(a) Field of the Invention
The present invention relates to an electrophotographic photosensitive product suitable for use in copying machines, printers, facsimile equipment, electronic photolithography, solar cells, photoelectric converting elements such as electroluminescent elements, light converting elements, and optical discs. Particularly, the present invention relates to an electrophotographic photosensitive product having a photosensitive layer formed of a charge generating substance and a charge transporting substance.
(b) Description of the Related Art
Conventionally, inorganic substances such as selenium (Se), cadmium sulfide (CdS), zinc oxide (ZnO), and amorphous silicon (a-Si) are used as photoconductive materials of photosensitive products used in electrophotographic systems. Photosensitive products including the above inorganic photoconductive materials have many advantages, but also have various disadvantages such as a property of being harmful, problematic disposal, and a relatively high cost. In recent years, therefore, many organic photosensitive products free of these disadvantages have been proposed and practically used.
These organic photosensitive products are classified into functionally layered photosensitive products and single-layer photosensitive products in accordance with their structures. A functionally layered photosensitive product has a multi-layer structure in which a material for generating charged carriers (hereinafter referred to as a charge generating material) is layered separately from a material for transporting charged carriers generated in the charge generating material (hereinafter referred to as a charge transporting material). In a single-layer photosensitive product, generation of charged carriers and transport of the generated charged carriers are both performed within a single layer. The functionally layered photosensitive product is preferable because it provides wider selection of materials and can provide a higher sensitivity. Charge transporting media are classified into two types. One type uses polymeric photoconductive compounds such as polyvinylcarbazole while the other type uses low-molecular photoconductive compounds dissolved in a binder.
Since a polymeric photoconductive compound described above does not by itself have a sufficient film-forming property or bonding property, a plasticizer, a binder polymer, and other materials are added thereto for improving the properties. This addition may reduce sensitivity of a photosensitive product, thus hindering practical use of the photosensitive product.
By contrast, a photosensitive product using an appropriately selected low-molecular weight photoconductive compound provides excellent mechanical characteristics, while fails to provide a sufficiently acceptable sensitivity or stability against repeated processing. For example, a diaryl alkane derivative disclosed in U.S. Pat. No. 3,820,989 has substantially no particular problem regarding the compatibility with a binder polymer, while shows poor stability against light. Hence, when the derivative is used in a photosensitive layer of a product for electrophotographic use, the sensitivity of the photosensitive product gradually decreases as the material is repeatedly charged and exposed.
When either a stilbene compound described in Patent Publication No. JP-A-1983-65440 or N,N'-tetra (3-methylphenyl)-4,4'-diaminostilbene described in Journal of the Electrophotographic Society, vol. 25, No. 3, pp. 16-(1986), is used as a photosensitive product, sufficient sensitivity and repeatability are not provided due to an insufficient transporting capability.