Dynamical evolution of polaron to bipolaron in conjugated polymers

We investigate the insertion of holes in conjugated polymers bearing polarons. We use a Su-Schrieffer-Heeger model modified to include electron-electron interactions via an extended Hubbard Hamiltonian, a Brazovskii-Kirova symmetry breaking term, and an external electric field. We study the dynamics performing numerical calculations within the time-dependent unrestricted Hartree-Fock approximation. We find that adding a hole in a conducting polymer bearing a single positively charged polaron leads to the direct transition of polaron to bipolaron state. The transition produced is a single-polaron to biplaron transition whose excitation spectrum explains the experimental data. The competing mechanism of two polarons merging to form a bipolaron is not observed. We also find that depending on how fast the hole is inserted, a structure that contains a bipolaron coupled to a breather is created. The bipolaron-breather pair can be decoupled under the action of an external electric field. We determine the value of the critical electric field to untrap the bipolaron from the breather.