Microstructure-dependent electrochemical properties of chemical-vapor deposited poly(3,4-ethylenedioxythiophene) (PEDOT) films

Abstract

Conductive polymer electrodes hold exceptional promise in energy conversion technologies and bioelectronics due to the inherent mechanical flexibility and synthetic tunability of physical, chemical, and electronic properties. Solution-processing is favorable to retain low-cost but can often result in heterogeneity of physical and electronic structure due to non-conjugated side chains and polyionic dopants creating insulating domains. Such a complex landscape limits control and systematic understanding of fundamental properties including electrical and ionic transport and rates of electron transfer central to overall device efficiencies. Oxidative chemical vapor deposition (oCVD) offers a promising route to simultaneously synthesize and deposit conductive polymer films at low temperatures (<150°C) with controllable microstructure. Herein we use the oCVD technique to synthesize and deposit different paracrystalline films of poly(3,4-ethylenedioxythiophene). Using grazing incidence wide-angle x-ray scattering, we demonstrate three different orientations of the π-π packing direction, relative to the surface normal, to yield thin films with face-on, edge-on, and isotropic character. These different microstructures have direct impact on the electrochemical conditioning of the redox properties of the films. We show that electrochemical properties, as defined by energy and power density, are highest for crystalline materials with edge-on orientation to facilitate ion transport while still retaining reasonable electrical conductivity.