Lawrence Berkeley National Laboratory

This study reports on the thermoelectric properties of poly(3-alkylchalcogenophene) thin films (500 nm) as a function of heteroatom (sulfur, selenium, tellurium), and how these properties change with dopant (ferric chloride) concentration. UV–vis–NIR spectroscopy shows that polaronic charge carriers are formed upon doping. Poly(3-alkyltellurophene) (P3RTe) is most easily doped followed by poly(3-alkylselenophene) (P3RSe) and poly(3-alkylthiophene) (P3RT), where R = 3,7-dimethyloctyl chain is the pendant alkyl group. Thermoelectric properties vary as functions of the heteroatom and doping level. At low dopant concentrations (≈1 × 10−3 m), P3RTe shows the highest power factor of 10 µW m−1 K−2, while, at higher dopant concentrations (≈5 × 10−3 m), P3RSe shows the highest power factor of 13 µW m−1 K−2. Most notably, it is found that the measured properties are consistent with Mott's polaron hopping model and not consistent with other transport models. Additionally, temperature-dependent conductivity measurements show that for a given dopant concentration, the activation energies for electronic transport decrease as the heteroatom is changed from sulfur to selenium to tellurium. Overall, this work presents a systematic study of poly(chalcogenophenes) and indicates the potential of polymers beyond P3HT by tuning the heteroatom and doping level for optimized thermoelectric performance.