Transport band gap opening at metal–organic interfaces

The interface formation between copper phthalocyanine (CuPc) and two representative metal substrates, i.e., Au and Co, was investigated by the combination of ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The occupied and unoccupied molecular orbitals and thus the transport band gap of CuPc are highly influenced by film thickness, i.e., molecule substrate distance. Due to the image charge potential given by the metallic substrates the transport band gap of CuPc “opens” from (1.4 ± 0.3) eV for 1 nm thickness to (2.2 ± 0.3) eV, and saturates at this value above 10 nm CuPc thickness. The interface dipoles with values of 1.2 eV and 1.0 eV for Au and Co substrates, respectively, predominantly depend on the metal substrate work functions. X-ray photoelectron spectroscopy measurements using synchrotron radiation provide detailed information on the interaction between CuPc and the two metal substrates. While charge transfer from the Au or Co substrate to the Cu metal center is pres...