Since the early pioneering work on efficient Organic Light-Emitting Devices (OLEDs) that was based on both small molecules and polymers, OLEDs have attracted a great deal of research interest due to their promising applications in full-color flat-panel displays and solid-state lighting [1-5]. Intensive research has been conducted into the development of OLEDs for realizing strong and efficient electroluminescent (EL) emission. To date, almost all previous work carried out on organic EL emission has involved unpolarized EL emission. Nevertheless, a number of researchers have reported the results of experiments in which linearly polarized EL emissions have been observed [6-17]. This particular avenue of research has been considered to be important because polarized EL emission from OLEDs is of potential use in a range of applications, not just those limited to high-contrast OLED displays, but also in efficient backlight sources in liquid crystal (LC) displays, optical data storage, optical communication, and stereoscopic 3D imaging systems . In order to design and manufacture these novel light-emitting devices, a high degree of polarization ratio (
Here we introduce an approach different from the conventional methods using uniaxially oriented materials. As an alternative, for the purpose of improving device performance, we suggest a technique to control the polarization of light emitted from OLEDs that are achieved using an anisotropic photonic crystal (PC) film. It has been predicted that in anisotropic PCs, the photonic band structure splits with respect to the state of polarization of the interacting light, in contrast to the degenerated band structure of conventional isotropic PCs, in which a certain energy range of photons is forbidden, giving rise to a photonic band gap (PBG) [18-20]. Of these applications, the study of light emission at the PBG edge is particularly attractive, as a result of the fact that the group velocity of photons approaches zero and the density of mode changes dramatically at the PBG edge [21-24]. The combination of PCs with OLEDs has also been reported to achieve high out-coupling emission efficiency, as achieved in the micro-cavity OLEDs or multi-mode micro-cavity OLEDs [25-27]. Moreover, by employing the anisotropic photonic structure, one may also obtain the polarized emission of EL light.
In this chapter, we describe in brief a technique to control the polarization of EL light emitted from photonic OLEDs that make use of a
2. Polarized photonic OLEDs with GBO films
Three kinds of polarized photonic OLEDs are presented here to demonstrate the use of the GBO film in the highly polarized OLEDs, exhibiting high brightness and efficiency.
2.1. OLEDs on the GBO polarizer substrates
In this section, we describe the polarization of EL light emitted from OLEDs that use a flexible GBO multilayer reflecting polymer polarizer substrate, instead of the conventional isotropic glass substrate. By using such a substrate, we demonstrate the potential for highly polarized light emission from OLEDs. Luminous EL emissions are produced from the polarized photonic OLEDs, and the direction of polarization for the emitted EL light corresponds to the polarizing axis (transmission axis or passing axis) of the GBO reflecting polarizer. The estimated polarization ratio between the brightness of two linearly polarized EL emissions parallel and perpendicular to the polarizing axis can be achieved as high as 25 for the OLEDs on GBO substrates.
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 μm 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 . 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 electron-injecting 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 shadow-mask 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.
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, (
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 (
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
Figure 4(b) shows the relative polarized
Next, as shown in Figure 6 are photographs of the operating polarized OLED sample (3 × 3 mm2, 10 V) with the polarization along the
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.
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 . We apply a quarter-wave retardation plate (QWP) film and a GBO reflective polarizer to a non-uniaxial OLED. The QWP film used in our study is a sheet of a birefringent (double refracting) material, which creates a quarter-wavelength (λ/4) phase shift and can change the polarization of the light from linear to circular and
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 (
2.2.1. Device fabrication and materials used
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 μm 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 (
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.
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
We also measured the polarization characteristics and Fig. 10(a) shows the polarized EL emission spectra for polarizations along the passing (
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
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
Here we describe the third technique that can be used to achieve high linearly polarized white EL emission based on the 'photon recycling' concept  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 π/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 (λ/4), and can change the polarization of the broad EL emission from linear to circular, and
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 (
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 μm 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 . 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
2.3.2. Results and discussion
The phase retardation (
Figure 14(a) shows the
In Fig. 15 (a), we have shown the current efficiencies of S and R WOLEDs. For the
Next, we have estimated the relationship between polarization ratio and luminance
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
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
We have fabricated flexible, polarized, and luminous OLEDs using a flexible GBO reflecting polarizer substrate. We have also described the fabrication and investigation of a polarized and luminous OLED and WOLED using a combination of a QWP retardation film and a GBO reflective polarizer. Polarized EL brightnesses of over 10,000 cd/m2 can be produced from the polarized OLED, with high peak efficiencies in excess of 10 cd/A, which are almost double those obtained from the polarized WOLED with only the GBO polarizer. The polarization direction of the EL light emitted from the polarized OLED corresponds to the passing axis of the GBO polarizer used. Furthermore, we have also shown that a high polarization ratio of greater than 35∼40 is possible over the whole emission brightness 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 , 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 Korean Ministry of Knowledge Economy (A002200104).