AVS 46th International Symposium
    Organic Electronic Materials Topical Conference Tuesday Sessions
       Session OE+EM+AS-TuM

Paper OE+EM+AS-TuM6
Examination of Band Bending at Organic Semiconductor / Metal Interfaces Studied by Kelvin Probe Method

Tuesday, October 26, 1999, 10:00 am, Room 616/617

Session: Interfaces and Characterization of Organic Thin Films
Presenter: H. Ishii, Nagoya University, Japan
Authors: H. Ishii, Nagoya University, Japan
N. Hayashi, Nagoya University, Japan
E. Ito, Nagoya University, Japan
K. Seki, Nagoya University, Japan
Correspondent: Click to Email
How the energy levels of an organic semiconductor and a metal electrode align at the interface between them is a critical and basic issue for understanding organic electronic devices. So far, Mott-Schottky (MS) model has been applied for the estimation of the electronic structure at organic/metal interfaces. In the model, vacuum level alignment is assumed right at the interface, while the Fermi level alignment is achieved through band bending in space charge layer. In order to examine the validity of this model, the film-thickness dependence of the location of the vacuum level of TPD (N,N'-bis(3-methylpheyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine) on various metals(Au, Cu, Ag, Mg and Ca) were measured by Kelvin probe method in ultrahigh vacuum (UHV). At all the interfaces, an abrupt shift of the vacuum level was observed within 1 nm thickness, while further deposition of TPD up to 100 nm led to no change of the location of the vacuum level. These results indicate that the possible space charge layer is much thicker than 100 nm in UHV. The observed relation between the vacuum level shift and the work function of the metals suggests that (1) the Fermi level of TPD is located around the midgap in UHV condition and that (2) the Fermi level alignment is achieved only at TPD/Cu,Ag,Mg interfaces, not by band bending but by dipole layer formation right at the interface. These findings clearly demonstrate the invalidity of MS model for organic / metal interfaces in UHV. The results for the interface between Alq@sub 3@ (tris(8-hydroxyquinolino)aluminum) and metal will be also presented.