Infrared ultrafast optical probes of photoexcitations in ∏-conjugated organic semiconductors

We measured the ultrafast dynamics of photoexcitations in a variety of semiconductor ∏-conjugated polymer films and solutions, in the spectral range from 0.13 eV to 1.05 eV and time domain from 100 fs to 800 ps. The measurements were made in the low signal regime, where the relative changes in transmission, ΔT/T <10-5. In pristine poly [2-methoxy-5(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) solution we found that the primary photoexcitations are excitons with a photoinduced absorption (PA) band PA1 at 1.0 eV. In pristine MEH-PPV film, however we found two PA bands, PA1 at 1.0eV and P1 at 0.3eV. We consider P1 to be due to a polarons that are photogenerated intrinsically via defects and/or impurities. When following the transient dynamics of PA1 in C60-doped PPV-based films we could easily measure the dynamics of exciton dissociation from the polymer chains onto the C60 molecules. We found the charge transfer time to range from about 100 fs to 100 ps, depending on the concentration of C60 molecules in the films.