Why Organic Electronic Devices Comprising PEDOT:PSS Electrodes Should be Fabricated on Metal Free Substrates

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is up to date the most popular and commercially most successful conductive polymer. It is being used not only for antistatic, anticorrosion, and even antifouling coatings on one hand, but also, owing to its plasma frequency residing within the far-infrared region, for semi-transparent electrodes or charge extraction layers in solar cells on the other hand. The work function of an electrode plays an important role in the performance of any electronic device. Therefore, proper tuning of electrode work functions in semiconductor devices is crucial. It is worth mentioning that the work function of electrodes can be impacted by uncontrolled conditions during film processing because even monomolecular adsorbates may cause noticeable changes. Due to its ionic properties governed by the sulfonate group and its counterion, PEDOT:PSS is strongly hygroscopic, which may influence its functionality. Furthermore, the acidity of the same may lead to the release of additional ions from the neighboring layer. In this contribution, the work function of PEDOT:PSS films, cast from various commercial formulations, was monitored in dependence of the impact of (a) post-production thermal annealing and (b) storage under well-controlled relative humidity conditions for (c) certain durations. Indeed, work functions could be correlated with the surface concentration of metal ions, which clearly depended on the mentioned processing conditions. Finally, the impact of processing conditions on the performance of organic solar cells was demonstrated.