2(Ca,Sr)F4S2:Ce3+, was obtained by using solid-state methods in a sealed silica ampoule. The synthesized Y-2(Ca,Sr)F4S2:Ce3+ was characterized by powder X-ray diffraction and refined with Rietveld methods. Y-2(Ca,Sr)F4S2:Ce3+ can be excited by blue light (440-470 nm) and shows yellow-to-orange broadband emission peaking at 553-590 nm with a quantum efficiency of 16-31%. Non-radiative transitions between Ce3+ ions in Y2CaF4S2:Ce3+ and Y2SrF4S2:Ce3+ hosts have also been demonstrated to be attributable to dipole-dipole interactions, and the critical distances were calculated to be 18.9 and 19.3 angstrom. The possible mechanism of the tunable luminescence properties was described on the basis of band structure calculations. In addition, a white LED device was fabricated by using Y-2(Ca,Sr)F4S2:Ce3+ phosphor pumped with a 460 nm blue chip. The CRI value and CCT were measured to be 74-85 and 3500-8700 K, respectively, showing promising potential for solid-state lighting.
Defect Mode Lasing in metal-coated GaN Grating Structure at Room Temperature
(2014)
Chen, Kuo-Ju
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Hsu, Wan-Hai
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Liao, Wei-Chun
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Shih, Min-Hsiung
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Kuo, Hao-Chung
The lasing action from a metal-coated GaN with defect grating structure was observed and the wavelength is approximately 365 nm with the quality factor of 480 at room temperature.
Efficiency and Droop Improvement in GaN-Based High-Voltage Flip Chip LEDs
(2014-01-01)
Chiang, Yen-Chih
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Lin, Bing-Cheng
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Chen, Kuo-Ju
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Chiu, Sheng-Huan
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Lin, Chien-Chung
The GaN-based high-voltage flip chip light-emitting diode (HVFC-LED) is designed and developed for the purpose of efficiency enhancement. In our design, the distributed Bragg reflector (DBR) is deposited at the bonded substrate to increase the light extraction. After the flip chip process, the general current-voltage characteristics between the flip chip sample and the traditional sample are essentially the same. With the help of great thermal conductive silicon substrate and the bottom DBR, the HVFC-LED is able to enhance the power by 37.1% when compared to the traditional high-voltage LEDs. The wall-plug efficiencies of the HVFC-LED also show good droop reduction as high as 9.9% compared to the traditional devices.
Efficiency and Droop Improvement in GaN-Based High-Voltage Flip Chip LEDs
(2014-01-01)
Chiang, Yen-Chih
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Lin, Bing-Cheng
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Chen, Kuo-Ju
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Chiu, Sheng-Huan
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Lin, Chien-Chung
The GaN-based high-voltage flip chip light-emitting diode (HVFC-LED) is designed and developed for the purpose of efficiency enhancement. In our design, the distributed Bragg reflector (DBR) is deposited at the bonded substrate to increase the light extraction. After the flip chip process, the general current-voltage characteristics between the flip chip sample and the traditional sample are essentially the same. With the help of great thermal conductive silicon substrate and the bottom DBR, the HVFC-LED is able to enhance the power by 37.1% when compared to the traditional high-voltage LEDs. The wall-plug efficiencies of the HVFC-LED also show good droop reduction as high as 9.9% compared to the traditional devices.
Efficiency and Droop Improvement in Hybrid Warm White LEDs Using InGaN and AlGaInP High-Voltage LEDs
(2013-04-01)
Chen, Kuo-Ju
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Kuo, Hsuan-Ting
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Chiang, Yen-Chih
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Chen, Hsin-Chu
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Wang, Chao-Hsun
This study investigates the optical and electrical characteristics in hybrid warm white high-voltage light-emitting diodes (HV-LEDs). The luminous efficiency of the hybrid warm white LED in this study improved by 11% and 51%, compared to conventional cool and warm LEDs, respectively, solving the warm white gap in white LEDs. The efficiency droop of the hybrid warm white LED was reduced to 21.8% from 26.8% for the conventional cool white LED, and from 26.3% in the conventional warm white LED at 40 mA (35 A/cm(2)) the operated current. Furthermore, the color rendering index (CRI) and angular correlated color temperature (CCT) were analyzed, indicating a significant improvement in hybrid warm white HV-LEDs.
Efficiency enhancement of InGaN/GaN multiple quantum well solar cells using CdS quantum dots and distributed Bragg reflectors
(2013)
Tsai, Yu-Lin
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Lin, Chien-Chung
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Han, Hau-Vei
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Chen, Hsin-Chu
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Chen, Kuo-Ju
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 similar to 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.
Efficient hybrid white light-emitting diodes by organic-inorganic materials at different CCT from 3000K to 9000K
(2015-04-06)
Chen, Kuo-Ju
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Lai, Yi-Chun
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Lin, Bin-Cheng
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Lin, Chien-Chung
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Chiu, Sheng-Huan
The hybrid white light-emitting didoes (LED) with polyfluoren (PFO) polymer and quantum dot (QD) was investigated using dispensing method at the different correlated color temperature (CCT) for cool and warm color temperature. This result indicates that the hybrid white LED device has the higher luminous efficiency than the convention one, which could be attributed to the increased utilization rate of the UV light. Furthermore, the CIE 1931 coordinate of high quality white hybrid LED with different CCT range from 3000K to 9000K is demonstrated. Consequently, the angular-dependent CCT and the thermal issue of the hybrid white LED device were also analyzed in this study. (C) 2015 Optical Society of America
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.
Enhancement of angular CCT by hybrid phosphor structure in White Light-emitting Diodes
(2013)
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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Chien, Shih-Hsuan
The hybrid phosphor structure for white light-emitting diodes application was reported. It was demonstrated that the improvement of the luminous efficiency as well as the good uniform angular correlated color temperature (CCT) in hybrid phosphor structure compared with the conventional dispense method. The experimental results showed the CCT deviation was improved from 453K to 280K in the range of -70 to 70 degrees, which could be attributed to the large divergent angle of blue light. Moreover, the high lumen output was also achieved, as compared with dispense and conformal phosphor structures. Therefore, the experimental results indicated that the hybrid phosphor structure was feasible to use in the solid state lighting.
Enhancement of power conversion efficiency in GaAs solar cells with dual-layer quantum dots using flexible PDMS film
(2012-09-01)
Chen, Hsin-Chu
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Lin, Chien-Chung
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Han, Hau-Vei
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Chen, Kuo-Ju
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Tsai, Yu-Lin
"This study demonstrates the high performance of GaAs solar cells with dual-layer CdS quantum dots (QDs) carried by flexible polydimethylsiloxane (PDMS) film. Several parameters were enhanced, including the short circuit current, fill factor, and power conversion efficiency, which were measured under white light illumination similar to the solar spectrum. The flexible PDMS film was fabricated using the spin-coating technique, and was then used to embed dual-layer CdS QDs. Different concentrations of QDs were applied in this design to test the optimal combination. The proposed scheme enhances power conversion efficiency by 22%, compared to a GaAs solar cell without CdS QDs. The photon down-conversion capability in the ultraviolet (UV) range and additional antireflection capability in the longer visible-IR range were verified through the external quantum efficiency. (C) 2012 Elsevier B.V. All rights reserved."
Excellent Color Quality of White-Light-Emitting Diodes by Embedding Quantum Dots in Polymers Material
(2016-01-01)
Lin, Huang-Yu
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Wang, Sheng-Wen
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Lin, Chien-Chung
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Chen, Kuo-Ju
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Han, Hau-Vei
This study employs the polyfluorene composite (called Green B) with several colors of quantum dots to generate the high-color-rendering index (CRI) white LEDs. The Green B polymer is a good candidate to manufacture the hybrid w-LED because of its good quantum efficacy. The hybrid w-LEDs are fabricated by the mixing of Green B with yellow quantum dots and red quantum dots or orange quantum dots to obtain the correlated color temperatures of 3500 and 5500 K. The use of the hybrid polymer/QDs w-LEDs is proved to be beneficial as QD w-LEDs can improve the CRI up to 90 in 3500 K. The luminous efficiency of such devices can be as high as 17.2l m/W.
Fabrication and characterization of back-side illuminated InGaN/GaN solar cells with periodic via-holes etching and Bragg mirror processes
(2014-08-25)
Chang, Yi-An
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Chen, Fang-Ming
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Tsai, Yu-Lin
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Chang, Ching-Wen
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Chen, Kuo-Ju
In this study, the design and fabrication schemes of back-side illuminated InGaN/GaN solar cells with periodic via-holes etching and Bragg mirror processes are presented. Compared to typical front-side illuminated solar cells, the improvements of open-circuit voltage (V-oc) from 1.88 to 1.94 V and short-circuit current density (J(sc)) from 0.84 to 1.02 mA/cm(2) are observed. Most significantly, the back-side illuminated InGaN/GaN solar cells exhibit an extremely high fill factor up to 85.5%, leading to a conversion efficiency of 1.69% from 0.66% of typical front-side illuminated solar cells under air mass 1.5 global illuminations. Moreover, the effects of bottom Bragg mirrors on the photovoltaic characteristics of back-side illuminated solar cells are studied by an advanced simulation program. The results show that the J(sc) could further be improved with a factor of 10% from the original back-side illuminated solar cell by the structure optimization of bottom Bragg mirrors. (C) 2014 Optical Society of America
High Performance of Double-layer Quantum Dots with PDMS Film for Novel GaAs Solar Cells
(2012)
Chen, Hsin-Chu
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Lin, Chien-Chung
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Han, Hau-Vei
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Chen, Kuo-Ju
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Tsai, Yu-Lin
This study demonstrates the high performance of GaAs solar cells with double-layer quantum dots carried by flexible PDMS film. Several parameters were enhanced. Different concentrations of QDs were also applied to test the optimal combination.