The EU-funded PLASTRONIX project has made a series of breakthroughs in polymeric semiconductor design for versatile flexible integrated circuits that can be used in mass-produced electronics devices, from smart labels to flexible TV displays.
Plastics, or polymers, can be readily shaped and manufactured, and their properties tailored to a particular application. Conventional plastics are electrical insulators, but the discovery of a remarkable class of polymeric semiconductors that can conduct electricity has opened up a new era of plastics science and technology.
Semiconductors form the basis of electronics and optoelectronics, and have countless applications in information technology, telecommunications, entertainment and medical equipment. Surprisingly though, this rainbow of applications involves the precise control of only a handful of semiconductor materials – rigid crystalline inorganic materials such as silicon, which must be extremely pure, and which need very precise processing under highly demanding conditions.
Combining plastics and semiconductors
Now polymeric semiconductors bring together plastics and semiconductors in materials whose electronic properties can easily be shaped and manufactured just like plastic. There are a nearly infinite variety of these organic materials and their properties can be tuned by changing their chemical structure. This makes them very versatile. Such a remarkable combination of properties means that organic semiconductors open up new opportunities for electronic and optoelectronic materials, which can be used to make a wide range of electronics devices such as transistors, light-emitting diodes, solar cells and even lasers. They offer special properties and are much simpler to manufacture than conventional inorganic semiconductors.
Polymer semiconductor light emitting diodes (P-LEDs) exploit a series of organic macromolecules, called conjugated polymers, for use in optoelectronic devices. They combine optical and electrical properties with mechanical robustness, offering a low cost approach for backlighting, illumination, and display applications. The relative simplicity of their architecture and production technology offers advantages over conventional silicon-based semiconductor LEDs and liquid crystal displays (LCDs). It is possible to design very thin light sources offering high brightness at low voltage with high power efficiency.
Organic semiconductors are poised to offer flat and even flexible electronics devices. Light emission from these materials is particularly promising – when a voltage is applied to a thin film of a polymeric semiconductor, it emits light, providing the basis for new display technologies. This could provide curved or flexible displays, TV screens that can be rolled up or folded like newspapers, light-emitting clothing for fashion or safety applications, and even light-emitting wallpaper – the range of applications seems to be limited only by imagination.
Just as remarkable is the way that such devices can be made. It is possible to deposit polymeric semiconductors by printing. This will greatly simplify the manufacturing process, increasing flexibility, reducing cost, and opening up new markets. The two-year EU-funded PLASTRONIX project built on this potential to design and control a manufacturing process for a polymeric semiconductor, and to develop an industrial process technology for flexible polymeric integrated circuits.
"We are continuing to explore the potential for semiconducting polymer material in electronics to try and take advantage of the specific properties of polymers, in particular their flexibility and low-cost manufacturing potential," says project coordinator, Dr Dago de Leeuw of Philips Research in the Netherlands "The project worked towards creating industrial processes which would design and fabricate robust, reliable and low-cost polymer electronic circuits. Lots of patents and disclosures have been filed. The market is huge both for radio frequency (RF) identity tags – so-called ‘smart labels’ – and electronic paper.
"PLASTRONIX was fully supported by the European Commission and co-operation has been a pleasant process. The project has also created a new business opportunity for the companies involved, and the research institute and university have gained a great deal of knowledge," adds Dr de Leeuw. "Philips sees polymer electronics as a promising future technology. Our current areas of interest are active-matrix display driving – as an enabler for flexible displays – and low-cost, flexible electronics."
Wide range of skills available
Philips Research played the central role of technology developer, bringing in state-of-the-art competence and facilities for fabricating the new devices. Philips Semiconductors in Austria wanted to develop its position of market leader in RF identification.
The development of layers of patterned poly-3,4-ethylene-dioxy-thiophene (PEDOT) – a conducting polymer – expanded Agfa-Gevaert’s conducting polymer product range. Because it is water-based, PEDOT is more suited to the environment, and its properties make it easier to process. It can be spin-coated on to a huge variety of conducting and non-conductive substrates including glass, silicon, chromium, and gold.
Covion Organic Semiconductors – the displays business of Avecia, one of Europe's largest specialised chemicals companies – played a key role of making the materials suitable for industrial processes, which have to conform to stringent environmental, safety and health constraints. Dutch research institute TNO and the Limburgs Universitaire Centrum in Belgium were responsible for characterising the project's materials and devices.
Innovative products developed
The project created a breakthrough for the low-cost identification market with the concept of an electronic barcode transponder linked to the Internet. This is an ultra-thin, disposable label containing a polymer chip that carries a unique identification code. It can be read by a base station over an RF wireless link.
"The chips can be used to connect the world of products (parcels, goods, etc.) to the virtual world of the web," says Dr de Leeuw. For example, cooking recipes could be displayed and downloaded from the producers' websites when the chip on a food package is scanned.
PLASTRONIX also built a 256-grey level active matrix liquid crystal display, driven by polymer transistors. "This is a major step towards electronic paper," he adds.
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