Fabrication of spherical microlens array using needle coating for light extraction of OLEDs

Using needle coating, we fabricated a spherical microlens array (MLA) with poly(methyl methacrylate) (PMMA) for potential applications in light extraction of organic light-emitting diodes (OLEDs). To achieve high-density and high-aspect-ratio MLAs, we investigated the coating behaviors by varying material parameters such as solute concentration and wettability of the PMMA solution, and process parameters such as dwell time of the needle near the substrate, retract distance of the needle from the substrate, and coating gap between the needle and substrate. Under optimized coating conditions, we demonstrated that high-aspect-ratio MLAs can be obtained using a coating solution with high solute concentration and a small amount of hydrophobic solvent. We found that while the diameter and height of MLA decrease with increasing PMMA concentration, the overall aspect ratio is enhanced. By adding 7.5wt% hexylamine in 30 wt% PMMA solution, we achieved a spherical microlens with a height of 4.7 μm and a width of 53.8 μm (aspect ratio of 0.087), well-coated on a 4mmx4mm area glass with a dot spacing of 10um. Ray tracing simulations showed that the light extraction efficiency of OLED could be enhanced up to 24% by the MLA.