2, University of Maryland Baltimore County, Baltimore, Maryland, United States
The power factor (PF) of thermoelectric materials, S2σ, where S is Seebeck coefficient and σ is electrical conductivity, requires high charge density at an energy level ca. 0.1 eV below the transport level, and high mobility of charge carriers in that level. Semiconducting polymers are considered for this purpose because of their possible contribution to thermoelectric composites based on sustainable materials and fabrication processes. Creating stable charge carriers in a semiconducting polymer structure that maintains mobility is a materials chemistry challenge. This talk will discuss two approaches to this challenge. For hole conductivity, we modified a standard thiophene polymer structure (PQT12) with electron donating sulfur atoms between the dodecyl chains and thiophene rings, and with ethylenedioxy substitution on half the thiophene rings. Both of these modifications are intended to stabilize holes and achieve unusually high nonionic polymer conductivity. For each of the modifications, one particular dopant yielded the highest σ and PF. For electron conductivity, we employed an emerging n-type polymer with enhanced electron accepting properties and air-stable ionic dopants, one an inorganic salt and another a particle made from common elements to achieve the first step toward air stability of electron σ and PF. One notable aspect of both of these investigations is the consistent correlations of S and σ with predictions of recently published models, which indicates high mobility of doped forms of the polymers. A second aspect is the constancy of S over the minutes time scale following imposition of a temperature difference, decreasing the likelihood of a major ionic contribution to S. Spectroscopic measurements were used as alternate means of observing charge carriers, transistor data provided estimations of mobility, and x-ray scattering revealed the effects of doping on polymer chain packing.
 H. Li, M.E. DeCoster, R.M. Ireland, J.; Song, P.E. Hopkins, H.E. Katz,
J. Am Chem. Soc. 2017, 139, 11149-11157.
 X. Zhao, D. Madan, Y. Cheng, J. Zhou, H. Li, S.M. Thon, A.E. Bragg, M.E. DeCoster, P.E.; Hopkins, H.E.; Katz, Adv. Mater. 2017, 29, 1606928; D. Madan; X. Zhao; R.M. Ireland; D. Xiao; H.E. Katz, APL Materials 5, 086106 http://dx.doi.org/10.1063/1.4990139