After development of phosphorescence materials, the internal quantum efficiency of organic light emitting diodes (OLEDs) reaches close to unity. However, the external quantum efficiency is still as low as ~20%. This is caused by the use of metallic cathodes. In contrast to inorganic LEDs metal is almost unavoidable material for cathodes due to its conductivity and work function and the metal cathode must completely cover whole device area. Additionally, the distance between the metal cathode and the emitting layer is ~100 nm. Thus, most of the energy of excitons is transferred to surface plasmons and other guided modes. Figure 1 shows modes to which exciton energy dissipates and the fraction of the energy dissipation to each mode. As can be seen, only 14% of energy is extracted into free space, whereas more than half of energy dissipates to surface plasmons, which just turn into heat. Thus, it is important to extract energy from the surface plasmons as photons for improving the efficiency. Appropriate plasmonic structures improve the light extraction efficiency. The introduction of 2-D periodic surface corrugation and aperiodic surface corrugation improves the light extraction efficiency of OLEDs for monochromatic light and that for white light, respectively.

© 2013 Japan Society of Applied Physics, Optical Society of America

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