Metallic state of polymers with nondegenerate ground states

Recent photoemission experiments that followed the motion of the Fermi energy, and the changes in the density of states, with doping have led to the contention that the metallic state in polypyrrole and poly(3-hexylthiophene) is a polaron lattice. We show that these experiments, and others on these materials, are in agreement with expectations for a bipolaron lattice for the holes added to the chains or, more specifically, a bipolaron band merging with the valence band. Such a situation is stable, in contrast to the polaron lattice.