Photoexcitation dynamics and laser action in solutions and films of PPE-PPV copolymer

We used a variety of ultrafast and steady-state spectroscopy techniques to study the photoexcitation dynamics and laser action in dilute and concentrated solutions and in thin films of poly(phenylene-ethynylene)/poly(phenylene-vinylene) [PPE-PPV] copolymer. We found that the primary photoexcitations in PPE-PPV solutions are relatively long-lived singlet excitons with lifetime of about 600 ps, consistent with the high-photoluminescence quantum efficiency. The singlet excitons have three distinct photoinduced absorption (PA) bands spanning the mid-IR and near-IR spectral ranges, which are correlated with a strong stimulated emission (SE) band in the blue spectral range. In thin PPE-PPV films, however, the SE band is much weaker because of its overlap with a new PA band that is due to long-lived polaron pairs. As a result we found laser action in dilute PPE-PPV solutions but not in films; this represents the first laser action reported in PPE-type polymers. A study of laser action in solutions of various copolymer concentrations confirms the lack of lasing in concentrated solutions and explains its absence in films. The polaron pair species, triplet excitons that are generated via intersystem crossing, and isolated charged polarons that are generated from polaron pair dissociation are also studied using steady-state spectroscopies.