Enhanced thermoelectric properties of two-dimensional conjugated polymers

Organic thermoelectric materials have drawn great attention in the past years. In this study, we report the thermoelectric properties of ferric salt bis(trifluoromethane)sulfonimide (TFSI−) doped two-dimensional (2D) conjugated polymer, poly[[4,8-bis[(5-ethylhexyl)thienyl]benzo[1,2-b;3,3-b]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-DT). It is found that the electrical conductivities of TFSI− doped 2D PTB7-DT are nearly 100 times larger than those of TFSI− doped one-dimensional (1D) corresponding conjugate polymer, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) at the same doping levels. Moreover, TFSI− doped 2D PTB7-DT possess over 20 times larger power factor (79.8 μW/mK2) than that of TFSI− doped 1D PTB7. The enhancement of electrical conductivities of TFSI− doped 2D PTB7-DT is attributed to enhanced charge carrier densities and polaron densities. The studies of grazing incidence wide angle X-ray scattering further indicate that the π-π stacking distances of TFSI− doped 2D PTB7-DT along the in-plane direction are largely reduced compared to that of TFSI− doped 1D PTB7, which would facilitate the inter-chain and the intra-chain charge carrier transport, resulting in enhanced electrical conductivities. All these results demonstrate that 2D conjugated polymers are promising candidate materials for approaching high performance organic thermoelectric electronics. © 2018, Springer International Publishing AG, part of Springer Nature.