Bipolaron:  The Stable Charged Species in n-Doped p-Sexiphenyl

• Published in: The journal of physical chemistry. B 2000-02, Vol.104 (7), p.1434-1438
• Main Authors: Koch, N, Rajagopal, A, Ghijsen, J, Johnson, R. L, Leising, G, Pireaux, J.-J
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp991349f

The change in the valence electronic structure of thin films of the electroluminescent material p-sexiphenyl (6P) has been investigated as a function of the deposition of an alkaline earth metal under ultrahigh vacuum conditions. This was done by in situ evaporation of increasing amounts of calcium onto the 6P film and recording the photoemission spectra. Two different substrates for the 6P films were used, namely Au and Ca. New emissions within the formerly empty energy gap of the pristine organic material were observed upon the metal deposition for both substrates. For 6P/Au this was accompanied by a significant shift of the Fermi level (EF) toward lower binding energy for increasing amounts of Ca. For low metal coverage the density of occupied valence states (DOVS) of the intragap emission was observed at EF, whereas for higher amounts of Ca coverage no DOVS are observed at EF. Commonly, this would be interpreted that first polarons (radical anions) are formed, and later on bipolarons (dianions) for higher Ca concentration. In contrast, on the basis of the different experimental findings when Ca was used as substrate and a line shape analysis of the intragap emission, we conclude that in 6P bipolarons are formed throughout all doping levels.