Abstract
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.
Highlights
Due to the difference among the refractive indices of the substrate, anode, organic thin films, and the air, two waveguiding phenomena arise in the anode/organic layers and substrate of the organic light-emitting devices (OLEDs)
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
A microlens can effectively outcouple the light rays originally at incident angles larger than the critical angle, it can impede the outcoupling for the light rays originally at incident angles smaller than the critical angle
Summary
Due to the difference among the refractive indices of the substrate, anode, organic thin films, and the air, two waveguiding phenomena arise in the anode/organic layers and substrate of the organic light-emitting devices (OLEDs). The light rays at an incident angle larger than the critical angle in a planar OLED, as shown in the ray I of Fig. 1(a), can be coupled out by a microlens array attachment, as shown in the ray I of Fig. 1(b) This mechanism contributes to luminance (lm/sr⋅m2) enhancement. Some of the light rays at an incident angle smaller than the critical angle in a planar case, as shown in the ray II of Fig. 1(a), can be reflected back to the glass substrate due to the microlens array attachment, as shown in the ray II of Fig. 1(b) In such case, the light extraction efficiency will be decreased by the microlenses and it is especially worse in the regions directly upon the pixels. By removing the microlenses directly upon the pixel, an 87% increase was observed and it is much higher than the luminance enhancement of 33% by using a regular microlens array attachment on a large OLED pixel
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