Surface potential measurement of organic photo-diode consisting of fullerene/copper phthalocyanine double layered device

We investigated the surface potential built across the electrode/fullerene (C60) or copper phthalocyanine (CuPc) interface and C60/CuPc interface as a function of the thickness of the semiconductor film in the dark condition and under illumination. The surface potential of C60 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 C60/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 C60. On the other hand, the surface potential of CuPc on ITO changes to + 0.1 V, and the work functions of C60 and CuPc were estimated as 5.0 eV and 4.7 eV. C60 film also accepts electrons from CuPc at hetero-junction interface, and the Fermi-level alignment was again obtained at C60/CuPc interface under illumination. The built-in potential of ca. 0.3 V formed at C60/CuPc interface was considered as the origin of the reduction of open-circuit voltage in ITO/CuPc/C60/Au device compared with the optimum value of 0.6 V. On the other hand, the very high electric field formed at C60/Mg contact improved the photovoltaic properties.