Bottom-up Filling of Nanosized Trenches with Silver and Copper to Fabricate Transparent Conducting Electrodes

Transparent conducting electrodes (TCEs) are essential in many optoelectronic devices including solar cells, LEDs, sensors and displays. The most common approach for TCEs is the use of transparent metal oxide layers, in particular Indium tin oxide (ITO). However, ITO comes with numerous drawbacks such as the relatively high cost, fragility, rarity of Indium, strong UV absorption and relatively high sheet resistance. An exciting TCE alternative is the use of interconnected metallic nanowires, where their sub-wavelength cross-sections makes them transparent to visible light. Especially Ag nanowires are a good candidate because of their high conductivity. However, the high material cost of Ag limits the usage in large-scale applications and therefore Cu is used as an alternative.<br/>In this work, we show a promising cost-effective method to fabricate metal nanowire structures using selective-area electrochemical deposition (SAEC). SAEC is based on electrochemical deposition through an insulating template onto a conductive substrate. Next to the low fabrication cost, electrochemical depositions is a bottom-up method with numerous advantages such as scalability, ambient operation conditions, control over nucleation and high purity. While submicron sized features have been already demonstrated by SAEC, obtaining homogeneous void free filling of features below 100 nm using direct plating is a major challenge because of the terminal effect and the poor control of nucleation of the seed layer.<br/>Here, we have explored the SAEC fabrication of Ag and Cu nanowire grids over a large area by using soft-conformal imprint lithography as means to fabricate the template on ITO. Finally, we compare the optical and electrical properties of the SAEC-fabricated metal grids with those fabricated using conventional thermal or e-beam evaporation followed by lift-off. The SAEC- fabricated metal grids show high transmissivity (~90%) and low sheet resistance (&lt; 50 Ω/sq), whose can be easily tuned by adjusting the thickness of the wires by controlling the growth time. This work represents a low-cost and scalable strategy to fabricate sub 100 nm predefined metal structures, enabling the commercial viable fabrication of nanowire based transparent conductive electrodes for optoelectronic devices.