Bipolarons in poly(3-methylthiophene): Spectroscopic, magnetic, and electrochemical measurements

In poly(3-methylthiophene), P3MT, charge is stored in spinless bipolarons at all but the most dilute doping levels. From the combination of magnetic and electrochemical measurements, we find definitive evidence of the reversed spin-charge relation expected for bipolarons with $N_s$/$N_{\mathrm{ch}{<10\mathrm{}}^{\mathrm{-}3}}$; from the spectroscopic data we find the two subgap absorptions indicative of self-localized bipolaron levels located nearly symmetrically within the gap. For charge injection below the principal injection threshold, one observes polaronlike features with a maximum number of spin-carrying species of approximately 0.2 mol % (one per 500 rings). Our results imply that these features result from defect states in the energy gap, but they are not the self-localized polarons which would be characteristic of a charge on a nondegenerate ground-state polymer such as a perfect P3MT chain. The conclusion that the bipolaron is the lowest-energy charge-storage configuration indicates that effective electron-electron Coulomb repulsion is relatively weak in P3MT.