The present invention relates to an electrophotographic imaging member having an improved hole blocking layer.
Typical electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated, for example, in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. The disclosure of this patent is incorporated herein in its entirety. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer the photogenerating layer sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting conductive layer is utilized as an anode.
As more advanced, complex, highly sophisticated, electrophotographic copiers, duplicators and printers were developed, greater demands were placed on the photoreceptor to meet stringent requirements for the production of high quality images. For example, the numerous layers found in many modern photoconductive imaging members must be uniform, free of defects, adhere well to to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a drum or belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a charge blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an overcoating layer. Although excellent toner images may be obtained with multilayered photoreceptors, it has been found that the numerous layers limit the versatility of the multilayered photoreceptor. For example, these photoreceptors often comprise a metal substrate having a roughened surface to avoid imagewise constructive interference effects, known as plywooding, that can occur with laser exposure systems. This surface is coated with a typical film forming hole blocking layer such as nylon, zirconium silane, and the like, to provide the charge blocking function. These materials, especially nylons, depend on water content to provide sufficient conductivity to bleed off negative charge residual in the charge generating layer. Although many electrophotographic imaging members perform well under normal ambient atmospheric conditions, they are sensitive to relative humidity such that their performance degrades in low and high humidity conditions. This is due to insufficient bleeding off of charge under low humidity conditions. Under high humidity conditions, the layer becomes more conductive and too much charge bleeds off between the uniform charging step and image developing step leading to print defects which appear as black spots in the background areas with a discharge area development printer, copier or printer. For high quality precision copiers, duplicators and printers, it is important to have a photoreceptor which maintains the same excellent print quality throughout the entire range of ambient environmental conditions.
For electrophotographic imaging systems which utilize uniform negative polarity charging prior to imagewise exposure, it is important that the charge blocking layer bleeds off negative charge while preventing positive charge leakage.
Although insulating type polymers can efficiently block hole injection from the underlying ground plane, their maximum thickness is limited by the inefficient transport of the photoinjected electrons from the generator layer to the substrate. If a charge blocking layer is too thick, resistivity of the layer increases and blocks passage of both negative and positive charges. Thus, the charge blocking coating must be very thin and this thin blocking layer coating often presents still another problem, the incomplete coverage of the underlying substrate due to inadequate wetting on localized unclean substrate surface areas. Coating thickness non-uniformity will lead to charge leakage. Further, blocking layers that are too thin, e.g. less than about 0.5 micrometer in thickness, are more susceptible to the formation of pinholes which allow both negative and positive charges to leak through the charge blocking and result in print defects. Also, when charge blocking layers are too thin, small amounts of contaminants can adversely affect the performance of the charge blocking layer and cause print defects due to passage of both negative and positive charges through the layer. Defects in hole blocking layer which allow positive charges to leak through lead to the development of charge deficient spots associated with copy print-out defects.
Moreover, alteration of materials in the various photoreceptor layers such as the charge blocking layer can adversely affect overall electrical, mechanical and other electrophotographic imaging properties such as residual voltage, background, dark decay, adhesion and the like, particularly when cycled thousands or hundreds of thousands of times in environments where conditions such as humidity and temperature can change daily.