Modification of ionization potential of poly(ethylene dioxythiophene): Poly(styrene sulfonate) film by acid or base treatment

We studied the effects of surface treatments on the surface electronic state of the poly(3,4-ethylene dioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) film. The surface treatments performed were immersion of the PEDOT:PSS film in an acid or base solution, and the exposure of the PEDOT:PSS film to acid or base vapor. The ionization potential (I-p) of the PEDOT:PSS film is modified between 4.5 and 5.3 eV by surface treatments. I-p tends to be increased by acid treatment, although it tends to be decreased by base treatment. Because I-p is modified even by brief treatments, the modification is thought to result from surface condition changes. Measurements using X-ray photoelectron spectroscopy (XPS), resistivity, and optical absorbance revealed that an electric double layer might have been formed on the surface of the PEDOT:PSS film by the treatment. The electric double layer shifts the vacuum level. Assuming the formation of an electric double layer, we can describe I-p modification by the treatment.