Photoexcitation and doping studies of poly(3-hexylthienylene)

Comprehensive (0.05--3-eV) spectroscopic measurements are reported for poly(3-hexylthienylene), P3HT, during photoexcitation (i.e., photoinduced absorption) and after doping with NOPF₆. A one-to-one correspondence between the photoinduced and doping-induced spectral changes is observed, typical of conducting polymers. The results demonstrate that charges are predominantly stored in bipolarons in P3HT. We find two subgap electronic absorptions and a series of infrared-active vibrational (IRAV) modes. The four localized IRAV modes (1088, 1161, 1200, 1354 cm^-1) associated with bipolaron distortions of the P3HT chains and made infrared active through coupling to the uniform translation of the bipolaron were observed in both the doping-induced and photoinduced absorption spectra, as in polythiophene and poly(3-methylthienylene). An additional absorption (1396 cm^-1) was identified as arising from an ir-active localized mode associated with the nonuniform translation (shape oscillation) of the bipolaron. Comparison of the energies of the photoinduced and doping-induced electronic transitions yields an estimate of the change in Coulomb energy of the bipolaron on photoexcitation, UB{}0.25 eV; this relatively small value of UB is consistent with bipolaron formation in P3HT.