Optimization of Thermoelectric Properties of Polymers by Incorporating Oligoethylene Glycol Side Chains and Sequential Solution Doping with Preannealing Treatment

Two types of p-type thermoelectric (TE) polymers with alkyl (PCPDTSBT) and oligoethylene glycol (OEG) side chains (PCPDTSBT-A) on an sp2-hybridized olefinic bis(alkylsulfanyl)methylene-substituted cyclopentadithiophene backbone are synthesized. Interestingly, the OEG-substituted polymer, PCPDTSBT-A, exhibits significant self-doping compared to PCPDTSBT, where the polaron density of the former is 2.3 × 1016 mm-3 (vs 7.9 × 1014 mm-3 for PCPDTSBT) without external doping. Changing the side chains also induces a completely different polymer chain orientation in the PCPDTSBT-A (face-on) and PCPDTSBT (edge-on) films. The effect of doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) on the morphological and TE properties of the polymers with different side chains is studied. Sequential solution doping (SQD) is performed by overcoating the preannealed polymer films with F4TCNQ solution, which affords highly effective doping without disrupting the morphology of the crystalline films, especially for PCPDTSBT-A with OEG side chains. Resulting from the synergistic effect of the OEG side chains and SQD, PCPDTSBT-A exhibits remarkably improved electrical conductivity (53.8 S cm-1) with a higher power factor (40.4 μW m-1 K-2), compared to PCPDTSBT, for which the maximum electrical conductivity is 1.4 S cm-1 and the power factor is 1.8 μW m-1 K-2. In addition, the transport coefficient of PCPDTSBT-A, determined by applying the Kang-Snyder model (2.40 × 10-2 S cm-1), is superior to that of PCPDTSBT (3.59 × 10-3 S cm-1), thereby showing the excellence of the developed strategy for improving the performance of TE polymers.