In the art of xerography and/or electrophotography, an imaging member is imaged by first uniformly electrostatically charging the imaging member. The imaging member is then exposed to a pattern of activating radiation, for example light, which selectively dissipates the charge in the illuminated areas of the imaging member while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles, for example from a developer composition, on the surface of the imaging member. The resulting visible toner image can be transferred to a suitable image receiving substrate such as paper. This imaging process may be repeated many times with reusable photosensitive members.
Typical imaging members (for example, photoreceptors) comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer is illustrated, for example, in U.S. Pat. No. 4,265,990, which describes a photosensitive member having at least two electrically operative layers. The first layer comprises a photogenerating layer, which is capable of photogenerating holes and injecting the photogenerated holes into the second layer, which comprises a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photogenerating layer is sandwiched between the contiguous charge transport layer and the supporting conductive layer, and the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge.
Layered imaging members have been described in numerous U.S. patents, such as U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference, wherein there is illustrated an imaging member comprised of a photogenerating layer and an arylamine hole transport layer. Examples of photogenerating layer components include trigonal selenium, metal phthalocyanines, and metal free phthalocyanines. Additionally, there is described in U.S. Pat. No. 3,121,006, the disclosure of which is totally incorporated herein by reference, a composite xerographic photoconductive member comprised of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder.
Although excellent toner images may be obtained with multilayered belt photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators and printers continue to be developed; there is a greater demand on copy quality. A delicate balance in charging image and bias potentials, and characteristics of toner/developer, must be maintained. This places additional constraints on the quality of photoreceptor manufacturing. In certain combinations of materials for photoreceptors or in certain production batches of photoreceptor materials, localized microdefects (for example, of sizes varying from about 50 to about 200 microns) may occur. These microdefect sites appear as print defects in the final imaged copy. In charged area development, where the charged areas are printed as dark areas, the microdefect sites may print out as white spots. These microdefects are called microwhite spots. In discharged area development systems, where the exposed area (discharged area) is printed as dark areas, these microdefect sites may print out as dark spots in a white background. All of these microdefects that exhibit inordinately large dark decay are referred to as charge deficient spots (CDS). Since the microdefect sites are fixed in the photoreceptor, the spots may be registered in every cycle of the imaging member.
Charge generating compositions may be comprised of charge generating pigment dispersed in a polymer binder. However, such formulations may exhibit dispersion settling and aggregation of pigments. This may adversely result in charge deficient spots and other printing defects.
A stable charge generating composition, for example that may exhibit non-Newtonian, thixotropic rheology, and substantially free of aggregation of the charge generating pigments therein, is desired. Also desired is a charge generating composition that may be used in forming an imaging member exhibiting excellent quality, for example with substantial elimination of charge deficient spots, and capable of producing high quality images.