2, Georgia Institute of Technology, Atlanta, Georgia, United States
3, Purdue University, West Lafayette, Indiana, United States
Conjugated polymers can be used in mechanically flexible and low cost thermoelectric (TE) devices, but their thermoelectric performance must be improved to make them commercially viable. The performance of thermoelectric materials depends on the electrical conductivity, Seebeck coefficient and thermal conductivity. In polymer based TE materials the polymer needs to be doped to become electrically conductive. The higher the doping concentration, the more electrically conductive the material becomes, but generally at the cost of a decrease in the Seebeck coefficient. Blending of π-conjugated polymers has been proposed as a method to minimize the tradeoff between electrical conductivity and the Seebeck coefficient, thus potentially allowing higher power factors to be reached. By blending polymers, the total density of states (D.O.S.) in the material will be manipulated, which may be used to alter the energy dependence of charge transport in the TE material. Manipulating the D.O.S., particularly by introducing highly conductive states at higher energies with respect to the Fermi energy, allows the energy dependence of charge transport to be enhanced and the Seebeck coefficient increased. The major parameters that we expect to impact the power factor in polymer blends are the mobility ratios between the pure polymer and the shape of D.O.S. Here, we use a model introduced by Bässler and Arkhipov to theoretically probe how the mobility ratio and the shape of the D.O.S. impact the Seebeck coefficient and thermoelectric performance. These calculations are then used to fit experimental data of various polymer blends with varying mobility ratios and D.O.S. distributions. We find that a narrower D.O.S. and higher mobility of the added polymer with respect to host polymer can lead to an enhancement in the Seebeck coefficient of the TE material, but we do not observe increases in the power factor.