Impact of Oligoether Side-Chain Length on the Thermoelectric Properties of a Polar Polythiophene

Abstract

Conjugated polymers with oligoether side chains make up a promising class of thermoelectric materials. In this work, the impact of the side-chain length on the thermoelectric and mechanical properties of polythiophenes is investigated. Polymers with tri-, tetra-, or hexaethylene glycol side chains are compared, and the shortest length is found to result in thin films with the highest degree of order upon doping with the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). As a result, a stiff material with an electrical conductivity of up to 830 ± 15 S cm–1 is obtained, resulting in a thermoelectric power factor of about 21 μW m–1 K–2 in the case of as-cast films. Aging at ambient conditions results in an initial decrease in thermoelectric properties but then yields a highly stable performance for at least 3 months, with values of about 200 S cm–1 and 5 μW m–1 K–2. Evidently, identification of the optimal side-chain length is an important criterion for the design of conjugated polymers for organic thermoelectrics.

Impact of doping on the mechanical properties of conjugated polymers

“… 118 Shorter triethylene glycol side chains give rise to a stiffer polymer, p(g 3 2T-T), with E = 76 MPa, which upon doping with 20 mol% F 4 TCNQ increased to 826 MPa. 84 …”