Explicit evidence for bipolaron formation: Cs-doped biphenyl

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Abstract
Thin condensed layers of biphenyl have been studied by ultraviolet photoelectron spectroscopy (UPS) and electron-energy-loss spectroscopy (EELS) as a function of Cs doping in order to model the situation in a doped conducting polymer. The UPS spectra show the growth of gap states which are assigned to bipolaron formation. This is the first direct observation of emission from states thought to be important for conduction in conducting polymers. While EELS also implies the formation of gap states their evolution with doping cannot be simply interpreted. The valence-band emissions clearly show shifts in the Fermi level due to pinning by the bipolaron states and movements due to increased polarizability on the formation of the Cs-biphenyl complex.

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