1. Field of the Invention
This invention relates to an image holding member for electrophotography sensitive to electromagnetic waves such as light (here referred to a light in a broad sense including ultraviolet ray, visible light, infrared ray, X-ray, .gamma.-ray and the like).
2. Description of the Prior Art
Heretofore, as photoconductive materials constructing image holding members for electrophotography, there have been known generally inorganic photoconductive materials such as Se, Cds, ZnO and the like and organic photoconductive materials (hereinafter referred to as "OPC") such as PVK (polyvinylcarbazole), TNF (tetranitrofluorenone) and the like. However, at present the known photoconductive materials can not simultaneously satisfy the characteristics required for image holding members for electrophotography such as high sensitivity, good S/N ratio (photo-current/dark current), wide range absorption spectrum, high light response, high dark resistance, stability upon repeated use and the like.
There have been recently proposed various practical OPC type image holding members since they are of low cost and do not cause any environmental pollution, and as a result of improvement, they have now excellent electrophotographic fundamental characteristics such as latent image formation, photosensitivity and the like which there is much room for improvement as to stability upon repeated use such as resistance to mechanical, optical or electrical external action. In particular, with respect to mechanical action, the surface of the OPC type image holding member is easily scratched even with a slight action force such as, for example, the action force of a blade used for cleaning the remaining toner (such cleaning is essential to electrophotographic processes) since the surface of the image holding member is made of an organic material. While the member is repeatedly used, the toner attaches to the scratched portions and therefore, the copied images become unclear gradually. Thus, the image holding member can be used only a limited number of times.
In order to supplement the mechanical strength of the OPC image holding member, there is proposed to form a surface protective layer of various kinds, for example, laminating with an organic material layer having a higher hardness by coating, depositing an inorganic layer by means of vacuum vapor deposition, laminating with a material capable of being surface-hardened by applying a thermal hysteresis, and the like.
However, these methods can give a sufficient function of a protective layer, but there are caused side effects undesirable for fundamental characteristics of electrophotography such as lowering of resolution, lowering of image quality, change of characteristics during using for a long time and the like, and when it is tried to make the characteristics compatible, all the characteristics often become insufficient resulting in impractical image holding members.
As mentioned above, any satisfactory results are not obtained. This results from the fact that there are very few materials which are excellent in mechanical strength and satisfy the function of the most surface layer of electrophotographic image holding members with respect to electric and optical characteristics. Even if there are obtained the above-mentioned desirable materials, there is not easily available a means suitable for laminating the material to form freely a thin and uniform layer without damaging the photoconductive layer. This is one of the reasons retarding solving the above problems.
Among the various methods proposed heretofore, the most popular method is a coating method in which a constant viscosity of a coating slurry is required so as to coat uniformly, but when a high viscosity is employed, the resulting coated layer is apt to be thick. Most of the materials having high hardness are non-photoconductive so that they retard photoconductive action and the image quality is lowered when the protective layer is unnecessarily thick.
Alternatively, for example, when an inorganic thin film such as SiO.sub.2 and the like is produced by means of a vacuum vapor deposition, the substrate temperature should be elevated to some extent to produce a good quality film, but OPC layers are usually weak to heat and are subjected to thermal damage upon forming a protective layer. In addition, vacuum vapor deposition, sputtering and the like have a directivity with respect to film forming atoms incident upon the substrate surface so that uniform and large area can be disadvantageously obtained with difficulty.