**2.2. OLEDs with a quarter waveplate film and a GBO polarizer film**

We present here an alternative approach to achieving highly linearly polarized EL emission by resorting again on GBO films. We present a simple polarized OLED that can be driven by a 'photon recycling' concept, which is similar to that developed by Belayev et al [34]. We apply a quarter-wave retardation plate (QWP) film and a GBO reflective polarizer to a nonuniaxial OLED. The QWP film used in our study is a sheet of a birefringent (double refracting) material, which creates a quarter-wavelength (/4) phase shift and can change the polarization of the light from linear to circular and *vice versa*. Our combination of the QWP film with a GBO reflective polarizer has enabled us to achieve a high degree of linear polarization with high brightness and efficiency.

**Figure 7.** Schematic structures of polarized EL emitting OLEDs. Type 1: simple structure of a polarized OLED with a GBO reflective polarizer, Type 2: Combined structure of a polarized OLED with a QWP (/4 plate) film and a GBO reflective polarizer.

A schematic configuration of the device structure, designed to achieve highly linearly polarized EL emission is shown as Type 2 in Figure 7. For comparison, we have also shown the Type 1 device in Fig. 7, which is presented above in section 2.1. In this Type 2 device a QWP film and a GBO reflective polarizer are assembled on an OLED device, at an angle of 45o between the fast optic axis of the QWP film and the passing axis (↕) of the GBO polarizer, as shown in Fig. 7. Then the unpolarized EL light generated from the OLED gets linearly polarization state by QWP and GBO polarizer, as follows; The EL emission that is

polarized along the direction parallel to the passing axis (↕) of the GBO polarizer is transmitted through GBO, whereas the other EL polarized perpendicular (☉) to the passing axis of the GBO polarizer is reflected back selectively as a result of the photonic band of the GBO polarizer. This reflected light changes its polarization to circular (*i.e.*, right-handed circularly polarized light) after its transmission through the QWP film. The sense of the rotation of this right-handed circularly polarized EL light is then changed by reflecting it from the surface of the metal cathode, *i.e.,* it now becomes left-handed circularly polarized light. Finally, by retransmitting it through the QWP film, the polarization of the light is again changed from left-handed circularly polarized to linearly polarized (↕). Now as the direction of polarization becomes parallel to the passing axis of the GBO it is transmitted through the GBO reflective polarizer. By this method, all generated EL light can be transmitted through the GBO reflective polarizer, has linear polarization (↕) along the passing axis of the GBO polarizer.

#### *2.2.1. Device fabrication and materials used*

50 Organic Light Emitting Devices

OLEDs with highly polarized luminescence emissions.

polarization with high brightness and efficiency.

(/4 plate) film and a GBO reflective polarizer.

**2.2. OLEDs with a quarter waveplate film and a GBO polarizer film** 

In this section, we have presented the results of a flexible, polarized, and luminous OLED using a flexible GBO substrate. It is shown that EL brightnesses over 4,500 cd/m2 can be produced using the sample OLED, with high peak efficiencies in excess of 6 cd/A and 2 lm/W. The polarization of the emitted EL lights from the sample OLED corresponds to the passing axis of the GBO polarizer substrate used. Furthermore, it is also shown that a high polarization ratio of up to 25 can possibly be achieved over the whole emission brightness range. These results show that use of GBO reflector enables the development of flexible

We present here an alternative approach to achieving highly linearly polarized EL emission by resorting again on GBO films. We present a simple polarized OLED that can be driven by a 'photon recycling' concept, which is similar to that developed by Belayev et al [34]. We apply a quarter-wave retardation plate (QWP) film and a GBO reflective polarizer to a nonuniaxial OLED. The QWP film used in our study is a sheet of a birefringent (double refracting) material, which creates a quarter-wavelength (/4) phase shift and can change the polarization of the light from linear to circular and *vice versa*. Our combination of the QWP film with a GBO reflective polarizer has enabled us to achieve a high degree of linear

**Figure 7.** Schematic structures of polarized EL emitting OLEDs. Type 1: simple structure of a polarized OLED with a GBO reflective polarizer, Type 2: Combined structure of a polarized OLED with a QWP

A schematic configuration of the device structure, designed to achieve highly linearly polarized EL emission is shown as Type 2 in Figure 7. For comparison, we have also shown the Type 1 device in Fig. 7, which is presented above in section 2.1. In this Type 2 device a QWP film and a GBO reflective polarizer are assembled on an OLED device, at an angle of 45o between the fast optic axis of the QWP film and the passing axis (↕) of the GBO polarizer, as shown in Fig. 7. Then the unpolarized EL light generated from the OLED gets linearly polarization state by QWP and GBO polarizer, as follows; The EL emission that is

*2.1.3. Summary* 

The polarized OLEDs are prepared by placing an organic EL layer between an anode and a cathode on a glass substrate, together with a QWP film and a GBO reflective polarizer, in the following sequence: GBO reflective polarizer / QWP film / glass substrate / transparent ITO (80 nm, 30 Ω/square) anode / hole-injecting buffer layer / EL layer / electron-injecting layer / Al cathode (Type 2). A commercial QWP film (Edmund Sci.) approximately 110 m thick and with a central operating wavelength of about 500 nm has been used. After a routine cleaning of the ITO substrate using wet (acetone and isopropyl alcohol) and dry (UV-ozone) processes, a solution of PEDOT:PSS is spin-coated onto the ITO anode in order to produce the hole-injecting buffer layer. Subsequently, in order to form an EL layer, a blended solution is also spin-coated onto the PEDOT:PSS layer. This blended solution consisted of a host PVK polymer, an electron-transporting butyl-PBD, a hole-transporting TPD and a phosphorescent guest dye of Ir(ppy)3. A mixed solvent of 1,2-dichloroethane and chloroform (mixing weight ratio 3:1) was used for the solution. The thicknesses of the PEDOT:PSS and EL layers were adjusted to about 40 nm and 80 nm, respectively. In order to form the electron-injecting layer, a ~2 nm thick Cs2CO3 interfacial layer was formed on the EL layer using thermal deposition (0.02 nm/s) at a base pressure of less than 2 × 10-5 Torr. Finally, a pure Al (~50 nm thick) cathode layer was formed on the interfacial layer using thermal deposition by means of a shadow-mask that had square (3 mm × 3 mm) apertures under the same vacuum conditions. After the Al cathode had been formed, the QWP and the GBO films were attached sequentially to the ITO glass substrate using index-matching oil. In order to assess the effectiveness of our device, we also fabricated unpolarized conventional reference devices, using exactly the same method as for the polarized OLEDs but without the GBO and QWP films (1st reference device). For further comparison, 2nd reference device was also fabricated using only the GBO film Type 1, Fig. 7). It may be noted that in both type 1 and type 2 devices, the organic layer structure and organic materials used are identical, and thus, electrical characteristics such as the current density-voltage (*J-V*) curve are identical.

Polarized Light-Emission from Photonic Organic Light-Emitting Devices 53

V (1 cd/m2 in Fig. 9(a)) with sharp increases in the *L-V* curve for polarization parallel to the passing axis. The polarized EL brightness (*EL||*) reaches ~13,400 cd/m2 at 17.0 V. This performance with respect to luminescence approaches the luminescence of ca. 18,500 cd/m2 at 17.0 V for the unpolarized 1st reference device, in which the QWP film and the GBO polarizer are omitted. The polarized *L-V* curves shown here also give quantitative results for the polarized light emissions observed along both the passing and blocking axes. As shown in Fig. 9(a), the highly polarized *L-V* characteristics for the polarized OLEDs give a high average *PR* of at least 40 over the whole voltage range (4.0 ~ 17 V). This ratio is significantly higher than that of the 1st and 2nd reference devices, which show *PR* of 1 (*i.e*., unpolarized light) and 7.53, respectively. Fig. 9(a) also shows that the EL emission polarized along the passing axis reaches only ca. 5,000 cd/m2 at 17.0 V for the 2nd reference OLED which only has the GBO polarizer. This performance of the 2nd reference OLED with respect to polarized luminescence along the passing axis of the GBO is only about the half. This relatively low brightness of the 2nd reference device is due to the absence of the 'photon recycling' effect mentioned above. It may also be seen that the EL brightness polarized along the blocking axis for the polarized OLEDs is further reduced compared with that of the 2nd reference OLEDs, as shown in Fig. 9(a). This is due to the reduced light intensity polarized along the blocking axis in the polarized device, following the change in the polarization to a direction parallel to the passing axis. Similarly, as shown in Figure 9(b), the peak efficiencies (10.3 cd/A and 3.63 lm/W) of the EL emission polarized along the passing axis for the polarized OLED are nearly double of that of the 2nd reference device (4.0 cd/A and 1.71 lm/W), while the efficiency of the EL emission polarized along the blocking axis for the polarized device is

We also measured the polarization characteristics and Fig. 10(a) shows the polarized EL emission spectra for polarizations along the passing (*EL||*, blue solid curves) and blocking (*EL┴*, red solid curves) axes at normal incidence, for an applied voltage of 10 V. It may be seen that the broad emission spectra are almost the same as those of the reference devices and conventional OLED devices reported elsewhere [32]. This figure also shows that the polarized EL emission spectrum depends very much on the polarization state, and that the polarized OLED shows highly polarized EL emission over the whole emission spectrum range. For the polarized device, *PR* of the integrated intensities of the *EL||* and *EL┴* lights is always greater than 40. It may therefore be concluded that our polarized OLEDs a QWP film and GBO reflective polarizer incorporated perform extremely well. Fig. 10(b) shows the *PR*-*L* characteristics of our polarized OLEDs. As shown in Fig. 10(b), in comparison with *PR* = 7.5 of the 2nd reference device our polarized device has a *PR* of over 40 in the whole

The operation of the 2nd reference and polarized OLEDs (3 mm × 3 mm, 10 V) for polarizations along the passing and blocking axes of the GBO reflective polarizer is shown in Fig. 11. It may be seen from the figure that under a rotation of linear dichroic polarizer, right OLED is more luminous (left fig.) and more highly polarized along the passing axis of the GBO polarizer in comparison to the left 2nd reference device (right fig.). All these results

further reduced compared with that of the 2nd reference OLED.

brightness range.

**Figure 8.** (a) Photographs of the transparent QWP film (left) and GBO reflective polarizer (right). (b) Polarized microphotographs at four angles of sample rotation of the QWP film.

#### *2.2.2. Results and discussion*

Figure 8(a) shows a photograph of the QWP film and the GBO reflective polarizer used in this study. As it can be seen the QWP film and GBO reflective polarizer are quite transparent. In Fig. 8(b), the optical anisotropy of the QWP film is shown in the polarized microphotograph obtained between crossed polarizers for four angles of rotation. Figure 8(b) shows that the QWP film has a clear optical birefringence. The two darker views of the polarized microphotographs enable us to obtain the orientation of the two optical axes for the QWP film.

**Figure 9.** (a) Polarized *L-V* characteristics. (*EL||* blue and *EL┴* red) (b) Current efficiency-voltage characteristics of the polarized OLEDs (solid curves). The dotted curves show the characteristics of the 2nd reference device.

The performance of the polarized OLEDs thus fabricated with the QWP film and the GBO reflective polarizer are presented here. Figure 9(a) shows the polarized *L-V* characteristics of the fabricated OLEDs for the polarizations along the passing (*EL||*, blue curves) and the blocking (*EL┴*, red curves) axes. The figure indicates that the turn-on voltages are below 4.0 V (1 cd/m2 in Fig. 9(a)) with sharp increases in the *L-V* curve for polarization parallel to the passing axis. The polarized EL brightness (*EL||*) reaches ~13,400 cd/m2 at 17.0 V. This performance with respect to luminescence approaches the luminescence of ca. 18,500 cd/m2 at 17.0 V for the unpolarized 1st reference device, in which the QWP film and the GBO polarizer are omitted. The polarized *L-V* curves shown here also give quantitative results for the polarized light emissions observed along both the passing and blocking axes. As shown in Fig. 9(a), the highly polarized *L-V* characteristics for the polarized OLEDs give a high average *PR* of at least 40 over the whole voltage range (4.0 ~ 17 V). This ratio is significantly higher than that of the 1st and 2nd reference devices, which show *PR* of 1 (*i.e*., unpolarized light) and 7.53, respectively. Fig. 9(a) also shows that the EL emission polarized along the passing axis reaches only ca. 5,000 cd/m2 at 17.0 V for the 2nd reference OLED which only has the GBO polarizer. This performance of the 2nd reference OLED with respect to polarized luminescence along the passing axis of the GBO is only about the half. This relatively low brightness of the 2nd reference device is due to the absence of the 'photon recycling' effect mentioned above. It may also be seen that the EL brightness polarized along the blocking axis for the polarized OLEDs is further reduced compared with that of the 2nd reference OLEDs, as shown in Fig. 9(a). This is due to the reduced light intensity polarized along the blocking axis in the polarized device, following the change in the polarization to a direction parallel to the passing axis. Similarly, as shown in Figure 9(b), the peak efficiencies (10.3 cd/A and 3.63 lm/W) of the EL emission polarized along the passing axis for the polarized OLED are nearly double of that of the 2nd reference device (4.0 cd/A and 1.71 lm/W), while the efficiency of the EL emission polarized along the blocking axis for the polarized device is further reduced compared with that of the 2nd reference OLED.

52 Organic Light Emitting Devices

*2.2.2. Results and discussion* 

the QWP film.

2nd reference device.

**Figure 8.** (a) Photographs of the transparent QWP film (left) and GBO reflective polarizer (right). (b)

Figure 8(a) shows a photograph of the QWP film and the GBO reflective polarizer used in this study. As it can be seen the QWP film and GBO reflective polarizer are quite transparent. In Fig. 8(b), the optical anisotropy of the QWP film is shown in the polarized microphotograph obtained between crossed polarizers for four angles of rotation. Figure 8(b) shows that the QWP film has a clear optical birefringence. The two darker views of the polarized microphotographs enable us to obtain the orientation of the two optical axes for

**Figure 9.** (a) Polarized *L-V* characteristics. (*EL||* blue and *EL┴* red) (b) Current efficiency-voltage characteristics of the polarized OLEDs (solid curves). The dotted curves show the characteristics of the

The performance of the polarized OLEDs thus fabricated with the QWP film and the GBO reflective polarizer are presented here. Figure 9(a) shows the polarized *L-V* characteristics of the fabricated OLEDs for the polarizations along the passing (*EL||*, blue curves) and the blocking (*EL┴*, red curves) axes. The figure indicates that the turn-on voltages are below 4.0

Polarized microphotographs at four angles of sample rotation of the QWP film.

We also measured the polarization characteristics and Fig. 10(a) shows the polarized EL emission spectra for polarizations along the passing (*EL||*, blue solid curves) and blocking (*EL┴*, red solid curves) axes at normal incidence, for an applied voltage of 10 V. It may be seen that the broad emission spectra are almost the same as those of the reference devices and conventional OLED devices reported elsewhere [32]. This figure also shows that the polarized EL emission spectrum depends very much on the polarization state, and that the polarized OLED shows highly polarized EL emission over the whole emission spectrum range. For the polarized device, *PR* of the integrated intensities of the *EL||* and *EL┴* lights is always greater than 40. It may therefore be concluded that our polarized OLEDs a QWP film and GBO reflective polarizer incorporated perform extremely well. Fig. 10(b) shows the *PR*-*L* characteristics of our polarized OLEDs. As shown in Fig. 10(b), in comparison with *PR* = 7.5 of the 2nd reference device our polarized device has a *PR* of over 40 in the whole brightness range.

The operation of the 2nd reference and polarized OLEDs (3 mm × 3 mm, 10 V) for polarizations along the passing and blocking axes of the GBO reflective polarizer is shown in Fig. 11. It may be seen from the figure that under a rotation of linear dichroic polarizer, right OLED is more luminous (left fig.) and more highly polarized along the passing axis of the GBO polarizer in comparison to the left 2nd reference device (right fig.). All these results demonstrate a successful fabrication of a highly polarized OLED with a high *PR* (> 40), using a QWP film and a GBO reflective polarizer.

Polarized Light-Emission from Photonic Organic Light-Emitting Devices 55

**2.3. Polarized white OLEDs with achromatic QWP films on GBO substrates** 

to circular, and *vice versa*.

WOLEDs.

parallel to the passing axis of the GBO polarizer.

Here we describe the third technique that can be used to achieve high linearly polarized white EL emission based on the 'photon recycling' concept [34] for a wide visible wavelength range including red, green, and blue light. We apply a GBO reflective polarizer to a WOLED with a broadband (achromatic) QWP film whose phase retardation is maintained at /2 for a wide range of wavelengths, in contrast to the narrow band QWP used in section 2.2. The applied achromatic QWP film also creates a phase shift of a quarter of a wavelength (/4), and can change the polarization of the broad EL emission from linear

The configuration of the device is shown in Figure 12(a), which is nearly identical to Type 2 in presented in section 2.2 as shown in Figure 7. Here an achromatic QWP film and a GBO reflective polarizer are attached to a WOLED with an angle of 45° between the fast optic axis of the QWP film and the passing axis (↕) of the GBO polarizer. From the unpolarized EL light generated from the WOLED EL (*EL||*) polarized along the direction parallel to the passing axis (↕) of the GBO polarizer is transmitted through the GBO polarizer. The EL (*EL┴*) polarized perpendicular (☉) to the passing axis of the GBO polarizer is reflected selectively by the wide photonic band of the GBO polarizer. The polarization of this reflected light is changed to right-handed circular (*R*) after its transmission through the achromatic QWP film. The sense of rotation of this circularly polarized EL light is then reversed to lefthanded circular (*L*) by reflecting it from the surface of the metal cathode. Then by retransmission of this light through the achromatic QWP film changes its polarization again from circularly to linearly polarized (↕), which can be transmitted through the GBO reflective polarizer. This method allows nearly all the generated white EL light to be transmitted through the GBO reflective polarizer with a direction of linear polarization (↕)

**Figure 12.** (a) Polarized WOLED (*S*) combined with an achromatic QWP (/4 plate) film and a GBO reflective polarizer: Type 2 and (b) Unpolarized EL spectra of the WOLED used for the polarized

**Figure 10.** (a) Polarized EL emission spectra along the passing and blocking axes. (b) Polarization ratios of the polarized OLED (blue curve) against luminance. The points in red show the characteristics of the 2nd reference device.

**Figure 11.** Photographs of the operating 2nd reference (left) and polarized (right) OLEDs for polarizations along the passing (*EL||*) (a) and the blocking (*EL┴*) (b) axes of the GBO reflective polarizer under a rotating linear dichroic polarizer film. The white arrow represents the transmission axis of the linear dichroic polarizer and the blue arrow represents the transmission axis of the GBO polarizer.

#### *2.2.3 Summary*

In summary, we have described the fabrication and operation of a polarized and luminous OLED using the combination of a QWP retardation film and a GBO reflective polarizer. A peak polarized EL brightness of over ca. 13,000 cd/m2 is produced from the polarized OLED, with high peak efficiencies in excess of 10 cd/A and 3.5 lm/W. The polarization direction of the EL light emitted from the polarized OLED corresponds to the passing axis of the GBO polarizer used. Furthermore, it has also been shown that a high polarization ratio greater than 40 is possible over the whole emission brightness range. These results show that using the QWP film and GBO reflective polarizer we can develop bright OLEDs with highly polarized luminescence emissions.

#### **2.3. Polarized white OLEDs with achromatic QWP films on GBO substrates**

54 Organic Light Emitting Devices

2nd reference device.

*2.2.3 Summary* 

polarized luminescence emissions.

using a QWP film and a GBO reflective polarizer.

demonstrate a successful fabrication of a highly polarized OLED with a high *PR* (> 40),

**Figure 10.** (a) Polarized EL emission spectra along the passing and blocking axes. (b) Polarization ratios of the polarized OLED (blue curve) against luminance. The points in red show the characteristics of the

**Figure 11.** Photographs of the operating 2nd reference (left) and polarized (right) OLEDs for

polarizations along the passing (*EL||*) (a) and the blocking (*EL┴*) (b) axes of the GBO reflective polarizer under a rotating linear dichroic polarizer film. The white arrow represents the transmission axis of the linear dichroic polarizer and the blue arrow represents the transmission axis of the GBO polarizer.

In summary, we have described the fabrication and operation of a polarized and luminous OLED using the combination of a QWP retardation film and a GBO reflective polarizer. A peak polarized EL brightness of over ca. 13,000 cd/m2 is produced from the polarized OLED, with high peak efficiencies in excess of 10 cd/A and 3.5 lm/W. The polarization direction of the EL light emitted from the polarized OLED corresponds to the passing axis of the GBO polarizer used. Furthermore, it has also been shown that a high polarization ratio greater than 40 is possible over the whole emission brightness range. These results show that using the QWP film and GBO reflective polarizer we can develop bright OLEDs with highly Here we describe the third technique that can be used to achieve high linearly polarized white EL emission based on the 'photon recycling' concept [34] for a wide visible wavelength range including red, green, and blue light. We apply a GBO reflective polarizer to a WOLED with a broadband (achromatic) QWP film whose phase retardation is maintained at /2 for a wide range of wavelengths, in contrast to the narrow band QWP used in section 2.2. The applied achromatic QWP film also creates a phase shift of a quarter of a wavelength (/4), and can change the polarization of the broad EL emission from linear to circular, and *vice versa*.

The configuration of the device is shown in Figure 12(a), which is nearly identical to Type 2 in presented in section 2.2 as shown in Figure 7. Here an achromatic QWP film and a GBO reflective polarizer are attached to a WOLED with an angle of 45° between the fast optic axis of the QWP film and the passing axis (↕) of the GBO polarizer. From the unpolarized EL light generated from the WOLED EL (*EL||*) polarized along the direction parallel to the passing axis (↕) of the GBO polarizer is transmitted through the GBO polarizer. The EL (*EL┴*) polarized perpendicular (☉) to the passing axis of the GBO polarizer is reflected selectively by the wide photonic band of the GBO polarizer. The polarization of this reflected light is changed to right-handed circular (*R*) after its transmission through the achromatic QWP film. The sense of rotation of this circularly polarized EL light is then reversed to lefthanded circular (*L*) by reflecting it from the surface of the metal cathode. Then by retransmission of this light through the achromatic QWP film changes its polarization again from circularly to linearly polarized (↕), which can be transmitted through the GBO reflective polarizer. This method allows nearly all the generated white EL light to be transmitted through the GBO reflective polarizer with a direction of linear polarization (↕) parallel to the passing axis of the GBO polarizer.

**Figure 12.** (a) Polarized WOLED (*S*) combined with an achromatic QWP (/4 plate) film and a GBO reflective polarizer: Type 2 and (b) Unpolarized EL spectra of the WOLED used for the polarized WOLEDs.

#### *2.3.1. Device fabrication and materials used*

The polarized WOLEDs were prepared by fabricating organic layers between an anode and a cathode on a glass substrate, together with a commercially available achromatic QWP film and a GBO reflective polarizer. The QWP film was approximately 110 m thick, and the range of its operating wavelengths were approximately 420 ~ 650 nm. After routine cleaning of the ITO (150 nm, 10 Ω/square) substrate using both wet (acetone and isopropyl alcohol) and dry (O2 plasma) processes, the organic layers were deposited on the ITO anode to form the structure of the tandem hybrid WOLED: ITO anode / short reduction layer (5 nm) / hole injection layer 1 (10 nm) / hole injection layer 2 (25 nm) / fluorescent blue-light emitting material layer (10 nm) / 8-hydroxy-quinolinato lithium (Liq)-doped electron injection layer (20 nm) / Li doped electron injection layer (20 nm) / hole injection layer 3 (10 nm) / hole transporting layer (55 nm) / phosphorescent green- and red-light emitting material layer (25 nm) / hole blocking layer (10 nm) / Liq doped electron injection layer (30 nm) / Al cathode. This is similar to the structure reported in reference [35]. The organic layers and Al cathode (150 nm) were deposited consecutively by thermal evaporation in a chamber with a base pressure of less than 1 × 10-6 Torr by means of a shadow-mask with square (1 mm × 1 mm) apertures. When the cathode was ready, the achromatic QWP and the GBO films were combined sequentially to the fabricated WOLEDs (device *S*). For comparison, we also fabricated reference devices, using exactly the same method as for the WOLEDs, but with only the GBO film (reference device *R*). The structure of the organic layer and the organic materials used were identical for each of the devices described herein. Figure 12(b) shows the white-light EL spectra (unpolarized) observed for the fabricated WOLED, in which three balanced emission peaks may be seen at 463 (blue), 503 (green), and 563 (red) nm. The spectral shape of the EL spectrum emitted from the device did not change significantly with applied voltage, and the color coordinates varied by less than 10% for the applied voltages between 7 ~ 14 V.

Polarized Light-Emission from Photonic Organic Light-Emitting Devices 57

represents the angle between a fast axis of the

between crossed polarizers. Here,

light.

solid curves) WOLEDs.

achromatic QWP film and a transmitting axis of the polarizer. The measured results are shown in Figure 13. This figure shows clearly that the phase retardation of the achromatic QWP film is about /2. Although the retardation decreases slightly as the wavelength increases, the QWP film has a nearly uniform phase retardation of a quarter of a wavelength (/4) in a wide visible range of wavelengths (420 ∼ 650 nm) that includes blue, green, and red

**Figure 14.** (a) *J-V* and (b) polarized *L-V* characteristics of Reference (*R*, dotted curves) and Sample (*S*,

Figure 14(a) shows the *J-V* curves of the fabricated WOLED devices *S* and *R*. For all the WOLEDs described herein, the organic layers used are the same, and the electrical characteristics (such as *J-V* curves) are therefore found to be identical for each device as shown in Fig. 14 (a). Figure 14(b) shows the *L-V* characteristics of the WOLEDs for *EL||* (blue curves) and *EL┴* (red curves). Figure 14(b) shows clearly that the devices operate at relatively low turn-on voltages (~ 6 V) and have bright EL emissions, which indicate the efficient emission of white EL from the WOLEDs. It is noteworthy that even without full operational optimization of the polarized WOLED, its performance shows its potential attractiveness. In particular, the WOLED with both the GBO and achromatic QWP films (device *S*) exhibits excellent performance, in which operating voltages of about 7.0 and 8.7 V are required to obtain brightnesses (*EL||*) of 100 cd/m2 and 1,000 cd/m2, respectively, with a peak luminescence of ca. 14,600 cd/m2 at 14.5 V. It may be seen that the peak luminance (*EL||*) of the device *S* under test is much higher than that of a previously reported polarized WOLED (ca. 850 cd/m2 in Ref. 8) that used a uniaxially oriented polymeric material as an EL layer. Figure 14(b) also shows that the *EL||* reaches only ca. 8,400 cd/m2 at 14.5 V for device *R*,

whose performance with respect to *EL||* is only about half as good as that of device *S*.

In Fig. 15 (a), we have shown the current efficiencies of S and R WOLEDs. For the *EL||* of device *S*, a current efficiency (C) of 16.5 cd/A is obtained at 100 cd/m2 (7.0 V), reaching C = 18.3 cd/A at 1,000 cd/m2 (8.7 V) and C = 16.5 cd/A at 14,600 cd/m2 (14.5 V). We have also determined the power efficiency P for the *EL||* of device *S*, which increases and reaches a maximum of 7.4 lm/W before slowly decreasing, with increasing bias voltage as shown in

**Figure 13.** Phase retardation () of the broadband achromatic QWP film used in this study.

#### *2.3.2. Results and discussion*

The phase retardation () of the achromatic QWP film used in this study is measured by observing the transmission *T* (*T* = 1/2 sin2(2 sin2(/2)) through the QWP film placed between crossed polarizers. Here, represents the angle between a fast axis of the achromatic QWP film and a transmitting axis of the polarizer. The measured results are shown in Figure 13. This figure shows clearly that the phase retardation of the achromatic QWP film is about /2. Although the retardation decreases slightly as the wavelength increases, the QWP film has a nearly uniform phase retardation of a quarter of a wavelength (/4) in a wide visible range of wavelengths (420 ∼ 650 nm) that includes blue, green, and red light.

56 Organic Light Emitting Devices

**Figure 13.** Phase retardation (

The phase retardation (

*2.3.2. Results and discussion* 

observing the transmission *T* (*T* = 1/2 sin2(2

*2.3.1. Device fabrication and materials used* 

The polarized WOLEDs were prepared by fabricating organic layers between an anode and a cathode on a glass substrate, together with a commercially available achromatic QWP film and a GBO reflective polarizer. The QWP film was approximately 110 m thick, and the range of its operating wavelengths were approximately 420 ~ 650 nm. After routine cleaning of the ITO (150 nm, 10 Ω/square) substrate using both wet (acetone and isopropyl alcohol) and dry (O2 plasma) processes, the organic layers were deposited on the ITO anode to form the structure of the tandem hybrid WOLED: ITO anode / short reduction layer (5 nm) / hole injection layer 1 (10 nm) / hole injection layer 2 (25 nm) / fluorescent blue-light emitting material layer (10 nm) / 8-hydroxy-quinolinato lithium (Liq)-doped electron injection layer (20 nm) / Li doped electron injection layer (20 nm) / hole injection layer 3 (10 nm) / hole transporting layer (55 nm) / phosphorescent green- and red-light emitting material layer (25 nm) / hole blocking layer (10 nm) / Liq doped electron injection layer (30 nm) / Al cathode. This is similar to the structure reported in reference [35]. The organic layers and Al cathode (150 nm) were deposited consecutively by thermal evaporation in a chamber with a base pressure of less than 1 × 10-6 Torr by means of a shadow-mask with square (1 mm × 1 mm) apertures. When the cathode was ready, the achromatic QWP and the GBO films were combined sequentially to the fabricated WOLEDs (device *S*). For comparison, we also fabricated reference devices, using exactly the same method as for the WOLEDs, but with only the GBO film (reference device *R*). The structure of the organic layer and the organic materials used were identical for each of the devices described herein. Figure 12(b) shows the white-light EL spectra (unpolarized) observed for the fabricated WOLED, in which three balanced emission peaks may be seen at 463 (blue), 503 (green), and 563 (red) nm. The spectral shape of the EL spectrum emitted from the device did not change significantly with applied voltage, and the

color coordinates varied by less than 10% for the applied voltages between 7 ~ 14 V.

) of the broadband achromatic QWP film used in this study.

 sin2(

) of the achromatic QWP film used in this study is measured by

/2)) through the QWP film placed

**Figure 14.** (a) *J-V* and (b) polarized *L-V* characteristics of Reference (*R*, dotted curves) and Sample (*S*, solid curves) WOLEDs.

Figure 14(a) shows the *J-V* curves of the fabricated WOLED devices *S* and *R*. For all the WOLEDs described herein, the organic layers used are the same, and the electrical characteristics (such as *J-V* curves) are therefore found to be identical for each device as shown in Fig. 14 (a). Figure 14(b) shows the *L-V* characteristics of the WOLEDs for *EL||* (blue curves) and *EL┴* (red curves). Figure 14(b) shows clearly that the devices operate at relatively low turn-on voltages (~ 6 V) and have bright EL emissions, which indicate the efficient emission of white EL from the WOLEDs. It is noteworthy that even without full operational optimization of the polarized WOLED, its performance shows its potential attractiveness. In particular, the WOLED with both the GBO and achromatic QWP films (device *S*) exhibits excellent performance, in which operating voltages of about 7.0 and 8.7 V are required to obtain brightnesses (*EL||*) of 100 cd/m2 and 1,000 cd/m2, respectively, with a peak luminescence of ca. 14,600 cd/m2 at 14.5 V. It may be seen that the peak luminance (*EL||*) of the device *S* under test is much higher than that of a previously reported polarized WOLED (ca. 850 cd/m2 in Ref. 8) that used a uniaxially oriented polymeric material as an EL layer. Figure 14(b) also shows that the *EL||* reaches only ca. 8,400 cd/m2 at 14.5 V for device *R*, whose performance with respect to *EL||* is only about half as good as that of device *S*.

In Fig. 15 (a), we have shown the current efficiencies of S and R WOLEDs. For the *EL||* of device *S*, a current efficiency (C) of 16.5 cd/A is obtained at 100 cd/m2 (7.0 V), reaching C = 18.3 cd/A at 1,000 cd/m2 (8.7 V) and C = 16.5 cd/A at 14,600 cd/m2 (14.5 V). We have also determined the power efficiency P for the *EL||* of device *S*, which increases and reaches a maximum of 7.4 lm/W before slowly decreasing, with increasing bias voltage as shown in

Figure 15(b). These results indicate that the peak efficiencies (18.3 cd/A and 7.41 lm/W) for the *EL||* of device *S* are nearly double those of device *R* (9.63 cd/A and 3.71 lm/W). These relatively high brightness and efficiency values of the *EL||* of device *S* are achieved by the 'photon recycling' effect. It is noted that the brightness and efficiency of *EL┴* for device *S* are further reduced compared with those of device *R*, as shown in Figures 14 and 15. This is due to the reduced *EL┴* in device *S* that occurred after the change in polarization to the direction parallel to the passing axis.

Polarized Light-Emission from Photonic Organic Light-Emitting Devices 59

**Figure 16.** (a) Polarization ratios of the polarized WOLEDs against luminance. (b) Polarized EL spectra

Finally, we have shown in Fig. 17 the photographs of the performance of WOLEDs (1 mm × 1 mm) operating under the same bias voltage of 8 V for polarizations along the passing (upper) and blocking (lower) axes of the GBO reflective polarizer. Fig. 17 shows clearly that under a rotating linear dichroic polarizer, the EL emission from device *S* is fairly brighter and more highly polarized along the passing axis of the GBO polarizer, compared to that of

In summary, we have described the fabrication and investigation of the properties of a polarized WOLED using a combination of an achromatic QWP and a GBO reflective polarizer. By applying the achromatic QWP and the GBO polarizer to the WOLED, polarized EL brightnesses in excess of ca. 14,600 cd/m2 can be obtained from the polarized WOLED, together with high peak efficiencies of more than 18 cd/A (7.4 lm/W), which are almost double of those obtained from the polarized WOLED with only the GBO polarizer. We have also found that a high polarization ratio of ca. 35:1 is possible over the whole range of brightness of the emissions. Although the *PR* value of the polarized WOLED is slightly lower than that of polarized narrow band (green) OLED in section 2.2, it may be noted that only the polarized WOLED can provide a polarized light source for a wide range of

of *EL||* (blue curve) and *EL┴* (red curve) of the sample WOLED *S* at 10 V.

wavelengths from the blue through to the green and red color regions.

device *R*.

*2.3.3. Summary* 

**Figure 15.** (a) Current efficiency-voltage and (d) power efficiency-voltage characteristics of Reference (*R*, dotted curves) and Sample (*S*, solid curves) WOLEDs.

Next, we have estimated the relationship between polarization ratio and luminance *PR-L* for the polarized WOLED *S*, thereby presenting quantitative results for the polarized emissions. Figure 16(a) shows that the highly polarized characteristics of the polarized WOLED *S* give a high average value of *PR* (*EL||* : *EL┴*) of at least ~35:1 over the whole range of brightness. It should be noted that this value of *PR* is significantly higher than that of device *R* (8.21:1). In order to understand the characteristics of the polarized EL, we have measured the polarized emission spectra for *EL||* (blue curves) and *EL┴* (red curves) for the device *S* under an applied voltage of 10 V (Figure 16(b)). It may be noted that the spectral shape of the *EL||* for device *S* is very similar to that for device *R*. The observed color rendering index (CRI) of *EL||* for device *S* is about 80.0, and the CIE XYZ color space is (0.285, 0.363, 0.353), with a correlated color temperature (CCT) of about 7,600 K. These characteristics are also similar to those of *EL||* from device *R*, which has a CRI of about 74.0, CIE XYZ color space of (0.275, 0.342, 0.383), and CCT of about 8,500 K. At the same time Figure 16(b) also shows that the polarized EL emission spectrum depends very much on the polarization state and that device *S* produces highly polarized EL emission over the whole spectrum. For the device *S*, the highest value of *PR* calculated from the integrated intensities of the parallel and perpendicularly polarized EL spectra is approximately 35:1, which is significantly higher than that of the white-light emitting devices that use uniaxially oriented materials [17]. These results prove that our polarized WOLED (*S*), which incorporates an achromatic QWP film with a GBO reflective polarizer, outperforms all other similar devices.

**Figure 16.** (a) Polarization ratios of the polarized WOLEDs against luminance. (b) Polarized EL spectra of *EL||* (blue curve) and *EL┴* (red curve) of the sample WOLED *S* at 10 V.

Finally, we have shown in Fig. 17 the photographs of the performance of WOLEDs (1 mm × 1 mm) operating under the same bias voltage of 8 V for polarizations along the passing (upper) and blocking (lower) axes of the GBO reflective polarizer. Fig. 17 shows clearly that under a rotating linear dichroic polarizer, the EL emission from device *S* is fairly brighter and more highly polarized along the passing axis of the GBO polarizer, compared to that of device *R*.

#### *2.3.3. Summary*

58 Organic Light Emitting Devices

parallel to the passing axis.

Figure 15(b). These results indicate that the peak efficiencies (18.3 cd/A and 7.41 lm/W) for the *EL||* of device *S* are nearly double those of device *R* (9.63 cd/A and 3.71 lm/W). These relatively high brightness and efficiency values of the *EL||* of device *S* are achieved by the 'photon recycling' effect. It is noted that the brightness and efficiency of *EL┴* for device *S* are further reduced compared with those of device *R*, as shown in Figures 14 and 15. This is due to the reduced *EL┴* in device *S* that occurred after the change in polarization to the direction

**Figure 15.** (a) Current efficiency-voltage and (d) power efficiency-voltage characteristics of Reference

Next, we have estimated the relationship between polarization ratio and luminance *PR-L* for the polarized WOLED *S*, thereby presenting quantitative results for the polarized emissions. Figure 16(a) shows that the highly polarized characteristics of the polarized WOLED *S* give a high average value of *PR* (*EL||* : *EL┴*) of at least ~35:1 over the whole range of brightness. It should be noted that this value of *PR* is significantly higher than that of device *R* (8.21:1). In order to understand the characteristics of the polarized EL, we have measured the polarized emission spectra for *EL||* (blue curves) and *EL┴* (red curves) for the device *S* under an applied voltage of 10 V (Figure 16(b)). It may be noted that the spectral shape of the *EL||* for device *S* is very similar to that for device *R*. The observed color rendering index (CRI) of *EL||* for device *S* is about 80.0, and the CIE XYZ color space is (0.285, 0.363, 0.353), with a correlated color temperature (CCT) of about 7,600 K. These characteristics are also similar to those of *EL||* from device *R*, which has a CRI of about 74.0, CIE XYZ color space of (0.275, 0.342, 0.383), and CCT of about 8,500 K. At the same time Figure 16(b) also shows that the polarized EL emission spectrum depends very much on the polarization state and that device *S* produces highly polarized EL emission over the whole spectrum. For the device *S*, the highest value of *PR* calculated from the integrated intensities of the parallel and perpendicularly polarized EL spectra is approximately 35:1, which is significantly higher than that of the white-light emitting devices that use uniaxially oriented materials [17]. These results prove that our polarized WOLED (*S*), which incorporates an achromatic QWP

film with a GBO reflective polarizer, outperforms all other similar devices.

(*R*, dotted curves) and Sample (*S*, solid curves) WOLEDs.

In summary, we have described the fabrication and investigation of the properties of a polarized WOLED using a combination of an achromatic QWP and a GBO reflective polarizer. By applying the achromatic QWP and the GBO polarizer to the WOLED, polarized EL brightnesses in excess of ca. 14,600 cd/m2 can be obtained from the polarized WOLED, together with high peak efficiencies of more than 18 cd/A (7.4 lm/W), which are almost double of those obtained from the polarized WOLED with only the GBO polarizer. We have also found that a high polarization ratio of ca. 35:1 is possible over the whole range of brightness of the emissions. Although the *PR* value of the polarized WOLED is slightly lower than that of polarized narrow band (green) OLED in section 2.2, it may be noted that only the polarized WOLED can provide a polarized light source for a wide range of wavelengths from the blue through to the green and red color regions.

Polarized Light-Emission from Photonic Organic Light-Emitting Devices 61

range. These results show that using the (achromatic) QWP film and the GBO reflective polarizer one can develop bright (W)OLEDs with highly polarized luminescence emissions. It is also noted that the polarization ratio of the polarized WOLED can be further improved by introducing a high quality achromatic QWP film for a wide range of wavelengths including red, green, and blue light. By combining the devices presented here with the luminous EL layers reported elsewhere [35], it may be possible to develop highly efficient polarized OLEDs with a wide range of optical applications. For example, the device structure used in this study can be applied to the design of special light-emitting devices, such as polarized backlights for LC displays. Such devices can also be used for the development of a new class of polarized OLEDs such as polarized surface emitting devices

for 3-D displays and/or the polarized light sources of optical waveguide devices.

This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Ministry of Education, Science and Technology, Republic of Korea (20120003831 and 2012015654). This research was also supported by the Converging Research Center Program through the Ministry of Education, Science and Technology (2012K001303) and the leading industry of NEW-IT and equipments of the Chungcheong Leading Industry Office of the

[1] C. W. Tang, S. A. Van Slyke, Organic electroluminescent diodes. Applied Physics

[2] R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradly, D. A. Dos Santos, J. L. Bredas, M. Logdlund, W. R. Salaneck,

[4] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Highly efficient phosphorescence from organic light-emitting devices with an exciton-block layer. Applied Physics Letters

[5] C. Adachi, M. E. Thompson, S. R. Forrest, Architectures for efficient electrophosphorescent organic light-emitting devices. IEEE Journal of Selected Topics in

Electroluminescence in conjugated polymers. Nature (London) 397, 121 (1999). [3] M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Very highefficiency green organic light-emitting devices based on electrophosphorescence.

*Department of Electrophysics, Kwangwoon University, Seoul, Korea* 

Korean Ministry of Knowledge Economy (A002200104).

**Author details** 

Byoungchoo Park

**4. References** 

Letters 51, 913 (1987).

79, 156 (2001).

Applied Physics Letters 75, 4 (1999).

Quantum Electronics 8, 372 (2002).

**Acknowledgement** 

**Figure 17.** Photographs of the brightness obtained from the reference (*R*, left) and sample (*S*, right) WOLEDs (at 8 V) for *EL||* (a) and *EL┴* (b) under a rotating linear dichroic polarizer film. The white and blue arrows represent the transmission axes of the linear dichroic polarizer and the GBO polarizer, respectively. The active areas of the polarized WOLEDs were 1 mm × 1 mm. (It may be noted that the device *R* appears to be brighter than device *S* in Fig (b) because device *S* is more highly polarized along the passing axis of the GBO polarizer, compared to that of device *R*.)
