We describe general requirements of organic photoreceptors used for electrophotographic applications. A typical organic photoreceptor consists of a charge generation layer (CGL) and a charge transport layer (CTL). The separation of the generation and transport functions has permitted the optimization of both layers in terms of sensitivity as well as device lifetime. Dual-layer and negative-charged/hole- transporting organic photoreceptors are of primary interest in device applications, which are the focus here. The spectral sensitivity, charge mobilities, and the general characteristics of charge acceptance and photoinduced discharge of these materials are discussed. In flash exposure of electrostatic latent image, reciprocity failures reduce the sensitivity of photoreceptors and are key issues associated with digital electrophotography. We discuss reciprocity failures relating to Langevin recombination and perhaps exciton-exciton annihilation at high exposure conditions. Other related issues such as latent image spread and fatigue are also discussed.
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