Competition between polarons and bipolarons in nondegenerate conjugated polymers

Abstract

The formation of polarons and bipolarons in conjugated polymers with nondegenerate ground states is studied theoretically. We use the Pariser-Parr-Pople model combined with the Su-Schrieffer-Heeger model of electron-lattice coupling and a Brazovskii-Kirova-type symmetry-breaking interaction. The relative stability of polarons and bipolarons is systematically studied within the unrestricted Hartree-Fock approximation as a function of the model parameters and the doping concentration. Coulomb interactions significantly suppress the stability of bipolarons, and for realistic parameters the doped carriers are most likely in the form of polarons at low doping levels. On the other hand, the high concentration of carriers tends to favor a bipolaron lattice, and our model predicts a possible doping-induced phase transition from a polaron lattice to a bipolaron lattice. The significance of these results is discussed in connection with experiments in polypyrrole and other polymers.