Bipolaron: The stable charged species in n-doped p-sexiphenyl

The change in the valence electronic structure of thin films of the electro luminescent material p-sexiphenyl (6P) has been investigated as a function of the deposition of an alkaline earth metal under ultrahigh vacuum conditi ons. This was done by in situ evaporation of increasing amounts of calcium onto the 6P film and recording the photoemission spectra. Two different sub strates for the 6P films were used, namely Au and Ca. New emissions within the formerly empty energy gap of the pristine organic material were observe d upon the metal deposition for both substrates. For 6P/Au this was accompa nied by a significant shift of the Fermi level (E-F) toward lower binding e nergy for increasing amounts of Ca. For low metal coverage the density of o ccupied valence states (DOVS) of the intragap emission was observed at E-F, whereas for higher amounts of Ca coverage no DOVS are observed at E-F Comm only, this would be interpreted that first polarons (radical anions) are fo rmed, and later on bipolarons (dianions) for higher Ca concentration. In co ntrast, on the basis of the different experimental findings when Ca was use d as substrate and a line shape analysis of the intragap emission, we concl ude that in 6P bipolarons are formed throughout all doping levels.