We study the ultrafast charge generation in a semiconducting polymer (MEH-PPV) by measuring the radiated THz field after photoexciting the biased polymer with a femtosecond visible pulse. The subpicosecond temporal characteristics of the emitted wave reflects the ultrafast photoconductivity dynamics and sets an upper limit for charge generation of 200 fs following photoexcitation, and reveals the dispersive nature of charge transport in MEH-PPV. A comparison of the fields radiated from MEH-PPV and the well-characterized model semiconductor system ͑GaAs͒ allows for an accurate estimate of the quantum efficiency for charge generation in the polymer, found to be less than 1%. Both observations are consistent with ultrafast charge generation in semiconducting polymers through hot exciton dissociation. Semiconducting conjugated polymers have recently received much interest owing to their potential in technological applications, particularly in electronics. 1 Despite this fact, the fundamentals of the photoexcitation physics in these materials have remained controversial: there is ongoing debate whether a band structure description is appropriate to these materials. 2,3 In this picture, interband excitations directly generate free charges, which may relax to lower energy bound states. This strongly contrasts the exciton model, in which molecular excitons are formed upon photoexcitation, so that secondary processes such as exciton-exciton annihilation 4 or hot exciton dissociation 5,6 are required to generate free charges.Although the observation of significant photoconductivity 7 demonstrates the presence of free charges, it is unclear whether carriers are generated directly on ultrafast time scales or produced on longer time scales through, for example, exciton-exciton annihilation. Unfortunately, conventional transient photoconductivity measurements 8,9 lack the resolution to resolve the ultrafast excitation process. Using time-domain THz spectroscopy, 10 we have recently shown that photoexcitation of the polymer poly(2-methoxy-5-(2Ј-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) leads to the generation of charges within 300 fs. 11 Additional information about the time scale of charge carrier generation and nature of the transport can be obtained by probing the radiation dispersed by the accelerating charges. 12,13 When changes in photocurrent occur on (sub) picosecond time scales, THz radiation is emitted. The shape of the THz pulse is determined by the rise and decay of the photocurrent, and thus provides a direct probe of the ultrafast transient photoconductivity. In this manner, information regarding the rate of charge carrier generation and time-dependent mobility can be obtained. This technique is ideally suited to the study of charge generation and cooling on sub-picosecond time scales, and has been successfully applied to study the photoexcitation dynamics in materials such as low-temperature grown GaAs. 13 Previous measurements on semiconducting polymer systems have also proven useful, 12 but suffered from...