*2.1.1. Device fabrication and materials used*

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

electrode is about 60 % in the visible wavelength region. A solution of PEDOT:PSS (poly(3,4 ethylenedioxythiophene): poly(4-styrenesulphonate), Clevios PVP. Al 4083, H. C. Starck Inc.) is spin-coated onto the Au anode in order to produce the hole-injecting buffer layer. Subsequently, to form an EL layer, a blended solution is also spin-coated onto the PEDOT:PSS layer. This blended solution consists of a host polymer of poly(vinylcarbazole) (PVK), an electron-transporting 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4 oxadiazole (Butyl-PBD), a hole-transporting N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1, 1'biphenyl-4,4' diamine (TPD), and a phosphorescent guest dye of Tris(2-phenylpyridine) iridium (III) (Ir(ppy)3), whose emission peak wavelength is ~510 nm with a full width at half maximum (FWHM) of ~85 nm [32]. A mixed solvent of 1,2-dichloroethane and chloroform (mixing weight ratio 3:1) is used for the solution. The thicknesses of the PEDOT:PSS and EL layers are adjusted to be about 40 nm and 80 nm, respectively. In order to form the electroninjecting layer, a ~1 nm thick Cs2CO3 interfacial layer is formed on the EL layer using thermal deposition (0.02 nm/s) at a base pressure of less than 2 × 10-6 Torr with a shadowmask that had 3 × 3 mm2 square apertures. Finally, a pure Al (~50 nm thick) cathode layer is deposited on the interfacial layer using thermal deposition under the same vacuum conditions. For comparison, we have also fabricated a reference device using a glass substrate in place of the GBO polarizer substrate. Apart from using different substrate materials, the reference devices are fabricated in exactly the same way as the sample OLED on the GBO polarizer substrate. Once the fabrication of OLEDs thus completed, the optical transmittance and reflectance spectra are measured using a Cary 1E (Varian) UV-vis spectrometer and a multichannel spectrometer (HR 4000CG-UV-NIR, Ocean Optics Inc., 0.25 nm resolution). A combination of a polarizer and an analyzer is also used to investigate the polarization of the light emitted from the sample device. A Chroma Meter CS-200 (Konica Minolta Sensing, INC.) and a source meter (Keithley 2400) have been used for measuring the EL characteristics.

Polarized Light-Emission from Photonic Organic Light-Emitting Devices 47

transmission spectra are thus quite different from each other. When measured in the *y* direction, the transmission spectrum shows a strong and broad reflection band, while in the *x* direction, there is no reflection band in the wide visible wavelength range (350 ~ 800 nm) that incorporates red, green, and blue light. This significant difference between the reflection bands clearly indicates that in a GBO reflecting polarizer film, the refractive indices of alternating layers are matched along both the *x-* and *z-* axes and mismatched along the *y*axis. It is thus evident that the birefringence causes the reflecting band structure to be polarized and that the *x* and *y* axes represent the ordinary (*o*) and extraordinary (*e*) axes, respectively. Note that the *o* axis is consistent with the polarizing axis (or passing axis) and the *e* axis represents the blocking axis of the GBO reflecting polarizer. The average extinction ratio of the GBO reflecting polarizer used was estimated to be about 16:1 in the wavelength

**Figure 2.** (a) Polarized microphotographs under crossed polarizers at four angles of sample rotation of the flexible GBO reflecting polymer polarizer film. (b) Polarized transmittance spectra for incident light

On the design outlined above, we have prepared samples of OLEDs on the GBO reflecting polarizer substrate. In order to study the EL characteristics of the sample OLEDs, we have observed the current density-luminance-voltage (*J-L-V*) characteristics, as shown in Figure 3(a). It is clear from this figure that both the charge-injection and turn-on voltages are below 4.0 V, with sharp increases in the *J-V* and *L*-*V* curves. The EL brightness reaches ~4,500 cd/m2 at 14.5 V. This performance of the sample OLED with respect to luminescence is nearly the same as that of the reference device using a conventional linear dichroic polarizer film, which shows ca. 5,000 cd/m2 at 14.5 V. In contrast, as shown in Figure 3(b), the peak efficiencies (6.1 cd/A and 2.0 lm/W) of the sample OLED are much higher than those of the reference device (2.3 cd/A and 0.6 lm/W). The relatively high efficiencies of the sample device may be caused by the improved transition probability of exciton (singlet and triplet) relaxation with respect to the polarization along the transmission axis due to the reduced transition probability of exciton relaxation with respect to the polarization perpendicular to

polarized linearly along the *x* (ordinary) and *y* (extraordinary) axes.

region between 470 and 700 nm.

the transmission axis [20, 33].

#### *2.1.2. Results and discussion*

Figure 1(a) shows a photograph of the flexible GBO reflecting polarizer substrate used in this study. As shown in Fig. 1(a), the GBO substrate is easy to bend and quite transparent, in contrast to conventional linear dichroic polarizer film made from light-absorptive materials. Figure 1(b) shows a scanning electron microscopy (SEM) image of the cross-sectional structure of the GBO polarizer film. The SEM image shows clearly that the uniform layers of two alternating layered elements [a/b] are formed in multiple stacks with different refractive indices, (*nax*, *nay*, *naz*) and (*nbx*, *nby*, *nbz*). The optical anisotropy of the GBO polarizer may be seen by inspecting the polarized microphotograph of the GBO film between crossed polarizers at four angles of sample rotation of the GBO film substrate, as shown in Figure 2(a). This figure shows that the GBO film has a clear optical birefringence. We can define the orientation of the two optical axes, *x* and *y*, for the GBO film from the darkest views of the polarized microphotographs. The polarized transmittance spectra from the GBO polarizer film have then been observed for the two incident lights polarized linearly along the *x* and *y* axes, as shown in Figure 2(b). From this figure, it is clear that the nature of the reflection bands depends strongly on the polarization of the incident light, and the polarized transmission spectra are thus quite different from each other. When measured in the *y* direction, the transmission spectrum shows a strong and broad reflection band, while in the *x* direction, there is no reflection band in the wide visible wavelength range (350 ~ 800 nm) that incorporates red, green, and blue light. This significant difference between the reflection bands clearly indicates that in a GBO reflecting polarizer film, the refractive indices of alternating layers are matched along both the *x-* and *z-* axes and mismatched along the *y*axis. It is thus evident that the birefringence causes the reflecting band structure to be polarized and that the *x* and *y* axes represent the ordinary (*o*) and extraordinary (*e*) axes, respectively. Note that the *o* axis is consistent with the polarizing axis (or passing axis) and the *e* axis represents the blocking axis of the GBO reflecting polarizer. The average extinction ratio of the GBO reflecting polarizer used was estimated to be about 16:1 in the wavelength region between 470 and 700 nm.

46 Organic Light Emitting Devices

EL characteristics.

*2.1.2. Results and discussion* 

electrode is about 60 % in the visible wavelength region. A solution of PEDOT:PSS (poly(3,4 ethylenedioxythiophene): poly(4-styrenesulphonate), Clevios PVP. Al 4083, H. C. Starck Inc.) is spin-coated onto the Au anode in order to produce the hole-injecting buffer layer. Subsequently, to form an EL layer, a blended solution is also spin-coated onto the PEDOT:PSS layer. This blended solution consists of a host polymer of poly(vinylcarbazole) (PVK), an electron-transporting 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4 oxadiazole (Butyl-PBD), a hole-transporting N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1, 1'biphenyl-4,4' diamine (TPD), and a phosphorescent guest dye of Tris(2-phenylpyridine) iridium (III) (Ir(ppy)3), whose emission peak wavelength is ~510 nm with a full width at half maximum (FWHM) of ~85 nm [32]. A mixed solvent of 1,2-dichloroethane and chloroform (mixing weight ratio 3:1) is used for the solution. The thicknesses of the PEDOT:PSS and EL layers are adjusted to be about 40 nm and 80 nm, respectively. In order to form the electroninjecting layer, a ~1 nm thick Cs2CO3 interfacial layer is formed on the EL layer using thermal deposition (0.02 nm/s) at a base pressure of less than 2 × 10-6 Torr with a shadowmask that had 3 × 3 mm2 square apertures. Finally, a pure Al (~50 nm thick) cathode layer is deposited on the interfacial layer using thermal deposition under the same vacuum conditions. For comparison, we have also fabricated a reference device using a glass substrate in place of the GBO polarizer substrate. Apart from using different substrate materials, the reference devices are fabricated in exactly the same way as the sample OLED on the GBO polarizer substrate. Once the fabrication of OLEDs thus completed, the optical transmittance and reflectance spectra are measured using a Cary 1E (Varian) UV-vis spectrometer and a multichannel spectrometer (HR 4000CG-UV-NIR, Ocean Optics Inc., 0.25 nm resolution). A combination of a polarizer and an analyzer is also used to investigate the polarization of the light emitted from the sample device. A Chroma Meter CS-200 (Konica Minolta Sensing, INC.) and a source meter (Keithley 2400) have been used for measuring the

Figure 1(a) shows a photograph of the flexible GBO reflecting polarizer substrate used in this study. As shown in Fig. 1(a), the GBO substrate is easy to bend and quite transparent, in contrast to conventional linear dichroic polarizer film made from light-absorptive materials. Figure 1(b) shows a scanning electron microscopy (SEM) image of the cross-sectional structure of the GBO polarizer film. The SEM image shows clearly that the uniform layers of two alternating layered elements [a/b] are formed in multiple stacks with different refractive indices, (*nax*, *nay*, *naz*) and (*nbx*, *nby*, *nbz*). The optical anisotropy of the GBO polarizer may be seen by inspecting the polarized microphotograph of the GBO film between crossed polarizers at four angles of sample rotation of the GBO film substrate, as shown in Figure 2(a). This figure shows that the GBO film has a clear optical birefringence. We can define the orientation of the two optical axes, *x* and *y*, for the GBO film from the darkest views of the polarized microphotographs. The polarized transmittance spectra from the GBO polarizer film have then been observed for the two incident lights polarized linearly along the *x* and *y* axes, as shown in Figure 2(b). From this figure, it is clear that the nature of the reflection bands depends strongly on the polarization of the incident light, and the polarized

**Figure 2.** (a) Polarized microphotographs under crossed polarizers at four angles of sample rotation of the flexible GBO reflecting polymer polarizer film. (b) Polarized transmittance spectra for incident light polarized linearly along the *x* (ordinary) and *y* (extraordinary) axes.

On the design outlined above, we have prepared samples of OLEDs on the GBO reflecting polarizer substrate. In order to study the EL characteristics of the sample OLEDs, we have observed the current density-luminance-voltage (*J-L-V*) characteristics, as shown in Figure 3(a). It is clear from this figure that both the charge-injection and turn-on voltages are below 4.0 V, with sharp increases in the *J-V* and *L*-*V* curves. The EL brightness reaches ~4,500 cd/m2 at 14.5 V. This performance of the sample OLED with respect to luminescence is nearly the same as that of the reference device using a conventional linear dichroic polarizer film, which shows ca. 5,000 cd/m2 at 14.5 V. In contrast, as shown in Figure 3(b), the peak efficiencies (6.1 cd/A and 2.0 lm/W) of the sample OLED are much higher than those of the reference device (2.3 cd/A and 0.6 lm/W). The relatively high efficiencies of the sample device may be caused by the improved transition probability of exciton (singlet and triplet) relaxation with respect to the polarization along the transmission axis due to the reduced transition probability of exciton relaxation with respect to the polarization perpendicular to the transmission axis [20, 33].

Polarized Light-Emission from Photonic Organic Light-Emitting Devices 49

that a flexible polarized OLED with a high polarization ratio can be fabricated successfully

**Figure 4.** (a) Polarized EL emission spectra along the *o* (*EL||*, blue solid curves) and *e* (*EL┴*, red solid curves) axes for the fabricated polarized OLED. The dotted curves show the total emission spectra (*o* + *e*). (b) The relative *L-V* characteristics for polarization along the *o* (*EL||*) and *e* (*EL┴*) axes of EL emission.

**Figure 5.** The polarization ratio characteristics obtained using the *L-V* characteristics shown in Fig. 4(b).

**Figure 6.** Photographs showing the operating polarized OLED sample (3 × 3 mm2, 10 V) for the polarizations along the *o* (*EL||*, left) and *e* (*EL┴*, right) axes of the flexible GBO reflecting polarizer substrate under a rotating linear dichroic polarizer film. The *passing axis* represents the polarizing axis

(or transmission axis) of the linear dichroic polarizer.

using the GBO reflecting polarizer substrate.

**Figure 3.** (a) Current density-voltage and luminance-voltage characteristics and (b) current efficiencyvoltage and power efficiency-voltage characteristics of the sample OLED on the flexible GBO reflecting polarizer. The dotted curves show the characteristics of the reference device.

In order to interpret the observed EL characteristics of our sample device, we have also measured its polarization characteristics, as shown in Figure 4. Figure 4(a) shows the polarized EL emission spectra for the polarizations along the *o* (*EL||*) and *e* (*EL┴*) axes at normal incidence (0o). The curves represented by the dotted lines show the total spectra (*o* + *e*). It may be seen that the broad emission spectra are quite similar to that of the reference device, which coincides with the EL emission spectra of conventional OLED devices that have been reported elsewhere [32]. This figure also shows that polarized EL emission spectra strongly depend on the polarization state (*EL||* and *EL┴*), and that the sample OLED exhibits highly polarized EL emission over the entire range of emission from 470 nm to 650 nm. The EL polarization ratio (*PR*) of the integrated intensities of the parallel (*EL||*) and perpendicularly (*EL┴*) polarized EL emission is approximately 25. This ratio is significantly higher than that of the reference device which shows a *PR* of 1 (unpolarized light emission). Here, the *PR* is deduced using the ratio of the intensities, which were measured with polarization parallel and perpendicular to the passing axis of the GBO film, respectively, *i.e. PR = EL||* / *EL┴*. These results show that this technique for assembling polarized OLEDs, which utilizes a GBO reflecting polarizer, is at least as good as the previous approach, which uses the alignment of uniaxially oriented polymers or oligomers.

Figure 4(b) shows the relative polarized *L-V* characteristics of the same OLED for the polarizations along the *o* and *e* axes. This figure also gives quantitative results for polarized light emissions that were observed along the *o* (||) and the *e* (*┴*) axes. The highly polarized *L-V* characteristics give a high averaged *PR* value of 25 over the whole brightness range. (See Figure 5)

Next, as shown in Figure 6 are photographs of the operating polarized OLED sample (3 × 3 mm2, 10 V) with the polarization along the *o* (*EL||*, left) and *e* (*EL┴*, right) axes of the flexible GBO reflecting polarizer substrate. It may be seen from the figure that under a rotatable linear dichroic polarizer (left), the OLED is relatively more luminous and highly polarized along the ordinary axis of the GBO polarizer substrate. From these results, we may conclude that a flexible polarized OLED with a high polarization ratio can be fabricated successfully using the GBO reflecting polarizer substrate.

48 Organic Light Emitting Devices

Figure 5)

**Figure 3.** (a) Current density-voltage and luminance-voltage characteristics and (b) current efficiencyvoltage and power efficiency-voltage characteristics of the sample OLED on the flexible GBO reflecting

In order to interpret the observed EL characteristics of our sample device, we have also measured its polarization characteristics, as shown in Figure 4. Figure 4(a) shows the polarized EL emission spectra for the polarizations along the *o* (*EL||*) and *e* (*EL┴*) axes at normal incidence (0o). The curves represented by the dotted lines show the total spectra (*o* + *e*). It may be seen that the broad emission spectra are quite similar to that of the reference device, which coincides with the EL emission spectra of conventional OLED devices that have been reported elsewhere [32]. This figure also shows that polarized EL emission spectra strongly depend on the polarization state (*EL||* and *EL┴*), and that the sample OLED exhibits highly polarized EL emission over the entire range of emission from 470 nm to 650 nm. The EL polarization ratio (*PR*) of the integrated intensities of the parallel (*EL||*) and perpendicularly (*EL┴*) polarized EL emission is approximately 25. This ratio is significantly higher than that of the reference device which shows a *PR* of 1 (unpolarized light emission). Here, the *PR* is deduced using the ratio of the intensities, which were measured with polarization parallel and perpendicular to the passing axis of the GBO film, respectively, *i.e. PR = EL||* / *EL┴*. These results show that this technique for assembling polarized OLEDs, which utilizes a GBO reflecting polarizer, is at least as good as the previous approach, which

Figure 4(b) shows the relative polarized *L-V* characteristics of the same OLED for the polarizations along the *o* and *e* axes. This figure also gives quantitative results for polarized light emissions that were observed along the *o* (||) and the *e* (*┴*) axes. The highly polarized *L-V* characteristics give a high averaged *PR* value of 25 over the whole brightness range. (See

Next, as shown in Figure 6 are photographs of the operating polarized OLED sample (3 × 3 mm2, 10 V) with the polarization along the *o* (*EL||*, left) and *e* (*EL┴*, right) axes of the flexible GBO reflecting polarizer substrate. It may be seen from the figure that under a rotatable linear dichroic polarizer (left), the OLED is relatively more luminous and highly polarized along the ordinary axis of the GBO polarizer substrate. From these results, we may conclude

polarizer. The dotted curves show the characteristics of the reference device.

uses the alignment of uniaxially oriented polymers or oligomers.

**Figure 4.** (a) Polarized EL emission spectra along the *o* (*EL||*, blue solid curves) and *e* (*EL┴*, red solid curves) axes for the fabricated polarized OLED. The dotted curves show the total emission spectra (*o* + *e*). (b) The relative *L-V* characteristics for polarization along the *o* (*EL||*) and *e* (*EL┴*) axes of EL emission.

**Figure 5.** The polarization ratio characteristics obtained using the *L-V* characteristics shown in Fig. 4(b).

**Figure 6.** Photographs showing the operating polarized OLED sample (3 × 3 mm2, 10 V) for the polarizations along the *o* (*EL||*, left) and *e* (*EL┴*, right) axes of the flexible GBO reflecting polarizer substrate under a rotating linear dichroic polarizer film. The *passing axis* represents the polarizing axis (or transmission axis) of the linear dichroic polarizer.

### *2.1.3. Summary*

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 OLEDs with highly polarized luminescence emissions.

Polarized Light-Emission from Photonic Organic Light-Emitting Devices 51

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

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

passing axis of the GBO polarizer.

are identical.

*2.2.1. Device fabrication and materials used*
