Energetic study of polarons and bipolarons in polythiophene: Importance of Coulomb effects

We study the effect of Coulomb interactions on the formation of polarons and bipolarons in polythiophene. First, the interaction of an ionized dopant is considered and these defects are found to be always energetically favored upon doping, bipolarons being the lowest-lying charged excitations. The influence of the Coulomb repulsion associated with the double occupation of the gap levels that occurs in bipolarons is then investigated within a phenomenological model. This interaction leads to a modification of the different equilibrium parameters and reduces appreciably the difference between the creation energy of a pair of polarons and that of a bipolaron. However, this contribution is not strong enough to destabilize bipolarons that remain the most stable excitation. Implications of these results for the existence of polarons are discussed depending on whether the excitations are photogenerated or created by doping.