hrvatski

Ion-electron exchange at the semiconducting polymer film interface studied by means of electrochemical spectroscopic ellipsometry

For the semiconducting polymer (SCP) thin films deposited as electrode material at the interface with ionic solution, the ion-electron exchange involves simultaneous electron transfer at the metal/film interface, and ion transfer within the diffusion layer extending from the SCP "surface" throughout the hydrated film into the solution1, 2. An important feature of this process is that infusion of a small amount of ions into the SCP film is able to dramatically alter its conductivity, which presents the working principle for the organic electrochemical transistors. The aim of the paper is to present the in situ electrochemical spectroscopic ellipsometry as powerful method for studying the ion-to-electron mechanism in the case of poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin film, deposited at the planar Au electrode3. The electrochemical potential of EDOT is controlled by applying an external equilibrium potential at the PEDOT:PSS film with respect to the reference electrode in a three electrode cell. Fitting the Tauc-Lorentz-Drude dielectric function from the experimental ellipsometric spectra, enabled resolving the free electron absorptions proportional to the conductivity, from the interband transitions4. These two contributions, as functions of the applied electromotive force, were successfully fitted by applying the Nernst law for the EDOT redox equilibrium. The obtained Nernst slope of only 12 mV presents evidence of a very strong electrostatic interaction between the ionized EDOT sites, whereby an analogy can be drawn with the cooperative ion binding at polyelectrolytes such as poly(ethyleneimine)5. The resulting low capacitance of the SCP/electrolyte interface is responsible for the electrochemical potential range (about 0.5 V wide) in which the ion-to-electron exchange is dominated by charge separation in PEDOT, rather than a redox reaction. Consequently, the charging mechanism of PEDOT:PSS as transistor channel material, is in this range similar to that observed in the field effect transistors.

conducting polymer film ; dielectric function ; electrochemical ; doping ; model ; ellipsometry

nije evidentirano