Studies of the electrical and structural properties of organic semiconducting thin films of thermally evaporated cobalt phthalocyanine

Structural and electrical measurements have been made on organic semiconducting thin films of cobalt phthalocyanine (CoPc). Thermally evaporated CoPc thin films deposited onto substrates maintained at room temperature are primarily of the α-form and show a single dominant Bragg peak, indicating a strong preferential orientation in either [001] or [200] directions, depending on whether the structure is tetragonal or orthorhombic. Much of the material undergoes a phase transition to the stable monoclinic β-form after annealing at approximately 300 deg. C, and shows a preferential orientation in the [001] direction. Optical absorption spectra in the visible and ultra-violet regions showed well-defines differences between the α- and β-phases. Phase transformation has also been observed in both purified and unpurified CoPc films by means of electrical measurements. The results were compared with those obtained from various other metal phthalocyanines. Evaporation of unpurified CoPc films of the same thickness under the influence of different magnetic fields showed deviations in X-ray intensity maxima, with increased reflection intensities observed at higher Bragg angles for a field of 0.03 T. Ohmic and space-charge-limited currents controlled by exponential trap distribution have been observed respectively in low and high applied voltages when gold electrodes are used. The permittivity, hole mobility, thermally generated hole concentration, detailed parameters of trap distribution, including the temperature parameter and the total trap concentration, have been calculated. Ac dark current measurements revealed that the ac conductivity σ(ω) varied as ωs where ω is the angular frequency and s an index less than unity. Ac activation energy measurements were interpreted in terms of hopping through a band of localised states at lower temperatures and by free-band conduction at higher temperatures. Measurements of capacitance and loss tangent showed a well-defined decrease with increasing frequency and an increase with increasing temperature. A comparison of samples both before and after heat treatment showed a decrease in both dc and ac conductivities, capacitance and loss tangent. (author)