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) approximate to 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 an the carrier recombination dynamics. The quantum efficiency is the same (phi(0) approximate to 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;ib the higher photon energy, providing a natural explanation for the increase in the photoconductivity at photon pump energies above 3 eV.