thin films; transparent conducting oxides; metal; core-shell particles; chemical solution deposition; metal oxide
Deshmukh Rupali, Calvo Micha, Schreck Murielle, Tervoort Elena, Sologubenko Alla S., Niederberger Markus (2018), Synthesis, Spray Deposition, and Hot-Press Transfer of Copper Nanowires for Flexible Transparent Electrodes, in ACS Applied Materials & Interfaces
, 10(24), 20748-20754.
Deshmukh Rupali, Tervoort Elena, Käch Julian, Rechberger Felix, Niederberger Markus (2016), Assembly of ultrasmall Cu3N nanoparticles into three-dimensional porous monolithic aerogels., in Dalton transactions (Cambridge, England : 2003)
, 45(29), 11616-9.
Deshmukh Rupali, Niederberger Markus (2015), Nonhydrolytic Sol-Gel Methods, in Levy David (ed.), Wiley-VCH, Weinheim, 29-69.
Deshmukh Rupali, Zeng Guobo, Tervoort Elena, Staniuk Malwina, Wood David, Niederberger Markus (2015), Ultrasmall Cu3N Nanoparticles: Surfactant-Free Solution-Phase Synthesis, Nitridation Mechanism, and Application for Lithium Storage, in Chemistry of Materials
, 27, 8282-8288.
Transparent conducting oxides (TCOs) are unique materials in the sense that they combine two opposing properties: Transparency in the visible light range and high electrical conductivity. They found widespread technological applications as transparent electrodes in solar cells, flat-panel displays, electrochromic windows and as low-emissivity coatings, just to mention a few. According to a recent IDTechEx report, the transparent conductive film and glass markets will grow from $1.63 billion in 2012 to $6.3 billion in 2014. With the emergence of transparent displays in 2012, market estimations even reached $87.2 billion by 2025.With the development of flat-panel display technology around 1970, indium tin oxide (ITO) became the most commonly used TCO material for transparent electrodes due to its low resistivity of the order of 1-2 x 10-4 O cm. The vast majority of TCO films are technologically produced by vapor phase techniques such as chemical vapor deposition or magnetron sputtering, and these processes have been fully optimized over the years. Nevertheless, they remain expensive and technically highly complex. Thin film deposition by solution routes, on the other hand, would be cheap, because no vacuum is required, no material is lost, complex compositional mixtures and a wide compositional range of materials could be used. However, whereas gas phase processes produce continuous, dense and homogeneous films with uniform thickness and low surface roughness, solution routes struggle with impurities and porosity in the films, which strongly affects their electrical conductivity. The only way to solve these problems is to apply a heat treatment to the films, burning off volatile impurities and enhancing the intergrain connectivity to improve the conductivity. Unfortunately, for thermally labile substrates like flexible plastics such a heat treatment is not an option.From these explanations it is obvious that liquid-phase deposition of TCO films at low temperature would technologically be extremely attractive, if the films could be prepared in sufficient quality. To solve the major problems of nanoparticle processing into films, we propose a new concept based on the combination of metals with metal oxides in a core-shell particle configuration. Both components specifically contribute to the TCO performance. The metal provides the conductive path for the electrons, and the metal oxide suppresses the color of the metal to make the film transparent. The use of metal oxide - metal composites avoids the temperature problem, because good electrical conductivity can be obtained without substrate heating. In addition, such a low temperature process would lead to strong reduction in energy consumption during thin film preparation.Specifically, the proposal suggests the development of a liquid-phase route to metal oxide particles coated with a thin layer of a metal, preferentially copper. These core-shell particles are deposited on flexible substrates by spin- or dip-coating. An additional metal oxide surface layer deposited on top of the film protects the copper from oxidation, decreases the surface roughness and increases the mechanical stability. Finally, these films will be tested as electrodes in solar-cell and organic light-emitting diode applications as a proof-of-concept for the success of this new fabrication technology.