Beyond doping: Self-Induced Anisotropy Breaks the σ-S Coupling Towards High Performance N-type Conjugated Polymer Thermoelectrics.

Diego Rosas Villalva^a, Dennis Derewjanko^b, Yongcao Zhang^a, Ye Liu^a, Andrew Bates^c, Anirudh Sharma^a, Soyeong Jang^a, Xiaodan Gu^c, Martijn Kemerink^b, Derya Baran Baran^a

^aKing Abdullah University of Science & Technology (KAUST), KAUST Solar Center (KSC), Physical Science and Engineering Division (PSE), Thuwal, 23955-6900, Kingdom of Saudi Arabia

^bInstitute for Molecular Systems Engineering and Advanced Materials, Heidelberg University, Germany

^cSchool of Polymer Science and Engineering, University of Southern Mississippi, USA

Materials for Sustainable Development Conference (MATSUS)
Proceedings of MATSUS Fall 2023 Conference (MATSUSFall23)

#OMIECs - Fundamentals of mixed ionic-electronic transport in polymers

Torremolinos, Spain, 2023 October 16th - 20th

Organizer: Simone Fabiano

Oral, Diego Rosas Villalva, presentation 052
DOI: https://doi.org/10.29363/nanoge.matsus.2023.052
Publication date: 18th July 2023

In recent years, there has been continuous progress in the development of n-type organic thermoelectrics (OTE) due to the synthesis of new conjugated polymers and dopants. The primary focus of most designs has been to enhance doping efficiency and increase electrical conductivity, resulting in values surpassing 102 S cm^-1. Although effective, doping encounters limitations because of the inherent inverse relationship between the properties that determine the figures of merit in thermoelectric materials. To advance the field further, alternative approaches are needed beyond optimizing doping efficiency and the host:dopant ratio. In this study, we propose the concept of self-induced anisotropy as a solution to overcome the inverse coupling between electrical conductivity and Seebeck coefficient in n-type OTEs. Previous research has suggested methods like rubbing and drawing to induce anisotropy in p-type OTEs based on thiophenes. In our work, we exploit the interactions between solvent, host, and dopant to induce a preferential orientation. This orientation increases the in-plane delocalization length, thereby improving electrical conductivity whitout hindering the Seebeck coefficient. By adopting this approach, we have achieved promising results in the 2DPP-2CNTVT:N-DMBI system. At room temperature, we obtained a Power Factor of 115 μW m^-1 K^-2 and a figure of merit of 0.17, which further increased to 188 μW m^-1 K^-2 and 0.36, respectively, at 105 ºC. To explain the origin of self-induced anisotropy, we utilized Hansen Solubility Parameters and established guidelines for formulating solvent-host-dopant systems that can achieve a similar effect. This discovery opens new possibilities for advancing n-type OTEs.

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