INVESTIGATION OF LOCAL TRAPPING STATES IN ORGANIC MOLECULAR-CRYSTALS BY METHOD OF THERMALLY MODULATED SPACE-CHARGE LIMITED CURRENT

The method of thermally modulated space-charge limited current has bee n 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 polyc rystalline layers of pentacene four sets of local trapping states in e nergy range from E(t) 0.35 to 0.06 eV were observed. In evaporated thi n layers of dimethylaminobenzylidene indandione-1,3 five sets of trapp ing states from E(t) = 0.59 to 0.28 eV were detected. In the evaporate d layers of indandione-1,3 pyridinium betaine only a single set of ver y shallow trapping states close to the conductivity level was observed . In the Langmuir-Blodgett multilayers of vanadyl phthalocyanine and i ndandione-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 oscillator y shape. Such character of the E(a)(U) dependence may be explained by the specific quantum well structure of the Langmuir-Blodgett multilaye rs and spatially non-uniform trap distribution. However, for in-plane conductivity of indandione-1,3 pyridinium betaine films the E(a)(U) de pendence 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 co nductivity 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.