Space charges in solid electrolytes detected by the scanning Kelvin probe

The scanning Kelvin probe is used for the contactless determination of the surface potential of the ion-conducting polymer polyethylene oxide (PEO). The potential distribution in the samples after application of an external voltage between two metal electrodes is strongly non-linear in the steady state. This is attributed to a space charge polarization which prevails in the PEO. Furthermore, the time dependent evolution of the surface potential at several fixed positions after a voltage step is measured. Here, the behavior is influenced by the movement of the mobile internal charges and also by injected charges from the electrodes. A comparison with simulations performed with a three-dimensional hopping model shows qualitatively the same results and yields negative mobile charges in the system. Additionally, it is observed that the dc current through the samples is independent of the sample length. This is caused by a depletion zone of mobile charges near the negative electrode which determines the resistance of the samples.