Energy Level Alignment at Fullerene/Phthalocyanine Interface Studied by Electron Spectroscopies
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Energy Level Alignment at Fullerene/Phthalocyanine Interface Studied by Electron Spectroscopies Hisao Ishii1 , Atsushi Seko2, Akira Kawakami2, Kazunori Umishita2, Yukio Ouchi2, and Kazuhiko Seki3 Research Institute of Electrical Communication, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, JAPAN 2 Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusaku, Nagoya 464-8602, JAPAN 3 Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, JAPAN 1
ABSTRACT Interfacial band offset and band bending of organic semiconductors are critical to understand and improve organic photovoltaic cells. In this study, the energy level alignment of fullerene(C60) / metal-free phthalocyanine (H2Pc) interface which is one of the model interfaces of organic photovoltaic cells has been investigated using UV and X-ray photoemissions. For both 'H2Pc on C60' and “C60 on H2Pc' interfaces, 0.3 eV downward energy level shift was observed in XPS at the interface formation. This energy shift is quite steep in contrast to the band bending observed for C60/metal interfaces in our previous study, where thickness of 500nm was required to achieve 0.21eV band bending to get Fermi level alignment between metal electrode and C60. To clarify the origin of the band bending, the effect of the insertion of C60-H2Pc co-deposited layer between C60 and H2Pc layers was also investigated. The result suggested that possible doping of H2Pc to C60 is not main origin of the observed energy shift. We also found that the vacuum level shift at H2Pc/C60 interface is strongly dependent on the deposition sequence of the interface formation. INTRODUCTION Organic hetero-junctions with contact of p-type and n-type organic semiconductors have attracted much attention in relation to possible applications to organic solar cells. So far, the energy level alignment at such organic hetero-junctions has been understood by assuming (i) vacuum level alignment at the interface, and (ii) band bending leading to Fermi level alignment between the two organic semiconductors as shown in figure1. On the basis of the former assumption, the band offset of valence band top (conduction band bottom) can be estimated as the difference of ionization potential (electron affinity) between the two organics. The latter assumption is critical to discuss the internal electric field in the junction in relation to the separation of electron-hole pair formed by photon absorption.
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Figure1. Traditional model of the energy level alignment at organic/organic hetero-junction. (a) before contact and (b) after contact. The assumption of
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