TRANSITION BETWEEN BIPOLARON AND POLARON STATES IN DOPED HETEROCYCLE POLYMERS

On the basis of a model similar to that of Su, Schrieffer, and Heeger, we investigate transitions between bipolaron and polaron states in do ped heterocycle polymers such as polypyrrole, polythiophene, and polys elenophene. The dopant impurity is assumed to be located between chain s, and the interchain coupling is due to the electron hopping which is assumed to be confined to nearest-neighboring carbon groups around th e counterion. The electron-electron interaction described by a Hubbard -type Hamiltonian is also included in our calculation. It is shown tha t the system may be in a bipolaron state or a state of a polaron pair of spin triplet (singlet) nature. We find that the interchain coupling has little influence on the stability of a bipolaron. Transitions fro m a bipolaron to a pair of polarons can be induced by a strong electro n-electron interaction, which is also responsible for the energy diffe rence between the singlet and triplet polarons states.