Direct evidence of photoinduced electron transfer in conducting-polymer–C60composites by infrared photoexcitation spectroscopy

We report direct spectral evidence of photoinduced electron transfer from the excited state of poly(3-octylthiophene), P3OT, onto ${\mathrm{C}}_{60}$ by infrared photoexcitation spectroscopy, 0.01 eV (100 ${\mathrm{cm}}^{\mathrm{-}1}$) to 1.3 eV (11 000 ${\mathrm{cm}}^{\mathrm{-}1}$). The photoinduced absorption spectrum of P3OT, characterized by subgap electronic absorptions and associated infrared-active vibrational modes, is significantly enhanced in magnitude upon adding a few percent of ${\mathrm{C}}_{60}$. Two new peaks are found in the photoexcited spectrum of the P3OT/${\mathrm{C}}_{60}$ composite and assigned to the allowed HOMO (${\mathit{T}}_{1\mathit{u}}$)-LUMO (${\mathit{T}}_{1\mathit{g}}$) transitions of ${\mathrm{C}}_{60}^{\mathrm{-}}$ with energies, 1.15 and 1.25 eV, in good agreement with calculated values. The photoinduced charge transfer from P3OT onto ${\mathrm{C}}_{60}$ enhances the quantum efficiency for photogeneration of charge carriers resulting in charge separation in the excited state. As a result, the lifetime of the excited-state configuration, (P3OT${)}^{+}$ and ${\mathrm{C}}_{60}^{\mathrm{-}}$, is extended; thereby leading to both the enhanced photoinduced response of P3OT and the appearance of the 1.15- and 1.25-eV signatures associated with ${\mathrm{C}}_{60}^{\mathrm{-}}$.