Photoemission study of energy alignment at the metal/Alq3 interfaces

We have investigated the interface formation and the energy alignment between different metals and Alq3 using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) simultaneously. While, the UPS results show that the vacuum level changes during interface formation, implying the possibility of surface dipole layer, our XPS results show more detailed and complex behavior. At the low work function, metal/Alq3 interfaces a gap state is observed in UPS. At the same time, a new peak can be observed in the N 1s XPS core level at a lower binding energy. These results can be well characterized as charge transfer from the low work function metal to Alq3. Shifting of core levels is also observed, which may be due to charge diffusion or doping from metal atoms. These interfaces are drastically different from the Al/Alq3 interface, which has very poor electron injection. At the Al/Alq3 interface, there is a destructive chemical reaction and much smaller core level shifts is observed.