Coulomb effects on bipolarons and polarons in non-degenerate conjugated polymers

The formation of polarons and bipolarons in conjugated polymers with non-degenerate ground states is studied theoretically. The Pariser-Parr-Pople model is adopted which includes long range Coulomb interactions, the Su-Schrieffer-Heeger-type electron-lattice coupling, and the Brazovskii-Kirova-type symmetry-breaking energy. The unrestricted Hartree-Fock calculations demonstrate that Coulomb interactions significantly suppress the stability of bipolarons. For realistic parameters a polaron is the most stable configuration at low doping levels. In addition, we point out the possibility of a doping-induced phase transition from a polaron lattice to a bipolaron lattice since polarons is destabilized at high concentration of carriers. The relevance of these results is discussed in connection with experiments in polypyrrole and other polymers.