Large enhancement of the transient and steady-state photoconductivity of conducting polymer/C60 composite films

We present the transient and steady-state photoconductivity (PC) results obtained from poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly(octylthiophene) (P3OT), and from conducting polymer films sensitized with several concentrations of C60. Both the magnitude and the lifetime of the transient PC increase substantially on increasing the concentration of C60. The results imply that quantum efficiency for photogeneration of charge carriers is increased by ultrafast photoinduced electron transfer and that recombination is inhibited by the spatial separation of the electron and hole on the C60 and the conducting polymer, respectively. The greater enhancement of the transient PC in MEH-PPV (nearly two orders of magnitudes with addition of a few percent of C60) than in P3OT (nearly one order of magnitude with addition of a comparable amount of C60) suggests a higher probability of the early-time recombination in pristine MEH-PPV. As a result of the enhancement in the photogeneration efficiency and the carrier lifetime, the spectral response of the steady-state PC in both MEH-PPV/C60 and P3OT/C60 films shows significantly enhanced PC throughout the entire spectral range from the near infrared to the ultraviolet.