Evolution of the unoccupied states in alkali metal-doped organic semiconductor

The evolution of the electronic structure, especially the unoccupied states, induced by alkali metal doping has been investigated with photoemission and inverse photoemission spectroscopy for organic semiconductors, such as copper-phthalocyanine (CuPc) and tris(8-hydroxyquinoline) aluminum (Alq). The n-type doping leads to a downward shift for the lowest unoccupied molecular orbital (LUMO) of the organic semiconductors, until the edge of the LUMO reaches the Fermi level. After that, 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 organic molecules. The modification of the LUMO intensity, as well as that of the gap state, suggests the formation of multiply charged anions in heavily doped film.