Printed Silver: Well Worth it in Many Places
A Changing Landscape: Emerging Materials and Emerging Applications
In the printed electronics industry, no other material comes close to printed silver. Besides being the most conductive of the metals, the native oxide that forms on its surface is also conductive, minimizing the impact of the oxide on the conductivity of the final film. This is in contrast to most other metals, on which native oxide films are all but unavoidable as well as insulating. In fact, all of the reasonable metallic alternatives to silver for printing have insulating native oxide films and thus silver's better suitability for forming conductive, particulate films goes beyond its higher bulk conductivity. The numerous contact points between particles--with high contact resistance due to the native oxide--dramatically reduce the conductivity of the final films of other metals, making them far less suitable for printing.
For this reason, silver was the first material used for inks and conductive printing on a wide scale. Easily applied silver inks and pastes are widely used in the electronics industry, most often for electrodes and similar features. Now, silver nanoparticle inks are being developed and used with the promise of improvements in performance, cost of use, and functionality with inkjet printing.
Silver pastes for screen printing have a long history and there are several applications that use them--and have used them for a long time--including membrane switches, printed circuit boards (PCBs), and capacitors. Rapidly shifting into this category is crystalline (c-Si) photovoltaics, which use thick-film silver pastes for the fingers on the front surfaces of the cells. These are established applications, but NanoMarkets still sees them as growth markets for silver inks and pastes.
Newer on the scene are nanosilver inks that capitalize on nanosilver's high surface area--and conductivity--in relation to its volume. Besides opening new markets--where conventional screen-printing is unsuitable--to the use of printed silver, these nanosilver inks also challenge the conventional thick-film silver pastes for use in the same established applications, with potential advantages in cost and/or performance.
Silver's industrial use as a conductor competes with its uses as a store of value (in coinage and bullion) and as a decorative metal (in jewelry and silverware, for example), as well as other industrial uses such as in photography and as a bactericide. As a conductor, silver also has a wide variety of uses including as a component of some solders and in high-performance cables, but these applications are not part of the electronics industry.
In nearly all of the applications in which printed silver is used, its major drawback is simply its cost. As silver prices have ridden the up and down--and up again--roller coaster in recent years, so too has the impetus for alternatives to silver for printed electronics.
Silver prices reached a 25-year high in early 2008, but then came tumbling down along with the rest of the economy. Now, only a year later, silver prices are back near the highs of 2008 and the pressure is on again to reduce costs. Does this mean that we will see a full-scale shift away from silver and into other conductive materials? Unlikely, although there is progress in and potential for developments to contain the cost of silver inks and pastes.
Two important facts lie solidly in silver's favor. First is the fact that silver typically makes up only a small percentage of the cost of the final devices in which it is used. Even with rising silver prices nearing $20 per ounce and beyond, the high values of the end-use devices still make the relative cost of silver inks and pastes relatively insignificant. It should be noted, however, that this might not quite hold true for extremely low-cost, low-value-added devices such as RFIDs.
Second, silver inks and pastes are essentially in a class of materials by themselves. The high conductivity of silver, the minimal impact of its oxide, the ease and efficiency of printing, and the decades of knowledge about and experience with silver inks and pastes set them apart from any other conductive film in terms of performance and "bang for the buck." Where printed silver is used, it is used because of its performance and suitability for the application.
Even considered on a cost basis, printed silver can often be used more sparingly than other films. Thinner films can be used with good conductivity and the well-established printing processes minimize waste. The quantity of silver consumed for a certain application can be an order of magnitude less than what would be consumed if a different material, or a different form of silver, were used. This makes the cost incentive for using alternative materials somewhat less than it appears.
There is no question that the bulk of development in silver inks and pastes in recent years has gone into the use of nanosilver. Nanosilver inks, often first developed for inkjet printing but then applied to higher-throughput flexography and gravure printing processes, are carving growing niches into materials markets for newer categories and devices. There are displays and thin-film photovoltaics, both of which have been using printed silver for some time, but there are also newer applications like OLED lighting, sensors, and RFID devices. In these applications, nanosilver inks can offer higher resolution, improved performance, and reduced material usage compared to thick-film silver pastes; in many cases opening the door to substantial markets for silver inks that would not be available to conventional silver pastes.
Some of the most rapidly developing applications for silver inks are those that use silver as a transparent conductor. Thin-film and organic photovoltaics, displays of various sorts, and OLED lighting all share the need for wide-area transparent electrodes and have most often used transparent conductive oxides (TCOs) like ITO for this purpose. But growing dissatisfaction with ITO and similar materials has led to increased development of alternative transparent conductors, including silver-based ones. OLED lighting has led this charge but all of the applications that use transparent conductors are either supporting some of this development or being targeted by the developers as potential markets.
Silver ink-based transparent conductors have several potential advantages over ITO and similar TCOs. Perhaps most importantly, silver is a flexible, ductile material and may be better suited to flexible electronics and roll-to-roll manufacturing than ITO. Transparent films of silver networks also have at least the potential to be more conductive than ITO at similar transparency (or more transparent at similar conductivity). Another potential advantage is that these silver-based materials are typically printed--unlike ITO--and thus reap some additional benefits. They are not subject to the tremendous amount of waste involved in the traditional sputtering processes used to deposit ITO, allowing significant potential for cost savings even though these silver materials are typically priced in a similar range to ITO materials. The printing process is also generally more gentle than sputtering, making it easier to use flexible substrates and to apply films over sensitive underlying layers.
Another fairly recent development is the use of nanosilver in otherwise conventional silver pastes. These silver nanopastes can be used for screen printing but can yield the benefits of higher resolution, reduced material usage, improved conductivity, and lower-temperature curing.
Source: Silver Inks Keep Up in the Electronics Industry
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