Evidence of gap state formed by the charge transfer in Alq3∕NaCl∕Al 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 Alq3∕Al was shifted when the ultrathin 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 interac...