Random organic nano-textured microstructures formed by photoexcitation for light extraction of blue OLEDs

Abstract

Organic light-emitting diodes require efficient and inexpensive light extraction methods for applications toward lighting and displays. Here, a facile and effective method for fabricating random organic microstructures for efficient light extraction from blue OLEDs is presented. Simple drop casting of a TCTA and B4PyMPM mixed solution followed by UV curing results in films with irregular-shaped microstructures (DACMs), ideal for light extraction without diffraction patterns. An external quantum efficiency (EQE) of 44.3% is realized by attaching DACMs formed on a polymer film to a blue phosphorescent OLED. The efficiency is improved by 35% compared to a planar device without the light extraction layer, greater than the 22% improvement obtained by using hemispherical-shaped microlens arrays. The method is useful for OLED lighting and potentially in displays because of the simple fabrication method that is applicable to a large area on rigid or flexible substrates, the low material cost, the insolubility of the microstructure in alkyl halide solvents such as chloroform, and the controllability of the structure through the solution process. Furthermore, we fully discuss on the EQE, enhancement ratio and the efficiency of light extraction structure (ELOS). We show that enhancement ratio and ELOS should be obtained using the optimized device structure as the reference device to be useful indicators for the effectiveness of a light extraction structure.