A Scanning Kelvin Probe Investigation of the Interaction of PEDOT:PSS Films with Metal Surfaces

The study of conducting polymers (CPs) is of significant current interest due to their incorporation in a range of applications, including optical and electronic devices. Poly(3,4-Ethylenedioxythiophene)-Poly(Styrene Sulphonate) (PEDOT:PSS) is considered one of the most electrochemically and thermally stable CPs available at present ¹. In many applications there is a requirement for electrical contact to be made between an organic PEDOT:PSS layer and a metallic substrate and therefore it highly important to understand the interaction between the CP and various metals, in order to gauge the long term stability of the metal-ICP interface.  In this work an in-situ scanning Kelvin probe (SKP) technique is employed to measure the Volta potential differences of various PEDOT:PSS coated metal surfaces to identify instances where a reaction may be taking place at the buried interface. Previous studies have demonstrated the capability of SKP in elucidating the nature of the interaction of polyaniline and polypyrrole coatings with various metal surfaces ^{2- 4}.

Experiments have been carried out on a range of metals spanning a significant part of the electrochemical series. A threshold thickness of PEDOT:PSS of 3 μm was determined for more reactive metals (i.e. Ni, Zn, Al and Mg), at or above which a consistent Kelvin probe potential (E_KP) value of ca. -0.15 V vs. SHE was measured. A potential of -0.4 V vs. SCE (ca. -0.156 V vs. SHE) has been reported previously by Ghilane et al, at which a PEDOT:PSS polymer film converts from the oxidized form to the reduced form ⁵. The constant value of -0.15V vs SHE measured for coated Ni, Zn, Al and Mg surfaces strongly suggests that these metals are polarized to this potential by a PEDOT:PSS_(ox)/PEDOT:PSS_(red) redox couple, where (ox) and (red) refer to the oxidized and reduced form of the CP respectively. This in turn implies that each of these metals enters into a galvanic interaction with the PEDOT:PSS, causing oxidation at the metal-coating interface and  the reduction of a  fraction of the CP.

For a selection of more noble metals, (i.e. Pt, Ag and Cu) no consistent E_KP values are observed (see Figure 1) and this is attributed to the fact that no redox reaction with PEDOT:PSS occurs.  In such a case the PEDOT:PSS coating remains fully oxidised, resulting in potentials that are consistently higher than E^o(PEDOT:PSS_(ox)) /(PEDOT:PSS_(red)). The current investigation also demonstrates that cyclic re-oxidation of a PEDOT:PSS coating applied to a Al surface by atmospheric oxygen can occur.  Previous studies have reported that sustained substrate ennoblement with other  conducting polymers, such as Polyaniline emeraldine salt ² occurs on ferrous metals by this mechanism. Upon removal of oxygen, E_KP values measured for PEDOT:PSS coated Al decrease to ca. -0.38 V vs. SHE, a value similar to that that measured for uncoated Al. Upon the reintroduction of air, E_KP values return to the values recorded initially. Therefore it seems that the presence of oxygen is important in maintaining the PEDOT:PSS redox state. Under conditions of low oxygen availability, complete reduction of PEDOT:PSS by the underlying Al is possible. This is confirmed by colorimetric observations of the thin PEDOT:PSS layer adherent to the Al surface.

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