AC conductivity and dielectric measurements of bulk magnesium phthalocyanine (MgPc)

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Abstract

The AC conductivity, σac(ω) for bulk magnesium phthalocyanine (MgPc) in the form of compressed pellet in the frequency range of 1–500 kHz and in a temperature range of 303–443 K with evaporated ohmic Au electrodes have been investigated. The frequency dependence of the impedance spectra plotted in the complex plane shows semicircles. The Cole–Cole diagrams have been used to determine the DC conductivity. The AC conductivity, σac(ω), is found to vary as ωs in the frequency range of 1–500 kHz. At high range of frequency, s < 1 and it decreases with increasing the temperature. The variation of s with temperature suggests that the AC conduction is due to the correlated barrier hopping (CBH). The dielectric constant, ɛ′, and dielectric loss, ɛ″, for bulk MgPc were decreased with increasing frequency and increased with increasing temperature.

Introduction

Organic semiconductors are of steadily growing interest as active components in electronics and optoelectronics. Due to their flexibility, low cost and ease of production they represent a valid alternative to conventional inorganic semiconductor technology in a number of applications, such as flat panel displays and illumination, plastic integrated circuits and solar energy conversion although first commercial applications of this technology are being realized nowadays, there still the need for a deeper scientific understanding in order to achieve optimum device performance [1].

Phthalocyanines represent a large family of heterocyclic conjugated molecules with high chemical stability. The study of these compounds is very essential to understand the behaviour of their electronic physical properties under various conditions: such as changes in temperature, pressure, frequency, ambient gases, etc. [2]. The electronic properties of metal phthalocyanine compounds (MPc's) have received increasing attention because of their importance as prototype semiconductors and their uses in technological applications such as generate various types of switching devices [3]. MPc's although there is evidence that they may exhibit suitable properties for numerous applications [4].

Dielectric relaxation studies are important to understand the nature and the origin of dielectric losses, which, in turn, may be useful in the determination of structure and defects in solids. The magnitude of geometric and measured capacitance may differ if the electric field at the metal insulator interface varies with the insulator over the region. AC conductivity and dielectric measurements of metal phthalocyanine were the subject of several investigations in the last decades [5], [6], [7], [8], [9], [10], [11]. The data were analyzed within the frame of the ‘universal’ power law σ  ωs.

In the present work, AC conductivity, dielectric constant and dielectric loss measurements were performed for MgPc in bulk form. The temperature and frequency dependence of the electrical conductivity, the dielectric constants for MgPc were investigated and the results were analyzed to determine some related parameters and to suggest predict the operating electronic conduction mechanisms.

Section snippets

Experimental techniques

The powder of magnesium phthalocyanine (MgPc) was obtained from Kodak Company, UK. It was thoroughly grounded in a mortar to obtain very fine particles, and then it was compressed under a pressure of ∼1.96 × 108 N/m2 in the form of a pellet 1.1 × 10−2 m diameter and thickness of 0.71 × 10−3 m. MgPc pellet was sandwiched between two evaporated Au film electrodes to be ohmic contacts to the sample [12]. The electrical contacts were equipped with copper wires, which applied to the metal electrodes of the

Results and discussion

Analysis of dielectric properties as functions of frequency is generally known as impedance spectroscopy, and dielectric relaxation spectroscopy or, more appropriately, AC spectroscopy.

Certain dielectric quantities or functions are employed more often than others depending on the particular field of application. The complex dielectric constant ɛ* complex impedance Z*, and complex conductivity σ* which are used to calculate the corresponding time constants for the relaxation models of Cole–Cole

Conclusion

AC conductivity σac(ω)of bulk MgPc, with evaporated ohmic Au electrodes was measured as a function of frequency and temperature. The Cole–Cole diagrams were used to determine the DC conductivity which was explained according to the VRH mechanism. The AC conductivity σac(ω) was found to vary with ωs in the frequency range of 1–500 kHz. At high range of frequencies the frequency exponent, s, was ∼ equal to unity and decreased with increasing temperature indicating that conduction is due to the

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