The changing face of PEDOT:PSS films:: Substrate, bias, and processing effects on vertical charge transport

Poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) is widely used as a semitransparent anode layer in organic light-emitting diodes and polymer photovoltaics. We use conductive atomic force microscopy (c-AFM) to map the electronic properties of PEDOT:PSS films during a variety of processing steps to better explain how the observed changes in macroscopic electronic properties arise from local changes in charge transport. We observe only small conductive regions, similar to 20 nm in size, surrounded by more insulating regions in all of the films studied. We confirm that these features dominate the c-AFM measurements, independent of the substrate. We observe a marked increase in the density of the conductive regions with increasing annealing times, increasing applied bias (independent of polarity), and decreasing PSS concentration (achieved by altering PEDOT:PSS grades). We also find an increase in current flow following a chlorobenzene wash, suggesting the solvents used in processing the active semiconductor layers on top of PEDOT:PSS anodes may affect the quality of the interface and subsequently alter device performance.