Doping of donor-acceptor polymers with long side chains via solution mixing for advancing thermoelectric properties

Abstract

One-step doping of conjugated polymers by solution mixing is typically performed instead of sequential doping because of its simplicity. However, doped polymer solutions often exhibit poor solubility, and the presence of dopants in the produced films can disturb the molecular ordering of polymer structures. In this work, effective pairs of two donor-acceptor (D-A) type polymers and a molecular dopant characterized by high solution stability and good thermoelectric properties of the prepared thin films have been reported. The presence of long side chains in the polymer structures preserves their original solubilities and crystallinity in the solution and thin-film states, respectively, even at large amounts of added dopant (up to 38 mol%). Furthermore, the relatively shallow levels of the highest occupied molecular orbitals of the selected D-A polymers enable efficient charge transfer from the dopant species. Owing to their good charge transport properties, the doped D-A polymers exhibit outstanding thermoelectric properties with a maximum power factor of 31.5 μW m−1 K−2, which is more than an order of magnitude higher than those of the control samples prepared from donor-only poly(3-hexylthiophene).