Two-Step Design Rule for Simultaneously High Conductivity and Seebeck Coefficient in Conjugated Polymer-Based Thermoelectrics

The trade-off between enhancing conductivity (σ) through doping while concurrently observing a reduction in the Seebeck coefficient (S) presents a key challenge in organic thermoelectrics. Here, a two-step structural design strategy is developed, where the first step enhances the backbone planarity which enhances the conductivity by an improved ordering of conjugated polymers (CPs). The second step, which is fluorination of the backbone, improves the Seebeck coefficient by the controlled induction of energetic disorder, stemming from the fluorine's disruption of the homogeneous electrostatic potential across the CP backbone. This strategy is applied to two series of donor-acceptor (D-A) types of CPs based on the BDT donor unit and BDD and TT as acceptor units, respectively. A maximum power factor (PF) over 155 (142.7 ± 12.7) µW m^-1 K^-2, coupled with S ≈202 (194.6 ± 7.6) µV K^-1 is achieved, leading to up to 32-fold enhancement of the PF compared to the initial non-planar and non-fluorinated polymer. This study provides valuable conceptual insights for designing CPs with high conductivity and Seebeck coefficient.