Highly Simplified FeCl<sub>3</sub>-Assisted Copolymerization and Doping for Organic Thermoelectrics

Abstract

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.