Thermoelectric and Structural Properties of Free-Standing PEDOT:PSS Films after Solvent Treatments

Abstract

Free-standing poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) films provide an ideal platform for probing intrinsic thermoelectric (TE) transport properties, as substrate-induced effects are eliminated. However, a quantitative understanding of how chain ordering, phase segregation, and dopant concentration jointly determine the TE performance of free-standing PEDOT:PSS films is lacking, and only a few studies isolate these intrinsic structure-property links from substrate effects. In this work, the post-treatment process, involving exposure to various polar solvents, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and ethylene glycol (EG); acid treatments (sulfuric acid (H2SO4) and hydrochloric acid (HCl)); and alkaline treatments (sodium hydroxide (NaOH) and potassium hydroxide (KOH)), was systematically adopted to tune the degree of chain ordering, phase segregation, and dopant content. The structural changes reveal that polar-solvent treatments can induce enhanced pi-pi stacking and chain planarity through the effect of secondary doping and PSS extraction, whereas acid treatment facilitates increased protonation and chain packing. In contrast, the effect of alkaline treatment promotes de-doping and structural disorder. All of these changes have a direct impact on the TE performance, such that EG-treated films show high conductivity and power factor of similar to 105 mu W & centerdot;m(-1)& centerdot;K-2, while H2SO4-treated films possess a large Seebeck coefficient. Despite the decreased electrical conductivity and power factor compared to EG films, the KOH-treated film provides the largest power density of similar to 20 mu W & centerdot;cm(-2), resulting from de-doping-induced energy filtering. The fabricated 12-leg TE generators exhibit a linear relationship between voltage and temperature. These findings establish clear intrinsic structure-property-performance relationships in free-standing PEDOT:PSS films and provide a rational strategy for optimizing high-performance organic thermoelectric materials.