INTERACTING BIPOLARONS IN N-DOPED SEXIPHENYL FILMS

The formation and evolution of the gap states formed with increasing C s concentration in thin sexiphenyl (6P) films is followed with ultravi olet photoemission spectroscopy (UPS). Two distinct doping regimes are apparent, experimental considerations suggest that in the lower-dopin g regime bipolaron-like (dianionic 6P2-) states are formed. In the hig her-doping regime, approaching Cs saturation, a 0.4 eV splitting in th e gap states is observed which is attributed to interacting bipolaron states. VEH calculations for 6P2- and 6P4- are in very good agreement with the experimental spectra and lend strong support to the notion th at a bipolaron to intrachain interacting bipolaron transition occurs w ith increasing Cs concentration.