The Impact of Temperature and Phosphor Concentration on the Refractive Index of LED Encapsulants

When light travels from one medium to another, such as from air to water, the light bends(refracts) to some degree at the boundary of the two media, depending on their refractive indices. When two media have nearly identical refractive indices, very little refraction occurs. But when light travels from a medium with a high refractive index to one with a low refractive index, light loss can happen at certain angles: The light bounces back at the boundary point, caused by the principles of "total internal reflection" and "Fresnel reflection."

With LEDs, light must travel from the semiconductor chip, which has a high refractive index, to the encapsulant surrounding the chip, which has a low refractive index. Some light loss may also occur between the encapsulant and air. The refractive index mismatch between the semiconductor chip and the encapsulant is one factor affecting the light extraction efficiency of LEDs, one of the most important issues facing LED performance today. As much as 60% of the photons emitted by the chip can bounce back and may be lost inside the LED package.

Increasing heat is known to decrease the refractive index of encapsulants. Mixing in particles with higher refractive indices, such as phosphors, can increase the encapsulant's refractive index depending on the concentration of particles.

This study explored the refractive indices of commercially available encapsulant materials used for LEDs and how those indices change as a function of temperature and the amount of YAG:Ce phosphor mixed within the encapsulant. By conducting such a study, one can select a suitable encapsulant for high-power LEDs in order to maximize light extraction.

EXPERIMENT

LRC researchers measured the refractive indices of epoxy and silicone encapsulant materials using an Abbe refractometer, which measures refractive index based on the principle of total internal reflection. Measurements were made in a limited temperature range of 25°C to 70°C using different concentrations of YAG:Ce phosphor. The refractive index was measured as a function of temperature alone for epoxy and silicone, as a function of phosphor concentration alone for epoxy, and as function of temperature and phosphor concentration for epoxy.

RESULTS

Experiments investigating the function of temperature showed that the refractive indices of the materials tested had a large range, from 1.40 to 1.58. The refractive index of the encapsulant materials decreased as temperature increased, which could cause a significant loss in LED light output.

The results from the investigation of phosphor concentration generally indicate that an increase in refractive index occurs when increasing the concentration of YAG:Ce phosphor mixed into the epoxy encapsulant. A statistical increase was shown for certain concentrations, but not for others.

The results from the investigation of phosphor concentration plus temperature also demonstrate that the refractive index of an epoxy-phosphor mixture decreases with increasing temperature, but again only certain concentrations showed statistical significance. 

SPONSOR

Lighting Research Center