Light transmission from the large-area highly ordered epoxy conical pillar arrays and application to GaN-based light emitting diodes

To improve the light transmission from the surface, we report a facile and cost-effective approach for the formation of wavelength-scale conical pillar arrays on a large surface area of epoxy resin. The highly ordered epoxy conical pillar arrays with a pitch of about 460 nm and height of about 800 nm have been successfully fabricated by the technique of shape-controlled anodization of Al foil followed by hot embossing. By replicating the tapered pore arrays onto a transparent semi-cylindrical epoxy structure, the incident angular resolved light transmission of the epoxy conical pillar arrays has been obtained. The integrated transmission of conical pillar arrays as high as 62.2% has been achieved which is confirmed to be 223% and 11.3% higher than that of planar epoxy and the cylindrical pillar arrays, respectively. It is reasonable to consider the wavelength-scale conical pillar array as a particular multilayer consisting of a series of two-dimensional photonic crystals with gradually increasing filling factor towards the surface. It can therefore be treated as a multilayer with continuously reducing refractive index towards the air. The conical and cylindrical pillar arrays of epoxy have been directly employed as the encapsulant of a GaN based flip-chip LED. Compared to the LED encapsulated by planar epoxy, the enhancement of light extraction from the LED covered with conical and cylindrical pillar arrays have been demonstrated to be 46.8% and 34.9%, respectively.