Effect of Additives on the Surface Morphology, Energetics, and Contact Resistance of PEDOT:PSS

For a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate(PEDOT:PSS)film employed in a device stack, charge must pass through both thebulk of the film and interfaces between adjacent layers. Thus, chargetransport is governed by both bulk and contact resistances. However,for ultrathin films (e.g., flexible devices, thin-film transistors,printed electronics, solar cells), interfacial properties can dominateover the bulk properties, making contact resistance a significantdeterminant of device performance. For most device applications, thebulk conductivity of PEDOT:PSS is typically improved by blending additivesinto the solid film. Doping PEDOT:PSS with secondary dopants (e.g.,polar small molecules), in particular, increases the bulk conductivityby inducing a more favorable solid morphology. However, the effectsof these morphological changes on the contact resistance (which playa bigger role at smaller length scales) are relatively unstudied.In this work, we use transfer length method (TLM) measurements todecouple the bulk resistance from the contact resistance of PEDOT:PSSfilms incorporating several common additives. These additives includesecondary dopants, a silane crosslinker (typically used to stabilizethe PEDOT:PSS film), and multi-walled carbon nanotubes (conductivefillers). Using conductive atomic force microscopy, Kelvin probe forcemicroscopy, Raman spectroscopy, and photoelectron spectroscopy, weconnect changes in the contact resistance to changes in the surfacemorphology and energetics as governed by the blended additives. Wefind that the contact resistance at the PEDOT:PSS/silver interfacecan be reduced by (1) increasing the ratio of PEDOT to PSS chains,(2) decreasing the work function, (3) decreasing the benzoid-to-quinoidratio at the surface of the solid film, (4) increasing the film uniformityand contact area, and (5) increasing the phase-segregated morphologyof the solid film.

Automated summary

For a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate(PEDOT:PSS) film used in device stack, charge transport is influenced by both bulk and contact resistances. However, for ultrathin films, interfacial properties can dominate, making contact resistance a significant determinant of device performance. This study investigates the effects of additives on both the bulk and contact resistances of PEDOT:PSS films through various characterization techniques.