Interacting Bipolarons in n -Doped Sexiphenyl Films

The formation and evolution of the gap states formed with increasing Cs concentration in thin sexiphenyl (6P) films is followed with ultraviolet photoemission spectroscopy (UPS). Two distinct doping regimes are apparent, experimental considerations suggest that in the lower-doping bipolaron-like (dianionic 6P 2- ) states are formed. In the higher-doping regime, approaching Cs saturation, a 0.4 eV splitting in the gap states is observed which is attributed to interacting bipolaron states. VEH calculations for 6P 2- and 6P 4- are in very good agreement with the experimental spectra and lend strong support to the notion that a bipolaron to intrachain interacting bipolaron transition occurs with increasing Cs concentration.