Polarons and bipolarons in oligothiophenes: a first principles study

The shape and stability of polarons and bipolarons in oligothiophenes are s tudied systematically as a function of the oligomer length using first prin ciples calculations. It is shown that the polaron is the stable charge carr ier and that intrinsically the bipolaron is not stable with respect to sepa ration into polarons. The polaron is rather large; a lower bound for its lo calization length is 60 Angstrom and an upper bound for the associated latt ice relaxation energy is 0.04 eV. In actual materials its properties will b e strongly modified by disorder which induces a smaller effective conjugati on length.