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Posted: Jul 04, 2013

Plastic electronics made easy

(Nanowerk News) Scientists have discovered a way to better exploit a process that could revolutionise the way that electronic products are made.
The scientists from Imperial College London say improving the industrial process, which is called crystallisation, could revolutionise the way we produce electronic products, leading to advances across a whole range of fields; including reducing the cost and improving the design of plastic solar cells.
The process of making many well-known products from plastics involves controlling the way that microscopic crystals are formed within the material. By controlling the way that these crystals are grown engineers can determine the properties they want such as transparency and toughness. Controlling the growth of these crystals involves engineers adding small amounts of chemical additives to plastic formulations. This approach is used in making food boxes and other transparent plastic containers, but up until now it has not been used in the electronics industry.
The team from Imperial have now demonstrated that these additives can also be used to improve how an advanced type of flexible circuitry called plastic electronics is made.
The team found that when the additives were included in the formulation of plastic electronic circuitry they could be printed more reliably and over larger areas, which would reduce fabrication costs in the industry.
The team reported their findings this month in the journal Nature Materials ("Microstructure formation in molecular and polymer semiconductors assisted by nucleation agents").
Dr Natalie Stingelin, the leader of the study from the Department of Materials and Centre of Plastic Electronics at Imperial, says:
“Essentially, we have demonstrated a simple way to gain control over how crystals grow in electrically conducting ‘plastic’ semiconductors. Not only will this help industry fabricate plastic electronic devices like solar cells and sensors more efficiently. I believe it will also help scientists experimenting in other areas, such as protein crystallisation, an important part of the drug development process.”
Dr Stingelin and research associate Neil Treat looked at two additives, sold under the names IrgaclearÒ XT 386 and MilladÒ 3988, which are commonly used in industry. These chemicals are, for example, some of the ingredients used to improve the transparency of plastic drinking bottles. The researchers experimented with adding tiny amounts of these chemicals to the formulas of several different electrically conducting plastics, which are used in technologies such as security key cards, solar cells and displays.
The researchers found the additives gave them precise control over where crystals would form, meaning they could also control which parts of the printed material would conduct electricity. In addition, the crystallisations happened faster than normal. Usually plastic electronics are exposed to high temperatures to speed up the crystallisation process, but this can degrade the materials. This heat treatment treatment is no longer necessary if the additives are used.
Another industrially important advantage of using small amounts of the additives was that the crystallisation process happened more uniformly throughout the plastics, giving a consistent distribution of crystals. The team say this could enable circuits in plastic electronics to be produced quickly and easily with roll-to-roll printing procedures similar to those used in the newspaper industry. This has been very challenging to achieve previously.
Dr Treat says: “Our work clearly shows that these additives are really good at controlling how materials crystallise. We have shown that printed electronics can be fabricated more reliably using this strategy. But what’s particularly exciting about all this is that the additives showed fantastic performance in many different types of conducting plastics. So I’m excited about the possibilities that this strategy could have in a wide range of materials.”
Dr Stingelin and Dr Treat collaborated with scientists from the University of California Santa Barbara, and the National Renewable Energy Laboratory in Golden, US, and the Swiss Federal Institute of Technology on this study. The team are planning to continue working together to see if subtle chemical changes to the additives improve their effects – and design new additives.
They will be working with the new Engineering and Physical Sciences Research Council (EPSRC)-funded Centre for Innovative Manufacturing in Large Area Electronics in order to drive the industrial exploitation of their process. The £5.6 million of funding for this centre, to be led by researchers from Cambridge University, was announced earlier this year. They are also exploring collaborations with printing companies with a view to further developing their circuit printing technique.
Controlling crystals
Here are some of the technologies that could benefit from Drs Treat and Stingelin’s research:
Improving drugs
Most drugs work by blocking or activating proteins in our bodies. To develop better drugs, scientists must understand what these proteins look like. The work carried out by the Imperial team could enable researchers in the future to develop more accurate models of proteins, by converting them into a crystalline form.
More efficient solar technology
Solar cells are made from a solid mixture of electrically conducting crystalline chemicals. Currently these cells only convert about 10% of the Sun’s energy into electricity. Dr Treat and Stingelin’s additives may provide a way of improving crystal growth in solar cells, which could improve the amount of energy they convert.
New flexible electronics
Flexible semiconductor films can be made by methods such as inkjet printing. Using additives that control how inkjet-printed droplets of semiconductors crystallise will mean they crystallise in evenly distributed patterns that conduct electricity efficiently. This means industry can produce these printed electronics more easily and cheaply.
Source: By Joshua Howgego, Imperial College London
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