AVS 49th International Symposium
    Organic Films and Devices Wednesday Sessions
       Session OF+EL+SS+SC-WeM

Invited Paper OF+EL+SS+SC-WeM1
Interfaces between Metals and Conjugated Organic Materials: From Physisorption to Covalent Bonding

Wednesday, November 6, 2002, 8:20 am, Room C-102

Session: Metal-Organic Interfaces
Presenter: N. Koch, Princeton University
Authors: N. Koch, Princeton University
J. Ghijsen, Facultes Universitaires Notre-Dame de la Paix, Belgium
A. Rajagopal, Rutgers University
C. Chan, Princeton University
J.J. Pireaux, Facultes Universitaires Notre-Dame de la Paix, Belgium
J. Schwartz, Princeton University
A. Kahn, Princeton University
Correspondent: Click to Email
The electronic properties of interfaces formed between conjugated organic materials (polymers and small molecules) and other organic and inorganic materials are of paramount importance in terms of the performance of organic-based devices (e.g., light emitting diodes, thin film transistors). The alignment of energy levels at such interfaces is a direct consequence of the physical and chemical interactions between the materials. Using mainly photoemission spectroscopy (PES), we show that the nature of interaction between low work function metals and phenylene-based electroluminescent oligomers and polymers covers the whole range from physisorption (aluminum and samarium) to chemical reduction (calcium), and to charge transfer reactions (alkali metals). Although PES is a very powerful experimental tool to determine electronic properties of interfaces, great care must be taken in the interpretation of the data when wide band-gap materials, such as conjugated organic materials, are being investigated. We demonstrate that the observation by PES of a finite density of occupied states at the Fermi-level on an organic film in which alkali metal atoms have been intercalated does not necessarily imply metallicity nor the presence of negative polarons (radical anions), as previously proposed. From a combination of ultraviolet PES and Kelvin probe measurements, evidence is obtained that the substrate and the surface of the organic film are not necessarily in thermodynamic equilibrium, leading to potential misinterpretations of the Fermi level position at the surface of organic films.