Femtosecond exciton dynamics in DOO-PPV films

We studied the ultrafast exciton dynamics in DOO-PPV films using the technique of femtosecond photoinduced absorption decay. We found that the exciton dynamics contains an ultrafast decay component of about 2 ps that remains unchanged at intensities up to five times the threshold excitation intensity for the phenomenon of emission spectral narrowing. We argue that amplified spontaneous emission of waveguided or leaky modes, depending on film thickness and excitation geometry, is the dominant mechanism for both spectral narrowing and ultrafast decay component. In very thin films we found that the process of amplification of leaky waves may be responsible for the amplification of the 0-0 emission band rather than the 0-1 band, that has been usually observed at high excitation intensities in thicker films.