Optimization of thermoelectric performances of conjugated polymers containing Trans-1,2-di(2-thienyl)ethylene subunits via structural modulation and doping engineering

Abstract

Focusing on the thermoelectric (TE) properties of (E)-1,2-di(thiophen-2-yl)ethene (TVT)-based organic conjugated polymers, backbone modification via copolymerization with different subunits and chemical doping with variant dopants are implemented. Herein, two polymers named PTVTTT14 and PTVTDTT18 with subtle structural difference are synthesized by copolymerizing TVT with TT (thieno[3,2- b ]thiophene) and DTT (dithieno[3,2-b:2′,3′-d]thiophene), respectively. Cooperation with less extended fused thiophene units TT, PTVTTT14 displays better solubility and hence larger molecular weight even with shortening alkyl side-chains comparing to that of PTVTDTT18. All these factors facilitate its better thermoelectric conversion efficiency. Furthermore, three dopants (CuTFSI, CN 6 CP and F 4 TCNQ) were employed to optimize polymers' TE performances. PTVTTT14 and PTVTDTT18 display highest PF of 9.4 and 5.0 μW m −1 K −2 with F 4 TCNQ as dopant. The higher electron affinity (EA) of F 4 TCNQ and CN 6 CP than ionization energy (IE) of the two polymers is energetically favorable to charge transfer, but the poor solubility of CN 6 CP hindering its effective doping. Moreover, CuTFSI-doped polymers show highest conductivity but lowest Seebeck coefficient ( S ) and power factor (PF) on account of its strong doping ability. Prospectively, reasonable ideas for polymers design and dopants selection are provided to obtain TVT-based polymers with better TE properties. • Correlation between structure and thermoelectric performances of TVT-based polymers was investigated. • By reducing a thiophene unit of conjugated backbone and shortening the alkyl side-chain, doped PTVTTT14 performed better thermoelectric performances than doped PTVTDTT18. • Optimized PF of PTVTTT14 and PTVTDTT18 were obtained upon F 4 TCNQ doping via doping engineering. • Selecting dopants with well matched energy level well with polymers are critical for optimization of the thermoelectric performances.