Photoemission investigation on copper phthalocyanine:fullerene blend film

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