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Energy Level Alignment at the Metal/Alq3 Interfaces Investigated with Photoemission Methods

Published online by Cambridge University Press:  21 March 2011

Li Yan ,
C.W. Tang ,
M. G. Mason  and
Yongli Gao
Li Yan
Affiliation:
University of Rochester, Rochester, New York 14627, USA
C.W. Tang
Affiliation:
Eastman Kodak Company, Rochester, New York 14650, USA.
M. G. Mason
Affiliation:
Eastman Kodak Company, Rochester, New York 14650, USA. Deceased. Eastman Kodak Company, Rochester, New York 14650, USA.
Yongli Gao
Affiliation:
University of Rochester, Rochester, New York 14627, USA
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Abstract

Tris(8-hydroxyquinoline) aluminum (Alq3) based organic light emission diodes (OLED) have been a focus of material research in recent years. One of the key issues in searching for a better device performance and fabricating conditions is suitable electron-injection materials. We have investigated the energy alignment and the interface formation between different metals and Alq3 using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The interface is formed by depositing the target cathode material, such as Ca, Al or Al/LiF, onto an Alq3 film in a stepwise fashion in an ultrahigh vacuum environment. While the UPS results show the work function and vacuum level changes during interfaces formation, implying a possible surface dipole layer, XPS results show a more detailed and complex behavior. When a low work function metal such as Ca is deposited onto an Alq3 surface, a gap state is observed in UPS. At the same time, a new peak can be observed in the N 1s core level at a lower binding energy. These results can be characterized as charge transfer from the low work function metal to Alq3. The shifting of core levels are also observed, which may be explained by doping from metal atoms or charge diffusion. These interfaces are drastically different than the Al/Alq3 interface, which has very poor electron injection. At the Al/Alq3 interface there is a destructive chemical reaction and much smaller core level shifts are observed. Based on detailed analysis, energy level diagrams at the interface are proposed.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 660: Symposia JJ – Organic Electronic & Photonic Materials and Devices , 2000 , JJ8.36
Copyright
Copyright © Materials Research Society 2001

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