High-Contrast Organic LED Design Targets Displays

Researchers at National Taiwan University in Taipei and at Industrial Technology Research Institute’s Electronics Research and Service Organization in Hsin-chu, Taiwan, have demonstrated an organic LED device structure that improves contrast without the incorporation of optical layers in the active region of the emitter. As described in the Oct. 3 issue of Applied Physics Letters, the approach relies on the optical characteristics of the electrodes and an antireflection coating, and it should be compatible with both top- and bottom-emitting devices for display applications.

In the top-emitting structure investigated, a molybdenum reflective bottom anode and a multilayer semitransparent top cathode of lithium fluoride, aluminum and silver created a microcavity that increased the electroluminescence efficiency by approximately 50 percent compared with conventional organic LEDs that employ a polarizer. The cathode and an antireflection coating reduced the effective reflectance in the visible from the surface of the emitter to approximately 3.9 percent. Increasing the reflectivity of the bottom electrode would further improve the efficiency, the researchers note, but it also would increase the reflectance of the structure.

In a proof-of-principle demonstration, the scientists fabricated a 240 × 320-pixel, active-matrix display using the high-contrast top-emitting organic LEDs. Under approximately 1000-l× illumination, the display exhibited higher contrast than one comprising organic LEDs with a conventional, bottom-emitting device structure.