Origin of charge transfer complex resulting in Ohmic contact at the C₆₀/Cu interface

The conducting charge transfer complex formed on the C₆₀/Cu interface after heat treatment was investigated by using density functional theory. We optimized the geometries of the C₆₀ and C₆₀-Cu complexes. Then, a comparison of the valence band spectrum with the simulated density of states revealed detailed molecular properties. We confirm that the C₆₀-Cu complex layer formed at the C₆₀/Cu interface enhances charge transfer. The conducting complex creates both an occupied gap state and unoccupied states between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of C₆₀. These states lead to the formation of Ohmic contact between the C₆₀ layer and the Cu layer.