AC impedance spectroscopy on copper phthalocyanine thin films.

摘要

Impedance spectroscopy is used to study copper phthalocyanine thin films in order to disentangle the contributions of the crystal and the unavoidable grain boundaries. The spectroscopy data is fit with an equivalent circuit model to determine resistance, capacitance, and activation energy for different grain morphologies. The copper phthalocyanine thin films are deposited via thermal evaporation on platinum interdigitated electrodes on glass substrates at different temperatures from 300 to 530 K with constant thickness of 22 nm. AC measurements, implementing a precision LCR meter are taken from 20Hz - 2 MHz, and at measurement temperatures from 25 - 90 °C. Stabilizing current by subjecting samples to 5 days in the dark, the impedance spectrum can be represented by Cole-Cole plots, which show either one or two peaks. The two maxima may be attributed to the crystalline bulk and grain boundaries of the film. We observe that the grain boundary resistance component changes by three orders of magnitude when varying the grain morphology, and the capacitance changes by one order of magnitude. The resistance of the grain boundary shows a minimum near the phase transition temperature of 450 K, followed by an increase in resistance for samples deposited at higher temperatures. The capacitance on the other hand, shows a maximum near the phase transition temperature. Similarly, the activation energy for the grain boundary peaks at 1.29 +/- 0.12 eV at the same phase-transition tempemture, whereas the crystalline bulk component has a constant activation energy of 0.36 +/- 0.08 eV for all sample of different grain sizes. Additional data taken using a perpendicular configuration for a 30 nm thick cobalt phthalocyanine thin film shows a double peak. The low temperature measurements for these samples are interpreted to have two contributions from micro-shorts and crystalline bulk.