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.