Coulomb effects on bipolarons and polarons in non degenerate conjugate polymers

Summary form only given. The formation of bipolarons and polarons in conjugate polymers with non degenerate ground states is studied theoretically. Our model is the Pariser-Parr Pople model with an electron-lattice interaction of the Su-Schrieffer-Heeger type and with a symmetry-breaking transfer energy modulation (a tight-binding version of the Brazovskii-Kirova model) for a single chain. We investigate systematically the relative stability of a bipolaron and two separated polarons as functions of the Coulomb interaction strength and the transfer modulation within the unrestricted Hartree-Fock approximation. It is demonstrated that Coulomb interaction significantly suppresses the stability of bipolarons. When more carriers are doped in a chain, there is a competition between a polaron lattice and a bipolaron lattice, the latter being stabilized at high doping concentrations. Therefore, our model predicts a possible doping induced transition from a polaron lattice to a bipolaron lattice. We will also discuss the optical properties of bipolarons and polarons.