Chemistry and electronic properties of metal contacts on an organic molecular semiconductor

This paper gives a brief review of the electronic properties of the surface of an organic molecular semiconductor, i.e. PTCDA, and of the chemical and electrical properties of its contacts with to metals, i.e. Ti and Au. The combination of direct and inverse photoemission spectroscopies demonstrates that correlation effects are small in PTCDA. Synchrotron radiation photoemission spectroscopy shows that the Ti/PTCDA interface is reacted and that chemistry-induced electronic states in the PTCDA band gap lead to an Ohmic contact. The Au/PTCDA interface, on the other hand, is unreacted and abrupt and the Au/PTCDA contact is rectifyiing. This duality, which applies also to other metal/PTCDA interfaces, suggests the overwhelming importance of chemistry in defining electrical properties of contacts to organic molecular thin films.