Charge carrier trapping states in thin evaporated and Langmuir-Blodgett organic films

Abstract

Thermally modulated space-charge-limited current (TM-SCLC) and conventional SCLC methods have been used to determine the energy parameters of the local charge trapping states for three heterocyclic organic compounds in two different structural states — as thin vacuum-evaporated layers and Langmuir-Blodgett (LB) multilayer films. The TM-SCLC method provides higher resolution and accuracy of trap depth (Et) determination with error not greater than ± 0.02 eV. In sandwich-type samples Me/organic layer/Me of metal-free phthalocyanine (α-H2Pc), five sets of hole trapping states are determined in the energy range from 0.55 to 0.07 eV; for dimethylamino-benzylidene indandione-1,3 (DMABI) samples 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 Et ≈︂ kT emerged. In the case of evaporated layers the conductivity activation energy (Ea) dependence on voltage energy (U), Ea = Ea(U), decreases in a step-like manner, every step giving the trap depth Et of the corresponding set of trapping states. In the case of LB multilayers of vanadyl phthalocyanine (VOPc) and IPB films the Ea = Ea(U) dependences are of complex, oscillatory-type character, possibly caused by a spatially non-uniform trap distribution in the bulk of the sample.