We present results that show highly polarized electroluminescence (EL) from an organic light-emitting device (OLED) by using a quarter-wave (λ/4) retardation plate (QWP) film and a giant birefringent optical (GBO) photonic reflective polarizer. Polarized EL light of 13,400 cd/m2 with high peak efficiencies (greater than 10 cd/A and 3.5 lm/W) was obtained from an OLED in this way. These values are almost double those of a polarized OLED that only uses a polarizer. The direction of polarization of the emitted EL light from the polarized OLED corresponded to the passing axis of the GBO reflective polarizer. Furthermore, the degree of linear polarization obtained, i.e. the ratio between the brightness of two linearly polarized EL emissions parallel and perpendicular to the passing axis, is greater than 40 over the whole range of emitted luminance.
© 2010 OSA
Ever since the early pioneering work on Organic Light-Emitting Devices (OLEDs) using both small molecules and polymers, OLEDs have attracted a great deal of research interest due to their promise for applications such as full-color flat-panel displays and lighting [1–5]. To date, almost all the previous work carried out in organic electroluminescent (EL) emission has involved unpolarized light. Some studies, however, have demonstrated linearly polarized EL emissions [6–9]. This particular direction was explored because polarized EL emission from OLEDs is of potential use in a range of applications, not just those related to high-contrast OLED displays or efficient backlight sources in liquid crystal (LC) displays, but also for optical data storage, optical communication, and stereoscopic 3D imaging systems. In order to design and manufacture such novel optical devices, a high polarization ratio (PR) of over 30~40 between the brightness of two linearly polarized EL emissions parallel and perpendicular to the polarizing axis is required. In most cases, linearly polarized EL emissions have been demonstrated for uniaxially oriented materials such as liquid crystalline polymers or oligomers incorporated in emissive layers. The methods that are commonly used for the uniaxial alignment of such layers include the Langmuir-Blodgett technique , rubbing/shearing/stretching of the film [7–10], orientation on pre-aligned substrates [11,12], precursor conversion on aligned substrates , vapor phase epitaxy , and friction transfer deposition [15,16]. We recently described the polarization of light emission from OLEDs using a giant birefringent optical (GBO) [17,18] multilayer reflective polymer polarizer substrate. It was demonstrated that such a structure allows polarized light emission from OLEDs with a peak brightness of 4,500 cd/m2 and an efficiency of 6 cd/A, and a high PR of 25 (Type 1 in Fig. 1 ) . Although much effort has been expended in trying to achieve linearly polarized EL emission, the PR and device performance (in terms of brightness and efficiency) reported are still insufficient for most applications.
We herein propose an alternative approach to achieve highly linearly polarized EL without resorting to the use of uniaxially oriented materials. We demonstrate a simple polarized OLED that can be driven by a non-uniaxial OLED by using the ‘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 was 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 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.
The configuration of our device, which was designed to achieve highly linearly polarized EL from an OLED, is shown as Type 2 in Fig. 1. We attached a QWP film and a GBO reflective polarizer to an OLED, choosing an angle of 45° between the fast optic axis of the QWP film and the passing axis (↕) of the GBO polarizer, as shown in the figure. Following the generation of unpolarized light by OLED under electrical excitation, the EL emission that is polarized along the direction parallel to the passing axis (↕) of the GBO polarizer is transmitted through the GBO polarizer, whereas other EL emission that is polarized perpendicular (☉) to the passing axis of the GBO polarizer is reflected selectively as a result of the photonic band of GBO polarizer. This reflected light changes the polarization to circular (i.e. right-handed circularly polarized light) following 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 retransmission through the QWP film, the polarization of the light is again changed from left-handed circularly polarized to linearly polarized (↕), with the direction of polarization being parallel to the passing axis of the GBO, so that it is transmitted through the GBO reflective polarizer. By this method, all the generated EL light can be transmitted through the GBO reflective polarizer and the transmitted EL light has the linear polarization (↕) along the passing axis of the GBO polarizer.
2. Experimental methods
The polarized OLEDs were 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: a GBO reflective polarizer / a QWP film / a glass substrate / a transparent indium-tin-oxide (ITO, 80 nm, 30 Ω/square) anode / a hole-injecting buffer layer / an EL layer / an electron-injecting layer / an Al cathode (Type 2). A commercial QWP polymer film (Edmund sci.) was used, and a commercial GBO reflective polarizer film (3M) was used as the reflective polarizer. The QWP film was approximately ~110 μm thick and the operating wavelength was approximately in the range 450 ~650 nm. After routine cleaning of the ITO substrate using wet (acetone and isopropyl alcohol) and dry (UV-ozone) processes, a solution of PEDOT:PSS (poly(3,4-ethylenedioxythiophene): poly(4-styrenesulphonate), CLEVIOSTM 4083, H. C. Starck Inc) was spin-coated on the ITO anode in order to produce the hole-injecting buffer layer. Subsequently, in order to form an EL layer, a blended solution was also spin-coated on the PEDOT:PSS layer. This blended solution consisted of a host polymer of poly(vinylcarbazole), an electron-transporting 2-(4-biphenylyl)-5- (4-tert-butylphenyl) −1,3,4 oxadiazole, a hole-transporting N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1, 1'biphenyl-4,4'-diamine, and a phosphorescent guest dye of Tris(2- phenylpyridine) iridium (III), whose emission peak wavelength was ~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) 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 has 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 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. The structure of 2nd reference is shown in Fig. 1 (Type 1). Note that, in the devices, the organic layer structure and used organic materials were identical and thus, electrical characteristics such as the current density-voltage (J-V) curve were identical in every device.
Once the OLEDs have been fabricated, the optical transmittance and reflectance spectra were measured using a Cary 1E (Varian) UV-vis spectrometer and a spectrophotometer (HR 4000CG-UV-NIR, Ocean Optics Inc., 0.25 nm resolution). A combination of a polarizer and an analyzer was also used to investigate the polarization of the light emitted from all the devices. A Chroma Meter CS-200 (Konica Minolta Sensing, INC.) and a source meter (Keithley 2400) were used to measure the EL characteristics.
3. Results and discussion
Figure 2(a) shows a photograph of the QWP film and the GBO reflective polarizer used in this study. It may be seen that the QWP film and the GBO reflective polarizer are quite transparent, unlike the conventional linear dichroic polarizer that is made from light-absorptive materials. Figure 2(b) shows a scanning electron microscopy (SEM) image of the cross-sectional structure of the GBO polarizer films. The SEM image of the GBO polarizer film shows clearly that the uniform layers of the two alternating layered elements are formed of multiple stacks in order to produce the photonic effect. The polarized transmittance spectra from the GBO polarizer film were then observed for the two incident light beams polarized linearly along the passing and blocking axes, both of which are shown in Fig. 2(c). From this figure, it is clear that the nature of the reflection bands depends strongly on the polarization of the incident light. The polarized transmission spectra are thus quite different from each other; when measured in the direction of the blocking axis, the transmission spectrum shows a strong and broad reflection band, while in the direction of the passing axis, there is virtually no reflection band in the wide visible wavelength range (350 ~700 nm) that includes blue, green, and red light. The average extinction ratio of the GBO reflective polarizer used was estimated to be about 16:1 for wavelengths between 470 and 700 nm. Next, the optical anisotropy of the QWP film may be seen in the polarized microphotograph of the QWP film obtained between crossed polarizers for four angles of rotation of the QWP film (Fig. 2(d)). This figure shows that the QWP film has a clear optical birefringence. The two darker views of the polarized microphotographs enabled us to obtain the orientation of the two optical axes for the QWP film.
The foregoing information allowed us to investigate the performance of the polarized OLEDs with the QWP film and the GBO reflective polarizer. In order to study the EL characteristics of our OLEDs, we assessed their luminance-voltage (L-V) characteristics. Figure 3(a) shows the polarized L-V characteristics of the fabricated OLEDs for the polarizations along the passing (blue curves) and the blocking (red curves) axes. For our polarized OLED, the figure indicates that the turn-on voltages are below 4.0 V, with sharp increases in the L-V curve for polarization parallel to the passing axis. The polarized EL brightness reaches ~13,400 cd/m2 at 17.0 V. This performance with respect to luminescence is approaching the total luminescence (ca. 18,500 cd/m2 at 17.0 V, unpolarized) of the 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 the passing and blocking axes. As shown in the figure, the highly polarized L-V characteristics for the polarized OLEDs give a high averaged PR value of at least 40 over the whole voltage range. This ratio is significantly higher than those of the 1st and 2nd reference, which showed a PR of 1 (i.e., unpolarized light) and 7.53, respectively. We herein deduce the PR using the ratio of the intensities, measured in terms of polarization parallel (Ipara) and perpendicular (Iperp) to the passing axis of the GBO polarizer, i.e. PR = Ipara / Iperp. Figure also shows that the EL emission polarized along the passing axis reaches only ~5,000 cd/m2 at 17.0 V for the 2nd reference OLED that only had 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 half that of our polarized device. This relatively low brightness of the 2nd reference device is brought about by 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 OLED, as shown in the figure. This is due to the reduced intensity of polarized light 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 Fig. 3(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 those 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.
Next, in order to interpret the observed EL characteristics of the polarized device, we also measured the polarization characteristics. Figure 4(a) shows the polarized EL emission spectra for polarizations along the passing (blue solid curves) and blocking (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 OLEDs reported elsewhere . 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 used, the PR of the integrated intensities of the parallel and perpendicularly polarized EL lights was always greater than 40.These results show that our polarized OLEDs, which incorporate a QWP film with a GBO reflective polarizer, perform extremely well. Figure 4(b) also shows the PR-L characteristics of our polarized OLEDs, giving quantitative results for the polarized EL emissions. As shown in the figure, while the degree of linear polarization of the 2nd reference is only about 7.5, our polarized device had a very high PR value of over 40 for the whole brightness range.
Finally, we visually observed 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 (Fig. 5 ). It may be seen from the figure that under a rotating linear dichroic polarizer, the OLED sample is more luminous and more highly polarized along the passing axis of the GBO polarizer in comparison with the 2nd reference device. All these results demonstrate our successful fabrication of a highly polarized OLED with a high PR (> 40), using a QWP film and a GBO reflective polarizer.
In summary, we have described the fabrication and investigation of a polarized OLED using a combination of a QWP retardation film and a GBO reflective polarizer. Polarized EL brightnesses of over ~13,400 cd/m2 was obtained from the polarized OLED, with high peak efficiencies over 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, we have also shown that a high polarization ratio of more than 40 is possible over the whole emission brightness range. These results show that use of the QWP film and the GBO reflective polarizer enables the development of bright OLEDs with highly polarized emissions.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0005557 and 2010-0016549).
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