Electronic structure of C₆₀/phthalocyanine/ITO interfaces studied using soft X-ray spectroscopies

The interface electronic structure of a bilayer heterojunction of C ₆₀ and three different phthalocyanines grown on indium tin oxide (ITO) has been studied using synchrotron radiation-excited photoelectron spectroscopy. The energy difference between the highest occupied molecular orbital level of the phthalocyanine (donor) layer and the lowest unoccupied molecular orbital level of the C₆₀ (acceptor) layer (E _HOMO^D - E_LUMO^A) was determined. The E_HOMO^D - E_LUMO^A of a heterojunction with boron subphthalocyanine chloride (SubPc) was found to be much larger than those of copper phthalocyanine (CuPc) and chloro-aluminum phthalocyanine (ClAlPc). This observation is discussed in terms of the difference of the ionization energy of each donor material. Additionally, we have studied the molecular orientation of the phthalocyanine films on ITO using angle-dependent X-ray absorption spectroscopy. We found that the SubPc films showed significant disorder compared to the CuPc and ClAlPc films and also found that E _HOMO^D - E_LUMO^A varied with the orientation of the ClAlPc molecules relative to the ITO substrate. This orientation could be controlled by varying the ClAlPc deposition rate.