Photoemission Investigation of the Energy Level Alignment at the Rubrene/metal Interface
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1029-F04-03
Photoemission Investigation of the Energy Level Alignment at the Rubrene/metal Interface Huanjun Ding, and Yongli Gao Department of Physics and Astronomy, University of Rochester, Rochester, NY, 14627 ABSTRACT The electronic structure of the interfaces between rubrene and various metals, including Au, Ag, Al, and Ca, have been investigated with photoemission and inverse photoemission spectroscopy. The formation of the interface dipole is observed at all interfaces. The Fermi level shifts linearly within the band gap as a function of metal workfunction, until it is almost aligned with the lowest unoccupied molecular orbital (LUMO) by Ca. Strong interactions take place at the interface between rubrene and Ca, evidenced by the evolution of the valence features. INTRODUCTION Thin film transistors based on organic semiconductors are promising due to their advantages, such as low-cost, large-area application, and mechanical flexibility [1-3]. Currently, one of the main limitations is the mobility. A wide variety of organic materials has been employed as the active layer, such as Tetracene, Pentacene, and oligothiophenes [4-7]. However, the mobilities remain low, compared with that of Si. Recently, organic thin film transistors (OTFTs) based on rubrene single crystal have been reported to achieve a hole mobility of 30 cm2/Vs at 200K [8], which is one order of magnitude larger than that of pentacene and is comparable to that of amorphous Si. Thus, the investigations of the rubrene-based electronic devices in either the crystal or thin film form become one of the hottest topics in this field. However, most of the studies focus on the device measurements, and there are very few investigations about the electronic structures of the rubrene thin film, especially the energy level alignment at the rubrene/metal interface [9]. Unlike inorganic semiconductors, where schottky barriers are usually expected, the metal organic interface are far more complicated, which usually involves the formation of interface dipole, band bending, and sometimes, chemical interactions. Therefore, understanding of the electronic structures at the rubrene/metal interface becomes one of the key steps to improve the device performance. In the paper, we report our photoemission and inverse photoemission (PES and IPES) investigations about the electronic structure of the interfaces between rubrene and various metals, such as Au, Ag, Al, and Ca. With x-ray and ultraviolet photoemission spectroscopy (XPS and UPS), the evolution of energy levels during the interface formation was recorded. The formation of the interface dipole was observed at the interfaces. The Fermi level shifted linearly within the band gap as a function of the metal workfunction, until it was almost aligned with the lowest unoccupied molecular orbital (LUMO) by low workfunction metal, like Ca. Strong interactions took place at the interface between rubrene and Ca, evidenced by the evolution of the valence features.
EXPERIMENT The UPS, XPS, and IPES studies were performed usi
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