Thermoelectric Properties of Expanded Graphite: Semiconducting Polymer/Compound Composites

We report for the first time on the thermoelectric properties of composites fabricated from expanded graphite (EXG) mixed with three p-type conjugated polymers: Poly(3-hexylthiophene), Poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}), and Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] commonly known as P3HT, PTB7 and PCDTBT, respectively and one n-type semiconducting compound [6,6]-Phenyl-C71-butyric acid methyl ester commonly known as PC70BM. The low percolation threshold of EXG in addition to its high electrical conductivity enabled a perfect balance for observing optimum thermoelectric properties when combined with polymers and PCBM that commonly exhibit low electrical and thermal conductivity. To decouple the electrical and thermal contributions to the Seebeck coefficient, we also fabricated composites containing EXG mixed with two well-known insulating polymers Polycarbonate and Polystyrene. We show several orders of magnitude enhancement in the thermoelectric power factor and figure of merit (ZT) for EXG: conjugated polymer/PCBM composites compared to their pristine material values. Further, we show that the charge transfer between the EXG and conjugated polymer or PCBM increases the composite’s power factor such that the composites containing p-type polymers with stronger electron-donating character like PTB7 and P3HT exhibit larger power factors in comparison. In addition, we show that our composites show deviation from the expected heterogeneous model and the power factors of our composites are higher than their pristine components suggesting an interacting and tunable interface between the polymers or PCBM and EXG.