Polaron-pair binding due to interchain coupling in conjugated polymers

By including both interchain hopping and electron-electron interaction within the context of the Takayama-Lin-Liu-Maki continuum model, and using the (nearly) degenerate perturbation method, we have studied characteristics of the interchain polaron-pair state in conjugated polymers, namely, its binding energy and the transition rate from an intrachain polaron exciton to an interchain polaron pair. It is found that the polaron-pair binding energy due to interchain Coulomb correlation is typically smaller than that due to interchain hopping. We introduce a small energy difference between intrachain and interchain excitations to distinguish their different lattice relaxations; we are then able to establish a two-level model, which can be solved exactly to obtain the transition probability. When is small enough, the solution is just a first-order degenerate perturbation result, i.e., it is proportional to a t binding term; when is large, then it becomes dependent on a (t)2-binding term. In both cases, the interchain electron-correlation effect always appears in first order and is found not to influence the intrachain to interchain transition.