1. Field of the Invention
The present invention relates to a silicon-containing triarylamine compound, a method of its production and a high-sensitivity high-durability photoreceptor for electrophotography incorporating said silicon-containing triarylamine compound as the charge transport material.
2. Description of the Prior Art
In recent years, there have been noticeable improvements in copying machines and printers based on electrophotography. A wide variety of new models of different shapes, types and functions have been developed according to uses, and various photoreceptors for such new models have been developed accordingly.
Traditionally, photoreceptors for electrophotography have been based mainly on an inorganic compound from the viewpoint of its sensitivity and durability. Examples of such inorganic compounds include zinc oxide, cadmium sulfide and selenium. However, these substances are often toxic, thus posing problems of disposal and environmental pollution. In addition, when using selenium, which offers good sensitivity, it is necessary to form a thin film on a conductive substrate by vacuum deposition, etc., which results in poor productivity and increased production cost.
Amorphous silicon has recently drawn much attention as an inorganic photoreceptor free of environmental pollution, and research and development activities are in progress. However, the use of the plasma CVD method for film formation results in extremely poor productivity and increased photoreceptor production cost and running cost, though excellent sensitivity is obtained.
On the other hand, organic photoreceptors have the advantage of being free of environmental pollution since they are burnable, many of which permit easy mass production since they are coatable for thin films. These aspects are advantageous in that significant cost reduction is possible and a wide range of shapes can be obtained according to uses. It should be noted, however, that organic photoreceptors involve some problems to be solved as to sensitivity and durability; there are strong demands for the development of an organic photoreceptor with high sensitivity and high durability.
Various methods have been proposed to improve the sensitivity of organic photoreceptor, but the current main stream uses photoreceptors having a double-layered photoconductive structure in which functions are allotted to a charge generation layer and a charge transport layer. For example, the charge generated in the charge generation layer in response to exposure is injected to the charge transport layer, through which it is transported to the surface of the photoreceptor, where the surface charge is neutralized and an electrostatic latent image is formed thereon. Thus, in the double-layered photoconductive structure type in comparison with the single-layered structure type, the generated charge is less likely to be trapped, and the charge can be more efficiently transported to the surface of the photoreceptor (U.S. Pat. No. 2,803,541).
As the organic charge generation material for the charge generation layer, a compound which absorbs the energy of the irradiated light and efficiently generates a charge is selected and used. Examples of such compounds include an azo pigment (Japanese Patent Publication Open to Public Inspection No. 14967/1979), a metal-free phthalocyanine pigment (Japanese Patent Publication Open to Public Inspection No. 143346/1985), a metallophthalocyanine pigment (Japanese Patent Publication Open to Public Inspection No. 16538/1975) and a squarylium salt (Japanese Patent Publication Open to Public Inspection No. 27033/1978).
As the charge transport material for the charge transport layer, it is necessary to select a compound which offers a high efficiency of charge injection from the charge generation layer and a high charge mobility in the charge transport layer. To meet these requirements, a compound having a low ionization potential or a compound which easily releases a cationic radical is selected. Examples of such compounds proposed include a triarylamine derivative (Japanese Patent Publication Open to Public Inspection No. 47260/1978), a hydrazone derivative (Japanese Patent Publication Open to Public Inspection No. 101844/1982), an oxadiazole derivative (Japanese Patent Examined Publication No. 5466/1959), a pyrazoline derivative (Japanese Patent Examined Publication No. 4188/1977), a stilbene derivative (Japanese Patent Publication Open to Public Inspection No. 198043/1983), a triphenylmethane derivative (Japanese Patent Examined Publication No. 555/1970), and a 1,3-butadiene derivative (Japanese Patent Publication Open to Public Inspection No. 287257/1987).
As stated above, however, these organic photoreceptors are lower than inorganic photoreceptors in charge mobility, and their sensitivity remains unsatisfactory.
Also, in a series of electrophotographic processes of charging, exposure, development, toner transfer and discharging, the photoreceptor is subject to extremely severe conditions and its ozone resistance and abrasion resistance are of great concern. Although improvements in these durability properties are demanded, no satisfactory photoreceptors have been developed.
Above all, triarylamine has been investigated for a large number of derivatives, including monotriarylamine (Japanese Patent Publication Open to Public Inspection Nos. 195254/1982, 118147/1989 and 118142/1989), bis(diarylamino)benzene (Japanese Patent Publication Open to Public Inspection Nos. 144250/1980, 118144/1989 and 118146/1989) and bistriarylamine (Japanese Patent Publication Open to Public Inspection Nos. 27033/1978, 35140/1981, 52756/1981 and 142647/1989).
However, monotriarylamine has drawbacks including insufficient charge mobility and a lack of stability, though it is generally easy to synthesize. As for bis(diarylamino)benzene and bistriarylamine, they have drawbacks such as a lack of stability and difficult synthesis, though they offer a fair level of charge mobility. Also, these triarylamine derivatives all have been suggested to pose a problem regarding a lack of compatibility with binder resin.
As stated above, conventional organic photoreceptors such as triarylamine derivatives have many drawbacks, and there are strong demands for improvements to overcome these drawbacks in the relevant technical field.
It should be noted that a silicon-containing triarylamine compound which is similar to the present invention is disclosed in U.S. Pat. No. 2,960,517, but it is a monosilyltriarylamine compound and is unsuitable for use as a charge transport material for photoreceptor for electrophotography because it is liquid at normal temperature. In addition, it is produced by reaction of a bromophenylalkylsilane compound and an alkali metal salt of amine, and these starting materials are not easily available and this reaction requires severe conditions both for reaction temperature and for reaction time.