The optical characteristics of organic light-emitting diodes with various microlens arrays

It is important to increase the light extraction efficiency (LEE) of devices in which micrometer-sized spheroidal, conical, pyramidal, and truncated microlens arrays (MLAs) are coupled to organic light-emitting diodes (OLEDs). MLAs may be hexagonal or square. To specify the shapes of individual lenses and their arrays in MLAs, the respective variables x (newly defined in this paper) and the fill factor (FF) can be used. MLAs are characterised by their LEE enhancements (LEEEs) and angular intensity distributions (AIDs), and these are the key optical characteristics of MLAs coupled to OLEDs. Here, these characteristics were obtained via computational simulation using a ray-tracing method. The LEEEs of the spheroidal MLAs were the same regardless of whether the arrays were hexagonal or square. The spheroidal MLAs are suitable for light emission applications in forward direction and, for spherical MLAs of FF = 74.55%, the LEEE was 88.3%. For conical hexagonal and pyramidal MLAs, the LEEEs gradually decreased and the difference widened as the extent of truncation increased. For display applications, the largest LEEE for light rays with emission angles < 10° was 56.5% for spheroidal MLA/OLD devices. Considering all emission angles, the largest LEEE was about 120% for pyramidal MLA/OLED devices. For regular pyramidal MLAs, the intensities of emitted light by the azimuth emission angle evidenced four sawtooth shapes precluding applications in displays.