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

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 $\mathrm{Ti}{\mathrm{O}}_2$ nanoparticles in epoxy for LED encapsulation is demonstrated for surfactant-coated $\mathrm{Ti}{\mathrm{O}}_2$ nanoparticles by drying, mixing with a solvent, refluxing, centrifuging, and mixing with epoxy. The refractive index of surfactant-coated $\mathrm{Ti}{\mathrm{O}}_2$ -nanoparticle-loaded epoxy is 1.67 at $500\mathrm{nm}$ , 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, $k_s$ , 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.