Field-directed nanowire chaining enabling transparent electrodes
Date
2019-01-08
Authors
Xu, Manyan
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Abstract
Transparent electrodes (TEs) require materials that have both transparency and electrical conductivity, a combination not usually found in nature. They are in increasing demand for use in solar cells, touch screens, displays, transparent heating films and several other devices. Most TEs used today are made of indium tin oxide (ITO). However, it has several disadvantages, such as high fabrication cost, rigidity and brittleness. Many ITO alternatives are being pursued, among which metallic nanowire (NW) networks on transparent substrates such as glass or polymer, have received much attention. This thesis demonstrates ordered silver NW networks on polyimide, fabricated by the field-directed chaining technique. We achieved a sheet resistance of 27 Ω/sq and 95.4% transparency at 550nm, with a Figure of Merit (FOM) 0.023Ω-1, which is higher than the FOM of commercial ITO, 0.005Ω-1. We have demonstrated that ordered NW networks, directed by alternative current (AC) electric fields, are easy to fabricate over a large area and at low cost, on rigid and flexible substrates.
The AC electric field changes with different experiment setup. In this work, the effect of polymer thickness, electric field frequency, and gap size between electrodes are explored by COMSOL simulation and validated experimentally. By choosing the appropriate frequency and gap size, ordered NW networks are successfully created on a 23μm polyethylene terephthalate (PET) sheet. Fluid motion is one of the disruptors during NW chaining. We demonstrate control of this disruptor by the use of sandwiched channels for the NW suspension.
Post-fabrication treatments are important and necessary for improving the connectivity and conductivity of Ag NW networks. In this work, we explore Joule heating and show its potential to improve the conductivity over other post-treatment approaches. However, Joule heating can also cause failures of NW networks.
Ordered NW networks present better optical-electrical properties than random NW networks. Post-fabrication treatment can improve the properties, but there is a limit. In this work, a mathematical model is built for optical-electrical properties of perfectly ordered NW networks, which sets the upper bound of performance for transparent electrodes made of NW networks. A linear relationship is found between the transmittance and inverse sheet resistance. The model is then modified with factors to account for departure from the ideal.
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Keywords
Transparent electrodes, Field-directed, Flexible substrates, Sandwiched channels, Nanowire chaining, Ordered nanowire networks, Joule heating