In this paper, we would like to report the following two subjects:(1) Thermal
decay mechanisms of phosphor-doped silicone in high power phosphor-converted
white light emitting diode (PC-WLED) module and (2) Thermal aging variations of
light profile and output power of blue LED modules having a polycarbonate lens and
silicone as an encapsulant.
Although silicone degradation attributed to the final thermal degradation, it is not
a dominant factor until a much thicker silicone is employed in PC-LEDs. The major
degradation mechanism of the PC-LEDs results from the higher doping concentration
of Ce:YAG in silicone. However, the negligible difference of fluorescent lifetimes
among the test samples before and after thermal aging (at 150°C for 500hrs)
eliminated any significant quenching processes that existed in our aged samples. The
emission spectra suggest that a higher doping concentration in silicone causes a higher
degree of loss at the emission wavelength of Ce:YAG, namely 570nm. Therefore,
minimizing any mismatch of the refractive index, thermal expansion , and chemistry
between the phosphor and the silicone is a new sign of improving thermal reliability
for high power PC-LEDs.
Thermal aging variations of light profile and output power of LED modules
fabricated by three manufacturers (namely, Type I, II, and III) were investigated
experimentally and numerically. Both experimental results and simulation results
suggested that improving the lens/ encapsulant materials and packaging designs are
essential to not only greatly extend the product lifetime but also enhance the light quality of LED modules as illumination sources.
Index Terms -High-powered phosphor-converted white-light-emitting diodes
(PC-WLEDs), lumen loss, chromaticity shift, silicone, polycarbonate, thermal aging,