Tuning the Thermoelectric Properties of a Conducting Polymer through Blending with Open-Shell Molecular Dopants

Polymer thermoelectric devices are emerging as promising platforms by which to convert thermal gradients into electricity directly, and poly(3,4-ethylene dioxythiophene) doped with poly(styrenesulfonate) ( PEDOT: PSS) is a leading candidate in a number of these thermoelectric modules. Here, we implement the stable radical-bearing small molecule 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO-OH) as an intermolecular dopant in order to tune the electrical conductivity, thermopower, and power factor of PEDOT: PSS thin films. Specifically, we demonstrate that, at moderate loadings (∼2%, by weight) of the open-shell TEMPO-OH molecule, the thermopower of PEDOT: PSS thin films is increased without a marked decline in the electrical conductivity of the material. This effect, in turn, allows for an optimization of the power factor in the composite organic materials, which is a factor of 2 greater than the pristine PEDOT: PSS thin films. Furthermore, because the loading of TEMPO-OH is relatively low, we observe that there is little change in either the crystalline nature or surface topography of the composite films relative to the pristine PEDOT: PSS films. Instead, we determine that the increase in the thermopower is due to the presence of stable radical sites within the PEDOT: PSS that persist despite the highly acidic environment that occurs due to the presence of the poly(styrenesulfonate) moiety. Additionally, the oxidation-reduction-active (redox-active) nature of the TEMPO-OH small molecules provides a means by which to filter charges of different energy values. Therefore, these results demonstrate that a synergistic combination of an open-shell species and a conjugated polymer allows for enhanced thermoelectric properties in macromolecular systems, and as such, it offers the promise of a new design pathway in polymer thermoelectric materials.