The Electronic Structure and the Energy Level Alignment at the Interface Between Organic Molecules and Metals
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0965-S09-28
The Electronic Structure and the Energy Level Alignment at the Interface Between Organic Molecules and Metals Eiji Kawabe1, Hiroyuki Yamane1, Kenji Koizumi1, Ryohei Sumii2, Kaname Kanai1, Yukio Ouchi1, and Kazuhiko Seki1,3 1 Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusaku, Nagoya, 464-8602, Japan 2 Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan 3 Institute for Advanced Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan
ABSTRACT We investigated the electronic structure and the energy level alignment at the interfaces between perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and metals (Au, Cu and Co) by ultraviolet photoelectron spectroscopy (UPS). In the case of PTCDA/Au interface, the apparent interface states and charge transfer (CT) states were not observed in its UPS spectra. In the case of PTCDA/Cu and Co interfaces, however, we found that the CT from metal to PTCDA occurred, and that the apparent CT states were observed at PTCDA/Cu interface. INTRODUCTION In recent years, much intentional research has been carried out for applying organic materials to electronic devices such as organic light emitting diodes, organic solar cells and organic field effect transistors. It is important to understand how the energy levels align at the organic/metal (O/M) interfaces for understanding the mechanism of such devices and for improving their performance, since these factors strongly affect various phenomena such as charge injection. However, the detailed mechanism of the energy level alignment at O/M interface is not yet well understood even for nonpolar molecules. Narioka et al. [1] found the formation of the interface dipole at O/M interface by ultraviolet photoelectron spectroscopy (UPS), clearly demonstrating the breakdown of the Mott-Schottkey model assuming the vacuum level alignment. Subsequently several possible origins of the interface dipole have been proposed by Ishii et al. [2], including charge transfer, mirror effect, push-back of spilled-out electrons, chemisorption, and the formation of interface states. As for more specific model based on the interface states, recently Vázquez, Flores and Kahn proposed the induced density of interface states (IDIS) model [3, 4], in which the charge neutrality level (CNL) plays an important role to define the energy level alignment. In spite of its rather simple assumption, the calculated energy level alignment at several O/M interfaces shows fairly good agreement with the experimental data. Still it needs examination whether the energy level alignment at all O/M interfaces can be fully explained by this model. Recently we started to systematically investigate the energy level alignment at various O/M interfaces within a few molecular layers by UPS measurements, with focus on how the energy level alignment is affected by the interaction between the electronic
states of organic molecules such as HOMO and LUMO and the valence states
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