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.

Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current conductivities and field-effect mobilities

“…19 The results of these measurements are summarized in Table I. Similar studies of dc conductivity on IMEs have been reported by Neher and coworkers for rod-like silicon phthalocyanine polymers, [12][13][14] and in our own preliminary work for Pc 1. 20 Relatively thick (28 bilayers) multilayer thin films of Pcs 1 and 1' and 28 monolayer films of Pc 2 were deposited onto two IMEs oriented orthogonal to each other during the horizontal deposition process, producing comparable Pc thin films from the same Langmuir layer with parallel and perpendicular orientations of the Pc column rod axes with respect to the legs of the microcircuit (inset of Fig.…”

“…Many liquid crystalline discotic mesophase materials self-organize from common solvents into coherent columnar, rod-like aggregates, and have become of interest as potential solution-processed molecular electronic materials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Side-chain modified phthalocyanines, porphyrins, hexabenzocoronenes, and triphenylenes have been created that show reasonable liquid crystalline (LC) transition temperatures (K → D h ) and can be readily processed into thin films. The side chains surrounding the discotic molecular core help control self-organization into these rod-like columns, and because of their closepacking and electrically insulating nature, their presence may eventually provide for dense lateral integration of organic field-effect transistor (OFET) devices, preventing significant electrical cross-talk between adjacent columns.…”

“…These mobilities are also critically dependent on the coherence of the aggregate, and the length scale over which the electrical properties are measured. [1][2][3][4][5][12][13][14][17][18][19][20] As will be discussed in this report, there are significant challenges yet to be overcome in optimizing the electrical properties of discotic mesophase materials for OFET applications. First, the charge mobilities in these discotic materials are strongly affected by misalignments, bifurcations, and breaks in the columnar aggregates (reminiscent of grain boundaries in crystalline materials), [25][26][27][28][29] which constitute a high-resistance pathway for charge migration.…”

Selective Deposition of Rod-like Phthalocyanine Aggregates on Au Surfaces Patterned with a Combination of Microcontact Printing and Electropolymerization

“…These results suggest that comparable combinations of hydrophobic and hydrophilic channel walls and upper and lower surfaces can be used to create useful patterns of a wider range of hydrophobic monomeric and polymeric materials, and that the lCP-EP approach can be extrapolated to submicrometer dimensions. (2,3,9,10,16,17,23,24-Oktakis((2-benzyloxy)-ethoxy)phthalocyaninato) copper (1) was synthesized as reported previously [51]. Solutions of 1 were prepared using high-pressure liquid chromatography (HPLC) grade chloroform 99.9 % (Aldrich).…”

“…[15±26] We have recently shown that Pcs with benzyl-terminated ethylene oxide chains (i.e., (2,3,9,10,16,17,23,24-oktakis((2-benzyloxy)ethoxy)phthalocyaninato) copper, 1 (Fig. 1a)) adopt cofacial aggregate geometries in dilute solution.…”