Evidence of gap state formed by the charge transfer in Alq ₃/NaCI/AI interface studied by ultraviolet and x-ray photoelectron spectroscopy

Electronic structures of Alq₃/NaCl/Al and Alq₃/Al were studied by UV and x-ray photoelectron spectroscopy (XPS). The initial energy level of the highest occupied molecular orbital (HOMO) of Alq ₃/Al was shifted when the ultiathin NaCl layer was inserted between them, although the vacuum level was not changed. The measured interface dipole was 1.1 eV, identical for both Alq₃/NaCl/Al and Alq₃/Al. Our experiment shows that the dipole is formed in very short range (less than 0.1 nm) from the interface. The onset of the HOMO level of Alq₃ was shifted 0.2 eV toward high binding energy for the additional NaCl layer, which lowered the barrier height and improved injection characteristics of the device. Moreover, a gap state was observed at 1.1 eV below the Fermi level when the NaCl was inserted between Alq₃ and Al. The XPS core-level spectra revealed that the interaction on Alq₃ and NaCl was very weak, which generated an unusual gap state without breaking or forming chemical bonds. We suggest that the weak interaction would originate from the charge transfer from Alq₃ to NaCl.