Impedence Spectroscopy

In contrast to DC-measurements, the impedance spectroscopy (IS) uses ac-voltage to investigate the corresponding material. Therefore an ac-voltage, usually in the range from 10MHz down to mHz (?Hz), depending on the task, is applied to the electrodes. This allows investigating several processes, which take place at the same time but with different time constants

This technique can be used to investigate different kinds of materials; organic and inorganic ones (link zu Peters Beitrag).

In the case of organic materials, we can distinguish two further cases. In one case electrochemical reactions take place, for example when special polymers like polyacrylic acid are used, here we are talking about EIS (Electrochemical Impedance Spectroscopy), were usually ions are investigated, which show a long relaxation time compared to electronic processes. In Fig.1 we can see easily, that 3 (2) different processes with the corresponding relaxation times occur, and can be assigned to processes in the bulk-at high frequencies- and processes taking place at the electrodes- at low frequencies.

In the other case dielectric properties can be observed (Impedance Spectroscopy), e.g. in the case of organic semiconductors like metal phthalocyanines.

For the evaluation of the measured data, the course is simulated by an equivalent circuit; this is a combination of resistive and capacitive elements, in which every element can be assigned to a certain property of the system.

For example, the high frequency semicircle in the case of polyacrylic acid can be represented by a parallel circuit consisting of a resistance and a capacitance, where the resistance shows the charge transfer resistance and the capacitance is caused by a double layer at the electrodes.

[1] Water and ammonia influence on the conduction mechanisms in polyacrylic acid films, M. Sahm, A. Oprea, N. Barsan and U. Weimar, Sensors and Actuators, B 127, 2007, 204-209.

[2] Sahm, M., A. Oprea, et al. (2005). Empfindliche Prozesse bei dünnen Polyacrylsäure Schichten unter Ammoniak- Atmosphäre durch Impedanzspektroskopie aufgedeckt. Dresdener Sensor Symposium, Dresden, Universitätsverlag.

[3] Capacitive Humidity Sensors on Flexible RFID Labels, A.Oprea, N. Barsan, U. Weimar, M.-L. Bauersfeld, D. Ebling, J. Wöllenstein, Sensors & Actuators B, 132, 2008, 404-410.

Influence of ammonia and water sorption on the chemical and electrochemical properties of polyacrylic acid and its devices. Melanie Hörter, Dissertation, 2008