Improvement of Extraction Efficiency of LED with Surface Relief Nanotructure Fabricated by Self-Assembled Block Copolymer Pattern

A surface relief nanostructure was fabricated on the emission surface of light-emitting diodes (LEDs) using a self-assembled diblock copolymer pattern. The pattern of the nanostructure possesses moderate short-range order with slightly deviation in size and spacing, which is different from conventional extraction surface structures, such as photonic crystal and randomly textured surface. The dot pattern of a self-assembled polystyrene -polymethylmethacrylate diblock copolymer (PS-b-PMMA) was used as an etched mask. An average dot spacing was controlled by changing blend ratio of PS-b-PMMA, homo (h-) PS, and h-PMMA in a polymer solution. In the photoluminescence (PL) measurement, the light extraction efficiency of the nanostructure exceeded over twice, compared with a flat surface, by optimizing the average spacing of the nanostructure. It was also revealed that the nanostructure showed more than 10{\%} higher extraction efficiency than the highly ordered nanostructure fabricated by a self-assembled nanosphere pattern. These results can be interpreted as a contribution of structural fluctuation in the nanostructure for enhancement of extraction efficiency.