Formation of a Ca/LiF/Alq₃ cathode for an organic light-emitting diode: Evolution of the electronic structure as studied by photoemission spectroscopy

We have studied the interface electronic structures of Ca/LiF/ins-(8- hydorxyquinoline)Al (Alq₃) using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). While the UPS revealed that the deposition of Ca on LiF/Alq₃ created weak gap states between the Fermi level and the highest occupied molecular orbital (HOMO) peak of Alq₃, the N 1s core level peak measured with XPS exhibited a very large satellite peak with Ca deposition, eventually dominating the original N 1s peak at about 2.0 nm of Ca. This implies that the origin of the valence band gap states and the core level satellite peak might be different, which is in contrast to what has been observed in many other similar systems. Although it is clear that the presence of LiF plays an important role in charge transfer, we could not find any definitive evidence that the deposition of Ca liberated Li atoms in LiF and reacted directly with Alq₃.