Bipolarons in polypyrrole chains

First-principles calculations on the geometric and electronic structures of undoped and doped polypyrrole chains indicate the appearance upon high doping of bipolaron states in the gap. A picture of the band-structure evolution upon doping is presented, showing the presence of bipolaron bands in the gap for experimentally achieved doping levels. A conductivity mechanism based on motion of spinless bipolarons is consistent with the absence of ESR signal in electro-chemically cycled highly conducting polypyrrole.