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