The exciton binding energy (Eb) and the band gap energy (Eg) of poly(phenylene vinylene) are determined by high-resolution measurements of the photoconductivity excitation profile as a function of light polarization, applied electric field, and temperature. At high applied electric fields, a peak in the photoconductivity is observed when the sample is pumped at a photon energy just below the onset of the band-to-band -* absorption. This peak is interpreted as resulting from field ionization of a weakly bound exciton with E b Ϸ 60 meV. The binding energy is obtained from the energy of the exciton peak relative to the band edge and independently from analysis of the dependence of the exciton dissociation on field and temperature.A central issue in the field of conjugated polymers is the strength of the electron-electron interaction relative to the bandwidth (1): Is the attraction of a geminate electron-hole pair so strong that the photoexcitations are localized and strongly correlated Frenkel excitons? Or rather, are the charge carriers sufficiently well screened that a band picture supplemented by the electron-phonon interaction (polaron formation) and the electron-electron interaction (weakly bound excitons) is justified? Determination of the exciton binding energy (E b ) is critically important to answering these questions and thereby to understanding the electronic structure of semiconducting polymers.Because this issue has not been resolved, the extensive literature on the optical properties of semiconducting (conjugated) polymers contains two conflicting assignments for the lowest energy -* absorption (1).The lowest energy -* absorption results from the creation of tightly bound neutral singlet excitons with the onset of the interband transition at a significantly higher energy, as for example, in molecular crystals such as anthracene (2).The lowest energy -* absorption results from a direct band-to-band transition, as for example, in direct gap semiconductors such as GaAs.These two different assignments imply very different results for the photogeneration of charged excitations. When the lowest energy -* absorption results from a direct band-to-band transition, one expects to observe a threshold for photogeneration of charge carriers close in energy to the onset of absorption (E -); i.e., at[1a]If, however, E b is large, one expects to observe the threshold for photogeneration of charge carriers via the lowest band-to-band transition at energy greater than the onset of optical absorption by E b ; i.e., atFor poly(phenylene vinylene) (PPV) and several of its soluble derivatives, the quantum efficiency for photogeneration of charged excitations (polarons) has been measured, in zero external field, using ultrafast photo-induced absorption by infrared active vibrational modes (3-5). The results demonstrated charge carrier photogeneration with a single threshold that is close in energy to the onset of absorption, in agreement with Eq. 1a (5). Although photoconductivity data have been reported with a second thres...

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