Influence of the counterion on the electronic structure in doped phenylene-based materials

The mechanisms of charge storage and transport in doped conjugated organic materials are a topic of interest in both fundamental and application-oriented research. We investigated the valence electronic structure of n-doped p-sexiphenyl (6P), a model molecule for the electroluminescent polymer poly(p-phenylene). Different alkali metals and an alkaline earth metal were deposited stepwise in situ on thin films of 6P. After each step ultraviolet photoemission spectra were recorded using synchrotron radiation. The evolution of characteristic new features within the formerly empty energy gap of 6P was observed. We conclude from our observations that negative bipolarons (dianions) are the only charged species on a 6P surface. However, the lineshape and energetic positions of the new intragap features varied for each dopant used. By comparing the experimental data with quantum-mechanical calculations on model systems, we discuss the mechanisms that could explain this phenomenon.