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Polymers can behave like Insulators, Semiconductors, Metals and Semimetals

Polymers can behave like Insulators, Semiconductors, Metals and Semimetals

Polymers can behave like insulators, semiconductors and metals -- as well as semimetals. Twenty researchers, under the leadership of Xavier Crispin, Docent in organic electronics at Linköping University, are behind the breakthrough published in Nature Materials. Traditional plastics, or polymers, are electrical insulators. In the seventies, a new class of polymers that conduct electricity like semiconductors and metals was discovered by Alan J.Heeger, Alan G. MacDiarmid and Hideki Shirakawa. Now Xavier Crispin, Docent in organic electronics at Linköping University's Department of Science and Technology, has led a project where no fewer than twenty researchers from five universities worldwide have collaborated to prove that polymers can also be semimetals. A few years ago Xavier Crispin discovered that conductive polymers can be thermoelectric. A thermoelectric material undergoes a diffusion of electronic charge carriers to the cold region when the material is submitted to a temperature gradient. As a result an electric potential is created between the cold and hot side of the material. This thermo-voltage is the basis of thermo-couples used for instance in an everyday oven thermometer. The experiments yielded a high thermoelectric effect, a Seebeck effect, which indicated that we were dealing with semimetals. “But we needed proof," says Dr Crispin.

Twenty researchers from Sweden, Australia, Belgium, Norway and Denmark are co-authors of the article. The theoretical input as well as state-of-the-art polymer samples and morphology studies by research colleagues showed the exact same thing: the polymer, in this case a doped variant of the plastic PEDOT, behaves exactly like a semimetal, which also explains the high Seebeck effect. Thermoelectric generators are available on the market today, but these are made from alloys of bismuth and the semimetal tellurium. Unlike the polymers, these elements are both rare and expensive. "These polymers are both easy and inexpensive to produce. That we now have an understanding of these phenomena will really drive developments forward, and will open up a new research field in organic electronics," says Prof Berggren. The research was financed primarily by ERC, the European Research Council. In 2012 Dr Crispin was awarded an ERC Starting Grant of SEK 13 million.

A team led by Berkeley Lab Materials Sciences Division’s Jeffrey Urban and Rachel Segalman have discovered highly conductive polymer behavior occurring at a polymer/nanocrystal interface. The composite organic/inorganic material is a thermoelectric – a material capable of converting heat into electricity – and has a higher performance than either of its constituent materials. The results may impact not only thermoelectrics research, but also polymer/nanocrystal composites being investigated for photovoltaics, batteries and hydrogen storage. An efficient thermoelectric material made from polymers and nanocrystals is an attractive prospect as it would be significantly cheaper to fabricate than traditional thermoelectrics. Here the researchers synthesized tellurium nanowires with PEDOT:PSS, a common conducting polymer, and cast thin films of the resulting solution. Intriguingly, the team found that the composite films had higher thermoelectric performance than either the polymer or nanowires alone. The researchers rationalized their unusual results by modeling the films as a composite of three distinct materials: nanowires, bulk polymer, and a new interfacial polymer phase with increased electrical conductivity. The highly conductive interfacial polymer phase suggests new routes to enhancing electronic and thermal properties in hybrid materials and devices, for thermoelectric energy conversion and other energy applications.

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