The present invention generally relates to a photosensitive member or photoreceptor for use in an image forming apparatus of an electrophotographic system such as a copying machine, a printer, etc., and more particularly, to a photoreceptor to be electrically charged through contact by a contact type charging device and an image forming apparatus employing such photoreceptor.
Commonly, in an electrophotographic process, for example, in a copying machine, printer or the like, it has been a normal practice to electrically charge the surface of a photoreceptor which is an electrostatic latent image holding member, by a charging device, and the charged region is subjected to exposure to image light to form an electrostatic latent image thereon, which is then developed into a visible image to be transferred onto a transfer material such as a copy paper sheet or the like.
As the charging device as referred to above, besides the non-contact type charging devices represented by corona charging devices such as corotron, scorotron chargers, etc., there have been known contact type charging devices which effect charging by causing charging members to directly contact the photoreceptor surface as in brush charging devices of stationary or rotary type, and charging devices employing charging rollers. Among these charging devices, attention has been directed to the contact type charging device, since it is very small in the deterioration on the photoreceptor surface or in the generation of ozone which affect adversely to human bodies as compared with the corona charging device.
During the manufacturing process of the electrophotographic photoreceptor, pin holes or pin hole-shaped flaws or layer lacking portions are often generated in the photosensitive layer thereof. Spray coating method, dipping coating method, and blade coating method have been commonly used to form an organic photoconductor (OPC). In these methods, photosensitive liquid is obtained by dispersing or dissolving photosensitive substance such as charge-generating substance and charge-transporting substance in a binder resin. According to spray coating method, the photosensitive liquid is sprayed onto a base member such as an aluminum drum or a resin film. According to dipping coating method, the aluminum drum or the resin film is dipped in the photosensitive liquid. According to the blade coating method, the liquid is painted on the aluminum drum or the resin film with a doctor blade. The problem of these methods is that bubbles which are formed in the liquid and may cause the generation of pin holes are liable to be formed in the photoconductor. In addition, since the organic photoconductor manufactured by these methods includes a comparatively soft binder resin as its main composition, the photoreceptor is liable to be damaged in replacing the photoreceptor or in removing jammed copy paper from a printer or a copying apparatus by a user or servicing personnel, and such damaged portion may form a pin hole or photosensitive layer lacking portion.
In charging the photoreceptor by the non-contact type charger, a short circuit rarely occurs between the base portion of the photoreceptor and the charging member even in the presence of such photosensitive layer lacking portion. But when the photoreceptor is charged by the contact type charger, the charging member to which a high voltage is applied contacts the surface of the photoreceptor, with the result that a short circuit occurs between the charging member and the base portion of the photoreceptor through the defective portion of the photosensitive layer. Such a short circuit is liable to be generated particularly by a brush charger. The occurrence of the short circuit causes the charging member or a high voltage-applying power source to be damaged and in addition, the entire surface of the photoreceptor in contact with the charging member to be uncharged because the charging member has the ground electric potential. As a result, a black stripe-like image noise is generated in reversal development, while a white stripe-like image noise is produced in normal development.
The following proposals have been made to solve the above-described problem caused by the short circuit: The proposal disclosed, for example, in Japanese Patent Laid-Open Publication Tokkaisho No. 61-148468 is such that a thin film insulating layer having a volume resistivity larger than 10.sup.12 .OMEGA. cm is formed on a conductive base portion of the photoreceptor, and thereafter, a photosensitive layer is formed on said insulating layer. Another proposal described in Japanese Patent Laid-Open Publication Tokkaihei No. 2-67575 is such that a barrier layer is provided between the base portion of the photoreceptor and the photosensitive layer.
The researches made by the present inventors have revealed that even though the insulating thin film or the barrier layer is formed between the base portion of the photoreceptor and the photosensitive layer, the charging member, for example, the charging brush in particular may be brought into contact with the insulating thin film or the barrier layer through a defective portion such as a pin hole of the photosensitive layer. Therefore, the insulating thin film or the barrier layer is destroyed because the soft base portion of the photoreceptor is incapable of supporting them, and consequently, a short circuit takes place after all.