Theoretical study of the evolution of electronic band structure of polythiophene due to bipolaron doping

We study here theoretically the evolution of the electronic structure of polythiophene (PT) due to bipolaron doping after modifying the sigma-bond compressibility model. Since the electron-lattice coupling in this model alters both the site energies and the hopping integrals, effects due to the changes in the site energies on the formation of polarons and bipolarons in PT are incorporated. The inductive effect of sulfur is also considered. The ground state geometry of the neutral PT and the experimentally observed bipolaronic optical transitions are reproduced. We predict an insulating behavior of the ordered bipolaron lattice of PT. The bipolaron cluster model also fails to develop a near degeneracy of the highest occupied and the lowest unoccupied molecular orbitals at and above 20 mol % defect concentration. We attribute this to the altering of the site energies upon doping. Our model further predicts that the superlattice structure of doped PT's will be semiconductors at room temperature but with a high resistivity.