POLARON AND BIPOLARON-LIKE STATES IN N-DOPED BITHIOPHENE

Bithiophene is studied with varying Cs dosing as a model for the n-dop ing of thiophene-based conducting polymers. The evolution of the elect ronic structure with increasing Cs concentration is followed both indi rectly, by electron-energy-loss spectroscopy (EELS), and directly by u ltraviolet-photoemission spectroscopy (UPS). In both spectroscopies th e behavior of the states formed in the gap indicates two doping regime s. The EELS results parallel the optical-absorption results for the p- doping of longer thiophene oligomers and polythiophene, and suggest a polaron (anion) to bipolaronlike (dianion) transformation. The UPS res ults are consistent with this interpretation, showing first in the low -doping regime the appearance of two states in the band gap which move closer together in the higher-doping regime. This constitutes direct observation of a transition from polaron to bipolaron states, albeit i n a model system. Consideration of the photoionization cross sections indicates that the deeper (bi)polaron level has C 2p character while t he shallow level, filled by donation from the Cs, has significant S 3p character.