1. Field of the Invention
This invention relates to a process for preparing photoconductive elements and also to a process of preparing electrophotosensitive materials from such photoconductive elements.
2. Description of Prior Art
As is well known, in electrophotographic processes electrostatic charges are uniformly applied to a layer of a photoconductive substance, the photoconductive layer is imagewise exposed to thereby discharge the electrostatic charges in the exposed areas, and the images of the remaining electrostatic charges (electrostatic latent images) are developed with colored particles (toner) having charges of the opposite polarity to form visible images. As photoconductive substances used for electrophotosensitive materials, there are known inorganic substances such as amorphous selenium, a compound semiconductor, etc.; organic polymers such as polyvinylcarbazole, etc.; and compositions composed of various organic compounds and organic dyes or pigments as sensitizers. Among these materials, amorphous selenium is most frequently used.
Electrophotosensitive materials based on amorphous selenium are usually prepared by vapor depositing selenium on a conductive support. However, with this technique, the continuous production of the electrophotosensitive materials is difficult and production costs are high. Also, vapor deposited amorphous selenium layers are only sensitive to light of wavelengths shorter than 520 mm and have such demerits that they are liable to crystallize during use (for example if scratched) and thus their characteristics as photosensitive materials tend to deteriorate.
To overcome these drawbacks it has been proposed to disperse selenium particles and particularly trigonal selenium particles in a layer formed on a conductive support. Trigonal selenium is sensitive to substantially all visible light, however, due to its low dark electric resistance it cannot hold a sufficient charge unless it is dispersed in an insulating binder. U.K. Pat. No. 1,506,929 describes photoconductors prepared by mechanically pulverizing the crystalline (trigonal) selenium and dispersing the pulverized crystals in a polymer (PVK) having a charge carrier transporting ability. U.S. Pat. No. 3,787,208 also describes photoconductors prepared by mechanically pulverizing solid amorphous selenium in liquid nitrogen and dispersing it in a polymer. In the course of the preparation the layer of dispersed selenium is heated to 230.degree. C. which may convert the selenium to the trigonal form although this is not clear from the disclosure. It is still difficult even using these techniques to prepare photoconductive layers having fine selenium particles formed by mechanical pulverization uniformly dispersed therein.
On the other hand, a method of preparing layer of a dispersion of fine selenium particles utilizing light irradiation is described in U.S. Pat. No. 3,994,791. A layer comprising elemental selenium dispersed in a polymer is prepared by forming on a conductive support a layer of an organic selenium compound dispersed in an insulating polymer (e.g., poly-N-vinylcarbazole) and uniformly exposing the layer to light to decompose the organic selenium compound. However, in this method the organic selenium compounds used are generally unstable and expensive and the compounds, their decomposition products or by-products formed during their synthesis are for the most part toxic or bad smelling. Furthermore, when an organic selenium compound is photodecomposed in a polymer layer, the compound disposed nearer the surface of the layer is decomposed to a greater extent due to the nonuniform absorption of light. Consequently, it is difficult to produce photoconductive layers having elemental selenium uniformly dispersed therein by the aforementioned method. Also, the technique is undesirably restricted in that the polymers used in the method are limited to insulating polymers having charge carrier transporting characteristics.
U.S. Pat. No. 4,115,115 also describes a method for forming a layer of elemental selenium dispersed in a polymer by reacting an organic selenium compound and dibenzoyl peroxide in a polymer. This method, however, has the drawbacks of the aforementioned method attributed to the use of organic selenium compounds, and benzoyl peroxide has an explosive property so great care must be taken for its handling.