Interfacial Electrostatic Phenomena in Phthalocyanine Langmuir-Blodgett Films under Photoillumination

Surface potentials of Cu tetra-tert-butyl-phthalocyanine (CuttbPc) Langmuir-Blodgett (LB) films on metal electrodes were measured under photoillumination. The surface potential of CuttbPc LB films on Au electrode, positively charged as deposited, decreases by photoillumination, whereas the potential of films on Al and Cr electrodes, negatively charged as deposited, increases. A linear relationship was obtained between the work function of metal electrodes and the saturated surface potential of CuttbPc LB films in the dark, and a similar relationship was also suggested under photoillumination. These results indicate that electronic charges are exchanged at the interface between metal electrodes and the films until the surface Fermi level of films and the Fermi level of metals mutually coincide. The space charge distribution in CuttbPc LB films was determined on the order of nanometer film thickness, and the distribution of the electronic density of states was also determined assuming the presence of both electron donor and acceptor states at the film/metal interface. It was concluded that about 1% of CuttbPc molecules contribute the charge exchange and 9–27% of these contributed charges lose the charge exchange by photoillumination. Finally, the surface potential decay process was analyzed assuming thermodynamics quasi-equilibrium at the interface.