Direct evidence of photoinduced electron transfer in conducting-polymer-C60 composites by infrared photoexcitation spectroscopy

We report direct spectral evidence of photoinduced electron transfer from the excited state of poly(3-octylthiophene), P3OT, onto C60 by infrared photoexcitation spectroscopy, 0.01 eV (100 cm-1) to 1.3 eV (11 000 cm-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 C60. Two new peaks are found in the photoexcited spectrum of the P3OT/C60 composite and assigned to the allowed HOMO (T1u)-LUMO (T1g) transitions of C60- with energies, 1.15 and 1.25 eV, in good agreement with calculated values. The photoinduced charge transfer from P3OT onto C60 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 C60-, 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 C60-.