Influence of gap states on electrical properties at interface between bathocuproine and various types of metals

A systematic study on the energy level alignment, chemical interaction, and electron doping at interfaces between bathocuproine (BCP) and various types of metals (Au, Cu, Ag, Mg, and Ca) was carried out by performing ultraviolet photoelectron spectroscopy and electronic conductivity measurements. The energy level alignment at BCP/metal interfaces was found to depend on the metal work function (Phi(m)). For BCP on Au and Cu, whose Phi(m) exceeds 4.3 eV, the energy shift in the highest occupied molecular orbital (HOMO) level with respect to the metal Fermi level (E-F) almost accords with the variation in Phi(m). For BCP on Ag, Mg, and Ca, whose Phi(m) is below 4.3 eV, the HOMO energy level is fixed at 3.7 eV with respect to E-F regardless of Phi(m) and new electronic states, called gap states, appeared at BCP/metal interfaces. Since the appearance of gap states is correlated with the energy of the lowest unoccupied molecular orbital (LUMO) level with respect to E-F, these states appear to have formed mainly through the interaction with the LUMO. A clear correlation between the density of the gap states and the vacuum level shift suggesting a charge redistribution at BCP/metal interfaces was found. The energy shift in the gap states, which may originate from the variation in the electron occupation of the states, directly affected the electronic conductivity of metal-doped BCP layers (doping metal=Au, Ag, and Ca). These results suggest the electron transfer from the metal E-F to gap states plays an influential role in the electrical properties at BCP/metal interfaces.