Polymers can be semi-metals

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. This was the motivation for their Nobel Prize in Chemistry year 2000. Now Xavier Crispin, Docent in organic electronics at Link?ping 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.

The results are published in an article in the prestigious journal Nature Materials, with Dr Crispin as principal author. 

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.

Xavier Crispin“Our experiments yielded a high thermoelectric effect, a Seebeck effect, which indicated that we were dealing with semimetals. But we needed proof,” says Dr Crispin. This required talented people from various locations to gain an in-depth understanding of the phenomenon.

No less than 20 researchers from Sweden, Australia, Belgium, Norway and Denmark are co-authors of the article in Nature Materials. Ten of them are from Link?ping University, including Xavier Crispin, Professor Magnus Berggren and Igor Zozoulenko from the Laboratory of Organic Electronics, Department of Science and Technology, Campus Norrk?ping, as well as Professor Mats Fahlman, Division of Surface Physics and Chemistry, and Professor Weimin Chen, Division of Functional Electronic Materials, both at the Department of Physics, Chemistry and Biology.

“It has been very impressive to see how Xavier Crispin has led this. We have a fantastic environment for materials research at Link?ping University, with world-leading expertise. We are all friends and we’ve been able to produce this article without joint funding,” says Magnus Berggren, professor or organic electronics.

The theoretical input of Igor Zozoulenko, advanced spectroscopic analysis by Mats Fahlman and Weimin Chen at Link?ping University, as well as state-of-the-art polymer samples and morphology studies by research colleagues in Australia, Belgium, Norway and Denmark 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.

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