Browsing by Author "Wang, Chao-Hsun"

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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.
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 and Droop Improvement in InGaN/GaN Light-Emitting Diodes by Selective Carrier Distribution Manipulation
(2012-04-01)
Wang, Chao-Hsun
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Chang, Shih-Pang
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Ku, Pu-Hsi
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Lan, Yu-Pin
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Lin, Chien-Chung
Efficiency and droop behavior in InGaN/GaN light-emitting diodes (LEDs) are both improved using selectively graded composition multiple quantum barriers (SGQBs). Simulation results show that SGQBs could moderately improve the hole transport in the active region. In the meantime, the spatial distribution overlap between electrons and holes in the active region could also be well considered. Therefore, the radiative recombination of the SGQB LED is more efficient than that of the conventional LED. The overall efficiency and droop behavior are simultaneously improved in the SGQB LED, at both low and high current densities. (C) 2012 The Japan Society of Applied Physics
Efficiency and droop improvement in InGaN/GaN light-emitting diodes by selectively carrier-distribution manipulation
(2012)
Lin, Da-Wei
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Wang, Chao-Hsun
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Chang, Shih-Pang
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Ku, Pu-Hsih
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Lan, Yu-Pin
Efficiency and droop behavior in InGaN/GaN light-emitting diodes are both improved by selectively-graded-composition multi-quantum barriers (SGQB). Simulation results show that SGQB could moderately improve the hole transport in active region. In the meantime, spatial distribution overlap between electrons and holes in active region could be also well-considered. Therefore, the radiative recombination of SGQB LED is more efficient than that of conventional LED. The overall efficiency and droop behavior are simultaneously improved in SGQB LED, at both low and high current density.
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.
High thermal stability of correlated color temperature using current compensation in hybrid warm white high-voltage LEDs
(2013-03-11)
Chen, Kuo-Ju
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Kuo, Hsuan-Ting
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Chen, Hsin-Chu
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Shih, Min-Hsiung
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Wang, Chao-Hsun
This study experimentally and numerically examines the correlated color temperature (CCT) stability issue for hybrid warm white high-voltage light-emitting diodes (HV-LEDs) by using a current compensation method. This method could efficiently maintain the CCT stability factor at approximately 1.0 and yield greater color uniformity with Delta u'v' values ranging from 0.017 to 0.003 in CIE 1976 chromaticity coordinates. The simulation results show that the red chip intensity drop is the primary cause of CCT instability in the hybrid warm white system when the temperature increases. Therefore, Furthermore, results indicate that the relative lumen drop improves from 21% to 15% by using a current compensation method. (c) 2013 Optical Society of America
High Uniformity of Remote Phosphor Structure by ZrO2 Nano-particles for White LEDs
(2012)
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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Tsai, Hsin-Han
High luminous efficiency and superior uniformity of angular-dependent correlated color temperature white light-emitting diodes have been investigated by ZrO2 nano-particales in remote phosphor structure due to the higher capability of light scattering.
Hole injection and electron overflow improvement in InGaN/GaN light-emitting diodes by a tapered AlGaN electron blocking layer
(2014-01-13)
Lin, Bing-Cheng
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Chen, Kuo-Ju
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Wang, Chao-Hsun
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Chiu, Ching-Hsueh
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Lan, Yu-Pin
A tapered AlGaN electron blocking layer with step-graded aluminum composition is analyzed in nitride-based blue light-emitting diode (LED) numerically and experimentally. The energy band diagrams, electrostatic fields, carrier concentration, electron current density profiles, and hole transmitting probability are investigated. The simulation results demonstrated that such tapered structure can effectively enhance the hole injection efficiency as well as the electron confinement. Consequently, the LED with a tapered EBL grown by metal-organic chemical vapor deposition exhibits reduced efficiency droop behavior of 29% as compared with 44% for original LED, which reflects the improvement in hole injection and electron overflow in our design. (C)2014 Optical Society of America
Improvement in efficiency droop of GaN-based light-emitting diodes by optimization of active regions
(2012)
Lin, Da-Wei
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Wang, Chao-Hsun
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Chang, Shih-Pang
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Chiu, Ching-Hsueh
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Lan, Yu-Pin
In this paper, we propose several methods to improve the efficiency droop of GaN-based light-emitting diodes by optimization of active regions, such as alternative substrates, semi-polar MQWs, insertion layer, graded-thickness MQWs, and graded-composition EBL.
Improvement in uniformity of emission by ZrO2 nano-particles for white LEDs
(2012-07-05)
Chen, Hsin-Chu
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Chen, Kuo-Ju
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Lin, Chien-Chung
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Wang, Chao-Hsun
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Han, Hau-Vei
The high luminous efficiency and superior uniformity of angular-dependent correlated color temperature (CCT) white light-emitting diodes have been investigated by ZrO2 nano-particles in a remote phosphor structure. By adding ZrO2 nano-particles with silicone onto the surface of the phosphor layer, the capability of light scattering could be enhanced. In particular, the intensity of blue light at large angles was increased and the CCT deviations could be reduced. Besides, the luminous flux was improved due to the ZrO2 nano-particles with silicone providing a suitable refractive index between air and phosphor layers. This novel structure reduces angular-dependent CCT deviations from 1000 to 420 K in the range of -70 degrees to 70 degrees. Moreover, the enhancement of lumen flux was increased by 2.25% at a driving current 120 mA, compared to a conventional remote phosphor structure without ZrO2 nano-particles. Consequently, the ZrO2 nano-particles in a remote phosphor structure could not only improve the uniformity of lighting but also increase the light output.
Output Power Enhancement of Light Emitting Diodes with Defect Passivation layer
(2010)
Lo, Ming-Hua
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Tu, Po-Min
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Cheng, Yuh-Jen
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Wang, Chao-Hsun
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Hung, Cheng-Wei
We demonstrate high efficiency blue light emitting diodes with defect passivation layers. The defect passivation layers were formed by defect selective wet etching, SiO(2) deposition, and chemical mechanical polishing process. The process does not require photolithography patterning. The threading dislocation density of grown sample was reduced down to similar to 4x10(7) cm(-2). The defect passivated epi-wafer is used to grow light emitting diode (LED) and the output power of the fabricated chip is enhanced by 45% at 20 mA compared to a reference one without using defect passivation.
Patterned structure of REMOTE PHOSPHOR for phosphor-converted white LEDs
(2011-07-04)
Kuo, Hao-Chung
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Hung, Cheng-Wei
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Chen, Hsin-Chu
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Chen, Kuo-Ju
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Wang, Chao-Hsun
High efficiency white light-emitting diodes with superior color-mixing have been investigated. It is suggested that the patterned remote phosphor structure could improve the uniformity of angular-dependent correlated color temperature (CCT) and achieve high chromatic stability in wider operating current range, as compared to the conventional remote phosphor coating structure. In this experiment, we employed a pulse spray coating method to place the patterned phosphor on the package and to leave a window region. The window area, a clear space without coating of the phosphor not only increases the extraction efficiency of blue rays at large angle, but also improves the stability of angular-dependent CCT. Moreover, the CCT deviation could be reduced from 1320 K to 266 K by this patterned remote phosphor method, and the stray blue/yellow light within the package can be effectively reduced and controlled. The design was verified both experimentally and theoretically. (C) 2011 Optical Society of America
The Influence of the Thermal Effect on CdSe/ZnS Quantum Dots in Light-Emitting Diodes
(2012-07-15)
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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Kuo, Ming-Yen
This study investigates the effect of temperature on CdSe/ZnS quantum dots (QDs) in GaN-based light-emitting diodes (LEDs) using the phosphor conversion efficiency (PCE) and LED junction temperature. In our simulation, the blue chip and CdSe/ZnS QDs temperature are similar because of their minimal thickness. Furthermore, to verify the effect of temperature on CdSe/ZnS QDs, we use continuous wave and pulsed current sources to measure the relationship between the temperature and relative PCE. Higher junction temperatures are observed with greater CdSe/ZnS QD volume in LEDs. This is attributed to the thermal conduction and nonradiative energy between CdSe/ZnS QDs and blue chip. Therefore, if thermal management is improved, CdSe/ZnS QDs are expected to be used as color converting material in LEDs.