An investigation into effects of heteroatom-tailored donor engineering on thermoelectric performance of cyclopentadithiophene-based copolymers

The past years have witnessed the rapid development of organic conjugated polymer thermoelectric (TE) materials profiting from new combinations of popular donor and acceptor units as well as the exploration for novel classes of donors and/or acceptors. Nevertheless, the relationship between the structures and properties of those TE materials remains far less known due probably to the complexity of variables involved in this regard. Factually, minimizing structural variables is more helpful to gain an in-depth insight into the structure–property relationship. In this work, two DA alternative copolymers were designed and synthesized by the Stille coupling reaction of the difluorinated benzotriazole acceptor unit with the N- or Si-substituted cyclopentadithiophene (CPD) donor unit, and systematically compared with the previously reported C-substituted analogue to reveal the influence of the heteroatom-tailored donor engineering on the molecular configuration and the physicochemical properties of the polymers. As a result, the FeCl₃-doped N-substituted polymer harvested a largest Seebeck coefficient of approximately 162.4 µV K⁻¹ as well as a highest electrical conductivity of about 12.8 S cm⁻¹, profiting primarily from a higher carrier mobility resulting from the good ππ stacking and a shallower HOMO level. As anticipated, a best power factor was achieved to be around 11.6 µW m⁻¹ K⁻², which is around one order of magnitude higher than that of the Si-substituted one, even much higher than that of C-substituted P(CPD-BTA-2F). This research is expected to afford an important guidance for purposefully designing the high-performance TE polymers in the future.