Mechanism of Charge Transport in Anisotropic Layers of a Phthalocyanine Polymer

Abstract

Solid samples of phthalocyaninato-polysiloxanes (PcPS) have been analyzed by electrical dark conductivity experiments and by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) measurements. The in-plane conductivity of anisotropic PcPS Langmuir−Blodgett assemblies on gold surface-comb electrodes reveals an ohmic dependence at low voltages followed by a space-charge-limited current regime at higher electric fields. The conductivity displays a close to Arrhenius-type temperature dependence, with activation energies of 0.33−0.36 eV. The analysis of the space-charge limited currents (SCLC) experiments yields charge carrier mobilities of approximately 10-7−10-6 cm2/V-1 s-1 at room temperature. The conductivity perpendicular to the polymer chains was investigated by performing dielectric experiments on sandwich devices. These measurements reveal conductivities which are lower by several orders of magnitude than the corresponding in-plane values but the activation energy agrees quite well with that of the ohmic currents in the in-plane conductivity experiments. This result indicates that the same processes such as the hopping of charges in an energetically disordered material are involved in the transport. The intrachain mobility, as determined by PR-TRMC, is temperature-independent with a value of ca. 2 × 10-2 cm2/Vs. The lack of any pronounced temperature dependence of the PR-TRMC mobility supports our conclusion that the electrical conductivities in the dc-surface-comb and ac-dielectric spectroscopy experiments are limited by temporary trapping or hindrance of motion of the charge carriers on particular sites during their transport over macroscopic distances.