**4. Summary**

After summarizing methods for enhanced outcoupling in OLED devices, we reviewed enhancement methods using photonic structures at a surface. As explained in sections 2.3 and 2.4, corrugated EL devices with periodic or quasi-periodic nanostructures show enhanced light extraction performance compared with the reference flat device without such structures. The principle of light extraction is the same in both device configurations; however, periodically corrugated EL device shows higher light extraction only at a specific wavelength because of the well-defined corrugation pitch. This anisotropic angular dependence does not satisfy the Lambertian emission pattern, which is an important requirement in the lighting technologies. On the other hand, spontaneously formed buckling patterns on OLEDs towards air are effectively used as a quasi-periodic structure to extract light from the waveguide modes. The characteristics of the broad periodicity distribution and randomly oriented wave vectors of buckles provide an invaluable advantage of possible outcoupling of the waveguide light propagating along any direction with a wide spectral range. Namely a buckling device shows a Lambertian emission pattern with an increase in emission angle, which satisfies the requirement of OLED lighting. In particular, it enhances the outcoupling of waveguide modes at various wavelength ranges due to the broad distribution of the periodicity that can be applied to white OLEDs. We may conclude that the buckling device structure overcomes the limitation of periodic nanostructures and it can be used in the development of white OLEDs for lighting.

We have also demonstrated the generation of highly circularly polarized EL and its tunability using a liquid crystal phase retarder. A wide-band reflector has enabled us to obtain high ratio of brightness between R-CP-EL and L-CP-EL with the overall intensity ratio of about 10 and *g*-factor of about 1.6 over the whole emission band. Also, using a voltage dependent phase retarder, we have confirmed that there is no limitation for choosing emissive materials to obtain the tunable polarized EL light. We have also shown that the devices with buckles have double current and power efficiencies over the entire emission wavelengths and emission angles. This is achieved without any spectral changes even though ITO thickness is thin (40 nm) to outcouple the only SP (TM) mode which would otherwise be lost into the Al cathode layer of OLED devices. It is also found that the diffraction of the SP mode by buckles causes polarization conversion to the TE mode with a higher light intensity than in the TM mode, which occurs due to the random orientation of the buckling structure.
