Investigation of Local Trapping States in Organic Molecular Crystals by Method of Thermally Modulated Space-Charge Limited Current |
| I. Muzikante and Ε.A. Silinsh Institute of Physical Energetics, Latvian Ac. Sci., 21 Aizkraukles Str., 1006 Riga, Latvia |
| Received: March 29, 1995; in final form: May 15, 1995 |
| Dedicated to Professors Krzyszof Pigoń, Józef W. Rohleder and Zdzisław Ruziewicz on the occasion of their 70th birthdays |
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| 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 Et = 0.35 to 0.06 eV were observed. In evaporated thin layers of dimethylaminobenzylidene indandione-1,3 five sets of trapping states from Εt = 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 Ea dependence on applied voltage U, Ea(U), for bulk conductivity is not step-like as in case of evaporated layers, but of rather complicated, oscillatory shape. Such character of the Ea(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 Ea(U) dependence is step-like and two sets of trapping states at Et = 0.45 ± 0.03 eV and Et = 0.18 ± 0.03 eV were obtained. In case of bulk conductivity of the Langmuir-Blodgett layers the activation energy Ea at sufficiently high electric fields drops down to zero and the charge carrier transport switches to a "trap-free" regime. |
| DOI: 10.12693/APhysPolA.88.389 PACS numbers: 72.20.Jv, 72.80.Le, 73.50.Gr, 73.61.Ph, 73.40.Rw |