Dielectric characterization of semiconducting ZnPc films sandwiched between Gold or Aluminum electrodes

Abstract

The dependencies of complex dielectric functions (the dielectric constant, ε ₁, and the dielectric loss, ε ₂), on frequency and temperature of zinc phthalocyanine (ZnPc) thin films sandwiched between either gold or aluminum Ohmic-electrode contacts have been investigated in the temperature range of 93–470 K and frequency range 0.1–20 kHz. It is found that both values of ε ₁ and ε ₂ decrease with increasing frequency and increase with decreasing temperature. The rate of change depends greatly on the temperature and frequency ranges under consideration. Around room temperature, neither ε ₁ nor ε ₂ show any appreciable change through the whole range of frequencies. Thus, the dielectric dispersion is found to include of both dipolar and interfacial polarizations. The dependencies of both dielectric functions on frequency at different temperatures were found to follow a universal power law of the form ω ⁿ, where the index 0<n≤−1. This indicates that the correlated barrier hopping (CBH) model is a suitable mechanism to describe the dielectric behavior in ZnPc films. Furthermore, the results of the dielectric response indicate that polarization in these films could be in the form of non-Debye polarization. However, the Debye polarization can be traced below room temperature. The obtained results of the relaxation-time, τ, dependency on temperature have shown that a thermally-activated process may be dominated in ZnPc thin films conduction at high temperatures. Partial phase transition (from α- to β-phase) has been observed around 400 K in molecular relaxation-time, τ, and optical dielectric constant, ε _∞. Arrhenius behavior has been observed for all the dielectric loss and conductivity relaxation-times above room temperature and their activation energies are explained and reported. The optical dielectric constant ε _∞ was found to increase with temperature.