1. Introduction

Ever 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 [1–5]. Almost all previous work carried out on organic electroluminescent (EL) light emission has involved unpolarized EL light emission. However, a number of studies have demonstrated linearly polarized EL emissions [6–9]. Such work was done because polarized EL emission from OLEDs is potentially useful in a number of applications, not only those restricted to high-contrast OLED displays or more efficient light 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 degree of polarization ratio (PR) of emitting light is needed. In most cases, linearly polarized EL emissions were demonstrated for uniaxially orientated materials of 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 [6], rubbing/shearing of the film surface [7, 8], mechanical stretching of the film [9, 10], orientation on pre-aligned substrates [11, 12], precursor conversion on aligned substrates [13], epitaxial vapour deposition [14], and the friction-transfer process approach [15, 16]. Although there have been a number of such efforts to achieve linearly polarized EL emission, the polarization ratio and the device performance (in terms of brightness and efficiency) of the devices reported are still insufficient for most applications.

We herein propose an approach different from the conventional methods, which make use of uniaxially oriented materials. This has enabled us to achieve a high degree of linear polarization with high brightness and efficiency by using a simple Giant Birefringent Optical (GBO) [17] multilayer reflecting polarizer substrate, instead of an optically isotropic substrate such as glass. When a large degree of birefringence is introduced into the in-plane refractive index between adjacent material layers of a multilayer optical system, GBO effects begin to occur [17]. Pairs of groupings of adjacent layers (unit cells) can produce constructive interference effects when their thicknesses are scaled properly to the wavelengths of interest. These interference effects in multilayered structures result in the development of wavelength regions of high reflectivity (reflection bands) with adjacent wavelength regions of high transmission (pass bands) [17]. The significant optical feature of these multilayer interference stacks is the difference in the refractive index in the thickness direction (z axis) relative to the in-plane directions (x and y directions) of the film. By appropriate adjustment of the refractive indices of the adjacent layers, it is possible to construct a GBO multilayer reflecting polarizer using an interference stack that is composed of multiple layers of transparent polymeric materials [17]. The reflection band of the GBO polarizer exhibits a unique optical property, such that interference polarizers maintain or increase their reflectivity with increasing angle of incidence. Furthermore, a graded unit cell thickness profile is normally used to create a wider reflective band that accommodates wavelengths from the blue, through to the green and red, color regions [17, 18]. Such a multilayer polymer polarizer may routinely be used for optical applications that require high reflectivity and wavelength selectivity. As an example of its application, GBO multilayer polarizers have been used to create reflective polarizers that make LC displays brighter and easier to view. By using this property of the GBO polarizer, one might obtain highly linearly polarized-light EL emission over a wide range of optical wavelengths.

In this report, we describe the polarization of light emission from OLEDs that use a flexible GBO multilayer reflecting polymer polarizer substrate, instead of the conventional glass substrate. By using such a substrate, we demonstrate the potential for highly polarized light emission from OLEDs.

 

Fig. 1. (a). Photograph showing the flexible transparent GBO reflecting polymer polarizer film and (b). SEM image of the cross-section of the studied GBO reflecting polymer polarizer film.

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2. Experimental methods

The sample OLEDs were prepared by placing an organic EL layer between an anode and a cathode on a flexible GBO reflecting polarizer film in the following sequence: a GBO reflecting polarizer film substrate/a thin semi-transparent Au anode/a hole-injecting buffer layer/an EL layer/an electron-injecting layer/an Al cathode. For the GBO reflecting polarizer film, a commercial multilayer reflecting polymer polarizer film (3M) was used. The film was approximately ~90 µm thick and the wavelength of the reflection band was in an approximate range of 400~800 nm. This film is normally used in a 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 it on the GBO reflecting polarizer to form the anode. This Au anode was 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 was about 60 % in the visible wavelength region. A solution of PEDOT:PSS (poly(3,4-ethylenedioxythiophene): poly(4-styrenesulphonate), Bayer) was spin-coated on the Au 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 [19]. 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 be about 40 nm and 80 nm, respectively. In order to form the electron-injecting layer, a ~1 nm thick Cs₂CO₃ interfacial layer was formed on the EL layer using thermal deposition (0.02 nm/s) at a base pressure 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 was formed on the interfacial layer using thermal deposition under the same vacuum condition. For comparison, we also fabricated reference devices using a glass substrate in place of the GBO polarizer substrate. Apart from the different substrate material, the reference devices were fabricated in exactly the same way as the sample OLED on the GBO polarizer substrate. Once the OLEDs were complete, the optical transmittance and reflectance spectra were 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 was 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) were used for measuring the EL characteristics.

3. Results and discussion

Figure 1(a) shows a photograph of the flexible GBO reflecting polarizer substrate used in the study. As shown in the photograph, 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 the two alternating layered elements of [a/b] are formed of multiple stacks with different refractive indices, (n_ax, n_ay, n_az) and (n_bx, n_by, n_bz). 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 Fig. 2(a). This figure shows that the GBO film has a clear optical birefringence. We were able to 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 were then observed for the two incident lights polarized linearly along the x and y axes, as shown in Fig. 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 the 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 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.

 

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

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On the basis of the information provided above, we prepared sample OLEDs on the GBO reflecting polarizer substrate. In order to study the EL characteristics of the sample OLEDs, we observed the current density-luminance-voltage (J-L-V) characteristics, as shown in Fig. 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-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 showed ca. 5,000 cd/m² at 14.5 V. In contrast, as shown in Fig. 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, 21].

 

Fig. 3. (a). Current density-voltage and luminance-voltage characteristics and (b) current efficiency-voltage 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.

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In order to interpret the observed EL characteristics of the sample device, we also measured the polarization characteristics, as shown in Fig. 4. Figure 4(a) shows the polarized EL emission spectra for the polarizations along the o (blue solid curves) and e (red solid curves) axes at normal incidence (0^o). 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 coincided with the EL emission spectra of conventional OLED devices that have been reported elsewhere [19]. This figure also shows that polarized EL emission spectra depend strongly on the polarization state, and that the sample OLED exhibits highly polarized EL emission over the entire range of emission spectra. For the sample device used, the EL polarization ratio (PR) of the integrated intensities of the parallel and perpendicularly polarized EL emission was approximately 25. This ratio is significantly higher than that of the reference devices, which showed a PR of 1 (unpolarized light emission). Here, the PR was deduced using the ratio of the intensities, which were measured with a polarization parallel and perpendicular to the passing axis of the GBO film, respectively, i.e. PR=I_o/I_e. 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 sample OLED for the polarizations along the o (blue solid curves) and e (red solid curves) axes. This figure also gives quantitative results for polarized light emissions that were observed along the o and the e axes. As shown in the figure, the highly polarized L-V characteristics give a high averaged PR value of 25 over the whole brightness range. (See Fig. 4(c))

 

Fig. 4. (a). Polarized EL emission spectra along the o (red solid curves) and e (blue solid curves) axes for the fabricated polarized OLED. The dotted curves show the total emission spectra (o + e). The detection angle is fixed at normal incidence (0o). (b) The relative L-V characteristics for polarization along the o (red curves) and e (blue curves) axes of EL emission. (c) The polarization ratio characteristics obtained using the relative L-V characteristics.

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Fig. 5. Photographs showing the operating polarized OLED sample (3×3 mm², 10 V) for the polarizations along the o (left) and e (right) axes of the flexible GBO reflecting polarizer substrate under a rotating linear dichroic polarizer film. The passing axis represents the transmission axis of the linear dichroic polarizer.

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Finally, as shown in Fig. 5, we observed, with the naked eye, the operating polarized OLED sample (3×3 mm², 10 V) for the polarizations along the o (left) and e (right) axes of the flexible GBO reflecting polarizer substrate. It may be seen from the figure that under a rotatable linear dichroic polarizer, the fabricated sample OLED is fairly luminous and highly polarized along the ordinary axis of the GBO polarizer substrate. From the above results, we conclude that a flexible polarized OLED with a high polarization ratio was fabricated successfully using the GBO reflecting polarizer substrate.

4. Conclusions

In summary, we fabricated a flexible, polarized, and luminous OLED using a flexible GBO reflecting polarizer substrate. It is shown that EL brightnesses over 4,500 cd/m² were 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 was also shown that a high polarization ratio of up to 25 was possible over the whole emission brightness range. These results show that using the GBO reflector enables the development of flexible OLEDs with highly polarized luminescence emissions. By combining the device reported here with luminous EL layers reported elsewhere, it will be possible to develop highly efficient polarized OLEDs that have a wide range of optical applications. For example, the device structure used in this study could be applied to the design of special light-emitting devices, such as polarized backlights for LC displays. Such devices could 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.