1. Introduction

Recently, intensive research has been conducted to control light in photonic crystals (PCs), in which the refractive index changes regularly with periodicity of the order of the optical wavelength, since the discovery of a photonic band gap (PBG) in which a certain energy range of photons is forbidden.[1, 2, 3, 4] Using an understanding of the physical properties of PBGs, various applications of PCs have been proposed for photonic devices using three-[5, 6], two-[7, 8], and one-dimensional (1-D) PBG materials[9, 10, 11, 12]. Although 1-D PCs do not have the range of capabilities of complete 3-D PBGs, 1-D PCs are simple to make and could hence be a potentially inexpensive photonic material for use in a variety of interesting applications. 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 the photons approaches zero and the density of mode (DOM) changes dramatically at the PBG edge.[9, 10, 11, 12, 13] Various studies of light emissions related to 1-D PCs have been reported, including 1-D PC band-edge lasers and wavelength-tunable 1-D PC lasers.[9, 10, 11, 12, 13] Furthermore, the combination of 1-D PCs with light emitting devices, for example, organic light emitting devices (OLEDs), was also reported to achieve high out-coupling emission efficiency, i.e., micro-cavity OLEDs or multi-mode micro-cavity OLEDs.[14, 15, 16] However, despite the recent development of efficient 1-D micro-cavity OLEDs, there remains a clear need to refine their structure further, as a result of the ongoing difficulties of controlling the polarization of the emitted EL light in the devices that have so far been proposed. It may be noted that the control of the polarization of the EL light emitted could be of use in a number of optical 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.[17, 18, 19, 20]

 

Fig. 1. (a) SEM image of the cross-section of the structure and (b) polarized microphotographs under crossed polarizers at four angles of sample rotation of the studied 1-D PC film substrate. (c) Polarized transmittance spectra for the incident lights polarized linearly along the x (ordinary) and y (extraordinary) axes.

Download Full Size | PDF

In this report, we describe the polarization of the emitted EL light from polarized micro-cavity OLEDs that make use of an anisotropic 1-D PC substrate. It has been predicted that in anisotropic PCs, the photonic band structure would split with respect to the state of polarization of the interacting light, in contrast to the degenerated band structure of conventional isotropic PCs.[17, 18, 19, 20] By employing this structure, we demonstrate the sharp and polarized emission of EL light for the first time.

2. Experimental methods

The polarized micro-cavity OLEDs were prepared by placing an organic EL layer between an anode and a cathode on a flexible, anisotropic 1-D PC film in the following structure: an anisotropic 1-D PC 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 anisotropic 1-D PC film, a commercial 1-D photonic band gap film (Magical film, Tokyu Hands Inc.) was used. The film was approximately 28 μm thick and the wavelength of the middle of the selective reflection band was close to 660 nm. Further information about the structure of the film was unavailable. After routine cleaning of the 1-D PC film using ultraviolet-ozone treatment, a flexible semi-transparent thin Au layer was deposited (87 nm, 38 ohm/square) by sputtering on the 1-D film to form the anode electrode. This Au anode was used in preference to the typical solid indium-tin-oxide (ITO) anode in order to preserve the high flexibility of the substrate. The optical transmittance of the Au electrode was about 60 % in the visible wavelength region. On the Au anode, a solution of PEDOT:PSS (poly(3, 4-ethylenedioxythiophene): poly(4-styrenesulphonate), Bayer) was spin-coated in order to produce the hole-injecting buffer layer. Subsequently, 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) (PVK), an electron-transporting 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1, 3, 4 oxadiazole (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 iridium(III) bis-(phenylquinoline) acetylacetonate derivative (Ir(prpiq)2acac), whose emission peak wavelength was ~618 nm with a full width at half maximum (FWHM) of ~60 nm.[21] For the solution, a mixed solvent was used, consisting of 1, 2-dichloroethane and chloroform (mixing weight ratio 3:1), which have different volatilities. The thickness of the PEDOT:PSS and the EL layers was adjusted to be about 40 nm and 80 nm, respectively. In order to form the electron-injecting layer, a ~1 nm thick CsF 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 instead of the 1-D PC film substrate. It should be noted that, apart from the substrate, the method of fabricating the reference devices was exactly the same as that of fabricating the sample micro-cavity OLED on the 1-D PC film. 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. A Chroma Meter CS-200 (Konica Minolta Sensing, INC.) and a source meter (Keithley 2400) were used for measuring the EL characteristics.

 

Fig. 2. (a) Current density-voltage and luminance-voltage characteristics and (b) current efficiency-voltage and power efficiency-voltage characteristics of the anisotropic micro-cavity OLED sample. (c) Normalized electroluminescence spectra of the micro-cavity sample (red solid curve) and the reference (black dotted curve) OLEDs at an operating voltage of 13 V.

Download Full Size | PDF

3. Results and discussion

The structure and optical characteristics of the 1-D anisotropic PC film substrate used are shown in Fig. 1. In Fig. 1(a), a scanning electron microscopy (SEM) image of the cross-sectional structure of the PC film may be seen. The SEM image clearly shows that uniform layers of two alternating layered elements of [a/b] are well-formed in multiple stacks with different refractive indices, n_a and n_b. The optical anisotropy may be seen by inspecting the polarized microphotograph of the film between crossed polarizers at four angles of sample rotation of the film substrate, as shown in Fig. 1(b). This figure shows that the PBG film has a clear optical birefringence. We were able to define the orientation of two optical axes, x and y, from the darkest views of the polarized microphotographs. Polarized transmittance spectra from the PBG film were then observed for the two incident lights polarized linearly along the x and y axes, as shown in Fig. 1(c). From this figure, it is clear that the wavelength and the width of the PBG bands depend strongly on the polarization of the incident light. The high-energy PBG edges differ slightly from each other for the incident lights polarized along both the x and y axes. In contrast, the low-energy PBG edges are quite different from each other; there is a relatively narrow band gap for the incident lights polarized along the x axis, and a broad band gap for the incident lights polarized along the y axis. The difference between the band gaps indicates clearly that in the PBG film, the difference between the refractive indices of the alternative layers forming the 1-D PC for a light polarized along the y direction is greater than that along the x direction. Thus, it is evident that the anisotropy causes the photonic band structure to split and that the x and y axes represent the ordinary (o) and extraordinary (e) axes, respectively.

On the basis of this information, we prepared sample micro-cavity OLEDs on the anisotropic 1-D PC film. In order to study the EL characteristics of the sample OLED, we observed the current density-luminance-voltage (J-L-V) characteristics, as shown in Fig. 2(a). It is clear from this figure that both the charge injection and turn-on voltage are below 6.0 V, with sharp increases in the J-L-V curves. The EL brightness reaches 1, 526 cd/m² at 20 V with a maximum efficiency of about 1.1 cd/A and peak power efficiency of ca. 0.33 lm/W. (see Fig. 2(b)) This performance is somewhat inferior to that of the reference device, which showed ca. 9, 000 cd/m² at 14 V with peak efficiencies of about 2.8 cd/A and 0.73 lm/W. The relatively poor performance of the sample device may result from the difficulties of optimizing the efficiency of the micro-cavity structure on the flexible 1-D PC substrate. It is noted that the performance of the micro-cavity device is strongly dependent on the layer thickness, the micro-cavity length, and the interface position, as well as on the thickness of the emission region. [26, 27, 28] Furthermore, in order to ensure optimum efficiency, the emission region should be narrow and aligned with the antinode of the electric field in the resonant cavity. [26, 27, 28] Thus, if a slight deviation from the optimum condition for the most efficient micro-cavity structure occurs, a reduced EL performance of the device may ensue. In order to study the EL emission characteristics of the sample device, we also observed EL spectra at normal incidence, as shown in Fig. 2(c). Upon applying a voltage of 13 V to the sample device, clear, sharp spectral peaks of EL emissions were observed (red solid curve). It may be noted that the peak wavelength of the sharp EL spectra is close to that of the edge of the high-energy band (~600 nm) of the 1-D PC film. In contrast, a relatively broad emission was observed for the reference device (black dotted curve), which coincided with the EL emission spectra of conventional OLED devices as reported elsewhere.

 

Fig. 3. (a) The dependencies of reflection (upper panel) and EL emission spectra (lower panel) on polarization along the o (red solid curves) and e (blue solid curves) axes for the fabricated OLED sample. The dotted curves show the total spectra measured without any polarizer. The detection angle is fixed at normal incidence (0°). (b) The dependence of calculated DOM spectra (solid curve) and simulated transmission spectra (dotted curve) on polarization along the o (red curves) and e (blue curves) axes.

Download Full Size | PDF

In order to interpret the observed EL characteristics of the sample device, we measured the polarization characteristics, as shown in Fig. 3(a). This figure shows the polarized reflectance (upper panel) and polarized EL emission spectra (lower panel) for the polarizations along the o (red solid curves) and e (blue solid curves) axes at normal incidence (0°). The curves represented by the dotted lines show the total spectra measured without any polarizer. In the upper panel, the optical micro-cavity modes of the sample device may be seen. From this figure, it can be seen that the wavelength of the optical modes depends strongly on the polarization state of the interacting light. For the incident light polarized along the o axis, the o-modes occur at 588.3 and 603.1 nm for the longer high-energy band edge. In contrast, for the interacting light polarized along the e axis, the e-modes occur at 583.7 and 600.0 nm for the shorter high-energy PBG edge. These optical modes were coincident with the split polarized emission modes of sharp EL spectra (lower panel). The observed FWHM of the o or e mode emission is about 5 nm. Because the high DOM caused EL light to be emitted, we also calculated DOM spectra for the polarized micro-cavity modes, as deduced from the simulation of the split PBG band spectra in Fig. 1(b).[22] Figure 3(b) shows the calculated DOM spectra with the simulated split PBG band spectra. These results clearly show that polarized light emissions were observed at the split eigen-modes of the micro-cavity and that the increased DOM at the split band edge modes allows the emission spectra to be modified for the given polarizations. It is noted that the slight mismatch in wavelength may result from the difficulty in simulating the transmission spectra due to the high flexibility of the PC film used.

Using a rotating linear dichroic polarizer, we also investigated the performance of the polarized EL by determining the relative efficiency characteristics for the polarizations along the o (blue solid curves) and e (red solid curves) axes with respect to the total peak efficiency in the absence of a polarizer (Fig. 4). In the investigation, the luminance of both o and e modes shows an almost identical brightness in each case, which is slightly lower than half the total luminance. This result means that the emission probabilities for both o and e modes in the sample device are virtually identical. Thus, virtually the same efficiency characteristics result from both o and e modes, as shown in Fig. 4. It is noted that small differences in the efficiencies may be due to the deterioration of the sample device during successive measurements. More detailed results will be reported elsewhere.

 

Fig. 4. The relative efficiency characteristics with respect to the peak efficiency for polarization along the o (red curves) and e (blue curves) modes of EL emission. Solid curves: current efficiency, Dotted curves: power efficiency.

Download Full Size | PDF

From the above results, one may know that a flexible micro-cavity OLED with well-defined polarizations and optical micro-cavity modes was fabricated successfully using the anisotropic 1-D PC substrate. As shown in Fig. 5, the fabricated flexible, polarized micro-cavity OLED is fairly luminous. The polarized output can be selected in either ordinary or extraordinary mode, depending on the proposed application of the device. Furthermore, the device structure used in this study can be applied to the design of electrically pumped organic laser systems and/or special light-emitting devices such as polarized micro-cavity OLED lasers. Such devices can be used for polarized surface emitting lasers, 3-D displays, and the polarized light sources of optical waveguide devices.

 

Fig. 5. Photograph of the micro-cavity OLED sample (5 × 4.5 mm²) in operation on a flexible and anisotropic 1-D PC film substrate at 10 V.

Download Full Size | PDF

4. Conclusions

In summary, we fabricated a polarized micro-cavity OLED that consisted of a flexible, anisotropic 1-D PC. We demonstrated that luminous light was emitted at relatively low operating voltages below 20 V. The peak wavelengths of the emitted EL lights correspond to the split micro-cavity eigen modes of the high-energy band edges. Moreover, we showed that the polarization and emission mode of the emitted EL light can be selected by choosing the optical axis of the anisotropic 1-D PC film. A combination of the device reported here with optical devices reported elsewhere will lead to highly efficient polarized micro-cavity OLEDs that could have a wide range of applications.