Abstract
We investigated the surface potential built across the electrode/fullerene (C 60) or copper phthalocyanine (CuPc) interface and C 60/CuPc interface as a function of the thickness of the semiconductor film in the dark condition and under illumination. The surface potential of C 60 on Au, Al and Mg changes negatively with the increment of film thickness and it saturates at − 0.25, − 1.0 and − 1.5 V within 20 nm. The Fermi level alignment at C 60/electrode interface is established within ∼ 20 nm from electrode, and very high electric field exists due to the displacement of negative electronic charges from electrode into C 60. On the other hand, the surface potential of CuPc on ITO changes to + 0.1 V, and the work functions of C 60 and CuPc were estimated as 5.0 eV and 4.7 eV. C 60 film also accepts electrons from CuPc at hetero-junction interface, and the Fermi-level alignment was again obtained at C 60/CuPc interface under illumination. The built-in potential of ca. 0.3 V formed at C 60/CuPc interface was considered as the origin of the reduction of open-circuit voltage in ITO/CuPc/C 60/Au device compared with the optimum value of 0.6 V. On the other hand, the very high electric field formed at C 60/Mg contact improved the photovoltaic properties.
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