Polarons and bipolarons in oligothiophenes: a first principles study

The shape and stability of polarons and bipolarons in oligothiophenes are studied systematically as a function of the oligomer length using first principles calculations. It is shown that the polaron is the stable charge carrier and that intrinsically the bipolaron is not stable with respect to separation into polarons. The polaron is rather large; a lower bound for its localization length is 60 å and an upper bound for the associated lattice relaxation energy is 0.04 eV. In actual materials its properties will be strongly modified by disorder which induces a smaller effective conjugation length.