Metal–organic–metal cathode for high-contrast organic light-emitting diodes

Organic light-emitting diodes have been fabricated using a stacked metal–organic–metal cathode structure. The first metal layer is embedded approximately between an emission layer and an electron transport layer serving as a floating electron injection electrode. The second metal layer is a terminal layer serving as a contacting electrode. The optical transmission of the embedded electrode and the optical path between the two electrodes were used together to maximize destructive optical interference, thus rendering the cathode nonreflective. These devices also showed excellent diode characteristics; for example, a luminance of 100 cd/m2 at ∼3 V. Optical reflectance in the visible spectrum for the diodes was found to be reduced from ∼80% on conventional devices to ∼20% on devices with the metal–organic–metal cathode.