**2.1. OLEDs on the GBO polarizer substrates**

44 Organic Light Emitting Devices

OLEDs (WOLEDs).

obtain the polarized emission of EL light.

Here we introduce an approach different from the conventional methods using uniaxially oriented materials. As an alternative, for the purpose of improving device performance, we suggest a technique to control the polarization of light emitted from OLEDs that are achieved using an anisotropic photonic crystal (PC) film. It has been predicted that in anisotropic PCs, the photonic band structure splits with respect to the state of polarization of the interacting light, in contrast to the degenerated band structure of conventional isotropic PCs, in which a certain energy range of photons is forbidden, giving rise to a photonic band gap (PBG) [18-20]. Of these applications, the study of light emission at the PBG edge is particularly attractive, as a result of the fact that the group velocity of photons approaches zero and the density of mode changes dramatically at the PBG edge [21-24]. The combination of PCs with OLEDs has also been reported to achieve high out-coupling emission efficiency, as achieved in the micro-cavity OLEDs or multi-mode micro-cavity OLEDs [25-27]. Moreover, by employing the anisotropic photonic structure, one may also

In this chapter, we describe in brief a technique to control the polarization of EL light emitted from photonic OLEDs that make use of a *Giant Birefringent Optical* (GBO) [28] multilayer reflective polarizer [29-31] as the anisotropic PC film. When a large degree of birefringence is introduced into the in-plane refractive index between adjacent material layers of a multilayer photonic system, GBO effects begin to occur [28]. Pairs of groupings of adjacent layers (unit cells) can produce constructive interference effects when their thicknesses are scaled properly to the wavelength of interest. These interference effects in multilayered structures result in the development of alternating wavelength regions of high reflectivity (reflection bands) adjacent to wavelength regions of high transmission (pass bands) [28]. A significant optical feature of these multilayer interference stacks is the difference in the refractive index in the thickness direction (*z* axis) relative to the in-plane directions (*x* and *y* directions) of the film. By appropriate adjustment of the refractive indices of the adjacent layers, it is possible to construct a GBO multilayer reflecting polarizer using an interference stack that is composed of multiple layers of transparent polymeric materials [28]. The reflection band of the GBO polarizer exhibits a unique optical property, where the reflectivity of interference polarizers either remains constant or increases with increasing the angle of incidence. Furthermore, a graded unit cell thickness profile is normally used to create a wider reflective band that accommodates wavelengths from the blue through to the green and red color regions [28]. Such a multilayer polymer polarizer may routinely be used for optical applications that require high reflectivity and wavelength selectivity. As an example of this application, GBO multilayer polarizers have been used to create reflective polarizers that make LC displays brighter and easier to view. By using this property of the GBO polarizer, one might obtain highly linearly polarized EL light emission over a wide range of optical wavelengths. These anisotropic photonic effects of GBO cause the reflecting band structure to be polarized, and thus make it possible to show that such a combined OLED device can achieve polarized light-emission with high brightness and efficiency, resulting in a high *PR* value even for wideband EL emission from white light-emitting In this section, we describe the polarization of EL light emitted from OLEDs that use a flexible GBO multilayer reflecting polymer polarizer substrate, instead of the conventional isotropic glass substrate. By using such a substrate, we demonstrate the potential for highly polarized light emission from OLEDs. Luminous EL emissions are produced from the polarized photonic OLEDs, and the direction of polarization for the emitted EL light corresponds to the polarizing axis (transmission axis or passing axis) of the GBO reflecting polarizer. The estimated polarization ratio between the brightness of two linearly polarized EL emissions parallel and perpendicular to the polarizing axis can be achieved as high as 25 for the OLEDs on GBO substrates.

**Figure 1.** (a) Photograph showing the flexible transparent GBO reflecting polymer polarizer film and (b) SEM image of the cross-section of the studied GBO film.
