Browsing by Author "Singh, Preetpal"

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Degradation Mechanisms for CdSe Quantum dot down converted LEDs
(2018-01-01)
Singh, Preetpal
;
Tan, Cher Ming
;
Kuo, Hao-Chung
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光電工程學系
;
Department of Photonics
Lifetime of packaged white light emitting diodes (W-LEDs) using a blue LED and red quantum dot have been observed. A new phenomenon of light intensity increase during the initial lifetime is noticed followed by the rapid light output degradation. The initial increase in light output is believed to be due to photo -modification whereas the later rapid degradation is found to be due to photo oxidation mechanism.
Dependence of operating conditions on lifetime of Phosphor Quantum dots-based white LEDs
(2019-01-01)
Singh, Preetpal
;
Tan, Cher Ming
;
Kuo, Hao-Chung
;
Wang, Kai
;
Yang, Hongchen
Lifetime of packaged Phosphor- Quantum dot (QDs) based white LEDs is studied. Various stress parameters such as blue light luminance, temperature and moisture are considered and their effect on the performance of the Phosphor- QD based LED is investigated. The lumen degradation rate and degradation trend for LEDs is observed to be different under varying stress conditions. Lumen recovery process is also observed under light stress test.
Investigation of the Impact of Drive Current and Phosphor Thickness on the Reliability of High Power White LED Lamp
(2019-06-01)
Singh, Preetpal
;
Tan, Cher Ming
;
Zhao, Wenyu
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Kuo, Hao Chung
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交大名義發表
Lumen flux of white light-emitting diodes (LEDs) can be increased by increasing phosphor layer thickness and blue LED's drive current. Higher LED currents will lead to an increase in blue light in the output lumen along with higher temperatures which is not acceptable, and a corresponding decrease in the yellow light emission. The physical reason for this decrease is attributed to the finite phosphor particles in the layer and re-emission occurring due to scattered or back reflected blue light rays. A parameter called blue to yellow ratio (BYR) is used which can lead to a current with maximum phosphor utilization while preventing the LED output color to shift from white toward blue. As the degree of impact on reliability from drive current and thickness is different, a careful adjustment of the drive current and phosphor layer thickness shall be made for a given lumen. Response surface method is employed to obtain an optimal combination of the current and thickness where the temperature is the minimum for a given lumen level. The temperature difference for different combinations of current and thickness for a given lumen can be as high as 15 degrees C as found in this paper.
Physical Limitations of Phosphor layer thickness and concentration for White LEDs
(2018-02-05)
Tan, Cher Ming
;
Singh, Preetpal
;
Zhao, Wenyu
;
Kuo, Hao-Chung
;
光電工程學系
Increasing phosphor layer thickness and concentration can enhance the lumen flux of white LED (W-LED). In this work, we found that increasing the phosphor layer thickness and concentration can increase its temperature, and there is also a maximum thickness and concentration beyond which their increase will not lead to lumen increase, but only temperature increase. Higher thickness and higher concentration also results in warm light instead of White light. The maximum thickness and concentration are found to be limited by the scattering of light rays with higher % decrease of blue light rays than the yellow light rays. The results obtained in this work can also be used to compute the temperature and thermo-mechanical stress distribution of an encapsulated LED, demonstrating its usefulness to the design of encapsulated LED packages. Simulation software like ANSYS and TracePro are used extensively to verify the root cause mechanisms.