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We replace Indium Tin Oxide (ITO) with an MgO nano-facet Embedded WO3/Ag/WO3(WAW) multilayer for electrodes of high efficiency OLEDs. WAW shows higher values for transmittance (93%) and conductivity (1.3×105 S/cm) than those of ITO. Moreover, WAW shows higher transmittance (92.5%) than that of ITO (86.4%) in the blue region (<500 nm). However, due to the large difference in refractive indices (n) of glass (n=1.55) and WO3 (n=1.95), the incident light has a small critical angle (52°). Thus, the generated light is confined by the glass/WAW interface, resulting in low light outcoupling efficiency (~20%). This can be enhanced by using a nano-facet structured MgO (n=1.73) layer and a ZrO2 (n=1.84) layer as a graded index layer. Using these optimized electrodes, ITO-free, OLEDs with various emission wavelengths have been produced. The luminance of OLEDs using MgO/ZrO2/WAW layers is enhanced by 24% compared to that of devices with ITO.
©2012 Optical Society of America
1. Introduction
Organic Light Emitting Diodes (OLEDs) have attracted attention due to their potential applications in full color flat panel displays, flexible display devices, and solid-state lighting [1–3]. In bottom emitting OLEDs, generated light is coupled-out through the transparent anode. Indium-tin-oxide (ITO) coated glass is typically used as the transparent substrate and the anode for bottom emitting OLEDs, because of its high transmittance (> 90%) and good electric conductivity (> 103 S/cm) [4,5]. However, ITO is becoming increasingly problematic due to the world-wide scarcity of the element indium, its susceptibility to ion diffusion into organic layers, and its relatively low transmittance in the blue region (~80%) [6–8]. Thus, there is a clear need for an alternative to ITO. A promising electrode structure is the dielectric/metal/dielectric (DMD) structure [9–11]. The optical transmittance of a metal film can be enhanced by exploiting the high refractive index (n>1.9) of the layers of dielectric films on the substrate-side and air-side of a metal layer (dielectric/metal/dielectric structure) satisfying the ‘zero-reflection’ condition [12,13]. However, most of the light generated in an organic material is confined by the high dielectric layer and glass substrate (nglass = 1.55) due to the large difference in the refractive indices n of these layers [14]. As explained by classical ray optics theory (i.e., Snell’s law), this has resulted in outcoupling efficiencies (ηout) (expressed as the ratio of surface emission to all emitted light) of only around 20% [15]. The remaining 80% of the photons are trapped in the organic and substrate modes [14,15]. This low outcoupling efficiency is one of the main problems with replacing ITO. Hence, the greatest potential for finding an alternative to ITO lies in improving the ηout of OLEDs using dielectric/metal/dielectric electrodes. A number of techniques to extract the waveguided light confined with transparent electrodes (especially ITO)/glass have been studied to enhance ηout including micro-lens to the glass substrate [16], subwavelength photonic crystals [17], surface plasmons [18], high refractive index substrates [19], low-refractive index grids [20], low-index-silica aerogel [21] and so on. However, many of these methods often have limited enhancement, distorted of shifted output spectra, enhancement at limited viewing angles, and difficulties in large-area fabrication [22, 23]. Furthermore, although the DMD electrodes have yet been reported as promising electrodes, no works on enhancing the ηout of OLEDs with these electrodes have yet been conducted.
Hereby, we demonstrate a novel means of enhancing the ηout in OLEDs by using nano-facet structured refractive-index graded layers between the WO3 and glass substrate. The graded index layer can preserve the electric properties of OLEDs fabricated on WO3/Ag/WO3 layers. For graded-refractive-index layer materials, nano-facet structured MgO (n=1.73) and ZrO2 (n = 1.84) layers that can reduce the total internal reflection at the glass/WO3 interface were deposited using the electron beam evaporation method. Formation of MgO nano-facet is a simple and low cost process that does not require additional lithography or patterning, because it occurs spontaneously due to the material anisotropic characteristics of MgO [16]. This results in enhanced luminance by up to 24% compared with conventional OLEDs with ITO electrode at the emission wavelength of 460 ~525 nm.
2. Methods
A glass substrate was used as the starting substrate. The substrate was cleaned with acetone, iso-propyl alcohol and deionized water and then dried with high purity nigrogen gas. The cleaned glass substrates were loaded into an electron beam evaporator, followed by the deposition of MgO (20, 40, 80 nm thick) and ZrO2 (120 nm) layers using high purity MgO and ZrO2 pellets (99.99%) with a diameter of 3 mm as a refractive index modulation layer. The films were grown at a rate of 0.3 nm/s, at a base pressure on the order of 10−6 Torr. To form the nano-facet structured surface of MgO, the substrate temperature was set to room temperature. The samples were then transferred into a thermal evaporation system with single chamber tool under high vacuum condition (base pressure ~10−7 Torr). The WO3/Ag/WO3 (30 nm) anode was deposited from WO3 and Ag pellets (99.99% with a diameter of 10 mm and 3 mm, respectively) at a base pressure on the order of 10−6 Torr. The optimal thickness of the Ag and WO3 layers was calculated by MACLEOD essential software. A layer of photoresist (PR) was then spun on the WO3. Array with 3 mm × 3 mm square openings were patterned following standard photolithographic method. After a 5-min hard bake, the opend surface of WO3 layer was treated by ultraviolet-ozone for 1 min under air ambient. The organic layers and Al cathode for the OLEDs were also deposited in a high vacuum (10−6 Torr) by simultaneous thermal evaporation onto all samples to ensure consistent results. The structure of the OLEDs consisted of 4,4’-[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD, 70 nm), emissive layer, 2,9-dimethyl-4,7-diphenyl-phenanthroline (BCP, 5 nm), tris(8-hydroxyguinolinato)aluminium (Alq3, 20 nm) LiF (1 nm), and Al (150 nm). Coumarin 545 tetramethyl doped Alq3 (1%-C545T:Alq3, 40 nm), iridium (III) bis(4,6-difluorophenylpyridinato) picolate doped (4,4′,4″-tris(N-carbazolyl)-triphenylamine (Firpic:TCTA, 18 wt%, 50 nm), and iridium (III) bis(4,6-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate doped TCTA (FIr6:TCTA, 10 wt%, 50 nm) were used as the emissive layers for green, blue and deep blue OLEDs. All organic films were deposited at a rate of 0.1 nm/s, at a base pressure on the order of 10−6 Torr. Aluminum top contacts were thermally evaporated at a rate of 0.3 nm/s, without breaking the vacuum. The devices were encapsulated with an additional glass and epoxy resin in a nitrogen atmosphere before evaluation. The active area of the device was 3 × 3 mm2. The current density-voltage and luminescence-current density characteristics of the devices were measured with an HP-4156A semiconductor parameter analyzer and YOKOGAWA 3298F test equipment in nitrogen ambient. The refractive index and transmittance of the refractive index modulation layer were measured by using ellipsometry (J. A. Woollam Co., Inc M-44) and a tungsten-halogen lamp (Mikropack HL-2000) and a monochromator (Acton Research Co., SpectraPro-300i). Scanning Electron Microscopy (SEM) was performed using a PHILIPS XL30S apparatus with an accelerating voltage of 10 kV and a working distance of 5 mm. Atomic Force Microscopy (AFM) images were recorded with a Digital Instruments Nanoscope III Multimode equipment in tapping mode using silicon cantilevers.
3. Results and discussion
Figure 1 shows a schematic diagram of the nano-facet structured graded index layer embedded OLEDs. Figure 1(a) presents the mechanism used for improving the outcoupling efficiency from internal reflection in OLEDs with a graded index layer. The emission from OLEDs is in general assumed to follow a Lambertian distribution. Thus, generated light should propagate in all directions. According to the classical Snell’s law, the low-angle light (<52.14°, with nDMD=1.9, nglass=1.5, schematically indicated by ray 1) cannot be extracted in the direction of the glass substrate at the glass/interface. However, the high angle mode (>52.14°, for example, ray 2) induced total internal reflection at the interface, resulting in propagation within the glass substrate and ITO electrode. In the case of OLEDs with a nano-facet structured graded index layer, the high angle mode that would otherwise be trapped by the glass/ITO interface can enter the graded index layer region and be refracted towards the substrate normal (Indicated by Ray 3 in Fig. 1(b)). In addition, the graded index layer will not affect the rays that were originally emitted into the forward viewing cone.
Fig. 1 Schematic illustrations of OLEDs with (a) WAW, and (b) MgO/ZrO2/WAW structure and the schematic representations of the mechanism used for improving device outcoupling efficiency. Inset: The cross section transmission electron microscope (TEM) image of MgO nano-facet structure.
Figure 2(a) shows the measured refractive indices of glass, MgO, ZrO2, and WO3. These values were found to be 1.55, 1.95, 1.84 and 1.73, respectively, and these values are well matched with results reported in the literature [24–26]. These results show that the MgO and ZrO2 layer can modulate the refractive indices between WO3 and glass. We used these values for simulating the optical transmittance of the graded index layer and DMD structures. Figure 2(b) shows the calculated transmittance at 460nm of ZrO2 and MgO as a function of thickness. The transmittance of both films oscillates according to the thickness of films, and the transmittance is maximized at 127 nm and 430 nm for ZrO2 and MgO, respectively.
Fig. 2 (a) Experimental values of refractive indices for Glass, MgO, ZrO2, and WO3. (b) Calculated transmittance values of ZrO2 and MgO as a function of thickness.
Figure 3 shows the calculated contour plots of transmittance (λ=460 nm) for WO3 (W0)/Ag (A)/ WO3 (W1) and MgO/ZrO2/W0AW1 as a function of W1 and Ag thickness with the W0 thickness fixed at 100 Å. Since the Ag film displays an island-based growth, the thickness of Ag played a critical role in determining the sheet resistance (RS). It has been reported that in the case of Ag on WO3, at least 10-nm-thick-Ag is required for a low RS value (< 10 ohm/sq) [10]. The transmittance of WAW layer can be optimized to ~93% with a 20 nm thick WO3 layer and a 10-nm-thick Ag layer. Our calculations showed that using a high refractive index dielectric layer of thermally evaporated WO3 (n > 2.0) as an index-matching (antireflection) layer could enhance the transmission of the Ag metal layer and this result agrees well with previously reported values [9–11]. In the layer WAW with the 127 nm thick ZrO2 and 430 nm thick MgO layer, the maximum transmittance decreased to 90.6% for the 20 nm thick WO3 and 10 nm thick Ag layers. Although the MgO/ZrO2 layer decreased the transmittance of the electrode, a high transmittance of over 90% can be expected, and the MgO/ZrO2 layer does not substantially alter the thickness dependence of the transmittance of the WO3 and Ag layers. Thus, the WAW electrode with MgO/ZrO2 layer can be used as a transparent anode for OLEDs.
Fig. 3 (a) Simulated contour plots of transmittance for WO3/Ag/WO3, and MgO/ZrO2/WO3/Ag/WO3 multilayers with respect to thickness variation of the upper WO3 and Ag layers.
We measured the optical transmittance of ITO, WAW, MgO/WAW and MgO/ZrO2/WAW layers as a function of wavelength [Fig. 4(a) ]. The ITO layer showed a high transmittance of 97.1% in the green emission region (λ = 520 nm). However, as the wavelength decreased, the transmittance also decreased to 86.4% in the blue emission region (λ = 460 nm) [8]. Meanwhile, the WAW layer showed a high optical transmittance in both the green emission region (93.5% at λ=520 nm) and the blue emission region (92.5% at λ=460 nm). The addition of the MgO and MgO/ZrO2 layers did not greatly decrease the transmittance of the transparent electrode. The respective transmittances were 88.9% and 88.2% at 460 nm, and these values were still higher than that of the ITO film (86.4%). We measured the current density-voltage (J-V) characteristics of OLEDs with four kinds of transparent anodes [Fig. 4(b)]. The operational voltage of the device with ITO anode was 6.8 V at J = 1 mA/cm2. This value slightly decreased to 6.5, 6.3 and 6.4 V as the anode was replaced with WAW, MgO/WAW and MgO/ZrO2/WAW layers, respectively, without leakage current.
Fig. 4 (a) Transmittance of ITO, WAW, MgO/WAW, and MgO/ZrO2/WAW layers as a function of wavelength (b) Current density-voltage characteristics of blue OLEDs with ITO, WAW, MgO/WAW, and MgO/ZrO2/WAW. Inset: Schematic diagram of the blue OLEDs with the embedded graded index layer at the glass/WAW interface.
Figure 5 (a) shows the top-view SEM images for the 400 nm thick MgO layer grown on glass. A nanopyramid structure was formed when the MgO layers were deposited. The MgO pyramid is formed due to anisotropic properties of (111), (200) and (220) main plane of MgO. The (111) orientation of MgO with an alternating array of Mg cations and O anions is very unstable because of dipole energy accumulation induced by polarity (Mg2+ planes and O2- planes) [27]. So, MgO films tend to grow via MgO films tend to grow via {200} surface termination and family plane in order to acquire the most stable atomic arrangement [27]. The clear MgO pyramid shown by the rock-salt MgO crystal structure [Fig. 5(a), inset] is the result of randomly distributed hexahedrons enclosed by neutral (200), (020) and (002) planes, as shown in Fig. 5(a). As the thickness of the MgO layer increases from 0.1 μm to 10 μm, the size of the MgO nanopyramids increases and the RMS value increases from 1.3 to 11.7 nm [Fig. 5(b)]. Thus the size of the pyramids can be easily controlled by adjusting the thickness of the MgO layer.
Fig. 5 (a) Top-view SEM images of 400 nm thick MgO film on glass substrate. Inset: Schematic crystal structure of MgO (b) RMS and peak-to-valley roughness (RPV) values of MgO nano-facet structure as a function of thickness. Inset: AFM images of MgO nano facet structure (Scale bar: 500 nm).
Figures 6(a) and (b) show the current density-voltage and luminance-current density characteristics of Green OLEDs(Alq3:C545T, λ = 525nm) using WAW anodes for different MgO thicknesses. The operational voltage of the device without refractive index modulation layer (WAW) was 9.9 V at a current density of 100 mA/cm2. It negligibly changed to 9.6 and 9.5 V when the 200 nm and 400 nm thick MgO/ZrO2(120nm) layers were inserted. However, as the 800 nm thick MgO/ZrO2 layer was inserted, the operational voltage of the device was increased to 13.5 V, and the leakage current density at ‒3 V increased from 14.8 nA/cm2 to 324 μA/cm2. This leakage current originated in the increment in the surface roughness of nano-facet MgO layer together its thickness. It is previously reported that the leakage current of OLEDs highly depend on the peak-to-valley roughness (RPV) of the substrate [28, 29]. When the thickness of the MgO film increases to 800 nm, the RPV drastically increased over 61% (Fig. 5(b)), leading to large leakage current level. The luminance of WAW device is 37500 cd/m2 (at J=220 mA/cm2). With the refractive index modulation layer of 200 nm thick MgO/ZrO2, the value increased to 41000 cd/m2. As the size of the facet increased, the nano-facet structure became more effective for light extraction. Thus, the OLEDs with 400 nm thick MgO/ZrO2 layer showed improved luminance of 43500 cd/m2. However, when the 800 nm thick MgO/ZrO2 layer is inserted, the luminance was decreased to 36000 cd/m2 due to leakage current. We found that the optimal thickness of the MgO nano-facet layer was 400 nm.
Fig. 6 (a) Current density-voltage and (b) luminance-current density characteristics of green OLEDs with MgO/ZrO2/WAW as a function of thickness of MgO.
To evaluate the correlation between the electroluminescence (EL) spectra and types of graded index layers, we fabricated OLEDs with three different emissive layers; Alq3:C545T, TCTA:Firpic and TCTA:FIr6. Figure 7(a) shows the normalized EL spectra of three kinds of OLEDs with ITO and MgO/ZrO2/WAW layers. The main emission peaks of Alq3:C545T, TCTA:Firpic and TCTA:FIr6 were located at 525 nm, 480 nm and 460 nm, respectively. In all cases, compared to the device with ITO anode, the EL peak positions and shapes of the EL spectra remained unchanged after utilizing MgO/ZrO2/WAW layer, and only the intensity of the spectra increased. The normalized improved luminance of the three emissive layers as a function of current density is shown in Fig. 7(b). For the device with Alq3:C545T(λ=525 nm), the emission intensity is increased by a factor of 1.09 regardless of current density. However, it significantly increased with the decrease of emission wavelength. The maximum enhancement was found to occur with the device using TCTA:FIr6 (λ=460 nm) with a factor of 1.24. These results are due to the relatively higher transmittance of MgO/ZrO2/WAW films in the blue region compared with ITO.
Fig. 7 (a) Normalized EL emission spectra and (b) relative luminance-current density characteristics of devices with Alq3:C545T, TCTA:Firpic, and TCTA:Fir6 using MgO/ZrO2/WAW(GIL/WAW) layers compared with devices using ITO layers. Inset: Photograph of operating OLEDs with MgO/ZrO2/WAW(top) and ITO (bottom) layers.
Figure 8 shows the relative luminance enhancement factor defined as the luminance of OLEDs with various kinds of electrodes (WAW, MgO/WAW and MgO/ZrO2) divided by that of the device with ITO electrode. In the case of WAW electrode, the green OLEDs (Alq3:C545T, λ = 525 nm) showed a luminance enhancement ratio of 0.93 due to the relatively low transmittance of WAW layer. However, as the main emission peaks located at 480 nm (TCTA:Firpic) and 460 nm (TCTA:Fir6) were diminished, the ratios were increased to 1.01 and 1.03 as a result of the increasing transmittance [Fig. 4(a)] Even though the transmittance was decreased, employing MgO and MgO/ZrO2 layers could increase the luminance enhancement factor of all devices. When the 400 nm thick MgO (n = 1.73) layer was inserted between the WO3 and glass substrate, the enhancement factor was further increased to 0.96, 1.05, and 1.09 at 525, 480, and 460 nm, respectively. This enhancement originated in the increased critical angle of the glass/WO3 interface. Compared to the glass/WO3 interface (50.27 o), the MgO/ITO interface has a larger critical angle (63.47°). Thus, total internal reflection could be reduced, resulting in the improved luminance properties. The enhancement factor can be maximized by employing a ZrO2 (n = 1.84) layer between the WO3 (n = 1.95) and MgO (n = 1.84) layer. The blue OLEDs (FIr6, λ = 460 nm) with MgO and ZrO2 layers showed the most improved enhancement factor of 1.24. Using the MgO/ZrO2 layer, the critical angle of the interface between the glass and WAW layer could be increased to 75.56 o and more light could be extracted from the WAW layer to glass, resulting in the improved optical properties of OLEDs. In addition, nano-facet structures formed spontaneously at the MgO surface during deposition could reduce internal reflection and scatter light outwards. At the pyramid-shaped surface, the incidence angle of light is reduced at each trap step, after all, the incidence angle could become smaller than critical angle, resulting in the light escaping from OLEDs and enhanced light outcoupling. Therefore, the enhanced luminance in OLEDs depends on the thickness of the MgO layer due to the size variation of MgO nano-facets, as shown in Figs. 5 and 6.
4. Conclusions
We demonstrated enhaced light extraction in OLEDs with dielectric/metal/dielectric electrodes by utilizing a graded index layer with a nano-facet structure. The luminance of OLEDs with MgO/ZrO2/WO3/Ag/WO3 layers is enhanced by up to 24% compared to that of OLEDs with an ITO layer at a wavelength of 460 ~525 nm. The significant increase of luminance for the MgO/ZrO2/WO3/Ag/WO3 OLEDs could be attributed to the increased critical angle for total internal reflection as well as to the roughened surface caused by the MgO nano-facet structure. The nano-facet structured refractive index modulation layer redirects light confined in the WO3 layer and glass substrate towards the substrate normal, thereby allowing for the extraction of waveguided light into the glass and air modes. Thus, the graded index layer embedded WO3/Ag/WO3 electrodes are a promising alternative to ITO ones.
Acknowledgements
This research was financially supported from the Information Display R&D Center in part by a grant (F0004090-2009-31), one of the Knowledge Economy Frontier R&D Programs funded by the Ministry of Knowledge Economy (MKE) of the Korean government, in part by the WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (Project No. R31-2008-000-10059-0), and in part by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0029711).
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