Influence of the electronic structures of diketopyrrolopyrrole-based donor-acceptor conjugated polymers on thermoelectric performance

Abstract

Donor–acceptor (D–A) conjugated polymers (CPs) with various D–A pairs have been developed as thermoelectric (TE) materials because of their superior charge-carrier mobilities and Seebeck coefficient values. However, because D–A CPs alter their molecular packing structures through both the doping process and different donor and acceptor moiety chemical structures, the complex structure–property relationships of doped D–A CPs impede the understanding of the influence of the electronic structure on charge transport. Herein, the influence of the electronic structures on the TE properties of five random D–A copolymers were investigated. The diketopyrrolopyrrole-based D–A copolymers were synthesized by varying the feed ratio of two oligomeric building blocks that differed only in the introduction of a strong electron-withdrawing cyano group on the vinylene linkage flanked between two thiophene rings. The almost identical crystalline structures and film morphologies of these D–A copolymers were confirmed, even after doping with FeCl3. The charge-carrier mobility of the doped CPs increased with delocalization of the polarons with an increasing DPP-TVT composition, resulting in a superior TE performance with a maximum power factor of 79.8 μW m−1 K−2. The correlation between the charge-carrier mobilities and TE performance was further investigated using the Kang–Snyder theoretical model. This study provides an understanding of the impact of the electronic structure of donor and acceptor moieties on the TE performance, and offers useful guidelines for the selection of D–A pairs to develop excellent TE materials.