Fabrication of cylindrical microlens array using rotatable multi-arrayed needle coating module for light out-coupling of OLEDs

We have performed coatings of poly(methyl methacrylate) (PMMA) using a rotatable multi-arrayed needle coating module to achieve high-density and high-aspect-ratio cylindrical microlens arrays (C-MLAs) for potential applications in organic light-emitting diodes (OLEDs). It is shown that high-density C-MLAs can be fabricated by rotating the coating module in such a way that the spacing between microlenses can be reduced (tuned). To enhance the aspect ratio of C-MLAs, we have analyzed the coating behaviors by varying the material parameters such as the molecular weight of PMMA, solute concentration, and wettability of the PMMA solution. It is demonstrated that high-aspect-ratio C-MLAs can be achieved using a coating solution with a small amount of a hydrophobic solvent, in which a high concentration of low-molecular-weight PMMA is dissolved. To further enhance the aspect ratio, we have come up with a new process method; repetitive coating and pre-drying treatment in ambient air. Such a repetitive coating method is shown to boost the microlens thickness without a much compromise on the microlens width. With this scheme, we have fabricated the C-MLA with the average thickness of 22.6 μm and width of 125 μm (aspect ratio as high as 0.18) at a coating velocity of 3 mm/s. The light out-coupling efficiency of OLED with the C-MLA increases by 20%, which is in good agreement with the result by ray tracing simulations.