Effect of heat and short-wavelength visible radiation on phosphor-embedded led encapsulant

Light Emitting Diodes (LEDs) are used in many applications, owing to their energy saving and long service life potentials. One of the ways to produce white light from an LED is to use a GaN semiconductor emitter with YAG:Ce phosphor. In such phosphor converted (pc) white LEDs the phosphor is either dispersed in the LED encapsulant surrounding the die or is conformally coated over the semiconductor chip. The encapsulant plays a vital role in determining the lifetime of an LED. Commonly used encapsulants are epoxy and silicone. During the operation of a pc-white LED the encapsulant is subjected to short wavelength radiation and heat from the LED chip that can degrade the encapsulant material. The degrading encapsulant reduces light output and results in reduced service life. This thesis study evaluates the effect of short-wavelength radiation and heat on the encapsulant material by examining the light transmission through an epoxy medium with phosphor as a function of short-wavelength irradiance, heat, and phosphor concentration. It was found that the epoxy with phosphor sample temperature is the key factor in determining the degradation. Epoxy samples with and without embedded phosphor when subjected to heat have a constant degradation rate. However, the degradation rate of the samples with phosphor increased as a function of time when subjected to short-wavelength radiation. This study helped to provide an understanding of the individual contribution of heat and short-wavelength radiation to reducing the light output of a pc-white LED package.