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The future role of printing in plastic electronics

Sean Smyth - 17 Feb 2010

Dr Sean Smyth, a leading consultant for the digital and analogue printing industry, examines opportunities and challenges for printing electronics.

Roll-to-roll printed electronics coming off the pressOne of the few positive side effects of recessions is to change the playing field, making it susceptible to disruptive technological developments. One field that is fertile for a major change is electronics and the source of disruption is the application of printing techniques.

Interest in the potential use of printing and specialist coating technologies to produce electronically functional devices is building. Such processes involve laying down specialist conductive or optically active fluids, rather than graphics inks, to make devices such as LEDs and organic solar cells.

IntertechPira estimates the market for new printed electronics - transistors, RFID tags, photovoltaics (PVs) and OLED displays - accounted for some €1.8 billion in sales in 2009 (as the manufacturing value of many high-cost items) and of course the print portion of this market is still small. Some forecasts suggest it may be worth €240 billion by 2025, with companies such as Nanosolar, First Solar and OTB Solar reporting orders in the billions of dollars for their solar cells, which use printing as part of manufacturing. Philips, GE and Konica Minolta among others are developing commercial lighting systems based on OLED technology that involve printing.

Printed solar panels by NanoSolarThe global electronics industry is looking toward organic and solution processable semiconductor technology to make thin-film, flexible, low-cost electronic devices. Though inorganic semiconductors such as silicon and gallium arsenide dominate electronics - providing superb computational speed and other benefits - they are rigid and brittle, precluding their use where flexibility is required. Printed and organic semiconductors combine the virtues of plastics and semiconductors, providing new electronic properties with scope for easy shaping and manufacture of plastics.


Printing techniques

Printing offers simpler and cheaper manufacture than conventional inorganic semiconductors, making it possible to deposit semiconducting polymers and other materials onto a range of substrates, including paper and board.

table 1

Print can be used for either rigid or flexible final products. Gravure, flexo and offset are printed in web (roll-to-roll) methods with screen, offset and inkjet as sheetfed.

In terms of digital technology, electrophotography is unsuitable as there are significant electrostatic charges, while inkjet is being used for rapid prototyping and then potentially high-volume applications. While there is work into paper- and board-based printed electronics, difficulties in controlling the moisture content means that plastic substrates are more widely used.

Print consistency is continually improving, but adapting high-volume production techniques and avoiding contamination requires significant knowledge and expertise. Often corona or plasma treatment will be used to ensure consistent lay down of inks. The final structure may be prepared subtractively by deposition of an etch mask or by lifting-off part of a surface, for example electrodes for organic field-effect transistors.

The choice of print technology is governed by the substrate properties, film thickness, ink or fluid properties, the format and the cost. Hybrid manufacturing - mixing the print process - is becoming more commonplace to achieve desired characteristics. Indirect gravure pad printing is used in a similar manner but the resolution is limited. Occasionally transfer methods, where solid layers from a pre-printed carrier are transferred to the substrate, are used.

Beside conventional methods new printing processes are being developed, including micro-contact printing and nano-imprint lithography. Here micrometre- and nanometre-sized layers, respectively, are prepared using techniques similar to stamping with soft and rigid forms. Nanotechnology such as solutions developed by firms like Nanogap in Spain offer great promise for producing next-generation inks for printed electronics.

Quality control

Electronic thin-film devices are prepared in printed electronics by printing several functional layers on top of each other. While the resolution of graphics printing (except in some security applications) is determined by visual appearance, to the human eye features below ~20µm cannot be observed. For electronics higher addressing resolution capability and smaller structures are desirable to prepare higher integration densities and boost the functionality of devices. Registration of layers is also critical. Deviation in the application film thickness and absence of defects and holes is important.

As print processes are increasingly used, quality assurance and testing methods are being developed in tandem to ensure high yields of good product. This is particularly important in web-fed roll-to-roll production, where the final components are punched out of a roll later and any defects can be costly. The broad nature of functional fluids to be printed means their transfer characteristics, printability, wetting, adhesion and dissolving need to be considered, as well as the press performance.



Inkjet

Inkjet deposition could eventually replace some techniques of electronic component manufacture and assembly with a single complete process. Many developers and specialist manufacturers are using inkjet to produce transistors, printed circuits, PVs and displays. In 2008 Konarka announced the first organic solar cell fabrication by inkjet printing. As developers progress, printers and other industries can license materials and processes for making devices.

table 2

First Solar and Nanosolar have successfully commercialised production methods using inkjet roll-to-roll methods during 2009. Nanosolar reports printing up to 1,500mm wide on the cell line, at speeds of 40m per minute.

The companies have multi-billion order books for their solar panels - like many printed electronic projects, just a small portion of this is printing value. As these applications take hold, Pira forecasts significant growth in the use of inkjet in the production of electronics, rising from some €62 million in 2008 to a multi-billion market by 2013.



Printing displays

Flexible electroluminescent displays may cover many tens of square metres and emit a range of colours, or be incorporated into small watch faces and instrument displays. HP's Information Surfaces Lab is trying to make flexible electronic displays that can fold like a newspaper or bend around a building. Possible applications range from computer games to product promotions and HP is also working on what it calls 'Dick Tracy's watch' for the military use that might tell soldiers where an enemy is located, or how to clean a machine gun.

Printed EL display by LumozaResearchers at MIT demonstrated print deposition of high resolution, patterned, multi-coloured thin films of luminescent colloidal quantum dot (QD)-polymer composites. They claim this results in robust, bright, full-colour displays offering simple, low-cost fabrication and high reliability.

Printing technologies are being employed to produce RFID tags and labels, circuits and memory, OLED displays, PV solar cells and batteries, and new applications including sensors and drug delivery. These devices have significant market potential for added-value products.

Examples that have reached the market include event tickets with RFID to allow visitor entrance and track attendees, board games and trading cards, novelty greeting cards with moving displays and sound, smart packaging, displays and solar power.



Printed logic

For printed logic and memory applications the first products are RFID tags, though adoption of the technology is slower than many early forecasts. Conventional RFID technology, using silicon chips, has two disadvantages - extended production time and high cost. PolyIC, a joint venture between Siemens and Kurz, in Bavaria, Germany, was set up in 2003 to produce printed electronics. The company supplies printed RFID chips a few square centimetres in area with a thickness of ~1µm.

printed memories developed by PolyIC and ThinFilmThe electrodes and the semiconductor layer are just a few hundred nanometres and the distance between the two conductors is less than 50µm. Antennas are positioned at the edge of the chips to transmit and receive radio signals and convey the energy required to operate the unit.

The process of manufacture involves spin-coating then flexo to print the conductors, then coating the foil with the semiconductor and insulator using a type of screen process. Kovio is a start up from the MIT has developed inkjet technology to print RFID tags using silicon-based inks.

There has been a great deal of debate and discussion on the market for printed semiconductors. Some advocates have been proven to be too optimistic, but most commentators are convinced it will take off. Electronic component manufacturers and developers are exploring printing technology to produce new devices or to improve existing ones and increase the efficiency of manufacture. The global electronics industry is looking toward organic semiconductor technology to make thin-film, flexible, low-cost electronic devices and print will be a key part of the manufacturing processes.

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