Structural and transport properties of evaporated iron phthalocyanine (FePc) thin films

Abstract

The structural and transport properties of thin FePc films of various thickness deposited onto glass substrates have been studied at several temperatures. The structural studies show that the films belong to monoclinic system of β-phase. This structure is confirmed by infrared absorption analysis. The dark electrical resistivity was found to decrease with increasing the film thickness. Graphical representation of $\log \rho$ as a function of reciprocal temperature yields two distinct linear parts indicating in turn the existence of two activation energies $\Delta E_1$ and $\Delta E_2$. Measurement of the thermoelectric power showed that FePc thin films behave as p-type semiconductor over the temperature range $300{-}450$ K. Analysis of thermoelectric power connected with the resistivity results reveals some essential parameters such as: hole mobility $\mu_{\rm h} \approx 2 \times 10^{-6}$ m2 V-1 s-1, hole concentration $p \approx 5\times 10^{18}$ m-3 and the ratio $c = \mu_{\rm e}/\mu_{\rm h} \approx 0.25$. Capacitance-voltage data confirm that the Au/FePc interface does not form a Schottky barrier and measurements of the dependence of capacitance on film thickness indicate that the relative permittivity of the films is approximately 3.7. Room temperature current density-voltage characteristics showed ohmic conduction in the lower voltage range and space-charge-limited – conductivity (SCLC) in the relatively high voltage. The SCLC controlled by an exponential distribution of traps above the valence band edge. The temperature dependence of current density in accordance with the theory for the exponential trap distributions yielded some essential parameters such as: the hole mobility, the relative permittivity, the trap concentration, the characteristic temperature and the trap density.