The present invention is a layered electrophotographic photoconductor, i.e., a photoconductor having a metal ground plane member on which a charge generation layer and a charge transport layer are coated, in that order. Although these layers are generally separate from each other, they may be combined into a single layer, which provides both charge generation and charge transport functions. Such a photoconductor may optionally include a barrier layer located between the metal ground plane member and the charge generation layer, and/or an adhesion-promoting layer located between the barrier (or ground plane member) and charge generation layer, and/or an overcoat layer on the top surface of the charge transport layer.
In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of this surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor surface, either exposed or unexposed to light, depending upon the relative electrostatic charge on the photoconductor surface and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively- or positively-charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor's surface, pulling the toner from the photoconductor surface onto the paper or the transfer medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rolls melts and fixes the toner on the paper, subsequent to direct transfer or indirect transfer when an intermediate transfer medium is used, producing the printed image.
The electrostatic printing process, therefore, comprises an on-going series of steps in which the photoconductor surface is charged and discharged as the printing takes place. It is important to keep the charge voltage on the surface of the photoconductor relatively constant as different pages are printed to make sure that the quality of the images produced is uniform (cycling stability). If the charge/discharge voltage is changed significantly each time the drum is cycled, i.e., if there is fatigue or other significant change in the photoconductor surface, the quality of the pages printed will not be uniform and will not be satisfactory.
Hydrazone derivatives, which have frequently been employed as charge transfer molecules and organic photoconductors for electrophotography, possess interesting photochemical properties which are known to connect closely with the so-called fatigue phenomenon of photoconductors. A good deal of research supports the fact that photoisomerization and photochemical reactions are responsible in large part for the fatigue phenomenon. For example, p-(diethylamino) benzaldehyde diphenyl hydrazone (DEH) undergoes a photochemically-induced unimolecular rearrangement to the indazole derivative, 1-phenyl-3-(4-(diethylamino)-1-phenyl)-1,3-indazole. The following articles give an overview of the mechanism of photo-induced fatigue in electrophotographic conductors: J. Pacansky, et al., Chem. Mater. 4:401(1992); T. Nakazawa, et al, Chem. Lett. 1992, 1125; and E. Matsuda, et al., Chem. Lett. 1992, 1129.
In order to use hydrazones as charge transport molecules for electrophotographic applications, photo-induced fatigue has to be reduced to an acceptable level. There are two major paths to minimize photo-induced chemical changes in hydrazone molecules such that photo-induced fatigue of photoconductors can be improved: (1) introducing appropriate substitution on the hydrazone molecules to increase rigidity such that photo-induced cyclization or isomerization can be hindered; and (2) formulating with additives, e.g., a light absorber, in the charge transport layer to filter away the harmful wavelength light (See, for example, U.S. Pat. No. 4,362,798, Anderson, et al.). The former approach will inevitably increase the cost to produce the molecules as compared with the corresponding unsubstituted hydrazones. Therefore, the approach of current choice is the use of additives, such as Acetosol Yellow, to serve as a light filter. Although this approach is effective in reducing room light fatigue of the photoconductor to a certain degree, it also negatively effects the electrical properties of the photoreceptor by increasing discharge voltage and dark decay.
Azines, which are the product of condensing the remaining NH.sub.2 of a hydrazone with a carbonyl compound, have been disclosed for use in electrophotographic applications, both as transport molecules and as dopants in charge transport layers. Several series of hydrazones and azines are disclosed as charge transport materials in DE3716982, JP62006262 and JP61209456. In addition, some azines have been taught to be used in combination with hydrazones in electrophotographic conductors (see, for example, JP61043752, JP61043753, and JP61043754). It is important to note that these azines are not the fluorenyl-azine derivatives used in the present invention.
Fluorenyl-azines are known in the art. For example, 9-[p-(diethylamino) benzylidenehydrazono)] fluorene has been disclosed in JP57138644 and JP59195659 as a charge transport agent.
U.S. Pat. No. 4,415,640, Goto, et al, issued Nov. 15, 1983, discloses flourenyl-azines of the type utilized in the present development. The materials are disclosed as charge transport materials, not as adjunct materials used together with another charge transport molecule (see, for example, column 6, lines 52-54; column 7, lines 30-32; and column 8, lines 62-68). The use of these fluorenyl-azines as charge transport materials is taught to minimize photoconductor fatigue.
It has now unexpectedly been found that addition to a DEH-containing charge-transport layer of a flourenyl-azine material provides elimination of room light fatigue and cycling fatigue in the resulting photoconductor. For example, a photoconductor containing a DEH-charge transport layer doped with 2-5% azine, exhibits no fatigue after four hours of fluorescent light exposure, while the same photoconductor containing the standard Acetosol Yellow filtering agent exhibits negative fatigue. Increasing the Acetosol Yellow concentration in the charge transport layer, results in negative affects on the sensitivity of the photoconductor and dark decay, while no such effects are observed with the azine material.