Direct observation of charge-transfer dynamics in a conjugated conducting polymer poly(3-octylthiophene)-fullerene composite by time-resolved infrared spectroscopy

Transient infrared-active vibrational (IRAV) modes observed in the π-conjugated conducting polymers upon photoexcitation are usually used to characterize the dynamics of the photoinduced charge separation and recombination processes. In this paper, the dynamic behavior of photogenerated charge carrier in the poly(3-octylthiophene) (P3OT) conducting polymer doped with fullerene $\left({\mathrm{C}}_{60}\right)$ has been studied by using step-scan time-resolved Fourier-transform infrared spectroscopy at room temperature in the $2500–900-{\mathrm{cm}}^{-1}$ (0.31–0.11-eV) frequency region. Upon photoexcitation with a 10-ns laser pulses, it is observed that IRAV modes appear in the similar vibrational frequency regions for pure P3OT and ${\mathrm{C}}_{60}$-doped P3OT; however, the observed IRAV modes is enhanced by 800% in intensity in the presence of small amount of ${\mathrm{C}}_{60}$ (e.g., 10%). Our results also show that the charge separation occurs instantaneously within the laser pulse width (10 ns), whereas the charge recombination processes occur in the microseconds to a few milliseconds time domain. In addition, the observed enhancement in the IRAV modes upon ${\mathrm{C}}_{60}$ doping indicates an enhanced photogeneration efficiency of the charge separation.