Dual-layered organic photoreceptors, which consist of a carrier generation layer and a carrier transport layer, are widely utilized in photocopiers and laser-beam printers because of their high sensitivity. Photocarriers are believed to be generated in the carrier generation layer. We found that photocarrier generations in azo-pigment-based layered photoreceptors are highly sensitized by the carrier transport materials used in the carrier transport layer. This confirms that photocarriers are generated at the interface between the carrier generation layer and the carrier transport layer. In this article, this extrinsic mechanism will be reviewed. First, an overview of the sensitization phenomnon is presented, and then the overall photocarrier generation is divided into elementary processes. Next, the mechanism of each elementary process assumed is discussed. Finally, the photocarrier generation site at a molecular interface between the azo pigment and the carrier transport material is investigated from microscopic and structural viewpoints.