Influence of cation size on the thermoelectric behavior of salt-doped organic nanocomposite thin films

Developing and understanding novel doping strategies for thermoelectric materials is imperative to efficiently convert waste into a useful voltage. One such method for improving the power factor of polymer nanocomposites is through salt doping. The cation size of a monovalent salt dopant was varied in a layer-by-layer (LbL)-assembled film composed of poly(diallyldimethylammonium chloride) (PDDA) and double-walled carbon nanotubes (DWNTs) stabilized by poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) [PEDOT:PSS]. Doping a 20 bilayer PDDA/DWNT-PEDOT:PSS film doped with 3 mmol CsCl yields the maximum power factor of 485 ± 29 μW m⁻¹ K⁻². This value was obtained through a five times increase in the electrical conductivity with a minimal decrease in the Seebeck coefficient relative to the undoped film. Cs⁺ is believed to more heavily dope the carbon nanotube network due to its relatively larger hydrophobicity, while also separating PEDOT from PSS due to charge screening. This study demonstrates the significance of the salt dopant identity, and the insight herein can likely be applied more broadly to improve a variety of organic thermoelectric materials.