In recent years, piezoelectric drop on demand inkjet technology has garnered significant interest in the field of printed electronics. The basic process of the technology is as follows: every nozzle in an inkjet print head contains an ink chamber attached to the ink reservoir. At the back of the chamber is a piezoelectric crystal, which deforms when an electric charge is applied. The crystal deforms inwards, reducing the volume of the ink chamber and ejecting an ink droplet out of the nozzle. The crystal then returns to its original position, replenishing the ink in the chamber by pulling more in from the attached reservoir. This mechanism allows the printing of complex repeatable patterns as defined by a digital image file. While printing ink onto paper with a desktop digital printer (or onto many other substrates using an industrial system) is common knowledge, the same piezoelectric inkjet process can be used to deposit conductive materials instead, hence the term “printed electronics”. It should be also be noted that while piezoelectric is not the only inkjet technology theoretically useable for printed electronics, piezo technology’s resolution and robustness with a wide range of deposition fluids makes it the industry standard.
Drop on demand inkjet deposition of nano-particle conductive inks have already found niches in the fields of photovoltaics, OLEDs, displays and RFID. Flexibility sets inkjet apart from traditional methods; inkjet technology allows for the additive deposition of thin line circuits on a range of substrates including those that are three dimensional. This also cuts costs because materials are only deposited where necessary, rather than being deposited over a large area and then removed where not needed. This is especially significant since some conductive materials are very expensive (e.g. silver based inks). Inkjet is also non-contact which means fragile substrates will not be damaged as they may be with other methodologies. Printing electronics using inkjet is also significantly less labour intensive and removes the need for expensive tooling (production of physical precursors such as silk screens and photo masks) which is a necessary burden in other forms of circuit manufacture. The process of tooling is not only time consuming and expensive, but the need for storage and replacements add cost and inconvenience. Using inkjet, alterations for deposition patterns can be made immediately since there is no wait for a new mask or screen.
Demand for the use of conductive inks in electronic products is growing. Innovations in conductive ink technology have been particularly significant in the area of organic electronics, where printable conductors are required as a component in complete devices and displays. Most circuits are produced using expensive vacuum deposition and photolithographic patterning, which produce large amounts of environmentally damaging toxic waste. This issue does not apply with inkjet technology. Inkjet also enables the use of biodegradable organic materials which cause less harm to the environment. However, due to the lower stability of organic materials, printed electronic products typically have a shorter life time than their conventionally produced counterparts.
Due to the fact that inkjet nozzles are of a very small diameter, this leads to rheological constraints for the ink. The ink typically needs to have a surface tension >35 mN/m and a viscosity of 5 to 15 cP. These rheological constraints, along with curing temperature issues and expensive materials, currently restrict the adoption of inkjet conductive inks. However, it is expected that with developments in inks and printing hardware, greater opportunities will present themselves.
Conductive inks are diverse in terms of composition, functionality and cost. For this reason choosing the correct ink for your application is crucial. Currently, inks with the highest conductivity contain silver in the form of nanoparticles (small particles with diameters in the range 10-100nm typically). To achieve conductivity levels near to that of bulk silver, high temperature post process firing or ‘sintering’ is required. The requirement for high temperature sintering restricts the use of silver inks in applications using temperature sensitive substrates. Many groups worldwide are working on inks and process developments that allow lower sintering temperatures (as low as 70-150˚C, with the aim being sub-100˚C sintering). This enables printing circuits on plastic substrates which would be unable to withstand higher sintering temperatures. The sintering of silver particles at low temperature presents a difficult technical challenge for manufacturers in creating a successful ink formula. For this reason, inkjet conductive inks fireable at low temperatures are scarce in the market and come at a higher price. Fluctuating silver prices also make cost planning for long term projects problematic. For that reason, applications which utilise silver based conductive inkjet inks are typically expensive and low volume at present. Examples include flat panel displays, photovoltaics and OLEDS. Carbon based inks are a cheaper alternative but are typically two orders of magnitude less conductive than silver. Carbon based inks typically suffer from poor substrate adhesion, low flexibility and poor rub resistance. For this reason their applications are greatly reduced. One application where carbon based inks are prevalent is EMI/RF shields for monitor screens. Copper has been used as a lower cost alternative to silver but is prone to pyrophoric oxidisation which reduces conductivity significantly. Copper inks also require high temperature sintering which restricts applications. The copper particles can be encapsulated with a thin layer of silver to allow sintering but prevent oxidisation, which gives greatly enhanced performance. Conductive inks based on graphene are an interesting prospect, particularly for thin printed flexible substrates. Compared to silver, graphene is considerably less expensive and show promise for future applications.
Solar cells, also known as photovoltaics, are used to convert the sun’s energy into electricity which can be used in homes and businesses, especially useful for remote locations. Since this energy is sustainable and emission free, implementation of solar panels is expected to continue to grow as the earth’s natural resources become depleted and more emphasis is placed on sustainable energy and reducing carbon emissions. A current challenge for the mass commercialisation of photovoltaics is increasing the cell’s efficiency (the percentage of photons hitting the cells that are converted to electricity) while also bringing costs down to levels comparable to other sources of electrical energy generation. Modern day mass produced solar cells typically have around 15% efficiency - this has been steadily increasing since the 1950s when solar cell efficiency was around 4%. Ongoing research by the United States Department of Energy’s National Renewable Energy Laboratory (NREL) suggests that the utilisation of inkjet technology in the production of thin film and rigid silicon based solar cells could be extremely significant for the future.
The additive nature of inkjet technology could help streamline production and render expensive vacuum equipment (which is essential in other production techniques) unnecessary. Since photovoltaic production involves the deposition of expensive materials (silver inks can cost over $1000/kg), inkjet’s ability to drop on demand means significantly less wastage and thus reduced production costs. Resists for wet or dry chemical etching could also be inkjet deposited which would cut material costs by precisely controlling usage. Photovoltaic production also requires the creation of extremely fine contact lines. This can be achieved with other printing technologies but can lead to damage and breakage; since inkjet is non-contact these pitfalls are avoided. Fine contact lines can be produced using screen printing but inkjet allows conductive ink to be deposited in a finer pattern and is also potentially significantly cheaper. Finer lines are required since they leave more surface area on the panel to absorb and convert light.
Researchers including the Oregon State University and the EU NOVA-CIGS project have been experimenting with printing solar cells based on a compound called chalcopyrite (CIGS), which contains copper, indium, gallium and selenium elements. While over 90% of solar panels are produced using silicon, CIGS may offer an economical alternative. CIGS is different from most thin film materials (especially organics) as it doesn’t degrade from exposure to sunlight and is easy and cheap to produce. Although improvements in efficiency levels would be necessary before the cells could go to the market, the distinguishing feature of the process is a 90% reduction in raw material waste which would dramatically decrease the cost of producing solar cells if the process was adopted. Research such as this plus the tenacious efforts of inkjet system and ink manufacturers to improve ink performance and reliability is an indicator that inkjet could become a key factor in the expansion of photovoltaic production.
Radio Frequency Identification (RFID)
RFID is an emerging wireless network technology used primarily for identifying and keeping track of items. Since the 1970s, bar codes have been the dominant technology allowing manufacturers to manage their inventory. RFID has been promising for many years to allow product tracking without the need for line of sight reading of bar codes. Although uncompetitive manufacturing costs have stalled mass adoption of RFID, these hindrances are likely to become negated in the future as production processes become more effective. In terms of functionality, RFID offers several distinct advantages over barcodes. RFID is able both to read and write; barcodes are read only so are unable to send out important information regarding the tracked items. With barcodes, every item of inventory must be scanned individually in order to keep figures correct – with RFID this process is automatic. It has also been reported that the advanced product tracking offered by RFID allows for greater understanding of consumer buying trends and thus gives indicators on how to manage stock effectively. Barcodes will still be the preferred technology for some applications but it is clear that RFID will continue to expand in many areas.
Although for the most part inkjet is still an overlooked technology in the field of RFID, It has been used to deposit conductive tracks in some devices. However, the goal to lower material costs has led companies to start investigating the use of paper as a substrate for RFID/sensing applications. This will enable the use of direct printing technology (such as inkjet) instead of expensive and high material waste incurring metal etching processes. This is particularly crucial where additional efficient, low cost and rapidly deposited conductive tracks are required for devices which utilise sensors and batteries. Paper also has the advantage of being perceived as a “green” biodegradable substrate, and with environmental legislation continually becoming more stringent it becomes an extra incentive for manufacturers to research this area. The current roadblock preventing paper substrates from becoming prevalent in RFID is that paper cannot survive the high sintering temperatures required for conductive inks to adhere to the substrate. Significant effort is currently being put into resolving this issue (see the EU Lotus Project and if a solution is reached it will likely mean a massive expansion for the use of inkjet in printed electronics.
Organic Light Emitting Diodes (OLEDs)
OLEDS are flat layer light emitting devices composed of a series of thin film organic compounds positioned between two conductors. When an electrical signal is received the device emits a bright light. OLED devices are typically 100 to 500 nanometres thick; approximately 200 times smaller than a human hair. Since OLEDs do not require a backlight and are extremely thin they have found special niche in the fields of display and lighting. OLEDs can be used in flexible and transparent displays which will potentially make them valuable for applications where displays are required on unusual substrates. OLEDs could be embedded in curved surfaces, windows or car windshields more effectively than conventional displays. Another key characteristic of OLED technology is low power consumption. Currently OLEDs are primarily available as small displays on devices such as mobile phones due to manufacturing limitations. Larger mass produced displays are likely to appear on the market within the next few years but currently high manufacturing costs and reliability issues over a suitable product lifetime stand as road blocks.
All commercially available OLEDs are currently produced using vacuum deposition which involves heating the organic modules in a vacuum chamber until they condense as thin films onto the substrate. This method is not only expensive but also inefficient and difficult to scale up for larger substrates. Inkjet has been highlighted as a potential new deposition methodology which if successful will bring high quality OLED displays to the market at a lower price than competing display technologies. Due to the fact near to instant specification and design changes can be made on an inkjet production line, it will be possible for the same machine to switch between printing displays of different sizes quickly and easily. Panasonic, who have been researching using inkjet to deposit organic compounds onto the polymer surface, have suggested their new displays will offer a colour rendering index of 95 (currently fluorescent illumination technology offers figures in the high 80’s).
The term E-textiles or “smart textiles” refers to fabrics with integrated electrical components to create digital capabilities such as sensing, communication and information processing. Complex electronic networks containing devices such as pressure sensors and LEDs have already been prototyped successfully for a wide range of uses, both functional and decorative. This amalgamation of textiles and electronic technology has led to some truly unique and revolutionary applications. Wearable technology has garnered much interest in terms of personal fashion due to the limitless new options it gives us in asserting our identity. However, much of the impetus for innovations in the field of E-textiles has come from the military, where their implementation could prove vital for the lives of soldiers on the battlefield.
One prolific area of research has been in the development of bio-monitoring textiles which track the wearer’s vital signs such as heart rate and body temperature and then raise alerts if they move outside of designated safe parameters. This technology would not only be greatly valuable to the military but in medical, sporting and public safety sectors as well. The light cotton fabric of the “SmartShirt” (patented by Sensatex) is integrated with conductive fibres which acquire analogue physiological signals and through a small personal controller, digitize the data send the data on wirelessly to a remote source for analysis. Another area of military research for E-textiles is adaptive camouflage which will automatically change the colour of the soldier’s fatigues as he passes through different environments (of significant interest for missions in places such as Afghanistan where lush rural, desert and urban environments exist in close proximity). As in other applications, inkjet could be used to deposit conductive tracks for E-textiles, but the deposition of functional coatings would also be advantageous. Research has been undertaken into bandages that raise alerts on the early signs of infection and determine the correct course of medication to follow. Antimicrobial coatings could be deposited onto these bandages using inkjet to help reduce the spread of infection.
Tim Phillips, Catenary Solutions