Studies on the doping mechanism of conjugated thienothiophene polymer/MWCNT hybrids for thermoelectric application

Conducting polymer-based thermoelectric (TE) materials have great promise for fabricating lightweight modules that can directly convert waste heat into electricity. Doping is essential for these materials to tune the electrical conductivity and Seebeck coefficient, enhancing their overall TE response. However, the doping mechanism on polymer-based hybrid systems needs further clarification for materials design and selection. Herein, two different TE hybrids are fabricated using different side-chain groups containing benzodithiophene-thienothiophene (BDT-TTE) based conjugated polymers and multi-walled carbon nanotubes (MWCNT). The TE properties of the hybrids before and after p-doping are presented. Post-doping, a simultaneous increase in electrical conductivity and Seebeck coefficient is observed for the composites of alkylthiophene side-chain containing polymer, leading to ≈24× enhancements in the power factor (PF). The composite with alkoxy side-chain polymer showed a nominal increase in electrical conductivity after doping, and the Seebeck coefficient remained unaffected. Tracking of electronic structure and density of valence state (DOVS) revealed the presence of charge-transfer-complexes (CTC) with different coordination sites, along with repositioned Fermi level and valance band maximum (VBM) that contribute to the observed differences in the TE response. This work presents a general understanding of the doping mechanism and the underlying physics to design high-performance organic-based TE composites.