Comprehensive (0.05–3-eV) spectroscopic measurements are reported for poly(3-hexylthienylene), P3HT, during photoexcitation (i.e., photoinduced absorption) and after doping with ${\mathrm{NOPF}}_6$. 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 ${\mathrm{cm}}^{\mathrm{-}1}$) 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 ${\mathrm{cm}}^{\mathrm{-}1}$) 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, $U_B$≊0.25 eV; this relatively small value of $U_B$ is consistent with bipolaron formation in P3HT.

• Received 11 April 1988

DOI:https://doi.org/10.1103/PhysRevB.38.5490