IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Organic Films and Devices Thursday Sessions
       Session OF+EL+TF-ThA

Paper OF+EL+TF-ThA9
Direct Observation of Fermi Level Pinning at LUMO in Alkali Metal Doped Organic Films

Thursday, November 1, 2001, 4:40 pm, Room 131

Session: Electronic Properties of Organic Thin Films
Presenter: L. Yan, University of Rochester
Authors: L. Yan, University of Rochester
N.J. Watkins, University of Rochester
C.W. Tang, Eastman Kodak Company
Y. Gao, University of Rochester
Correspondent: Click to Email
The electronic structures of pristine and alkali metal doped organic films are investigated using the combination of photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) can be directly observed by IPES and UPS simultaneously. We found that the Fermi level position in the organic film can be modified by alkali metal doping. For example, in CuPc films, the observed LUMO of the CuPc film is shifted by the Cs doping to less than 0.2eV above the Fermi level. This pinning may be explained by the charge transfer process from Cs to CuPc. Our observation is the direct confirmation of widely used assumption that the LUMO can be inferred from HOMO position in organic films when a valance band shift is observed. The result indicates that energy alignment and charge injection properties of the organic materials can be modified by a simple doping process. Our findings could lead to various interface-engineering methods useful in organic optoelectronic devices. Possible interface dipoles, energy level shifts, charge transfer, metal diffusion and chemical reactions at the interface will be discussed.