**2. Light extraction**

As far as the material for organic light-emitting diodes (OLEDs) is concerned, semiconductor-based organic light emitters are the obvious choice because semiconducting organic light-emitting materials have reached a high level with internal quantum efficiencies of ~100% [1]. Unfortunately, however, most of this light is trapped inside OLEDs, and only 20% can be outcoupled because of the total internal reflection [2-7]. In this section, various light extraction technologies are reviewed to suppress guided light loss. In particular, the enhanced light extraction efficiency by means of photonic structures onto OLEDs is discussed in depth.
