In Situ Monitoring of Optical Constants, Conductivity, and Swelling of PEDOT:PSS from Doped to the Fully Neutral State

Abstract

Insights into the evolution of optical constants, conductivity, and swelling properties as a function of the doping level are provided for poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) films in an acetonitrile-based electrolyte by a comprehensive spectroelectrochemical study using cyclic voltammetry coupled with in situ transmission spectroscopy and by in situ spectroscopic ellipsometry from 400 to 1600 nm. Monitoring over a wide potential and wavelength range allows us to in situ access the optical, electronic, and swelling behavior during electrochemical cycling, from the as-cast doped conducting state via intermediate states to the fully reduced neutral non-conductive state. This knowledge will help optimize doping-level-dependent functional properties for a variety of applications, including photonics, electrochemical transistors, thermoelectric devices, and actuating soft robotics. Our main findings are as follows: (I) The films appear to be optically and electronically anisotropic, and the optical constants strongly vary as a function of applied potential. The maximum difference in the ordinary refractive index between the doped and neutral state amounts to Δn = 0.20 in the visible range and to Δn = 0.52 in the NIR. (II) A plateau conductivity over large potential ranges is found. When monitoring the plasma frequency as a function of potential, a continuous metal-to-insulator transition is obtained. (III) The as-cast doped films show remarkable swelling behavior. Passive swelling, that is, exposure of the as-cast dry films to the acetonitrile-based electrolyte, leads to swelling of plus 50%. In the electrochemically cycled state, the thickness of the doped film increases by plus 200% compared to the as-cast doped film. Upon reduction to the neutral state, a further swelling of ∼plus 60% is measured, which is fully reversible after initial electrochemical cycling.