Electronic structures of 8-hydroquinolatolithium on Au substrate: The organic electron injection layer having semiconducting properties

We studied the electronic structures of the 8-hydroquinolatolithium (Liq)/Au interface using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and theoretical calculation. It is revealed that Liq is an organic semiconducting material with electron and hole injection barriers of 1.28 and 1.32 eV in contact with Au, respectively. The theoretical calculations showed that the highest (lowest) occupied (unoccupied) molecular orbital [HOMO (LUMO)] of Liq mainly derive from the quinolate part of the molecule. The HOMO and LUMO levels are highly delocalized throughout the quinolate part, giving it good conducting properties unlike other insulating electron injection layers. Furthermore, the electronic structures of Liq are nearly identical to tris-(8-hydroquinoline) aluminum (Alq3), so that the energy level mismatching between the two molecules could be minimal. This explains why the Alq3 based device in combination with Liq shows good performance.