Polarons and Bipolarons in Doped Polythiophenes

The optical transitions induced by doping poly(3-methylthiophene) or polythiophene with BF4− or ClO4− have been investigated by in-situ spectroscopy in an opto-electrochemical cell. From the number of doping-induced bands and concentration dependence we found evidence that polarons are the primary electronic excitations at doping concentrations up to 1% and that an increasing number of bipolarons is formed at higher concentrations. Both the positions and the intensities of the optical transitions indicate that electron-electron interactions and the Coulomb potential of the dopant ions are relevant.