Interfacial electronic phenomena in phthalocyanine Langmuir-Blodgett films under photo-illumination

Surface potentials of Cu-tetra-tert-butyl-phthalo-cyanine (CuttbPc) Langmuir-Blodgett (LB) films on metal electrodes were measured under photo-illumination in a vacuum vessel. The surface potential of positively charged CuttbPc LB films on Au electrode decreases by photo-illumination, whereas the potential of negatively charged films on Au and Cr electrodes increases. A linear relationship between the work function of metal electrodes and the saturated surface potential of films was obtained in dark and under photo-illumination. These results indicate that electronic charges are exchanged at the film/metal interface until the surface Fermi level of films and the Fermi-level of metals coincide with each other. The space charge distribution in CuttbPc LB films and the distribution of the electronic density of states was also determined assuming the presence of both electron donor and acceptor states at the interface. Finally, the surface potential decay process was analyzed assuming quasi-equilibrium thermodynamics at the interface.