Insulator-to-metal transition in polythiophene

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Abstract
In the present work, the electronic structure of polythiophene at several doping levels is investigated by the use of the Hückel Hamiltonian with σ-bond compressibility. Excess charges are assumed to be stored in conformational defects of the bipolaron type. The Hamiltonian matrix elements representative of a bipolaron are obtained from a previous thiophene oligomer calculation, and then transferred to very long chains. Negative factor counting and inverse iteration techniques have been used to evaluate densities of states and wave functions, respectively. Several types of defect distributions were analyzed. Our results are consistent with the following: (i) the bipolaron lattice does not present a finite density of states at the Fermi energy at any doping level; (ii) bipolaron clusters show an insulator-to-metal transition at 8 mol % doping level; (iii) segregation disorder shows an insulator-to-metal transition for doping levels in the range 20–30 mol %.