Thermoelectricity: record efficiency for doped polymer semiconductor materials
Thermoelectric materials make it possible to recover unused heat and convert it into electricity. Their polymer-based semiconductor versions offer flexibility and lightness, enabling deployment in a wide range of environments, but they typically suffer from low electrical conductivity. However, research teams from ICPEES (CNRS/University of Strasbourg), ICS (CNRS), IPCMS (CNRS/University of Strasbourg), and Saarland University (Germany) have succeeded in achieving a new record in thermoelectric performance for these materials. Their results have been published in the journal Advanced Energy Materials.
Exactly 200 years ago, the German physicist Thomas Johann Seebeck showed that certain materials subjected to a temperature gradient generate electric currents. This phenomenon is harnessed in thermoelectric materials. They make it possible, for example, to convert energy lost as heat—such as in an engine or brake pads—into electrical current to power electronic devices and sensors. For optimal performance, these materials must combine low thermal conductivity, to maintain the temperature gradient, with high electrical conductivity, to efficiently transport charge carriers.
Conventional thermoelectric materials are mainly inorganic semiconductors. However, over the past decade, researchers have also been exploring polymer-based semiconductors. These offer greater mechanical flexibility, much lower weight, inherently low thermal conductivity, and the ability to operate near room temperature. On the downside, they often suffer from low electrical conductivity—a limitation that can be partially overcome by doping them with other molecules.
A collaboration within the Strasbourg organic electronics consortium—bringing together the Institute of Chemistry and Processes for Energy, Environment and Health ((ICPEES, CNRS/Université de Strasbourg), l’Institut Charles Sadron (ICS, CNRS), Saarland University (Germany), and the Institute of Physics and Chemistry of Materials of Strasbourg (IPCMS, CNRS/University of Strasbourg)—has achieved a new record in thermoelectric performance. With values close to 3 mW m⁻¹ K⁻², this represents twice the previous record—already held by the Strasbourg group—and up to fifteen times better than the most efficient comparable organic systems.
The team developed a thermoelectric polymer featuring carbon-based side chains incorporating a unique ether function. This design preserves optimal solid-state structuring of the semiconductor polymer while improving its compatibility with polar oxidizing molecules used as dopants. Doping generates a high density of charge carriers within the material, significantly enhancing its electrical conductivity. Once the polymer is deposited as thin films, the second innovation involves physically brushing the films to align the polymer chains in the same direction. This alignment increases electrical conductivity along the brushing direction without significantly altering other properties, enabling these materials to reach very high thermoelectric performance. These polymers hold strong potential for use in electrochemical transistors, which are key components in bioelectronic sensors.
Reference
Pablo Durand, Huiyan Zeng, Till Biskup, Vishnu Vijayakumar, Viktoriia Untilova, Céline Kiefer, Benoît Heinrich, Laurent Herrmann, Martin Brinkmann & Nicolas Leclerc
Single Ether-Based Side Chains in Conjugated Polymers: Toward Power Factors of 2.9 mW m-1 K-2
Adv. Energy Mater. 2022
https://doi.org/10.1002/aenm.202103049