Electrical impedance spectroscopy study of unsubstituted palladium (II) phthalocyanine

A pressed pellet of unsubstituted palladium (II) phthalocyanine (PdPc) was synthesized and characterized by UV-Vis transmittance and impedance spectroscopy. Optical absorbance analysis revealed that upon increasing the concentration, the absorption at 630 nm became more intense, demonstrating strong aggregation at high concentrations. For the synthesized PdPc pellet, the absorbance decreased beyond 800 nm, and the direct band gap energy was 1.95 eV. The electric properties of the sample were studied for a range of frequency (40-107 Hz) and temperature (300-660 K) using complex impedance analysis. The activation energy and the relaxation time values for both time and frequency ranges were close to each other, suggesting that the electrical conduction and relaxation phenomena were controlled by the same type of charge carrier in the sample. Notably, the sample exhibited a semiconductor-metal transition at temperature TSM = 560 K. The AC-conductance was satisfactorily explained using the Jonscher universal power law. The electrical transport mechanism was dominated by the overlapping large polaron tunneling (OLPT) model. Furthermore, impedance spectroscopy analysis revealed a non-Debye-type relaxation. The Nyquist plot indicates the presence of grains and grain boundary effects. The results demonstrate that this material may be a promising candidate for photovoltaic applications.