Alternatives to the most commonly used transparent electrodes, indium tin oxide (ITO), are highly required by the printed electronics industry, for applications such as touch screens and solar cells. The demand for low-cost materials and technologies is mainly due to the complexity of the currently used methods and the industrial trends towards flexible and wearable electronics, which ITO is not suitable since it is a brittle material. For this reason, the number of reports on new materials and methods to fabricate transparent conductive coatings (TCC), have increased dramatically for the past 5 years. A promising approach to obtain a TCC with a very low sheet resistance is based on using metallic nanomaterials. The intrinsic high conductivity of metals such as silver, copper and gold, and carbon nanotubes, enables to obtain TCC with sheet resistance as low as 1-10 Ohms per square. We have developed several new approaches which are based on self-assembly, controlled wetting and inkjet printing to fabricate transparent patterns composed of narrow lines (<10 µm).The various approaches will be briefly described in the following sections and more information can be found in: Nanoscale 6, no. 11 (2014): 5581-5591.
Coffee stain 2D ring array
We have developed an ink and a fabrication process for direct patterning of TCCs, by ink-jet printing silver nanoparticles, which spontaneously forms arrays of transparent rings due to the coffee ring effect (Fig. 1). The diameter, height and width of the rings can be tailored according to the specific requirements of conductivity and transparency of a device such as smart phone's touch screen. These findings were published in ACS nano 3.11 (2009): 3537-3542. This research led to the establishing a new start-up company, Clear-Jet, through an agreement with Yissum, the tech transfer company of The Hebrew University.
Fig. 1: A TCC composed of array of printed "coffee rings"
Transparent conductive grids
TCCs formed by a grid patterns are becoming commercially available. Among the many companies that are utilizing this approach are: POLYIC, Toray and Rolith. Typical technologies to achieve conductive transparent metal grids are based on photolithography, direct printing, embossing and as reported by our research group, by combining low temperature sintering and self-assembly. The method is based on evaporative lithography process which is performed directly onto plastic substrates. In essence, a droplet containing silver nanoparticles is placed on top of a mesh, instantaneously spreading over the mesh and the plastic substrate, forming a grid composed of the nanoparticles. The nanoparticles are sintered at low temperature by a chemical sintering process that we developed. For more information see: Journal of Materials Chemistry 21.39 (2011): 15378-15382 and Nanoscale 6.9 (2014): 4572-4576 and Journal of Materials Chemistry A2.38 (2014): 16224-16229.
These transparent grids were integrated into electroluminescent and electrochromic devices. Recently, this process was further utilized with a perovskite material to form a semi-transparent solar cell: Adv. Mater. Interfaces, 2: (2015) . doi: 10.1002/admi.201500118.
Fig 2: Transparent conductive grid
A new approach was developed to fabricate patterned transparent conductive electrodes by direct printing of “2D holes” with controllable diameters onto a wet thin film composed of metal nanoparticles (Fig.3). The holes are formed by inkjet printing a de-wetting liquid, which pushes away the metal nanoparticles, thus forming a transparent array of interconnected conductive rings. The array of interconnected rings can be printed in a macro patterns in any required shape, such as honeycomb, diamond shape or rectangular grids: ACS applied materials & interfaces 6.21 (2014): 18668-18672.
Fig. 3: TCC composed of array of printed "holes"