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Investigation of Local Trapping States in Organic Molecular Crystals by Method of Thermally Modulated Space-Charge Limited Current

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Muzikante I., Silinsh Ε. A.

Acta Physica Polonica A

1995

vol. 88

issue 2

389-399

The method of thermally modulated space-charge limited current has been used for studying local charge carrier trapping state energy spectra in vacuum evaporated thin films and Langmuir-Blodgett multilayers of different organic molecular crystals. In case of evaporated thin polycrystalline layers of pentacene four sets of local trapping states in energy range from E_{t} = 0.35 to 0.06 eV were observed. In evaporated thin layers of dimethylaminobenzylidene indandione-1,3 five sets of trapping states from Ε_{t} = 0.59 to 0.28 eV were detected. In the evaporated layers of indandione-1,3 pyridinium betaine only a single set of very shallow trapping states close to the conductivity level was observed. In the Langmuir-Blodgett multilayers of vanadyl phthalocyanine and indandione-1,3 pyridinium betaine the activation energy E_{a} dependence on applied voltage U, E_{a}(U), for bulk conductivity is not step-like as in case of evaporated layers, but of rather complicated, oscillatory shape. Such character of the E_{a}(U) dependence may be explained by the specific quantum well structure of the Langmuir-Biodgett multilayers and spatially nonuniform trap distribution. However, for in-plane conductivity of indandione-1,3 pyridinium betaine films the E_{a}(U) dependence is step-like and two sets of trapping states at E_{t} = 0.45 ± 0.03 eV and E_{t} = 0.18 ± 0.03 eV were obtained. In case of bulk conductivity of the Langmuir-Blodgett layers the activation energy E_{a} at sufficiently high electric fields drops down to zero and the charge carrier transport switches to a "trap-free" regime.

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1 Acta Physica Polonica A

1 1995

1 Muzikante I.

1 Silinsh Ε. A.

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Investigation of Local Trapping States in Organic Molecular Crystals by Method of Thermally Modulated Space-Charge Limited Current