Impedance Spectroscopy to Discern Grain Boundaries in Copper Phthalocyanine Thin Films

The thin film growth of phthalocyanine molecules, which play a key role in photovoltaic and gas sensing devices, is important to electronic and magnetic properties. Impedance spectroscopy is used to study copper phthalocyanine thin films in order to disentangle the contributions of the crystal and the unavoidable grain boundaries created under different growth conditions and provide complimentary data to atomic force microscopy. The spectroscopy data is fit with an equivalent circuit model to determine resistance, capacitance, and activation energy for different grain morphologies. The Cole-Cole plots show either one or two peaks, which are attributed to the crystalline bulk and the grain boundary regions. 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 growth temperature of 450 K. The capacitance and the activation energy peak at the same temperature. The impedance spectroscopy results help determining optimal deposition conditions for electronic applications of small molecular thin films.