One of the most exciting entrants in the field is a class of inks based on graphene.
By Sanjay Monie
In general, proper formulation includes a complex balance between the functional pigment dispersed in a suitable solvent system and the binders and other additives that ensure a printable, properly dispersed ink. In all cases, the ink should provide prints that have adequate film cohesion and adhesion to the substrate, in addition to possessing conductive characteristics.
High-performance, graphene-based conductive inks are formulated to work on a wide variety of screen, gravure, flex-ographic, and industrial inkjet printers, enabling a surface resistivity of 1 Ω/sq or below. This translates to bulk conductivities greater than 300 S/cm at 5 μm, depending on the print technology and substrate. Such inks are extremely flexible when printed (Figure 2) and allow only a minimal drop in conductivity after a multitude of folds over a mandrel. Another key property is excellent rub-resistance, demonstrated by a conductivity reduction of less than 10% after 10 rubs. These types of inks are able to achieve high conductivities at low film thicknesses with vastly improved flexibility and handling characteristics relative to carbon inks, thereby offering solutions that bridge the price-performance gap between silver and traditional carbon inks.
While the biggest market for printed electronics remains consumer electronics, the ultimate goal is not to merely to replace PCBs, but rather to eventually replace silicon-based integrated circuits (ICs). For example, a low-cost field-effect transistor could be formed by printing a conductor layer forming source and drain electrodes, followed by a semiconductor layer, a dielectric layer, and another conductor layer as a gate electrode. By direct writing of multilayered devices directly onto flexible substrates using semi-conductive, conductive, and dielectric inks with low curing temperatures, one could avoid the expensive photolithographic patterning and etching steps of silicon-wafer-based fabrication, which wastes materials and has a negative environmental impact. Transistor devices can be printed directly onto inexpensive substrates using inkjet devices.
The next generation of electronics, or more demanding applications that require multilayer printing, will challenge printers to achieve higher resolutions, greater accuracy in print thicknesses, more stringent requirements for defect-free printing, and higher precision in layer-to-layer registration than what they can currently realize with presses developed for graphics printing. Nevertheless, future advances in printer technologies that are specifically targeted towards printing electronics, along with new generations of conductive inks such as graphene-based formulations, may make even an economical alternative to silicon-based fabrication achievable in the foreseeable future.
Sanjay Monie, Ph.D.
Sanjay Monie is technology development manager at Jessup, MD-based Vorbeck Materials. He has actively researched and developed media for digital printing since 1996, and holds a Ph.D. in Materials Science from The Pennsylvania State University.
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