CHARGE-CARRIER TRAPPING STATES IN THIN EVAPORATED AND LANGMUIR-BLODGETT ORGANIC FILMS

Thermally modulated space-charge-limited current (TM-SCLC) and convent ional SCLC methods have been used to determine the energy parameters o f the local charge trapping states for three heterocyclic organic comp ounds in two different structural states - as thin vacuum-evaporated l ayers and Langmuir-Blodgett (LB) multilayer films. The TM-SCLC method provides higher resolution and accuracy of trap depth (E(t)) determina tion with error not greater than +/- 0.02 eV. In sandwich-type samples Me/organic layer/Me of metal-free phthalocyanine (a-H(2)Pc), five set s of hole trapping states are determined in the energy range from 0.55 to 0.07 eV; for dimethylamino-benzylidene indandione-1,3 (DMABI) samp les also, five sets of trapping states are detected in the 0.5-0.28 eV range; however, for indandione pyridinium betaine-1,3 (IPB) samples, only a very shallow set of traps at E(t) approximate to kT emerged. In the case of evaporated layers the conductivity activation energy (E,) dependence on voltage energy (U), E(a) = E(a)(U), decreases in a step -like manner, every step giving the trap depth E(t) of the correspondi ng set of trapping states. In the case of LB multilayers of vanadyl ph thalocyanine (VOPc) and IPB films the E(a) = E(a)(U) dependences are o f complex, oscillatory-type character, possibly caused by a spatially non-uniform trap distribution in the bulk of the sample.