- February 22, 2013, Dr. Dene Taylor
Attending the Printed Electronics 2012 Conference in Santa Clara, CA, was, for me, like going to a big candy store. I had not enough time by half to hear about all the new materials, the printed constructions for circuitry, sensors, energy generation, RFID and communications, and to see the devices designed to make or measure them. Other writers have already covered the novel materials and future of the industry presented at the conference. So as not to repeat them, I will discuss here other more practical subjects that could be of interest to you, especially those products which can be used confidently within our businesses today: that means conductive materials for inks and coatings.
Films and papers with conductive coatings or print represent the largest substrate market within printed electronics. Two materials are at the heart of this application: indium tin oxide (ITO) and metallic silver. According to Lux Research, silver ink represents $1.4 billion of the estimated $1.5 billion for printed electronics materials. Although we might not use silver ink directly, it is used extensively later on substrates we produce. By the names alone, we know these materials are expensive, so it is not surprising that major efforts are being made to develop replacements and alternatives.
Sputter-coated ITO is familiar to many of us: some of us produce it and others coat or print over it. It is found in a mainstay of applications such as touch screens, displays, and photovoltaic panels. ITO has been the premier transparent conductive coating with a number of reliable suppliers. But it is a mature product and improvements are coming relatively slowly. The primary drawbacks of using ITO are: cost, both the cost of indium and the cost to manufacture by sputter coating; concerns for the potential supply shortages of indium; and, technically, its brittleness, which inhibits the use of ITO for flexible applications—an area of great opportunity.
Regardless of these facts, ITO is well entrenched in its markets because it has an entirely satisfactory performance, with many years of proven service in the field. Today a large number of products are designed around it. Because no other conductive material matches its properties exactly, ITO cannot be directly substituted or replaced. Its greatest vulnerability will be the arrival of new designs or new uses on new production lines.
The conductive polymer complex PEDOT/PSS1 has raised a lot of interest as a conductive material for inks and coatings. For starters, it is produced in high volume—annual coating exceeds 2 billion sq ft of film and is often shipped in totes—and is modestly priced. Because it is transparent, water resistant, and flexible, it has been used as a high-performance movie film antistatic agent for over two decades,. Its relative humidity independence and flexibility are especially useful with paper, for obvious reasons.
Over the years, new complexes with increasing conductivity and greater stability have been introduced, and it is competing with ITO in some devices (i.e., in touch screens), for it has a real cost advantage. Furthermore, PEDOT/PSS is an aqueous dispersion: it can be applied and dried on films like polyester (PET) by slot or other precision method. Patterning by intermittent slot coating, printing, laser ablation, or etching is comparatively straight forward, again on roll-to-roll equipment. It is performing especially well with grids of silver ink for solar cells and organic light emitting diodes (OLED). And, as it is a complex of two separate types of polymer, we can expect to see chemical innovation continuing to bring better combined transparency and conductivity for years and, therefore, less need for the grids.
Silver metal costs about twice as much as indium. Because it is far more conductive than ITO and more tractable, the economics might favor very thin silver films. But transparency doesn’t come readily with conventional approaches. Silver nanowires, which have dimensions of about 30–100 nm diameter and 2–30 μm in length, are a demonstrated transparent conductive material. They are typically provided as dispersions in water or solvent, so they can be coated directly to films.
On drying, they form a 2D mat. There is good connectivity between over-lapping wires, and light passes through the open areas between them. The amount of silver used is small, so cost can be competitive with ITO. And they can be applied to substrates that are not suitable for vacuum processing. Some mobile phone manufacturers use this technology for their touch screens. On the other hand, typical wires are too long to print well.
Silver inks, made from either flakes or nanoparticles, are available for most print methods from good vendors. After drying the conductivity between particles is obtained by sintering. The effectiveness of this method varies. The ideal is to obtain a structure with equal average cross section and conductivity comparable to a wire. Nanoparticles are close to this objective. They also can be fused at comparatively low temperatures in very short times, on, for example polyester and paper. Applications include conductive links between components in solar constructions and RFID.
A great deal of effort has been made within the academic community and by new ventures to introduce conductive materials based on the graphene structure. This area deserves watching because it is likely to provide conductive materials for the next generation of batteries and ultra-capacitors, which I understand is of increasing importance to the PFFC community. Like diamond, graphene is an elemental form of carbon. In this case, the atoms are arranged in a two dimensional hexagonal matrix. The in-plane (x and y directions) electrical conductivity is extremely high. It is also unreactive, transparent in thin layers, and flexible. It should provide alternatives to silver and ITO, especially considering that it is widely available in crude form—it is the basic unit of graphite. However, it doesn’t show its extreme properties in this form—the conductivity of layers is not additive. Also, the out-of-plane (z-direction) conductivity is very low, electrons do not jump easily from sheet-to-sheet or particle-to-particle, which would be necessary if it were to be used in an ink.
Carbon nanotubes (CNTs) are a tubular form of the same hexagonal atom structure. They can be prepared with good consistency and are also being used for inks. Again we encounter the problem of low particle to particle conductance, although in this case, with micron lengths, there are fewer surface-to-surface connections needed. Vorbeck Materials is one of the few suppliers in this area with a proven record and tonnage capacity to provide CNT. The graphene from Vorbeck Materials is chemically modified so it’s tractable, and the basis for stable conductive inks and coatings. MeadWestvaco Packaging is using the ink in security-enhanced packaging.
A related subject is the role we manufacturers have in the development of these new technologies, especially the development of the materials. The new producers must demonstrate commercial feasibility with a useful prototype. To create this prototype, they are turning to paper, film, and foil converters with advanced laboratory or pilot equipment, i.e., us. To introduce a novel material into one of our own products is already difficult for us. For the people at a typical start up, it is an order of magnitude more difficult. Typically they lack expertise with every aspect of manufacturing, not just the processes, but also the logistics and management quality. Invariably they grossly underestimate the work that takes to make a uniform layer of their material on a substrate, and, if unguided, they will ship in un-runnable mixtures.
So if a company approaches you and ask you to assist them in this process, please, make sure that you become a real partner, one that is interested on their success and not merely looking to bill them for machine hours. And if your client doesn’t have the skills or the experience to organize a basic trial, help them do so or suggest they hire someone to do it for them. It might sound like tough love, but, on the long run, everyone will be better served that way. New venture survival frequently depends on scale-up partners. And as an industry, we are invested in its success because we continually need new materials.
Thank you for reading.
1PEDOT/PSS is poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate. A number of different varieties are available from Hereaus under the trade name Clevios and from Agfa under the trade name Orgacon.
On Print columnist and printing expert Dene Taylor, PhD, founded Specialty Papers & Films Inc. (SPF-Inc.), New Hope, PA, in 2000 for clients seeking consultation for technical management, new product design, development, commercialization, and distribution, as well as locating/managing outsourced manufacturing. Contact him at 215-862-9434; firstname.lastname@example.org; www.spf-inc.com.