Impact of structural imperfections on the energy-level alignment in organic films

Abstract

This paper reports that structural imperfection in an organic thin film modulates the electronic structure to result in a serious band bending and change in the energy-level alignment (ELA) at the organic-conductor interface. Ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) were adopted to investigate thickness dependences of the electronic structure of polar phthalocyanine (chlorogallium phthalocyanine) thin films grown on graphite with respect to the film structure. We observed a large band-bending--like shift of occupied molecular-orbital bands toward the Fermi level and a continuous increase in the vacuum level for the as-grown film, whereas these phenomena were considerably suppressed by annealing the film. Both the as-grown and annealed films were characterized as essentially the same stacked bilayer film structure; however, high-resolution UPS and MAES measurements evidenced that there are structural defects in the as-grown film but not clearly in the annealed film, indicating that the defects are the origin of the modulation of the ELA and the band bending. Controlling the structural imperfections is a key issue for the desired ELA in organic devices.