Impact of Substrate Hydrophobicity on Layer Composition and Work Function of PEDOT:PSS Thin Films

Ultraviolet and X‐ray photoelectron spectroscopy (UPS and XPS) are used to investigate the impact of surface hydrophobicity on the work function of spin‐coated poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films. Although O 2 –plasma or UV–ozone treatment of indium tin oxide (ITO) changes the substrate hydrophobicity, the work functions of subsequently spin‐coated PEDOT:PSS layers are not affected and stay constant at around 5 eV. ITO modification by a self‐assembled monolayer of p ‐(trifluoromethyl)phenylphosphonic acid leads to a pronounced contact angle of 90°, resulting in a PSS‐rich PEDOT:PSS surface (as shown by XPS) and a work function of 5.15 eV (as shown by ultraviolet photoelectron spectroscopy). This can be rationalized by an increased bulk chemical potential of PEDOT:PSS.