Electric Field Control of Charge Transport in Doped Polymers

Recent observation of a large “field effect” for conducting polymers is in conflict with a fact that the electric field cannot penetrate in a conductor further than the molecular scale Debye radius. We review the experimental results for gate voltage control of conductivity in a FET structure. We propose that the field effect is related to inhomogeneous structure and remnant ions. The free space within the polymer network enables ions to move in and out. The electronic screening at the dielectric/conducting polymer boundary transforms the electric potential into the shift of chemical potential for ions inside leading to ionically charging/discharging of the polymer film. The ionic charge is compensated by the variation of the electron concentration. In the linear response to the gate voltage, the charging capacity is c=εε0(ri2+re2), where ri,e is the Debye radius for ionic and electronic subsystems. For low concentration of ions (ri ≫ re) the charging response increases exponentially with applied voltage and approaches saturation c0 = e2N(EF) that is a few hundred F/cm3. Surprisingly, the experimental “off state” is reached with only a few percent of compensation. Therefore we propose that the main effect of the ionic invasion is suppression of the charge conduction between metallic grains of the inhomogeneous polymers.