Photoemission study of energy alignment at the metal/Alq(3) interfaces

We have investigated the interface formation and the energy alignment betwe en different metals and Alq(3) using X-ray and ultraviolet photoelectron sp ectroscopy (XPS and UPS) simultaneously. While, the UPS results show that t he vacuum level changes during interface formation, implying the possibilit y of surface dipole layer, our XPS results show more detailed and complex b ehavior. At the low work function, metal/Alq(3) interfaces a gap state is o bserved in UPS. At the same time, a new peak can be observed in the N Is XP S core level at a lower binding energy. These results can be well character ized as charge transfer from the low work function metal to Alq(3). Shiftin g of core levels is also observed, which may be due to charge diffusion or doping from metal atoms. These interfaces are drastically different from th e Al/Alq(3) interface, which has very poor electron injection. At the Al/Al q(3) interface, there is a destructive chemical reaction and much smaller c ore level shifts is observed. (C) 2001 Elsevier Science B.V. All rights res erved.