**1. Introduction**

Since the early pioneering work on efficient Organic Light-Emitting Devices (OLEDs) that was based on both small molecules and polymers, OLEDs have attracted a great deal of research interest due to their promising applications in full-color flat-panel displays and solid-state lighting [1-5]. Intensive research has been conducted into the development of OLEDs for realizing strong and efficient electroluminescent (EL) emission. To date, almost all previous work carried out on organic EL emission has involved unpolarized EL emission. Nevertheless, a number of researchers have reported the results of experiments in which linearly polarized EL emissions have been observed [6-17]. This particular avenue of research has been considered to be important because polarized EL emission from OLEDs is of potential use in a range of applications, not just those limited to high-contrast OLED displays, but also in efficient backlight sources in liquid crystal (LC) displays, optical data storage, optical communication, and stereoscopic 3D imaging systems [17]. In order to design and manufacture these novel light-emitting devices, a high degree of polarization ratio (*PR*) of emitting light is required, which has to be at least 30 ~ 40:1, between the brightness of two linearly polarized EL emissions that are parallel and perpendicular to the polarizing axis. Most cases of linearly polarized EL emission have been achieved through the use of uniaxially oriented materials, such as LC polymers or oligomers, incorporated within emissive layers. Methods that are commonly used for the uniaxial alignment of such layers include the Langmuir-Blodgett technique [6], rubbing/shearing of the film surface [7, 8], mechanical stretching of the film [9, 10], orientation on pre-aligned substrates [11, 12], precursor conversion on aligned substrates [13], epitaxial vapor deposition [14], and the friction-transfer process approach [15, 16]. Although there have been a number of such efforts to achieve linearly polarized EL emission, the polarization ratio and the device performance (in terms of brightness and efficiency) reported are still insufficient for most applications.

© 2012 Park, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Park, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 obtain the polarized emission of EL light.

Polarized Light-Emission from Photonic Organic Light-Emitting Devices 45

**2. Polarized photonic OLEDs with GBO films** 

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

(b) SEM image of the cross-section of the studied GBO film.

*2.1.1. Device fabrication and materials used* 

for the OLEDs on GBO substrates.

Three kinds of polarized photonic OLEDs are presented here to demonstrate the use of the

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

**Figure 1.** (a) Photograph showing the flexible transparent GBO reflecting polymer polarizer film and

Sample OLEDs were prepared by placing an EL layer between an anode and a cathode on a flexible GBO reflecting polarizer film in the following sequence: GBO reflecting polarizer film substrate / thin semi-transparent Au anode / hole-injecting buffer layer / EL layer / electron-injecting layer / Al cathode. For the GBO reflecting polarizer film, a commercial multilayer reflecting polymer polarizer film (3M) has been used. The film is approximately 90 m thick, and the wavelength of the reflection band is found to be in an approximate range of 400 ~ 800 nm. This film is normally used in an LC display backlight unit as a reflecting polarizer film. After routine cleaning of the GBO reflecting polarizer film using ultraviolet-ozone treatment, a flexible semi-transparent thin Au layer was deposited (90 nm, 40 ohm/square) by sputtering onto the GBO reflecting polarizer to form the anode. This Au anode is used in preference to the typical rigid indium-tin-oxide (ITO) anode in order to preserve the flexibility of the GBO polarizer substrate. The optical transmittance of the Au

GBO film in the highly polarized OLEDs, exhibiting high brightness and efficiency.

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 OLEDs (WOLEDs).
