- Nano Express
- Open Access
Fabrication and characterization of WO3/Ag/WO3 multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes
© Jeon et al.; licensee Springer. 2012
Received: 12 February 2012
Accepted: 20 April 2012
Published: 15 May 2012
The dielectric/metal/dielectric multilayer is suitable for a transparent electrode because of its high-optical and high-electrical properties; however, it is fabricated by an expensive and inefficient multistep vacuum process. We present a WO3/Ag/WO3 (WAW) multilayer transparent anode with solution-processed WO3 for polymer light-emitting diodes (PLEDs). This WAW multilayer not only has high transmittance and low resistance but also can be easily and rapidly fabricated. We devised a novel method to deposit a thin WO3 layer by a solution process in an air environment. A tungstic acid solution was prepared from an aqueous solution of Na2WO4 and then converted to WO3 nanoparticles (NPs) by a thermal treatment. Thin WO3 NP layers form WAW multilayer with a thermal-evaporated Ag layer, and they improve the transmittance of the WAW multilayer because of its high transmittance and refractive index. Moreover, the surface of the WO3 layer is homogeneous and flat with low roughness because of the WO3 NP generation from the tungstic acid solution without aggregation. We performed optical simulation and experiments, and the optimized WAW multilayer had a high transmittance of 85% with a sheet resistance of 4 Ω/sq. Finally, PLEDs based on the WAW multilayer anode achieved a maximum luminance of 35,550 cd/m2 at 8 V, and this result implies that the solution-processed WAW multilayer is appropriate for use as a transparent anode in PLEDs.
Recently, polymer light-emitting diodes (PLEDs) have been studied as next-generation light sources because they have high luminous efficiency and can be fabricated by an efficient solution process for high productivity [1–3]. However some layers of PELDs (the electron injection layer, cathode and anode) must be deposited by an expensive vacuum process; only the emitting and hole injection layers are coated by a solution process. Therefore, researchers have investigated extending the solution-processed layers [4–6]. The development of a solution-processed transparent electrode is particularly important because it is a fundamental component of organic electronics.
Indium tin oxide (ITO) is commonly used as the transparent anode for PLEDs because of its combination of high optical transmittance (>85% in the wavelength range of visible light) and low resistance (approximately 10 Ω/sq) . However, ITO has some disadvantages. The supply of indium is constrained by both mining and geopolitical issues, and ITO must be deposited by a vacuum process that is expensive to set up and maintain. Therefore, researchers have investigated solution-processed transparent anodes such as conducting polymers and carbon nanotube films [8–12]. These anodes can be inexpensively coated by a solution process in an air environment, but their sheet resistances are ten times higher than that of ITO while their transmittance is similar [8, 11]. Thus, conducting polymers and carbon nanotube films are unsuitable as transparent anodes for PLEDs.
Fan et al. reported dielectric/metal/dielectric (DMD) multilayers such as TiO2/Ag/TiO2, InZnO/Ag/InZnO , ZnS/Ag/ZnS [15–17], WO3/Ag/WO3 (WAW) , or ZnS/Ag/WO3[19, 20] as transparent anodes. A DMD multilayer has low sheet resistance in the metal layer (approximately 5 Ω/sq) and high transmittance (>85%) because the refractive index discrepancy between the dielectric layers and the thin metal layer improves the transmittance of the metal layer . However, it has low productivity because the vacuum process for a conventional DMD multilayer requires a high-degree vacuum for an extended time and has a limited chamber volume. This problem could be overcome by using a solution process. When dielectric layers are coated by a solution process, the productivity of the DMD multilayer is greatly improved, because the dielectrics form two layers in a DMD multilayer consisting of three layers. However, it is difficult to coat a thin and uniform dielectric layer using conventional sol-gel or nanoparticle (NP) solutions [22–24].
WO3 is one of the most suitable dielectric materials for the DMD multilayer. It has a high refractive index (n = 2.0 at wavelength of 580 nm) and high transmittance (>90% in the wavelength range of visible light) as well as high electrical conductivity . However, in the conventional solution process WO3 has high surface roughness and too large particle size to form a dielectric layer thinner than 100 nm [25, 26].
In this work, we develop a WO3/Ag/WO3 multilayer transparent anode with solution-processed WO3 for PLEDs. To coat thin WO3 layers by a solution process, we devise a novel method wherein WO3 NPs are synthesized from a precursor solution by rapid conversion that obstructs the growth of particles. Thin WO3 NP layers form WAW multilayer with a thermal evaporated Ag layer, and they improve the transmittance of the WAW multilayer without degradation of the Ag conductivity. The solution-processed WAW multilayer has excellent optical and electrical properties and higher productivity than the conventional multilayer because it can be fabricated by a high volume printing technologies. The optimal structure of the WAW multilayer is calculated by optical simulation, and the results are verified by comparison with experimental values. Finally, we evaluate the luminance of PLEDs based on the WAW multilayer transparent anode.
Fabrication of solution-processed WAW multilayer
Fabrication of PLEDs on WAW multilayer anode
The PLEDs on the WAW multilayer anode were composed of layers of glass, the WAW multilayer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (50 nm, PEDOT:PSS; Clevios P VP AI4083, Heraeus Precious Metal, Hanau, Germany), yellow light-emitting polymer (60 nm, ‘super yellow’ (SY), Merck & Co., Inc., Whitehouse Station, NJ, USA), ZnO NPs and ionic interlayer (20 nm, polyethylene oxide + tetra-n-butylammonium tetrafluoroborate), and Al, as shown in Figure 2b. The layers of WO3, PEDOT:PSS, super yellow, and ZnO NPs and the ionic interlayer were all fabricated by spin-coating, whereas the Ag and Al electrodes were thermally evaporated. PLEDs on a conventional ITO anode were fabricated as control devices for comparison. The resulting active area of each PLED is approximately 4.4 mm2. After the fabrication, the devices were measured without further encapsulations in air. The device characteristics of the current density, voltage, and luminance (J-V-L) curves were recorded with a source-measure unit (Keithley 2400, Keithley Instruments Inc., Cleveland, OH, USA) and a calibrated photodiode (CS-100A, Konica Minolta Optics, Inc., Tokyo, Japan).
Results and discussion
Preparation of solution-processed WO3
We passed an aqueous solution of Na2WO4 through a protonated cation exchange resin. The Na+ ions were replaced with H+ ions by the exchange resin, and we collected tungstic acid consisting of H+ and WO42− ions. The tungstic acid was converted to WO3 ·2H2O as a primary product, and then WO3 ·2H2O was calcined at 200°C for conversion to WO3 as a secondary product .
To verify the conversion of WO3 ·2H2O to WO3, we observed the crystallization of the WO3 NPs using an X-ray diffractometer (RIGAKU D/MAX-2500, Rigaku Corporation, Tokyo Japan). Solution-processed WO3 should be pure without WO3 ·2H2O. WO3 ·2H2O reduces the hole and the electron mobility of the dielectric layer of the WAW multilayer anode as impurity, and the low hole and electron mobility cause high turn-on voltage of PLEDs . Figure 4b shows the XRD pattern of the solution-processed WO3. The typical sharp peaks of the monoclinic phase of WO3 are recognizable in the XRD pattern. This suggests that most of the WO3 ·2H2O is converted to WO3, and solution-processed WO3 is formed as a crystalline phase. Figure 4c shows the real part (n) and imaginary part (k) of complex refractive indices of the solution-processed WO3 layer. A 45-nm thick WO3 layer has higher refractive index than 2.0 in the wavelength range of visible light as in the case of a WO3 layer deposited using the evaporation process .
Properties of WAW multilayer
We fabricated a WAW multilayer with solution-processed WO3 and investigated its optical and electrical properties as a transparent electrode. The transmittance of the multilayer is a function of the thicknesses and refractive indices of the materials . Since the refractive indices of WO3 and Ag are fixed, the transmittance is determined by the thicknesses of the layers. It has been reported that a Ag layer has optimal electrical and optical properties when its thickness is 15 nm [14, 16, 19]. Therefore, the transmittance of the WAW multilayer should be optimized by controlling the thickness of the bottom and top WO3 layers.
Transmittance and sheet resistance of Ag, ITO, and WAW multilayers.
Transmittance (% at 580 nm)
Sheet resistance (Ω/sq)
Ag 15 nm
WO3 45 nm/Ag 15 nm/WO3 15 nm
WO3 45 nm/Ag 15 nm/WO3 30 nm
WO3 45 nm/Ag 15 nm/WO3 45 nm
WO3 45 nm/Ag 15 nm/WO3 60 nm
ITO 150 nm
Consequently, the solution-processed WAW multilayer had a sheet resistance of 4 Ω/sq, an rms of 0.588 nm, and a transmittance of 85%, which is considerably high as compared to the 50% transmittance of a Ag layer. As a transparent anode for PLEDs, the solution-processed WAW multilayer is superior to ITO. This is because the WAW multilayer has better electrical conductivity and surface roughness than ITO, although its transmittance is slightly lower.
Performance of PLEDs based on solution-processed WAW anodes
We have demonstrated the applicability of a highly productive WAW multilayer anode formed by a solution process as an alternative to conventional ITO anodes for PLEDs. To coat a thin and uniform WO3 layer by a solution process, we devised a new method in which WO3 NPs less than 2 nm in size were synthesized from an aqueous solution of Na2WO4. We then analyzed the crystallization characteristics of the WO3 NPs and the morphology of the WO3 NP layer. The solution-processed WAW multilayer, which was optimized via a simulations and experiments, had a sheet resistance of 4 Ω/sq and a transmittance of 85% at a wavelength of 580 nm. Moreover, WAW-based PLEDs had a maximum luminance of 35,550 cd/m2 at 8 V and a luminous efficiency of 6.24 cd/A. They were superior to ITO-based PLEDs because of the high conductivity of the WAW multilayer and the good contact quality of the WAW/PEDOT:PSS interface. Therefore, a solution-processed WAW multilayer is appropriate for both large-area displays and lighting applications because of its high productivity and excellent performance as a transparent electrode.
This work was supported by grant no. EEWS-2011-N01110035 from Energy, Environment, Water, and Sustainability (EEWS) Research Project of the office of KAIST EEWS Initiative.
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