High-refractive-index TiO2-nanoparticle-loaded encapsulants for light-emitting diodes

Abstract

A high-refractive-index (high-n) encapsulant is highly desirable because it can result in enhancement of light-extraction efficiency from high-n semiconductor light-emitting diode (LED) chips. A uniform dispersion of TiO2 nanoparticles in epoxy for LED encapsulation is demonstrated for surfactant-coated TiO2 nanoparticles by drying, mixing with a solvent, refluxing, centrifuging, and mixing with epoxy. The refractive index of surfactant-coated TiO2-nanoparticle-loaded epoxy is 1.67 at 500nm, significantly higher than that of conventional epoxy (n=1.53). Theoretical analysis of optical scattering in nanoparticle-loaded encapsulants reveals that the diameter of nanoparticles and the volume loading fraction of nanoparticles are of critical importance for optical scattering. Quasispecular transparency of the encapsulant film can be achieved if the thickness of the film is kept below the optical scattering length. A graded-refractive-index multilayer encapsulation structure with the thickness of each layer being less than the mean optical scattering length is proposed in order to reduce optical losses from scattering and Fresnel reflection. Furthermore, three-dimensional optical ray-tracing simulations demonstrate that encapsulants with an optimized scattering coefficient, ks, benefit from optical scattering by extracting deterministic trapped modes. Theoretical light-extraction enhancements larger than 50% are found when comparing scattering-free to scattering encapsulation materials.

ZrO2/epoxy nanocomposite for LED encapsulation

“…Scattering is welcomed for LED encapsulant. According to the 3D-ray-tracing simulation made by Mont [4], an optimized scattering could enhance the light-extraction efficiency by as much as 50%.…”

“…The RI of the encapsulant is important since the light extraction efficiency is limited by the total internal reflection at the encapsulant/chip interface [3,4]. A reduced thermal resistivity helps to dissipate the heat.…”

Effect of Glass Refractive Index on Light Extraction Efficiency of Light‐Emitting Diodes

“…The light trapped by total internal reflection because of a large refractive index difference between the encapsulant and phosphor is a fundamental limiting factor of light extraction efficiency in LED packages . The critical angle of total internal reflection can be written using Snell's law aswhere n 1 and n 2 are the refractive indices of the phosphor and encapsulant, respectively.…”

On the Thermal Load of the Color‐Conversion Elements in Phosphor‐Based White Light‐Emitting Diodes

“…Such an increase of the thermal conductivity of the silicone encapsulation layer may be beneficially achieved by mixing it with non‐emitting particles of higher thermal conductivity. For example, Mont et al and Ma et al have reported, that the light‐extraction from LED dice can be improved by increasing the refractive index of the encapsulation layer, e.g., by adding TiO 2 particles . Since TiO 2 also has, e.g., for its most common form, rutile, a thermal conductivity in the range of 9–13 W/mK, such a concept in addition to light‐extraction enhancement could also increase the thermal conductivity of the encapsulation layer and therefore could contribute to an additional reduction of the thermal load of the CCEs.…”