Fabrication of high-refractive-index microstructures and their applications to the efficiency improvement of...

This study employed a UV-assisted roller imprinting technique using PDMS soft molds to imprint periodically inverted-pyramid structures on the surface of GaN-based LEDs. The refractive indices of the structures were 1.5, 1.7, and 1.9, which enhanced light output power by 26%, 43%, and 50%, respectively, compared to conventional LEDs. Materials with a greater refractive index indicate a larger critical angle of total internal reflection from the semi-conductor to the imprinted material. Once photons are extracted into the imprinted structure, they are prone to be extracted into the air through the structured surface. The numerical simulation performed using the ray tracing method proved useful for identifying the microstructure with optimal light extraction efficiency. Simulation results showed that LEDs with imprinted structures of varying refractive indices enhance optical efficiency in a manner similar to that demonstrated in these experiments.