Evolution of defect states in doped polythiophene: A study based on the method of simulated annealing

A modified Su-Schrieffer-Heeger Hamiltonian-based model is used to compute the electronic and geometric structures of fairly long polythiophene (PT) chains, neutral as well as doped. The geometry optimization is carried out by the simulated annealing method. Both Metropolis and Glauber functions are used for sampling. It is shown that a bipolaron can be structurally represented by a fragment of the PT chain containing 14 thiophene units. As a series of bipolaronic defects are introduced in a long PT chain (50-100 rings), the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap energy (Delta) becomes vanishingly small, a feature not present in the PT chains of similar sizes containing polaronic defects. The Fermi energy level (E-F) also moves into the valency band and nonzero density of states at epsilon = E-F are created. Once again, this feature is shown to be missing in PT chains containing polaronic defects. Implications of these findings are analyzed. (C) 2003 Wiley Periodicals, Inc.