**3. Polarized electroluminescence**

The polarization control of light is important for optical information processing, display and storage devices. Although linearly polarized light has already been applied to various optical devices, there are only a few reports on circularly polarized devices. However, the potential applications of circularly polarized light have been suggested for optical data storages and flat panel displays. Recently, the research for active devices that can emit polarized light has gained attention [51-58]. Peeter *et al.* [56] have first demonstrated circularly polarized (CP) EL from a polymer LED using a chiral π-conjugated poly(*p*phenylenevinylene) (PPV) derivative as an active layer, although the degree of circular polarization was very low. Later, Oda *et al*. [57] have succeeded in obtaining a high CP-EL using main-chain polymer liquid crystals (LCs) and chiral-substituted polyfluorenes (PF) as an active layer. However, the degree of circular polarization was still insufficient for applications in optical devices. More recently, Grell *et al.* [58] have proposed a new idea for CP-EL without using chiral active materials and succeeded in achieving high degree of circular polarization. They used a simple CP-EL device that can be driven by nonchiral polymer LED using "photon recycling" concept developed by Belayev *et al.* [59]. Belayev *et al*. and Grell *et al*. used a chiral nematic liquid crystal (cholesteric liquid crystal; CLC) cell attached to the glass side of polymer LED and obtained a high degree of circular polarization at the center of the stop band. However, the degree of circular polarization outside of the stop band rapidly decreased, because the emissive material had wider emission band than the stop band width formed.

86 Organic Light Emitting Devices

the wavelength dependence of the enhanced emission by considering the intensity ratio of the two spectra in the devices with and without buckling (Fig. 19(b)). The calculated peak wavelengths of the TE0 and TM0 modes for the first-order diffraction are consistent with the broad peak intensities in Fig. 19(b), although the enhancement due to the TM0 mode is not distinct because of the weak emission intensity above 700 nm. The relatively flat enhancement by a factor of ~2.2 around λ0=525 nm in the devices with triple buckling is partially due to the relatively weak first- and second-order diffraction TE0 and TM0 modes, whereas the remarkable enhancement (factor of 4.0) around 655 nm is mainly due to the strong first-order diffraction in TE0 and TM0 modes (see Fig. 19(b)). These results indicate that a further enhancement of more than a factor of at least 2.2 can be expected if the peak wavelength of the buckles is optimized for the TE0 and TM0 modes to be diffracted at around 525 nm in the normal direction. Moreover, the broad distribution of periodicity in the buckling structure suggests that the entire emission wavelength range over blue, green and red in white OLEDs can be simultaneously outcoupled by only one grating structure. The angular dependence of the light intensity for the devices is shown in Fig. 19(c). It is interesting to note that all devices with and without buckling show a Lambertian emission pattern with a maximum intensity in the normal direction. According to the Bragg equation, the first-order diffraction angles of the TE0 and TM0 modes around the main emission wavelength of 525 nm by the grating period of 410 nm are expected to be between 20° and 40°. However, because *k*G has random orientation and broad periodicity due to the buckling, it is distributed over all azimuthal directions in contrast to one- or two-directional *k*G in conventional corrugated OLEDs [21-24,27,28,33]. Thus, the outcoupled emission

concentrates into the normal direction, resulting in the Lambertian emission pattern.

The polarization control of light is important for optical information processing, display and storage devices. Although linearly polarized light has already been applied to various optical devices, there are only a few reports on circularly polarized devices. However, the potential applications of circularly polarized light have been suggested for optical data storages and flat panel displays. Recently, the research for active devices that can emit polarized light has gained attention [51-58]. Peeter *et al.* [56] have first demonstrated circularly polarized (CP) EL from a polymer LED using a chiral π-conjugated poly(*p*phenylenevinylene) (PPV) derivative as an active layer, although the degree of circular polarization was very low. Later, Oda *et al*. [57] have succeeded in obtaining a high CP-EL using main-chain polymer liquid crystals (LCs) and chiral-substituted polyfluorenes (PF) as an active layer. However, the degree of circular polarization was still insufficient for applications in optical devices. More recently, Grell *et al.* [58] have proposed a new idea for CP-EL without using chiral active materials and succeeded in achieving high degree of circular polarization. They used a simple CP-EL device that can be driven by nonchiral polymer LED using "photon recycling" concept developed by Belayev *et al.* [59]. Belayev *et al*. and Grell *et al*. used a chiral nematic liquid crystal (cholesteric liquid crystal; CLC) cell attached to the glass side of polymer LED and obtained a high degree of circular polarization at the center of the stop band. However, the degree of circular polarization

**3. Polarized electroluminescence** 

For evaluating the degree of circular polarization at a certain wavelength λ, a *g*-factor is used which is defined as:

$$\log(\lambda) = 2\frac{I\_L(\lambda) - I\_R(\lambda)}{I\_L(\lambda) + I\_R(\lambda)} = 2\frac{r(\lambda) - 1}{r(\lambda) + 1} \tag{14}$$

where *I*L/R is the intensity of left/right-handed CP (L-CP, R-CP) light, and *r* is the left/righthanded intensity ratio, *I*L(λ)/*I*R(λ). It is evident that |*g*(λ)| is zero for nonpolarized light (*r*()=1) and is equal to -2 for pure, single-handed circularly polarized light (*r*()= ∞ or 0). The *g*(λ) values found were 0.001 [56], 0.25 [57], and 1.6 [58], but only in a narrow wavelength range. Woon *et al.* and Geng *et al.* respectively reported circularly polarized PL [60] and EL [61] with a constant *g*(λ) value over a wide spectral range covering most of the emission band. However, the bandwidth [60] and *g*(λ) value [61] were still insufficient for application to commonly used emissive materials with wide emission band.

To achieve a tunable polarization of electroluminescence, we have used combination of voltage dependent nematic liquid crystal (NLC) phase retarders and photon recycling concept [62,63]. The phase retardation arises between two optical eigenmodes during light propagation in an anisotropic medium as a phase retarder. Upon emerging from the phase retarder, the relative phase of the two eigenmodes is found to be different from that at the incidence, and thus their polarization state becomes different as well [64-66]. Now suppose we apply a voltage (*V*) across the cell filled with NLC, by which the liquid crystal molecules change their orientation toward the field direction, if the NLC has positive dielectric anisotropy. With increasing the voltage, the birefringence e 0 *nE n E n* – decreases, where *ne* and *no* are refractive indices for extraordinary-(*e*-) and ordinary-(*o*-) light waves, respectively, and the retardation () decreases as well. Hence, as the *e*- and *o*-waves propagate through the NLC cell, their relative phase difference changes, and the state of polarization of the wave also changes.

We have introduced another polarization characteristics, namely polarization conversion in surface plasmon (SP) coupled emission by buckling structures. In section 2.4, we have demonstrated that the quasi-periodic buckling structures with broad distribution and directional randomness can effectively enhance the light-extraction efficiency by outcoupling the waveguide modes without introducing spectral changes and directionality [36]. In this study, however, we could not differentiate the outcoupling of transverse electric (TE) mode from that of the surface-plasmon (SP) mode (transverse magnetic (TM) mode) by buckles because of the broad periodicity of the buckling structure and the similar propagation vectors of the TE and SP modes. The explanation of polarization conversion in the surface-plasmon-coupled emission presented here is based on a trial method for distinguishing TE and TM modes in light enhancement in OLEDs with buckling pattern. However in this trial approach, an interesting phenomenon of polarization conversion in SP coupling has been observed.

In this section, we have summarized and introduced our studies regarding not only circularly polarized EL and its tunability but also the polarization conversion in surface coupled emission from corrugated OLEDs with buckling structures.

Effect of Photonic Structures in Organic Light-Emitting Diodes

The fabrication method of a tunable phase retarder is as follows. The single-layered PCLC films are fabricated by spin coating the solution onto ITO glass substrates coated with PI rubbed unidirectionally at room temperature. The coated PCLC films are cured for 30 min at a temperature over 160 °C in a bake oven, and then quenched to room temperature. The sample cell is made of L-PCLC and PI coated glass substrates and is sustained by spacer. The NLC (ZLI2293, Merck) is introduced into an empty cell using capillary action. The

The OLED structure used here is fabricated in the same way as described in section 2.2. The vacuum evaporated OLEDs with structure of ITO/CuPc/TPD/Alq3/LiF/Al are described in section 3.2, an spin-coated OLEDs with structure of ITO/PEDOT:PSS/MEH-CN-PPV/LiF/Al

**Figure 21.** Schematic illustration of tunable phase retarder. 63 Copyright 2008, American Institute of

The device configuration for highly CP-EL from OLEDs is illustrated in Fig. 22. We have simply attached an L-PCLC reflector to an OLED device. After generating the unpolarized light by electrical pumping, the R-CP-EL transmits through the PCLC reflector, whereas L-CP-EL is reflected by the selective reflection of the L-PCLC. This reflected light by the PCLC is still L-CP and changes the polarization to R-CP by getting reflected at the metal surface, and gets transmitted trough the L- PCLC reflector. Thus all the transmitted light has the

In comparison with the previous work [58] here the band width of the reflector is wider. For fabricating a wide-band CLC reflector, the use of PCLCs has two advantages compared with general low-molecular-weight CLCs. First advantage is that PCLCs used here have higher optical anisotropy (*n*e-*n*o=0.22), resulting in a wider photonic band gap (PBG). The second advantage is that PCLC films can be easily stacked to multi-layered films by spin-casting. Using these technical advantages we have fabricated a wide-band PCLC reflector using

The structure of a three-layered PCLC film with a wide stopband width is shown in Fig. 23(a). The fabrication method of multi-layered PCLC films is already explained in section 3.1. Figure 23(b) shows the reflectance spectra of single-layered and three-layered PCLC

illustration of the fabrication of the final cell is shown in Fig. 21

**3.2. Highly circularly polarized electroluminescence** 

multi-layered PCLC films with different selective reflection bands.

are given in section 3.3.

same sense of rotation, R-CP.

Physics.

– Light Extraction and Polarization Characteristics 89
