ABSTRACT
In electronic devices based on organic materials, and for light-emitting diodes (LEDs) in particular, the characteristics of carrier injection from a metal contact to a polymer layer is a crucial issue in determining performance. The details of energy level alignment at interfaces between organic layers and metal electrodes, a fundamental issue for designing better devices, is still the subject of controversy [1-4]. Most detailed spectroscopic work in this area has been performed on the interfaces prepared under the ultrahighvacuum conditions (UHV), by means of the vapor deposition of an organic overlayer onto ultraclean metallic substrates, thereby minimizing potential contamination. As has been shown for these "ideal" contacts, the systems studied are not necessarily in the Schottky-Mott limit [5], and alignment of the vacuum levels of the organic film on the metal substrate does not occur in most cases. The reported results of recent studies using photoelectron spectroscopy [1,2,4,5] emphasize the importance of the influence of several factors that may lead to the formation of interfacial dipole layers at inter-
faces. These factors include charge transfer through the interface, interfacial electronic states, chemical interactions, and image forces, among others.
