Energy-level alignment at metal-organic and organic-organic interfaces

This article reports on the electronic structure at interfaces found in organic semiconductor devices. The studied organic materials are C-60 and poly (para-phenylenevinylene) (PPV)-like oligomers, and the metals are polycrystalline Au and Ag. To measure the energy levels at these interfaces, ultraviolet photoelectron spectroscopy has been used. It is shown how the energy levels at interfaces deviate from the bulk. Furthermore, it is demonstrated that the vacuum levels do not align at the studied interfaces. The misalignment is caused by an electric field at the interface. Several effects are presented that influence the energy alignment at interfaces, such as screening effects, dipole layer formation, charge transfer, and chemical interaction. The combination of interfaces investigated here is similar to interfaces found in polymer light-emitting diodes and organic bulk heterojunction photovoltaic devices. The result, the misalignment of the vacuum levels, is expected to influence charge-transfer processes across these interfaces, possibly affecting the electrical characteristics of organic semiconductor devices that contain similar interfaces. (C) 2003 Wiley Periodicals, Inc.