Triplet spin densities of the dimer of pBBT-2T-TT (above) and pBBT-2T-2T (below).

© A*STAR’s Institute of Materials Research and Engineering (IMRE)

Helping devices take the heat

7 Sep 2020

A new class of organic conductors shows high thermoelectric efficiency, thanks to an unusual arrangement of electrons inside its bonds.

Imagine a T-shirt that could charge a power bank from your body heat—or draw power from a battery to keep you cool. Such inventions could become a reality thanks to the thermoelectric effect, in which solid-state semiconductors can convert thermal energy into electricity and back again.

However, scientists working on thermoelectric devices such as these have struggled with a fundamental trade-off—materials that generate higher voltages between hot and cold temperatures are unfortunately also poor electrical conductors, and vice versa. This limits the rate at which heat and electricity can be interchanged.

Now, Jianwei Xu and Dexter Tam, scientists at the A*STAR Institute of Materials Research and Engineering (IMRE), have shown that a new class of organic semiconductors they discovered previously, called proquinoidal polymers, avoid this constraint, thanks to an unusual arrangement of electrons that give them excellent electrical conductivity.

“Scientists around the world are trying different tricks to increase the power factor of materials, by increasing conductivity without compromising too much on the Seebeck coefficient (thermoelectric sensitivity). However, our previous work on the highly-conductive proquinoidal polymers suggests that they might naturally have a high Seebeck coefficient as well, thanks to how electrons behave within them,” Tam said.

As Tam explains, proquinoidal structures imbue polymers with rapidly switching double bonds, giving them high electrical conductivity. Together with Shuo-Wang Yang at A*STAR's Institute of High Performance Computing (IHPC), the researchers showed how these rapid switches also occasionally separate the tightly-bound pairs of electrons that make up chemical bonds.

When these separated electrons ‘spin’ in the same direction, forming a ‘high-spin triplet state,’ they respond more effectively to thermal energy, resulting in high Seebeck coefficients in addition to high conductivity. Under testing conditions, the doped proquinoidal semiconductors showed power factors exceeding 50 µW m-1 K-2—among the highest ever reported for organic semiconductors.

“Our findings provide design strategies to improve both conductivity and Seebeck coefficient simultaneously for thermoelectric applications,” Tam said, adding that they hope to further explore the application of proquinoidal polymers in spintronics and thin-film soft magnets.

The A*STAR researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE) and the Institute of High Performance Computing (IHPC).

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Tam, T.L.D., Wu, G., Chien, S.W., Lim, S.F.V., Yang, S.W. and Xu, J. High spin pro-quinoid benzo[1,2-c;4,5-c']bisthiadiazole conjugated polymers for high-performance solution-processable polymer thermoelectrics. ACS Materials Letters 2 (2), 147-152 (2020) | article

About the Researcher

Dexter Tam obtained his PhD in 2013 from Nanyang Technological University. He was awarded the World Future Foundation PhD Prize in Environmental and Sustainability Research, as well as the A*STAR International Fellowship. He did his post-doctoral studies in Northwestern University before returning to Institute of Materials Research and Engineering (IMRE) in 2015 where he was the Assistant Department Head for Molecular Materials in 2016-2017. His research interest lies in structure-property relationship in conjugated materials, and their applications in electronic devices.

This article was made for A*STAR Research by Wildtype Media Group