Molecular level alignment at organic semiconductor-metal interfaces

In order to clarify the electronic structure of metal-molecular semiconduct or contacts, we use photoemission spectroscopy to investigate the energetic s of interfaces formed by vacuum deposition of four different molecular thi n films on various metals. We find that the interface electron and hole bar riers are not simply defined by the difference between the work functions o f the metals and organic solids. The range of interface Fermi level positio ns is material dependent and dipole barriers are present at all these inter faces. The results demonstrate the breakdown of the vacuum level alignment rule at interfaces between these organic molecular solids and metals. (C) 1 998 American Institute of Physics. [S0003-6951(98)02831-9].