Ultrafast detection of charged photocarriers in conjugated polymers

Ultrafast measurements of transient excited-state absorption in the spectral region spanning the infrared-active vibrational active (IRAV) modes in the prototypical luminescent polymers, poly(phenylene vinylene) (PPV) and poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV), reveal charge carrier generation within 100 fs after photoexcitation. The photocarrier quantum efficiency in MEH-PPV is ${\phi }_0\approx 0.1$ in zero applied electric field. There is no correlation between the temporal behavior of the photoinduced IRAV signals and the exciton lifetime. Thus, carriers are photoexcited directly and not generated via a secondary process from exciton annihilation. Comparison of the recombination dynamics in MEH-PPV and PPV demonstrates the importance of the strength of interchain interaction on the carrier recombination dynamics. The quantum efficiency is the same $({\phi }_0\approx 0.1)$ when the system is pumped either at photon energies well above the first $\pi -{\pi }^{*}$ transition (at 267 nm, 4.7 eV) or when pumped into the first $\pi -{\pi }^{*}$ transition (at 400 nm, 3.1 eV). The carrier lifetime, however, increases at the higher photon energy, providing a natural explanation for the increase in the photoconductivity at photon pump energies above 3 eV.