Anisotropic polymer thermoelectric materials – Anisotherm

This project has developped an original methodology for the fabrication of new efficient thermoelectric materials based on i) innovative macromolecular engineering, ii) fabrication of highly oriented thin films by high temperature rubbing, iii) controlled doping of aligned films, iv) in-depth structural analysis by TEM and v) evaluation of thermorlectric properties.

ANISOTHERM project has shown a unique enhancement of the thermoelectric performances of p-type doped polymer semiconductors thanks to the polymer chain alignment by using the method of high temperature rubbing.The combination of this unique alignment method with optimized doping protocols and newly designed polymer semiconductor architectures led to world-leading thermoelectric power factors beyond 2.9 mW/mK2. Moreover, p-type donor-acceptor polymers show a change of the sign of the Seebeck coefficient, giving rise to a new fabrication method for n-type thermoelectric polymers of interest for the design ot new thermoelectric modules.

ANISOTHERM project has demonstrated the importance of polymer alignment methods for the enhancement of thermoelectric figure of merit, the gain is by a factor of 25 (P3HT doped with F4TCNQ). The high temperature rubbing method used for alignment can be applied to new polymers of n and p type. The polarity switching evidenced in heavily p-doped donor-acceptor polymers opens new perspectives in terms of all-polymer thermoelectric module fabrication.

Anisotherm project generated more than 12 publications , two publications in Advanced Energy Materials and a highlight on INC CNRS webpage. (https://www.inc.cnrs.fr/fr/cnrsinfo/thermoelectricite-un-rendement-record-pour-des-materiaux-semi-conducteurs-polymeres-dopes). The key publication (V. Vijayakumar et al. Adv. Energu Mater. 2019, 9, 1900266) was cited more than 100 times.

This project focuses on the fabrication of new polymer thermoelectric materials and to explore how their processing in thin films by crystallization, orientation and soft doping can improve their thermoelectric (TE) properties. It is organized around four tasks: i) the synthesis of new macromolecular architectures of p- and n-type polymers that allow an efficient and controlled doping, ii) to develop alignment methods and soft doping processes to obtain highly structured and oriented conducting polymer films, iii) characterize precisely the structure and orientation of the aligned conducting polymer films by transmission electron microscopy and grazing incidence X-ray diffraction and iv) to determine charge transport and TE properties of the anisotropic polymer films. Thanks to this work, correlations between structure and TE properties will be obtained for model systems and will help obtain a better understanding of the physical mechanisms governing TE properties in highly ordered conducting polymers. Ultimately, this project will generate polymer TE materials with an efficiency ZT=0.1-0.2.
The originality of the project stems from the fact that the processing of the conducting polymer films is split in separate and well controlled steps that help fabricate conducting polymer films with high crystallinity and orientation to enhance charge conductivity and TE properties. The proof of concept of this fabrication method of highly oriented, crystalline conducting polymer films has been validated recently by one of the partners of the project. From the material’s point of view, the project will yield new efficient, air-stable p- and n-type conjugated polymers whose doping shall preserve crystalline perfection. Soft-doping from vapor or solution phase as well as electrochemical doping will aim at reaching a controlled intercalation of dopants in the host matrix of the polymer. Most importantly, the correlations between processing, structure and TE properties will be established in order to have a better understanding of the parameters that can enhance TE properties of polymer thin films.

This pluridisciplinary project involves four laboratories located in Strasbourg and Tours with highly complementary experties : macromolecular engineering (ICPEES and PCM2E), growth control of polymer thin films and structural investigations (ICS), controled electrochemical doping (PCM2E) and determination of charge transport and thermoelectric properties (GREMAN).