Charge transport in ultrathin Langmuir-Blodgett film devices: HOMO-mediated tunneling?

Symmetrically contacted sandwich junctions incorporating very thin Langmuir-Blodgett films of perylenetetracarboxyldiimide and octasubstituted phthalocyanine derivatives have been prepared. At 4.2 K current/voltage curves display abrupt current increases at reproducible voltage thresholds. To estimate the ionization potential of these π-systems, cyclic voltammetry studies were conducted on the molecules in a nonpolar solvent. The ionization potential of the phthalocyanine compound is directly accessible through its oxidation potential. In the case of the perylenetetracarboxyldiimide derivatives, the ionization potential is approximated by using the electron affinity (obtained from the reduction potential) and the ‘HOMO-LUMO’ gap from absorption spectra. The obtained values are close to the measured voltage thresholds and emphasize the picture of HOMO-mediated tunneling processes.