A single-layer organic photoreceptor for electrophotography with relatively low concentrations of phthalocyanine pigments dispersed in a suitable insulating binder polymer, which exhibits the induction effect, is generally referred to as a high-gamma photoreceptor. Surface charges from corona ions can be injected into the photoconductive layer through chemicals adsorbed on the free surface generated by corona discharge. It has been confirmed experimentally and theoretically that an overcoat layer is effective in protecting the surface of the high-gamma photoreceptor from surface-charge injection. In this study, a high-gamma photoreceptor with X-type metal-free phthalocyanine pigment dispersed in polyester binder polymer is overcoated with isocyanate. The theoretical model, which takes into account the distributed structural-trap depth, demonstrates well the photoreceptor’s photoinduced-discharge and dark-discharge characteristics.