Effects of interacting ionic and electronic space charges

Thin films of ion conducting polyethylene oxide (PEO) are investigated as model systems for a solid electrolyte. Metal-PEO-metal capacitance structures show a pronounced relaxational behavior with a sample thickness (length) dependent effective dielectric permittivity caused by a space charge polarization of mobile ions. For high applied fields we find experimentally two effects: (i) During the polarization charge injection sets in resulting in nearly constant currents in the long time range which are independent of the sample length for same applied voltage. (ii) After switching off the external field the transient short-circuit current is not monotonically decreasing but shows a pronounced maximum in time. This behavior depends on the height and on the duration of the previously applied electric field step, the doping concentration, the temperature, the sample length and the electrode material. Dielectric measurements in the time and frequency domain are supported by AFM investigations in the Kelvin mode using an appropriate geometry. For interpretations a discrete three-dimensional hopping model is developed to describe the charge motion of ions and electrons. Here, discrete charges are considered. Due to the interaction between electrons, ions and electrodes we find a space charge region close to the electrodes where nearly the complete applied voltage drops down. Therefore, the injection currents are independent of the sample length at constant voltage. Furthermore, the simulations yield a maximum in the depolarization current. It is shown that this effect is caused by interaction between ionic and electronic space charges.