ELECTRODE MATERIAL EFFECT ON CAPACITANCE AND LOSS TANGENT OF THE ORGANIC ZnPc SEMICONDUCTING THIN FILMS

Abstract

The capacitance and loss tangent of thermally evaporated zinc phthalocyanine, ZnPc , semiconducting thin films were measured in the temperature range of 180–430 K and frequency between 0.1 and 20 kHz. Aluminum and gold electrical contact electrodes were employed to sandwich ZnPc films. For both electrode types, the capacitance and loss tangent showed strong dependence on both temperature and frequency. Such dependence is related to the relevant temperature and frequency range under consideration. The capacitance has strong temperature dependence for T>240 K and frequency below 3 kHz, while it becomes a constant at higher frequencies and all temperatures. The loss tangent dependence on temperature is more evident at low frequencies and a minimum or an indication of a minimum was observed in tan δ versus f curves. Loss tangent variation with temperature was not monotonic for all frequencies. An anomaly (maximum) in tan δ was observed approximately between 300 and 360 K. This maximum was attributed to the presence of oxygen molecules in the sample and their subsequent exhaustion as the temperature is increased. The behavior of capacitance and loss tangent (for both Al and Au -electrodes) may be explained qualitatively and successfully in terms of an equivalent circuit model.