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Donor-Acceptor Conjugated Copolymer with High Thermoelectric Performance: A Case Study of the Oxidation Process within Chemical Doping

The doping process and thermoelectric properties of donor-acceptor (D-A) type copolymers are investigated with the representative poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b] dithiophene]3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl)) (PTB7-Th). The PTB7-Th is doped by FeCl3 and only polarons are induced in its doped films. The results reveal that the electron-rich donor units within PTB7-Th lose electrons preferentially at the initial stage of the oxidation and then the acceptor units begin to be oxidized at a high doping concentration. The energy levels of polarons and the Fermi level of the doped PTB7-Th remain almost unchange with different doping levels. However, the morphology of the PTB7-Th films could be deteriorated as the doping levels are improved, which is one of the main reasons for the decrease of electrical conductivity at the later stage of doping. The best electrical conductivity and power factor are obtained to be 42.3 S·cm−1 and 33.9 μW·mK−2, respectively, in the doped PTB7-Th film at room temperature. The power factor is further improved to 38.3 μW·mK−2 at 75 ◦C. This work may provide meaningful experience for development of D-A type thermoelectric copolymers and may further improve the doping efficiency.