Tailoring the Seebeck Coefficient of PEDOT:PSS by Controlling Ion Stoichiometry in Ionic Liquid Additives

Mixing simple additives into poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) dispersions can greatly enhance the thermoelectric properties of the cast films with little manufacturing cost, but design rules for many of these additives have yet to emerge. We show that controlling stoichiometry in ionic liquid (I.L.) additives can decouple morphological and electronic modifications to PEDOT:PSS and enhance its power factor by over 2 orders of magnitude. Blending I.L. additives with a 1:1 stoichiometry between cationic imidazolium (Im(+)) derivatives and anionic bis(trifluoromethane)sulfonamide (TFSI-) groups into PEDOT:PSS dispersions raised the film conductivity to similar to 1000 S/cm. The Seebeck coefficient, which gives insight into the electronic structure as well as thermoelectric performance, remained unchanged. This behavior mimics that of popular high-boiling solvent additives such as dimethyl sulfoxide and ethylene glycol, which restructure the film morphology to enhance carrier mobility. Blending I.L. additives with a 4:1 stoichiometry between Im(+) and TFSI groups raises the conductivity in a similar manner but also enhances the Seebeck coefficient. This selective Seebeck enhancement proceeds from the interaction of excess Im(+) with anionic poly(styrenesulfonate) (PSS-) groups, similar to previous studies using inorganic salts, that results in a shift in charge carrier populations. Inorganic salts by themselves cannot raise the conductivity of PEDOT:PSS to appropriate values since they lack the solvent restructuring effect. These I.L. additives combine the effects of high-boiling solvents and diffuse ions, with the ability to tailor the Seebeck coefficient through ion stoichiometry.