**3.2. Highly circularly polarized electroluminescence**

88 Organic Light Emitting Devices

**3.1. Device fabrication** 

fabrication of single-layer films.

PCLC (photonic band gap wavelength) is controlled.

**Figure 20.** Fabrication process of multi-layered PCLC films.

thus fabricated are cured for 30 min at 160 °C.

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

We have fabricated multi-layered polymer CLC (PCLC) films for using them as wide-band reflectors or single-layered films for polarization-tunable OLEDs. As an experimental method for fabricating single-layered PCLC films is a part of fabricating multi-layered PCLC films, we introduce here only the fabrication of multi-layered films and skip the

The fabrication process of multi-layered polymer PCLC films is shown in Fig. 20. Mixtures of two aromatic polyester liquid crystalline polymers (Nippon Oil Corporation; currently, JX Nippon Oil & Energy Corporation) are used to make PCLCs. One of the polymers (chiral polymer) contains 25% chiral units in its chemical composition and the other contains no chiral unit. By changing the ratio of the amounts of the two polymers, the helical pitch of

For fabricating three-layered PCLC films for use as a wide band reflector, the PCLC (λp=610 nm; chiral polymer 72 wt%) is spin-cast on glass substrates with unidirectionally rubbed polyimide (PI ; AL1256, JSR). Then, aqueous solution of polyvinyl alcohol (PVA) is spin-cast and the film surface is rubbed again unidirectionally. Another PCLC (λp= 510 nm; chrial polymer 87 wt%) is spin-cast on the rubbed PVA surface. The same procedure is repeated for preparing the third PCLC film (λp= 530 nm; chrial polymer 82 wt%). Finally PCLC films

coupled emission from corrugated OLEDs with buckling structures.

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 same sense of rotation, R-CP.

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 multi-layered PCLC films with different selective reflection bands.

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

films and the emission spectrum of the active EL material, Alq3 (see below). A wider selective reflection band formed due to the overlap of the selective reflection bands of the three-layered PCLC films extends to the whole emission band, although the selective reflection band of the single-layered PCLC film covers only the emission peak region.

Effect of Photonic Structures in Organic Light-Emitting Diodes

**0.6 0.8 1.0 1.2**

**0.2 0.4 Refle**

**0 6 0.8 1.0 1.2**

**0.0 0.2 0.4 0.6** **ctance**

**Reflec**

**ectance**

It is also noted that the degree of circular polarization is high in the wide-PCLC device over the whole emission band. Figure 25 shows the wavelength dependence of the *g*-factor [eq. (14)] for light emitted from each device. At the center of the stopband, |*g*(λ)| approaches to 1.67 in both the devices with PCLC films. However, the difference is that |*g*(λ)| remains same over the whole emission band in the wide-PCLC device but it suddenly decreases

**R-CP-EL**

**400 500 600 700 800**

**Wavelength (nm)**

**R-CP-EL**

**1500.0**

**narrow PCLC**

**wide PCLC**

**L-CP-EL**

**Figure 24.** R- and L-CP-EL spectra from OLED devices with (a) narrow- and (b) wide-PCLC films. Reflection spectra for the narrow- and wide-PCLC films are also shown using dotted curves.62

**L-CP-EL**

**400 500 600 700 800**

**Wavelength (nm)**

**Figure 25.** Calculated *g*-factor values in each device over the whole wavelength range of the emission

outside of the stopband in the narrow-PCLC device.

**0 50 100**

**ty (a.u.)**

**Intensit**

(b)

**sity (a.u.)**

**Intens**

(a)

Copyright 2007, American Institute of Physics.

band of Alq3.62 Copyright 2007, American Institute of Physics.

– Light Extraction and Polarization Characteristics 91

**Figure 22.** Schematic illustration of a 'photon recycling' device. 62 Copyright 2007, American Institute of Physics.

**Figure 23.** (a) The structure of a three-layered PCLC film. (b) Normalized electroluminescence spectrum of Alq3 and reflectance spectra of single-layered and three-layered PCLC films. 62 Copyright 2007, American Institute of Physics.

To evaluate the degree of circular polarization quantitatively, R-polarizer and L-polarizer are inserted between the EL device and detector. We confirmed that R- and L-polarized EL intensities are almost the same in OLEDs without a PCLC reflector. In contrast, OLEDs with narrow (single-layered)-PCLC and with wide (three-layered)-PCLC reflectors emit high intensity R-circularly polarized EL within the stopband of PCLC as shown in Fig. 24(a) and 24(b). The R- and L-CP-EL spectra from OLEDs with the narrow-PCLC film are almost the same as that with the wide-PCLC film in the selective reflection region of narrow-PCLC (480nm–560nm) as a result of 'photon recycling'. Outside of the stopband of narrow-PCLC, however, both R- and L-CP components from the narrow-PCLC device do not show any prior circularly polarization characteristics due to the lack of 'photon recycling' (Fig. 24(a)), whereas the wide-PCLC device shows the highly R-circularly polarized light over the whole emission spectrum range, as shown in Fig. 24(b).

It is also noted that the degree of circular polarization is high in the wide-PCLC device over the whole emission band. Figure 25 shows the wavelength dependence of the *g*-factor [eq. (14)] for light emitted from each device. At the center of the stopband, |*g*(λ)| approaches to 1.67 in both the devices with PCLC films. However, the difference is that |*g*(λ)| remains same over the whole emission band in the wide-PCLC device but it suddenly decreases outside of the stopband in the narrow-PCLC device.

90 Organic Light Emitting Devices

Physics.

2007, American Institute of Physics.

emission spectrum range, as shown in Fig. 24(b).

films and the emission spectrum of the active EL material, Alq3 (see below). A wider selective reflection band formed due to the overlap of the selective reflection bands of the three-layered PCLC films extends to the whole emission band, although the selective

**Figure 22.** Schematic illustration of a 'photon recycling' device. 62 Copyright 2007, American Institute of

**(b)**

**Figure 23.** (a) The structure of a three-layered PCLC film. (b) Normalized electroluminescence spectrum of Alq3 and reflectance spectra of single-layered and three-layered PCLC films. 62 Copyright

**0.4 0.6 0.8 1.0**

**0.0 0.2**

**c**

**tance (%)**

**0.4 0.6 0.8 1.0**

**0.0 0.2**

**3-layer L-CLC 1-layer L-CLC Alq3 emission**

**400 500 600 700 800**

**Wavelength (nm)**

**InteReflec**

**e**

**nsity (a.u.)**

To evaluate the degree of circular polarization quantitatively, R-polarizer and L-polarizer are inserted between the EL device and detector. We confirmed that R- and L-polarized EL intensities are almost the same in OLEDs without a PCLC reflector. In contrast, OLEDs with narrow (single-layered)-PCLC and with wide (three-layered)-PCLC reflectors emit high intensity R-circularly polarized EL within the stopband of PCLC as shown in Fig. 24(a) and 24(b). The R- and L-CP-EL spectra from OLEDs with the narrow-PCLC film are almost the same as that with the wide-PCLC film in the selective reflection region of narrow-PCLC (480nm–560nm) as a result of 'photon recycling'. Outside of the stopband of narrow-PCLC, however, both R- and L-CP components from the narrow-PCLC device do not show any prior circularly polarization characteristics due to the lack of 'photon recycling' (Fig. 24(a)), whereas the wide-PCLC device shows the highly R-circularly polarized light over the whole

reflection band of the single-layered PCLC film covers only the emission peak region.

**Figure 24.** R- and L-CP-EL spectra from OLED devices with (a) narrow- and (b) wide-PCLC films. Reflection spectra for the narrow- and wide-PCLC films are also shown using dotted curves.62 Copyright 2007, American Institute of Physics.

**Figure 25.** Calculated *g*-factor values in each device over the whole wavelength range of the emission band of Alq3.62 Copyright 2007, American Institute of Physics.

#### **3.3. Polarization-tunable organic light-emitting diodes**

In this section, we examine the electro-tunable polarization of electroluminescence by combination of circularly polarized OLEDs (same concept as explained in section 3.2) and tunable phase retarder. A voltage controllable liquid crystal cell is adopted as a tunable phase retarder for tunable polarization characteristics.

The device configuration for polarization-tunable OLEDs with a phase retarder is shown in Fig. 26. For the phase retardation, NLC is filled between the glass substrates with a rubbed PI layer. The phase retarder is simply attached to one of the glass sides of OLEDs. After the generation of unpolarized light from OLED, the whole EL light is extracted as R-CPL by photon recycling. This R-CP-EL can be transformed into arbitrary polarizations by changing the orientation of NLC through applying a voltage. The phase retardation at a wavelength can be expressed by:

$$
\Delta \varphi = \frac{2\pi}{\lambda} d \Delta n
\tag{15}
$$

Effect of Photonic Structures in Organic Light-Emitting Diodes

V correspond to 3λ/2, 5λ/4, λ, 3λ/4 wave plates, respectively. When the R-polarizer is inserted, the spectrum shows a selective reflection band at 0 V (=3λ/2) as shown in Fig. 27(a). This is because the transmitted R-CPL changes its polarization to L-CPL through L-PCLC, and the transmittance decreases down to 0.15 within the selective reflection band. As the voltage increases up to 6 V (=λ), the spectral shape shows no selective reflection band because the phase retarder acts as a full-wave plate. On the other hand, the situation is

> **0.8 1.0**

> **0.0 0.2 0.4 0.6**

**0.8 1.0**

**0.0 0.2 0.4 0.6**

**nce**

**Transmittan**

**nce**

**Transmitta**

**Figure 27.** Polarization characteristics of voltage dependent transmittance spectra of a phase retarder. Transmittance spectra of (a) R-CPL, (b) L-CPL, (c) LPL(+45°) and (d) LPL(-45°) under fields of 0, 6, 4.5

Conversion to linearly polarized light is also possible by 4.5 (=5λ/4) and 7.5 V (=3λ/4) applications. If the phase retardation is a quarter-wave, R-CPL changes the polarization condition to linearly-polarized light (LPL). Figure 27(c) and (d) shows LPL with electric field direction of +45° and -45°, respectively. At 4.5 V, the phase retardation is 5λ/4 resulting in a LPL (+45°) as shown in Fig. 27(c). The transmitted R-CPL changes into LPL (-45°) and shows a selective reflection band when the direction of linear polarizer is +45°. On the other hand, if the phase retardation is 3λ/4 (=7.5 V), the transmitted R-CPL changes into LPL (+45°) after transmitted through the linear polarizer. As a result, no selective reflection is observed in the transmittance spectrum. Reversed situation is also observed when the direction of linear

and 7.5 V, respectively. 63 Copyright 2008, American Institute of Physics.

**500 600 700 800 Wavelength (nm)**

polarizer is -45°, as shown in Fig. 27(d).

reversed in L-polarizer as shown in Fig. 27(b).

(a) (b)

**3**λ**/4 (=7.5V)** λ **(=6V) 5**λ**/4 (=4.5V)**

**3**λ**/4 (=7.5V)** λ **(=6V) 5**λ**/4 (=4.5V)**

**3**λ**/2 (=0V)**

**3**λ**/2 (=0V)**

**500 600 700 800**

(c) (d)

**Wavelength (nm)**

**0.8 1.0**

**0.0 0.2 0.4 0.6**

**0.8 1.0**

**0.0 0.2 0.4 0.6**

**nce**

**Transmittan**

**nce**

**Transmitta**

– Light Extraction and Polarization Characteristics 93

**500 600 700 800**

**500 600 700 800 Wavelength (nm)**

**Wavelength (nm)**

**3**λ**/4 (=7.5V)** λ **(=6V) 5**λ**/4 (=4.5V)**

**3**λ**/4 (=7.5V)** λ **(=6V) 5**λ**/4 (=4.5V)**

**3**λ**/2 (=0V)**

**3**λ**/2 (=0V)**

where *d* is cell thickness and *n* is birefringence of NLC.

**Figure 26.** Schematic illustration of the principle of polarization-tunable OLED and polarized light with different polarization.63 Copyright 2008, American Institute of Physics.

If the wavelength and cell thickness are constant, phase retardation between *e*- and *o*-waves can be varied by applying an electric field. Then the effective birefringence of NLC *n*() is determined by the angle between the director and the substrate surface; i.e., *n*(=0°)=*n* and *n*(=90°)=0. If *dn*() is equal to (2 1) / 2 *m* (*m* = 0, 1, 2…), NLC layer acts as a halfwave plate. On the other hand, if *dn*() is equal to (4 1) / 4 *m* (*m* = 0, 1, 2…), the NLC layer acts as a quarter wave plate. It should be noted, however, that the maximum phase retardation must be over half-wavelength (λ/2) to realize four kinds of different polarizations. Hence we have fabricated a cell with the thickness satisfying 3λ/2 retardation condition in the absence of a field.

Figure 27 shows the voltage dependence of transmittance spectra through R-, L- circular and linear polarizers with the direction of +45° and -45°. The applied voltages of 0, 4.5, 6 and 7.5 V correspond to 3λ/2, 5λ/4, λ, 3λ/4 wave plates, respectively. When the R-polarizer is inserted, the spectrum shows a selective reflection band at 0 V (=3λ/2) as shown in Fig. 27(a). This is because the transmitted R-CPL changes its polarization to L-CPL through L-PCLC, and the transmittance decreases down to 0.15 within the selective reflection band. As the voltage increases up to 6 V (=λ), the spectral shape shows no selective reflection band because the phase retarder acts as a full-wave plate. On the other hand, the situation is reversed in L-polarizer as shown in Fig. 27(b).

92 Organic Light Emitting Devices

**3.3. Polarization-tunable organic light-emitting diodes** 

phase retarder for tunable polarization characteristics.

where *d* is cell thickness and *n* is birefringence of NLC.

different polarization.63 Copyright 2008, American Institute of Physics.

wave plate. On the other hand, if *dn*() is equal to (4 1) / 4 *m*

and *n*(=90°)=0. If *dn*() is equal to (2 1) / 2 *m*

condition in the absence of a field.

wavelength can be expressed by:

In this section, we examine the electro-tunable polarization of electroluminescence by combination of circularly polarized OLEDs (same concept as explained in section 3.2) and tunable phase retarder. A voltage controllable liquid crystal cell is adopted as a tunable

The device configuration for polarization-tunable OLEDs with a phase retarder is shown in Fig. 26. For the phase retardation, NLC is filled between the glass substrates with a rubbed PI layer. The phase retarder is simply attached to one of the glass sides of OLEDs. After the generation of unpolarized light from OLED, the whole EL light is extracted as R-CPL by photon recycling. This R-CP-EL can be transformed into arbitrary polarizations by changing the orientation of NLC through applying a voltage. The phase retardation at a

> <sup>2</sup> *d n*

(15)

(*m* = 0, 1, 2…), NLC layer acts as a half-

(*m* = 0, 1, 2…), the NLC

**Figure 26.** Schematic illustration of the principle of polarization-tunable OLED and polarized light with

If the wavelength and cell thickness are constant, phase retardation between *e*- and *o*-waves can be varied by applying an electric field. Then the effective birefringence of NLC *n*() is determined by the angle between the director and the substrate surface; i.e., *n*(=0°)=*n*

layer acts as a quarter wave plate. It should be noted, however, that the maximum phase retardation must be over half-wavelength (λ/2) to realize four kinds of different polarizations. Hence we have fabricated a cell with the thickness satisfying 3λ/2 retardation

Figure 27 shows the voltage dependence of transmittance spectra through R-, L- circular and linear polarizers with the direction of +45° and -45°. The applied voltages of 0, 4.5, 6 and 7.5

**Figure 27.** Polarization characteristics of voltage dependent transmittance spectra of a phase retarder. Transmittance spectra of (a) R-CPL, (b) L-CPL, (c) LPL(+45°) and (d) LPL(-45°) under fields of 0, 6, 4.5 and 7.5 V, respectively. 63 Copyright 2008, American Institute of Physics.

Conversion to linearly polarized light is also possible by 4.5 (=5λ/4) and 7.5 V (=3λ/4) applications. If the phase retardation is a quarter-wave, R-CPL changes the polarization condition to linearly-polarized light (LPL). Figure 27(c) and (d) shows LPL with electric field direction of +45° and -45°, respectively. At 4.5 V, the phase retardation is 5λ/4 resulting in a LPL (+45°) as shown in Fig. 27(c). The transmitted R-CPL changes into LPL (-45°) and shows a selective reflection band when the direction of linear polarizer is +45°. On the other hand, if the phase retardation is 3λ/4 (=7.5 V), the transmitted R-CPL changes into LPL (+45°) after transmitted through the linear polarizer. As a result, no selective reflection is observed in the transmittance spectrum. Reversed situation is also observed when the direction of linear polarizer is -45°, as shown in Fig. 27(d).

In order to apply this concept to OLEDs, we have attached a phase retarder to an EL device. This situation is different from the transmittance measurement system because here the EL device has a metallic mirror as a cathode. The output of EL light is R-CP-EL, as explained in Fig. 26. Hence different polarization states are also possible by controlling the birefringence of the NLC layer. To evaluate the degree of polarization quantitatively, R-, L-circular or linear polarizer with the direction of +45° and -45° is inserted in the emissive EL devices between the phase retarder and detector. The output of EL light transmitted from the L-PCLC is R-CP-EL within the wavelength range corresponding to the stopband. The emitted R-CP-EL can be changed into a different polarization by the phase retardation. Figure 28 shows the polarized EL spectra with different polarizations as applied voltage increases from 0 (Fig. 28(a)) to 4.5 (Fig. 28(c)), 6 (Fig. 28(b)), and 7.5 V (Fig. 28(d)). Thus EL light with different polarizations can be selectively emitted by varying the voltage. Outside of the stopband of PCLC, the intensity of opposite polarized light becomes higher because the stopband of PCLC cannot cover a wide wavelength range. It should be noted, however, if a multilayered-PCLC with different pitches is used, the polarization rate can be high over all wavelength [62,67].

Effect of Photonic Structures in Organic Light-Emitting Diodes

**3.4. Polarization conversion in surface-plasmon-coupled emission from** 

nm) to extract TM mode preferentially by a surface plasmon coupled emission [68].

~690 nm for 0° to ~580 nm, ~490 nm, and ~440 nm for 20° , 40° , and 60° , respectively.

We have measured the linearly polarized electroluminescence spectra of the devices with and without buckles at the emission angles of 0°, 20°, 40°, and 60°, and then calculated the lightenhancement ratio (the intensity ratio of the two spectra in the devices with and without buckles) as a function of emission wavelength. Figure 29(b) presents the enhancement ratio of the TM-polarized light. The broad peak intensities for each emission angle are consistent with the main diffraction wavelengths calculated in Fig. 29(a), as indicated by arrows. It is very interesting to note that the TE-polarized light also gets enhanced by buckles as shown in Fig. 29(c). This enhancement is even greater than that for TM-polarized light, particularly at larger emission angles, although generally the SP mode is considered to be excited only by TMpolarized light and the diffraction gratings do not convert the polarization state of an incident light upon diffraction. However, it is also known that the polarization conversion can occur if the grating wavevector is not parallel to the plane of incidence [69-73]. So-called conical diffraction occurs at 0°–90° azimuthal angles by the grating with different wavevectors with respect to the incidence plane, where even TE-polarized light may excite the SP mode because of the existence of the electric field component parallel to the grating vector. In other words, the SP mode excited by a TM-polarized light can be outcoupled to the TE- as well as TMmodes radiation. As the azimuthal angle increases from 0° to 90°, the outcoupled TM mode decreases and the outcoupled TE mode increases by the conical diffractions [69,70]. As far as we know, this was the first report on the polarization state of the extracted SP mode, although a qualitative description on the polarization state can be found for the outcoupled SP mode

Because the grating vectors in a buckling structure are random over all azimuthal angles, the SP mode in the device with buckles also experiences conical diffractions at all azimuthal angles and then the polarization conversion of the outcoupled light occurs. For example, *k*0sinθ, *k*SP, and *k*G for the emission wavelength at 600 nm are graphically presented in Fig. 30. Here only one grating wavevector from a 1-D grating with a periodicity of 410 nm is assumed. The radius of the solid circle (blue) corresponds to *k*SP, the momentum space

The fabrication process of buckling and OLED devices is almost the same as described in section 2.2. The only difference is the use of a thinner ITO (40 nm) than previous one (120

To characterize the outcoupled SP mode by buckles, we have calculated its in-plane propagation vectors and plotted the grating period for the emission angles of 0°, 20°, 40°, and 60° as a function of the wavelength of the outcoupled light in Fig. 29(a). Considering the distribution maximum of the buckling periodicity at ~410 nm, it is reasonable that the main diffraction of the SP mode for the normal direction occurs at the emission wavelength of ~690 nm. In addition the FWHM of the periodicity distribution from 300–600 nm allows outcoupling of the SP mode over the entire emission wavelengths by the first- and secondorder diffractions. As the emission angle increases, the main diffraction wavelength shifts from

**corrugated OLEDs with buckling structures** 

from a silver cathode with a 2-D corrugated structure [74].

– Light Extraction and Polarization Characteristics 95

**Figure 28.** Measured polarized electroluminescence from OLED. Selectively emitted light of (a) L-CP-EL, (b) R-CP-EL, (c) LP-EL(+45°) and (d) LP-EL(-45°) under fields of 0, 6, 4.5, and 7.5 V, respectively. 63 Copyright 2008, American Institute of Physics.
