Ultrafast photoinduced charge generation in conjugated polymers

Transient excited-state absorption measurements in the spectral region spanning the infrared active vibrational active (IRAV) modes in 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 φo≈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. Our data indicate that the carrier recombination rate is sensitive to the strength of the inter-chain interaction.