Ultrafast detection of charged photocarriers in conjugated polymers

Ultrafast measurements of transient excited-state absorption in the spectra l region spanning the infrared-active vibrational active (IRAV) modes in th e prototypical luminescent polymers, poly(phenylene vinylene) (PPV) and pol y[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV), reveal ch arge carrier generation within 100 fs after photoexcitation. The photocarri er quantum efficiency in MEH-PPV is phi(0) approximate to 0.1 in zero appli ed 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 excit on annihilation. Comparison of the recombination dynamics in MEH-PPV and PP V 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 explan ation for the increase in the photoconductivity at photon pump energies abo ve 3 eV.