Electronic and vibrational spectra of positive polarons and bipolarons in regioregular poly(3-hexylthiophene) doped with ferric chloride.

We studied the carriers generated in regioregular poly(3-hexylthiophene) (P3HT) upon FeCl3 vapor and solution doping using visible/near-infrared (VIS/NIR) absorption, infrared (IR), and Raman spectroscopy. Upon doping with an FeCl3 solution in air, the main carriers that were generated were positive polarons. Upon doping with FeCl3 vapor, positive polarons also formed initially, but at higher doping levels, positive bipolarons formed with the concomitant disappearance of the positive polarons. The Raman and IR spectra of the positive bipolarons and the positive polarons were obtained. Raman spectroscopy is very useful for characterizing positive polarons and bipolarons. The Raman results indicated that the positive bipolarons were converted to polarons upon heating to 85 °C, indicating that the positive bipolarons formed a metastable state. The temporal changes in the electrical conductivity of a P3HT film upon doping with FeCl3 vapor were measured. The conductivity reached a maximum and then decreased by 2 orders of magnitude. This result suggests that the mobility of the polarons is approximately 100 times as high as that of the bipolarons.