Photoemission investigation on copper phthalocyanine:fullerene blend film

The electronic structure of thin blend films composed by copper phthalocyanine (CuPc) and fullerene (C-60) have been studied by soft x-ray photoemission spectroscopy. Several samples with different C-60/CuPc concentration ratios were prepared by thermal evaporation in ultrahigh vacuum conditions. As the C-60 concentration increases, strong shifts of the CuPc highest occupied molecular orbital (HOMO) level towards the Fermi level and of the C-60 HOMO level to higher binding energies have been observed. This experimental result suggests that there is a sizeable interaction between CuPc and C-60 molecules that can be interpreted in terms of a charge transfer from C-60 to CuPc. These shifts play an important role in the application of the CuPc:C-60 blend films for instance in the construction of solar cells on flexible substrates.