Unoccupied states in copper phthalocyanine/fullerene blended films determined by inverse photoemission spectroscopy

Blends of copper phthalocyanine (CuPc) and fullerene (C60) have successfully been used for organic field-effect transistors and photovoltaic cells. In this work we investigate the unoccupied states of CuPc/C60 blends by inverse photoemission spectroscopy. The energies of the lowest unoccupied molecular orbitals of CuPc and C60 vary linearly with the mixing ratio, which is consistent with the energy shifts of the vacuum level, the highest occupied molecular orbitals and core levels. The observed results can be interpreted as unchanged electron affinities of CuPc and C60 in the blended films due to nano-phase separation.