Patterned microlens array for efficiency improvement of small-pixelated organic light-emitting devices

In this paper, we experimentally and theoretically investigated the optical characteristics of organic light-emitting devices (OLEDs), having different pixel sizes and attached with patterned microlens array films. For a regular microlens array, though it can extract the waveguiding light and increase luminous current efficiency for a large-pixelated OLED, we observed that it decreases the luminance to an even lower level than that of the planar OLED as its pixel size is close to the microlens dimension. Although a microlens can effectively outcouple the light rays originally at incident angles larger than the critical angle, it also can impede the outcoupling for the light rays originally at incident angles smaller than the critical angle. Enhancement or reduction of the light extraction depends on the relative positions of the light emitting point and the microlens. Therefore, we proposed a center-hollowed microlens array, of which the microlenses directly upon the pixel are removed, and proved that it can increase the luminous current efficiency and luminous power efficiency of a small-pixelated OLED. By attaching this patterned microlens array, 87% of luminance enhancement in the normal direction was observed for a 0.1x0.1 mm2 OLED pixel. On the other hand, a regular microlens array resulted in 4% decrease under the same condition.