Interfacial band offset and band bending of organic semiconductors are critical to understand and improve ogranic photovoltatic cells. In this study, the energy level alignment of fullerene (C_(60))/ metal-free phthalocyanine (H_2Pc) interface which is one of the model interfaces of organic photovoltaic cells has been investigated using UV and X-ray photoemissions. For both 'H_2Pc on C_(60') and "C_(60) on H_2Pc' 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 C_(60)/metal interfaces in our previous study, where thickness of 500nm was required to achieve 0.21 eV band bending to get Fermi level alignment between metal electrode and C_(60). To clarifty the origin of the band bending, the effect of the insertion of C_(60)-H_2Pc co-deposited layer between C_(60) and H_2Pc layers was also investigated. The result suggested that possible doping of H_2Pc to C_(60) is not main origin of the observed energy shift. We also found that the vacuum level shift at H_2Pc/C_(60) interface is strongly dependent on the deposition sequence of the interface formation.
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