INTER-IMPURITY INTERACTION AND STRUCTURE OF A TRAPPED BIPOLARON IN DOPED CONJUGATED POLYMERS

The effects of an inter-impurity interaction are studied by the extended Peierls-Hubbard model. The Coulomb interactions in the chain are treated by the Hartree-Fock approximation. There are two acceptor-type impurities expressed by long-ranged Coulomb potentials with anisotropic dielectric constants. Stationary positions of the impurities as well as stationary lattice configurations and electronic wave functions are numerically determined self-consistently. Trapping of a bipolaron around impurities is found. The distance between impurities is of the order of the coherence length. Two mid-gap levels, which would exist for a free bipolaron, move to higher energies due to the strong dopant potential. The Coulomb interactions in the chain effectively weaken the strength of the dopant potential. The energy levels shift to lower energies with increasing interaction strengths. Dynamical conductivity (optical absorption) is calculated. The isolated peak in the lower frequencies is associated with the position of the localized level in the energy gap. The consequences for experimental observation of the impurity clustering are discussed.