1. Introduction

The introduction of a two-dimensional (2D) photonic crystal (PC) structure into a semiconductor slab liberates the photons trapped within the high index waveguide layer, and provides an effective way of solving the light-trapping problem [1–4]. Recently, one-dimensional (1D) and two-dimensional (2D) periodic structures were introduced onto the glass substrate of an organic light-emitting diode (OLED) in order to enhance the light extraction [5–10]. Recent research into 2D photonic crystal nanolasers composed of organic luminescent materials has also attracted much attention [11–13]. The operation of band-edge lasers is based upon the enhancement of the optical density of states at the photonic band edge [14–18]. These advances motivated us to design a 2D array of layered OLEDs embedded in sub-wavelength scale holes. There has been much interest in fabricating sources of light that are smaller than the emitted wavelength, because of their possible applications in quantum computing, nano-size optical sensors, and optical microscopes [19]. The fabrication and characterization of discrete, structurally regular, nanoscopic OLEDs have recently been reported [20]. The challenge now is to use nanoscopic lighting in nano-optoelectronic devices. In this paper, we report the fabrication and optical characteristics of an array of subwavelength scale OLEDs embedded in a predefined 2D nanohole array template. We discuss our procedure for fabricating the nanohole-templated OLEDs and present optical analyses of the resulting OLED array, a so-called “moth-eye like OLED”.

Figure 1 shows a schematic diagram of the simple structure we used in the fabrication of the moth-eye like OLED. An array of nanoholes in a SiO₂ thin film on a patterned indium-tin oxide (ITO) glass substrate is used as the structural and optical template. As shown in Fig. 1, conventional OLED structures with a multi-layer sandwich architecture comprised of several organic emitting layers and a metal cathode are deposited inside the predefined nanoholes on top of the ITO anode. Each sub-wavelength scale OLED deposited on the glass substrate consists of the ITO anode, the SiO₂ template (n=1.47) embedded with several organic emitting layers (n=1.67~1.75), and a metal cathode.

The patterning of the 2D nanohole array into the SiO₂ film is carried out on top of ITO glass patterned with 3 mm stripes. Considerable progress has now been made in constructing 2D nanostructures using various fabricating methods, such as conventional lithography, interfering lithography, electron-beam lithography, nano-contact printing, Dip-Pen lithography, and nanoimprinting. We have previously demonstrated the suitability and simplicity of a method using two-step irradiated laser-interfering lithography and reactive ion etching (RIE) for the fabrication on a glass substrate of a 2D SiO₂ nanorod array with a size larger than the order of ~cm² [5,6]. Interfering hologram lithography is also an excellent procedure for fabricating 2D nanohole arrays that are larger than ~cm², and enables us to fabricate novel light sources that are smaller than the emitted wavelength. In this study, we investigated the possibility of fabricating nanoscale OLEDs using the simple method of inserting organic layers into 2D nanoholes between two electrodes, and determined the optical properties of a high density (~7.7×10⁶/mm²) 2D array of such OLEDs with an active area of 3×3 mm².

 

Fig. 1. Schematic diagram of the nanohole OLEDs embedded in the 2D periodic SiO2 nanohole array. Inset is view from above the 2D array of nanohole OLEDs.

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2. Experimental methods

The fabrication of this array was carried out as follows. In the first step, an ITO layer of up to 130 nm in thickness was deposited on a glass substrate by rf magnetron sputtering with heating at 200°C. The sheet resistance of the ITO layer was found to be approximately 50Ω/□, and its transmission was ~85% in the visible range. Subsequent photo-lithographical patterning and etching of the ITO layer were used to produce 3 mm wide ITO stripes. Next, a SiO₂ film was deposited onto the patterned ITO-coated glass substrate by using plasma enhanced chemical vapor deposition (PECVD). A negative photoresist film was then spin-coated on top of the SiO₂ film. Two-step irradiated laser-interfering lithography and reactive ion etching (RIE) were used to generate a square-lattice 2D nanohole pattern in the SiO₂ film [5,6]. Figure 2 shows SEM images taken from the side and from above the 2D array of nanoholes which have a pitch of ~360 nm, a diameter of ~220 nm, and a height of ~120 nm. An SEM microscopic profile was determined along the horizontal line (A-B) shown in Fig. 1, before the organics were evaporated. This profile indicates that the nanoholes in the array are regular and uniform; their calculated fill factor is about ~29.3%. The SiO₂ layer is etched by RIE during the formation of the holes until the surface of the ITO layer is exposed to the air. The profile also indicates that a small amount of nanoscale SiO₂ debris is present on the skin of the ITO electrode after etching.

 

Fig. 2. SEM images taken from the side and from above the 2D SiO₂ nanohole array of nanoholes with a pitch of ~360 nm, a diameter of ~220 nm, and a height of ~120 nm on ITO glass.

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This ITO electrode can be utilized as an anode electrode for the nanoscale OLEDs, in order to test their basic electro-optical properties. The following materials were vacuum sublimed on top of both conventional ITO glass and nanohole-templated ITO glass: a 65 nm thick N,N ^’-di(naphthalene-1-yl)-N,N ^’-diphenyl-benzidine (NPB) layer, a 50 nm thick tris(8-hydroxyquinoline)aluminum: 1% coumarin-6 (Alq₃:C6) layer, and a 30 nm thick Alq₃ layer. Then 30–50 nm thick LiF/Al cathodes and a 200 nm thick Al capping layer were formed by thermal evaporation using a shadow mask with 3 mm wide stripes making crosses with the ITO stripes (equivalent to an active area of 3×3 mm²). The OLEDs were encapsulated with 0.7 mm Corning cover glass. Figure 3 shows a focused ion beam-type SEM (FIB-SEM) image of a cross-sectional view of the 2D array of nanohole OLEDs. This image shows that the 2D array of nanohole OLEDs was successfully created at the interface between the ITO anode and the Al cathode, resulting in a moth-eye like OLED with the design shown in Fig. 1.

 

Fig. 3. FIB-SEM image of a cross-sectional view of the 2D nanohole OLEDs.

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3. Results and discussion

Electroluminescence analyses were carried out along the normal of the surface (θ=0°) to study the electro-optical properties of the moth-eye like OLED, using a PSI DARSA-II fluorescence spectrometer [5]. Figure 4 shows the applied voltage-luminance (V-L, left axis) characteristics of the 2D array of nanohole OLEDs. The luminance was found to increase rapidly after the turn-on voltage, before reaching a plateau. The luminance of the 2D array of nanohole OLEDs measured along the normal of the surface (θ=0°) under 10 V DC excitation was found to be 7,500 cd/m² at a current density of 220 mA/cm². When we corrected the EL luminance for the fill factor of the nanoholes (~29.3%), the luminance of the moth-eye like OLED with a 2D array of nanoholes of radius (r) 110 nm and period (Λ) of 360 nm was found to be 25,600 cd/m².

 

Fig. 4. Applied voltage-luminance (left axis) dependence of both the conventional (filled squares) and the 2D nanohole OLEDs (half-filled circles). The luminance data for both OLEDs are the measuring data before correction. The current density-applied voltage (I-V, right axis) responses of the conventional and 2D nanohole OLEDs.

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Figure 5 shows the corrected EL luminance-current density (L-I) characteristics of both the moth-eye like OLED and the conventional OLED. It is usually found that the efficiency of an OLED increases linearly with increasing current density. Figure 4 also shows the relationship between current density and applied voltage (I–V, right axis) for the nanohole OLEDs. In agreement with a previous result [20], we found upon comparison of the device I–V responses of the nanoscale and conventional millimeter-scale OLEDs with the same layer dimensions and composition that there is almost no evidence for quantization effects in the moth-eye like OLED. The shapes of both the current density and luminance plots are in excellent agreement with those of standard OLEDs, such as reported for small molecule OLEDs [21]. The similarity of the electron injection characteristics of the conventional and moth-eye like OLEDs demonstrates that the techniques used to create the 2D array of nanohole OLEDs with the SiO₂ nanohole template are electrically acceptable. The plotted IV-L relationships show that there are only minimal effects of device shrinkage on the properties of the nanohole OLEDs. These results confirm that the device efficiency produced by charge injection, transport, and recombination (exciton formation) in the organic layers is independent of the emitting dimension of the device in the range from nanometers to millimeters.

 

Fig. 5. Plot of corrected EL luminance-current density for the conventional (filled squares) and 2D nanohole OLEDs (half-filled circles). The data for the moth-eye like OLED are corrected with the fill factor. Inset: Emission spectra measured along the normal of the surface (θ=0°) under an applied voltage of 10.0 V

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The emission spectra of the conventional and nanohole OLEDs along the normal of the surface (θ=0°) under an applied voltage of 10.0 V are shown in the inset of Fig. 5. This figure shows that at three different wavelengths the shape of the emission spectrum of the moth-eye like OLED is slightly different to that of the conventional OLED. These differences in the EL spectrum of the moth-eye like OLED indicate that periodic scattering is occurring [5–10]; the Bragg scattering characteristics of such systems can be used as a guide to alterations in their emission spectra.

We have measured the angular dependence of the emission of the 2D moth-eye like OLED in order to determine the effects of periodic scattering in this system. The perceived colors of the emission spectra were expressed in terms of the chromaticity coordinates (x,y) developed by the Commission Internationale d’Eclairage (CIE), and used to assess the differences between the spectra of the conventional and moth-eye like OLEDs. Figure 6 shows the angular dependences of the CIE color coordinates for the conventional OLED and for the 2D moth-eye like OLED (Λ=360 nm). The results show that as the viewing angle varies by up to ±70° to the normal, the color change ratio of the 2D moth-eye like OLED (Δx=24.7%, Δy=10.8%) is larger than that of the conventional OLED (Δx=6.2%, Δy=2.6%). These results show that the angular dependence of the peak shape of the conventional OLED is negligible, but that large variations in the emission spectrum of the moth-eye like OLED occur with variation in the viewing angle. The strong angular dependence of the emission of the moth-eye like OLED confirms the occurrence of a periodic emission effect due to the subwavelength pitch [6,8]. It is difficult to examine an individual nanoscale OLED (220 nm in diameter and 360 nm in pitch) with an optical microscope or with the naked eye, because of the limits of resolution of optical microscopes [20,22]. Although we cannot see direct images of the nanoscopic emissions of the moth-eye like OLED, both the changes in spectrum shape and the strong angular dependence of the moth-eye like OLED are characteristic of a periodic array of distinct, nanoscopic OLEDs, because of the Bragg scattering that results from 2D periodic emissions with sub-wavelength pitch.

 

Fig. 6. Angular dependences of the CIE color coordinates for the conventional (filled squares) and 2D nanohole OLEDs (half-filled circles, pitch (Λ)=360 nm).

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4. Conclusion

We have demonstrated that a two-dimensional periodic array of nanoscopic OLEDs can be constructed by embedding organic layers and cathodes into a predefined sub-wavelength scale 2D SiO₂ nanohole array template on top of patterned ITO-coated glass. Two-step irradiated hologram lithography and reactive ion etching were used to create the 2D SiO₂ nanohole array pattern on the ITO glass substrate, which enables the fabrication of a moth-eye like OLED with a large substrate area of ~cm². We found by analyzing the electro-optical characteristics of the OLEDs that there was minimal variation of their optical properties due to their shrinkage down to the sub-wavelength scale, and confirmed the production of 2D periodic nanohole emission resulting from millions of 220 nm OLED dots by comparing the angular dependence of its spectrum with that of a conventional OLED. This nanohole-templated OLED offers simple possibilities and methodologies for designing nanoscale lighting devices, including inorganic and organic LEDs.