Fullerenes enhance the photoconductivity of photoconductive polymers. This paper studies the mechansim of enhancement both experimentally and theoretically. The effects of fullerene doping on the spectroscopic, charge generation, and charge transport properties of the polymer are reported. Electrical field effects and magnetic field effects are examined. On the basis of these data a charge generation mechansim involving a singlet of weak charge-transfer complex is proposed for fullerene-doped N-poly (vinylcarbazole). A new theoretical model combining both the Onsager and Marcus theory is developed to quantitatively account for the field-dependent charge generation efficiency in nonpolar medium. Fullerenes, C 60 and C 70 , 1 are known to be good electron acceptors. 2,3 They can form charge-transfer complexes 4,5 or charge-transfer salts 6 with electron donors such as aromatic amines. Ferromagnetism 6 and enhanced second-order optical nonlinearity 7 have been observed from these charge transfer complexes. The direct optical excitation of fullerene can lead to excited state electron transfer reactons, The first fullerene-based polymeric photoconductor was demonstrated by doping fullerenes into polymers containing electron-donating moieties such as N-poly(vinylcarbazole) (PVK). The purpose of this paper is to study the effects of fullerene doping on the charge transport and charge generation properties of the host photoconductive polymer. It will be shown that the effects of fullerene doping on the hole transport property of the polymer are minimal. So the main focus of the paper will be on the charge generation mechanism of fullerene-doped polymeric photoconductors, in particular, fullerene-doped PVK. It is known that, upon photoexcitation, the singlet state of C 60 and C 70 undergoes efficient intersystem crossing to generate a long-lived triplet state (lifetime varies from micro-to milliseconds depending on the conditions) with a quantum yield of almost 1. This paper also address a more general issue regarding the quantitative treatment of charge generation processes in polymers. The field-dependent charge separation theory of Onsager 29 has been the standard model to use for analyzing the charge generation efficiency of polymeric photoconductors for many years, in spite of its generally recognized deficiencies. The theory predicts the probability of an electron-hole (e-h) pair separating to infinity by solving the diffusion equation of the relative motion of the e-h pair in the potential provided by their Coulomb interaction and applied external field. The origin of the e-h pair and the pathway by which it is generated are not considered in the model. An important boundary condition (and an assumption) for this model is that if the e-h pair separation distance reaches zero, the pair annihilates (with an infinitively fast rate). For low dielectric constant solids, the theory predicts a strong field-dependent charge generation efficiency depending on the initial e-h separation distance. The shorter the initial separation distance, the stronger the field dependence. Over the past decades, the field-dependent charge generation efficiency of many polymeric photoconductors have been fitted to the Onsager theory with apparent success
