Enhancement of Thermoelectric Properties of PEDOT:PSS Films Prepared Under an Electric Field

Poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(4-styrenesulfonate) (PSS) molecules have positive and negative charge in aqueous solution, respectively. Therefore, PEDOT:PSS film crystal growth is expected to be affected by an external electric field. We have prepared PEDOT:PSS films under an electric field and investigated their thermoelectric properties. PEDOT:PSS films were synthesized by aqueous solution casting on a quartz substrate under electric fields E (pr) up to 4 kV/cm applied perpendicular to the substrate. With increase of E (pr), the electrical conductivity sigma increased from 1.5 S/cm for E (pr) = 0 kV/cm to 8.0 S/cm for E (pr) = 1 kV/cm, then saturated for E (pr) above 1 kV/cm. On the other hand, the Seebeck coefficient S was almost constant at 16 mu V/K for all E (pr) values. The power factor S (2) sigma improved from 0.04 mu W/m-K-2 for E (pr) = 0 kV/cm to 0.20 mu W/m-K-2 for E (pr) = 1 kV/cm. The transient photocurrent was also increased by applying E (pr). X-ray photoelectron spectroscopy (XPS) analysis showed that the molecular ratio of PEDOT to PSS molecules was significantly increased by applying an electric field during film formation. These results suggest that the electric field promoted PEDOT:PSS crystal growth near the film surface, resulting in improved thermoelectric properties due to enhanced carrier mobility.