Electrode effects in solid electrolyte capacitors

Thin films of ion-conducting polyethylene oxide (PEO) are prepared as a model system for solid polymer electrolytes. Plane-parallel capacitance structures of the type Al-PEO-Al (Metal-Insulator-Metal, MIM) with blocking electrodes are produced. In the short time range t <= 1 s after the application of an external field, an ionic space-charge polarization with an exponential time dependence prevails. In the long time range the samples exhibit a Kohlrausch behavior of the polarization current, i.e., j proportional to t(-alpha). This time dependence can be explained by image charges in the electrodes which attract the mobile ions. This attraction of ions is responsible for the static and the dynamic behavior of the system. The remanent polarization appearing in hysteresis measurements is an affirmation of the presence of these image charges. Computer simulations of a three-dimensional hopping model yield qualitatively the same results. Here the attraction of ions at the electrodes due to the image charges can be observed, as well as the experimentally determined t(-alpha) currents and the hysteresis of the polarization. This behavior cannot be simulated with a standard continuum model. (c) 2005 American Institute of Physics.