Imperial College London scientists say improving “crystallization,” an industrial process for making plastics, could revolutionize the way we produce electronic products,  reducing the cost and improving the design of solar cells and other electronic devices.

The process of making many well-known products from plastics involves controlling the way that microscopic crystals are formed within the material.

That allows engineers to determine the exact properties they want, such as transparency and toughness. To grow these crystals, engineers add small amounts of chemical additives to plastic formulations. This approach is used in making transparent plastic containers.

Advanced flexible circuits

The Imperial team has 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.

“Essentially, we have demonstrated a simple way to gain control over how crystals grow in electrically conducting ‘plastic’ semiconductors,” says Dr. Natalie Stingelin, the study leader from the Department of Materials and Center of Plastic Electronics at Imperial.

“This will 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 crystallization, an important part of the drug development process.”

Stingelin and research associate Neil Treat looked at two additives, sold under the names Irgaclear XT 386 and Millad 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.

It’s all about the additives

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 crystallization happened faster than normal. Usually plastic electronics are exposed to high temperatures to speed up the crystallization 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 crystallization 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.

The scientists collaborated with scientists from the University of California Santa Barbara, the National Renewable Energy Laboratory in Golden, Colorado, and the Swiss Federal Institute of Technology on this study.

They will be working with the new Engineering and Physical Sciences Research Council (EPSRC)-funded Center 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 center, 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.

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Technologies that could benefit from additives

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. The 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 crystallize will mean they crystallize in evenly distributed patterns that conduct electricity efficiently. This means industry can produce these printed electronics more easily and cheaply.

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