Evolution of the unoccupied states in Cs-doped copper phthalocyanine

The evolution of both the occupied and unoccupied states for Cs-doped copper phthalocyanine (CuPc) has been investigated with photoemission and inverse photoemission spectroscopy. As the Cs doping ratio increases, the lowest unoccupied molecular orbital (LUMO) of CuPc shifts downwards, reaching the Fermi level. After the saturation, the LUMO intensity decreases monotonically, while a gap state grows in the valence spectra, which gives direct evidence for the origin of the doping-induced gap state in CuPc molecules. The intensity of the LUMO, as well as the gap state, suggest the formation of multiply charged CuPc spices in heavily doped film.