Tailoring Polar Side Chain Ratio and Distribution in Conjugated Polymers—A Path to High-Performance Thermoelectrics

Thermoelectric (TE) devices, which convert waste heat into electricity through the Seebeck effect, have garnered considerable attention as an emerging strategy for enhancing energy efficiency. Among the various materials explored for TE applications, conjugated polymers (CPs) hold promise due to their lightweight nature, mechanical flexibility, and compatibility with low-cost fabrication methods. However, compared to inorganic TE materials, the relatively lower Seebeck coefficient and electrical conductivity of CPs pose a critical challenge, necessitating innovative strategies to overcome these performance gaps. In this work, we address this challenge by engineering the ratio and distribution of tetraethylene glycol (TEG)-based polar side chains in difluorobenzothiadiazole (DFD)-based CPs. Two series of CPs were synthesized named as PDFD-T-DFED(M)-T and PDFD-T-DEED(M)-T where M denotes the feed ratio of TEG-containing units. Specifically, in PDFD-T-DFED(M)-T, a single TEG moiety is introduced into the difluorobenzothiadiazole unit, whereas in PDFD-T-DEED(M)-T, two TEG moieties are introduced, leading to differences in the distribution of TEG side chains among these two series. All CPs were designed with comparable molecular weights to isolate the side-chain effects on doping efficiency and TE properties. Upon doping with FeCl3, CPs with TEG side chains showed substantially enhanced TE performance compared to a reference PDFD-T series which have no polar side chains. For instance, PDFD-T-DFED(2.5)-T reached an electrical conductivity of 460.3 S cm⁻¹, a Seebeck coefficient of 51.2 μV K⁻¹, and a power factor of 120.7 μW m⁻¹ K⁻² at 1.00 wt% FeCl₃. Similarly, PDFD-T-DEED(5)-T exhibited electrical conductivity of 228.0 S cm⁻¹, a Seebeck coefficient of 67.3 μV K⁻¹, and a power factor of 103.3 μW m⁻¹ K⁻² at 0.50 wt% FeCl₃. UV-vis-NIR spectro-electrochemistry and Kelvin probe force microscopy (KPFM) confirmed that TEG substitution facilitated dopant penetration and increased carrier concentration. Moreover, a higher distribution of TEG side chain led to improved doping efficiency, as evidenced by the PDFD-T-DEED(M)-T series, which achieved higher carrier concentrations even at lower doping levels. While doping efficiency improved with higher TEG content, grazing incidence wide-angle X-ray scattering (GIWAXS) and Hall effect measurement revealed that excessive TEG side chains disrupted molecular packing and reduced charge carrier mobility. Consequently, there is a trade-off between maximizing carrier concentration through polar side-chain pathways and preserving crystallinity for efficient charge transport.
Overall, these findings underscore the critical importance of precisely tuning the ratio and distribution of polar side chains in CPs to optimize doping efficiency, charge carrier mobility, and TE performance. This approach offers a practical design framework for next-generation polymeric thermoelectrics with improved power factors and potential for flexible, low-cost energy-harvesting applications.