Characterization of metallophthalocyanine-metal contacts: Electrical properties in a large frequency range

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Abstract
The dark electrical properties of metallophthalocyanine‐metal contacts are studied in an extended frequency range (10−3–105 Hz). The measurements show that the standard Schottky model cannot be applied to metallo‐organic semiconductors. Three different contributions to the admittance may be distinguished. Over the very first few angstroms of the semiconductor extends a surface‐charge layer associated with a high capacitive term. The surface‐charge capacitance is only slightly dependent upon superimposed dc voltages. The so‐called space‐charge region extends below the surface‐charge layer over approximately 2000–4000 Å. The properties of this space‐charge region are, however, dramatically different from those found in monocrystalline inorganic semiconductors. The corresponding capacity is almost independent of any dc superimposed voltage. At the same time, the resistance of the space‐charge region is highly dependent on externally applied dc voltages. The I‐V relationship seems to indicate a Frenkel–Poole mechanism of charge detrapping. Finally, the rest of the metallo‐organic layer shows the properties of the bulk material. Due care must be taken when interpreting usual capacitance‐voltage relationships (1/C2‐vs‐V plots). Apparent variations of the overall capacitance do not correspond to properties of the space‐charge region. A close examination of the measurements over an extended range of frequencies is necessary to correctly derive the electrical properties of the metallo‐organic semiconductor.