Highly Simplified FeCl3-Assisted Copolymerization and Doping for Organic Thermoelectrics

The FeCl₃ is a well-known oxidizing agent for oxidative polymerization as well as a p-type dopant for making conducting polymers, but no studies have yet explored the simultaneous use of FeCl₃ for both polymerization and doping of organic soluble conjugated polymers (CPs) in organic thermoelectric (OTE) applications. In this study, soluble 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT)-co-3,4-ethylenedioxythio-phene (EDOT) polymers were successfully synthesized via a simple FeCl₃-assisted oxidative coupling reaction at room temperature. In addition, FeCl₃ doping of the synthesized polymers enabled them to exhibit thermoelectric properties. The addition of 10% ~ 20% EDOT in a polymer chain gave a synergetic effect on the electrical conductivity and power factor (PF) in the OTE devices. In that ratio, the strong electron-donating EDOT facilitates doping in the CPs, and the planar CPDT backbones stabilize the generated polaron/bipolaron species through efficient π-electron delocalization, resulting in the highest electrical conductivity. At the optimal EDOT ratio of 20%, a more than 10-fold enhancement in PFs was achieved reaching up to 0.182 ± 0.021 μW m⁻¹ K⁻². The EDOT moiety, except in PEDOT:PSS, is rarely found in conjugated copolymers, but its proper incorporation is expected to enhance thermoelectric properties of the organic soluble CPs.