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The conductivity of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films was significantly enhanced by preferential solvations of the hydrophobic PEDOT and hydrophilic PSS chains with cosolvents. When a PEDOT:PSS film prepared from the PEDOT:PSS aqueous solution was treated with water or a common organic solvent like ethanol, iso-propyl alcohol (IPA), acetonitrile (ACN), acetone, or tetrahydrofuran (THF), its conductivity did not change remarkably. But the conductivity was significantly enhanced when the PEDOT:PSS film was treated with a cosolvent of water and one of these common organic solvents. The conductivity enhancement was affected by several factors, including the ratio of the organic solvent to water, the dielectric constant of the organic solvent, and the temperature during the treatment. The conductivity enhancement from 0.2 S cm−1 to 103 S cm−1 was observed. The significant conductivity enhancement is attributed to the preferential solvation of PEDOT:PSS with a cosolvent. Water and the organic solvent of the cosolvent preferentially solvate the hydrophilic PSS and hydrophobic PEDOT chains, respectively. The preferential solvation of a PEDOT:PSS film with a cosolvent induces the phase separation of the insulator PSSH chains from the PEDOT:PSS film, aggregation of PSSH segments in the PEDOT:PSS film, and the conformational change of the PEDOT chains from coiled to linear. The cosolvent-treated PEDOT:PSS films were quite smooth and could be used to replace indium tin oxide (ITO) as the transparent electrode of electronic devices. Polymer photovoltaic cells (PVs) with the cosolvent-treated PEDOT:PSS films as the transparent electrode exhibited high photovoltaic performance.
Xia et al. (Sat,) studied this question.