Observation of filled states at the Fermi-level in alkali-metal intercalated organic films: Dependence on substrate work function

We present ultraviolet photoelectron spectroscopy studies on alkali metal-intercalated thin films of electroluminescent p-sexiphenyl (6P) and the organic hole transporting material N,N′-bis-(1-naphthyl)-N,N′- diphenyl1-1,1-biphenyl1-4,4′-diamine (α-NPD). The formation of new occupied electronic states in the otherwise empty energy gap of 6P and α-NPD is observed, accompanied by a significant shift of the Fermi-level. Depending on the work function φ of the substrate used for the organic thin films, we observe two different scenarios at low intercalation levels: the new electronic states are observed at (and even above) E_F for high φ substrates, whereas they are observed below E_F for low φ substrates. At high intercalation levels, all the states are observed below E_F. The observation of states at and above E_F is attributed to a lack of thermodynamic equilibrium across the organic thin film.