Understanding the Solution-State Doping of Donor-Acceptor Polymers Through Tailored Side Chain Engineering for Thermoelectrics

Abstract

Among the molecular doping methods widely utilized to control charge carriers in organic thermoelectric (TE) devices, solution mixing is attractive for the preparation of uniform and thick TE films to obtain high output powers. However, the effects of doping behavior in solution-state polymers on their film properties are still not well understood, particularly for donor-acceptor (D-A) polymers, key materials for high-performance (opto)electronic devices. Here, by preparing three D-A polymers with engineered side chains, the aggregation effects of D-A polymers on the doping behavior in the solution and film states are demonstrated. In a less aggregated polymer, mixing the solution with a p-dopant leads to the Lewis complex formation, which reduces doping efficiency and power factor. However, solution mixing of aggregated polymers induces the formation of ion pairs with the dopant, resulting in power factors that are two orders of magnitude higher at 23.7 mu W m(-1) K-2. Doping in the film state where aggregation is induced facilitates ion-pair formation, resulting in smaller deviation in TE properties (power factors = 9.0-16.4 mu W m(-1) K-2) despite differences in their ordering structures. These results provide an in-depth understanding of efficient solution-state doping of D-A polymers for TEs.