US3795513A - Method of storing an electrostatic image in a multilayered photoreceptor - Google Patents

Abstract

A XEROGRAPHIC MEMBER HAVING A SUPPORTING SUBSTRATE WHICH CONTAINS AN ELECTRICALLY INSULATING BLOCKING LAYER, A LAYER OF VITREOUS SELENIUM CONTAINING A THALLIUM DOPANT OVERLAYS THE BARRIER LAYER AND A LAYER OF A PHOTOCONDUCTOR TRANSPORT MATERIAL OVERLAYS THE THALLIUM DOPED LAYER, THE MEMBER IS IMAGED BY UNIFORMLY POSITIVE CHARGING THE TOP SURFACE WHILE SUMULTANEOUSLY EXPOSING TO LIGHT, CHARGING TO A NEGATIVE POLARITY AND EXPOSING TO IMAGING LIGHT WHEREBY A LATENT ELECTROSTATIC IMAGE IS FORMED WITHIN THE THALLIUM DOPED LAYER.

Description

A. .1. CIUFFINI 3,795,513

March 5, 1974 METHOD OF STORING AN ELECTROSTATIC IMAGE IN A MULTILAYERED PHOTORECEPTOR Filed Sept. 2, 1971 mm mm INVENTOR. ANTHONY J. CIUF F INI ATTORNEY METHOD OF STORING AN ELECTROSTATIC IMAGE IN A MULTILAYERED PHOTORE- CEPTOR Anthony J. Ciuflini, Rochester, N.Y., assignor to Xerox Corporation, Stamford, Conn. Filed Sept. 2, 1971, Ser. No. 177,246 Int. Cl. G03g 13/00 U.S. Cl. 96-1 R 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This application relates to xerography, and more specifically to a novel device and method of imaging. In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image may then be developed to form the visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. This concept is described by Carlson in U.S. Pat. 2,297,691, and is further amplified and described by many related patents in the field.

When used in the conventional xerographic mode, a xerographic photoreceptor, which is normally in the form of a drum, is usually cycled through at least six basic steps. These include: (1) uniformly electrostatically charging the surface; (2) imaging the charged drum in the dark by exposure to a pattern of light which results in the formation of a latent electrostatic image on the drum surface; (3) developing the latent electrostatic image by cascading the drum with electroscopic toner particles which adhere to the drum surface to form a powder image; (4) transferring the powder image to a sheet of plain bond paper; (5) fusing the transferred image to the paper to form a permanent and visible copy; and (6) cleaning the drum surface. In order to prepare the drum for a second or duplicate image, all of the above six steps are employed a second time.

Where the prior art has attempted to store an electrostatic image on the surface of the photoreceptor, and develop this image the plurality of times, a number of problems occur. First, the photoreceptor must have an exceptionally low dark decay rate and little if any lateral conductivity. During the transfer of the toner image to a sheet of paper, and the subsequent removal of the paper from the photoreceptor, high fields are high enough to cause air breakdown and the deposition of unwanted charge in the background areas of the xerographic drum. This charge prints out on the second and following copies so that background on the copies is continuously increasing. In addition, techniques of this type are normally limited to producing a relatively small number of usable copies before the process must be discontinued and the drum surface cleaned.

United States Patent 0 Patented Mar. 5, 1974 ice It can therefore be seen that imaging methods in which the latent electrostatic image is stored within the photoreceptor member, and which can be cleaned between each copy, would have obvious advantages over the above described process.

One alternative method in xerography which utilizes special xerographic techniques, and usually specially treated or constructed photoreceptors, employs the concept of persistent conductivity in the formation of useful images. Persistent conductivity may be defined as a latent image which exists as a state of electrical conductivity on a photoconductive layer, and persists as an after effect of the exposure radiation. An example of the phenomenon is as follows:

A photosensitive member which includes a layer of a suitable specially prepared photoconductive insulator contained on a conductive substrate is exposed to an optical image in the form of a pattern of light. Because of a special treatment or structure, the exposed image areas of the photoconductor become more or less conductive depending upon the amount of light impinging on the photoconductive surface during the exposure step. A conductivity pattern is formed in the exposed areas and persists in the dark after exposure, while on the other hand, the background or unexposed areas remain relatively non-conductive.

One way in which this concept may be used to form useful images is to first expose a suitable photoconductive surface to an optical image which causes the formation of a latent conductivity pattern. Using a corona charging device, an electrical surface charge is then uniformly applied to the photoconductive surface. The areas previously exposed to light, which are of course conductive, dissipate the surface charge, while the nonconductive areas will retain a surface charge. This results in the formation of an electrostatic image which is a reversal of the latent conductivity pattern. This electrostatic image may then be developed by any conventional developing technique used in xerography.

One example of this imaging process is more fully defined in U.S. Pat. 3,427,157 in which a photoconductor which exhibits persistent conductivity comprises amorphous selenium containing about 0.001 to 5 weight percent thallium. In one embodiment, this structure is imaged by exposure to a pattern of light which forms a pattern of persistent conductivity on the photoconductive surface. This plate is then uniformly charged to a negative polarity which results in a charge pattern being formed on the unexposed areas of the plate. The charge pattern is then developed with toner particles and the toner image transferred to a sheet of paper and fixed to form a charge pattern in the unexposed areas. It can readily be seen that this technique, unlike conventional xerography, allows a greater number of duplicate copies of an original to be made without re-exposing the photoconductor layer for each cycle of operation.

In related copending application Ser. No. 99,412 filed on Dec. 18, 1970, now U.S. Pat. No. 3,709,683, entitled, Infrared Sensitive Image Retention Photoreceptor, a pho toreceptor, a photoreceptor storage device and technique which is related to the instant invention is described. In this system, a photosensitive member which includes the layer of photoconductive vitreous selenium or selenium arsenic which contains thallium in a concentration of about 2 to 5000 parts per million is imaged as follows:

The member is uniformly electrostatically charged followed by uniformly exposing the photoconductive layer to a source of visible radiation below the red portion of the visible spectrum. The member is then exposed to infrared radiation in the form of an image which results in the formation of a developable latent electrostatic image contained within the photoconductive layer.

It can be seen from the above discussion that image storage photoconductors have certain advantages over conventional xerography in that multiple copies may be made, requiring only that the photoreceptor surface be cleaned between. the formation of each image. These types of imaging techniques, provide high quality images from the first to last copy, and result in greater speed and efficiency in forming a plurality of the same image.

OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide a method of imaging which overcomes the above noted disadvantages.

It is a further object of this invention to provide a novel imaging method and photosensitive member.

It is a further object of this invention to provide an improved method of imaging suitable for duplicating.

It is yet another object of this invention to provide a novel method of forming a stored image within a photoconductive layer.

SUMMARY OF THE INVENTION The foregoing objects and others are accomplished in accordance with this invention by providing a novel multilayered xerographic plate which is capable of storing a developable image for an indefinite period of time. The structure comprises a grounded conductive support member which is overlayed with a blocking layer which prevents charge injection from the grounded support. A storage layer in which an electrostatic image is eventually stored overlays the grounding contact. This storage layer comprises vitreous selenium or a vitreous seleniumarsenic alloy both of which are doped with a suitable amount of thallium. Overlaying the storage layer is a transport layer which comprises an undoped layer of vitreous selenium or a vitreous selenium-arsenic alloy. A storage image is formed by the following technique: A flushing corotron is activated which supplies positive charge which is uniformly distributed over the entire surface while activating radiation such as light simultaneously floods the top surface layer. This serves to fill hole trap sites which are provided in the layer containing the thallium dopant. A negative surface voltage is then uniformly applied to the top surface layer. At this point the structure is now activated and ready for imaging exposure. The structure is then imaged by exposure to a pattern of activating light which results in a latent image formed within the storage layer. In the exposure area, electron-hole pairs are generated which result in a recombina tion or removal of positive holes which are trapped in the light exposed areas. After the exposure or imaging step, the remaining negative surface charge is conducted away by any suitable technique, such as by running a conductive roller over the surface of the device. At this point, a positive electrostatic image remains stored within the storage layer of the photoreceptor device and may be developed by any conventional xerographic technique such as cascading toner particles over the top surface of the imaging member. The developed image may be transferred to a sheet of paper or any other media, and the member cleaned repeatedly in room lights if necessary and as many copies made as required. When it is desired to change images, the first step is repeated which results in a filling up the emptied hole trapping sites, thus eliminating the stored image. The additional steps outlined above are then repeated to form a new image.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of the instant invention will become apparent upon consideration of the following disclosure of the invention, especially when taken in conjunction with the following drawings;

FIG. 1 represents a schematic illustration of one embodiment of the xerographic member contemplated for use in the instant invention.

FIG. 2 represents the first step in the imaging process of the instant invention.

FIG. 3 represents the second step of the imaging process.

FIG. 4 represents the third step of the imaging process.

FIG. 5 represents the final step of the imaging process.

DETAILED DESCRIPTION OF THE DRAWINGS In FIG. 1 of the drawings, reference character 10 illustrates one embodiment of an improved photoreceptor device suitable for use in the instant invention. Reference character 11 designates a support member which is preferably an electrically conductive material. The'support may comprise any conventional metal such as brass, aluminum, steel, or the like. It may also be in any convenient thickness, rigid or flexible and in any form such as a sheet, web, cylinder or the like. The support may comprise other materials such as metalized paper, a plastic sheet covered with a thin coating of aluminum, or glass coated with a thin conductive layer of chromium or tin oxide.

Reference character 12 illustrates a barrier or blocking layer which is made in a thickness great enough to withstand the field contemplated by the instant invention. The barrier layer must be of an inert material and thick enough so that no charge carrier of either polarity can flow across this layer making it electrically isolated from the supporting substrate. Thicknesses in the range of about 2 microns to 5 mil have proven satisfactory. The material comprising this interface may be of any suitable material such as an insulating resin or an oxide coating. Typical materials include polycarbonate resin, aluminum oxide and high. resistivity epoxies. A charge storage or trapping layer 13 overlays said barrier layer. Layer 13 comprises either vitreous selenium or a vitreous selenium-arsenic alloy each of which contain thallium in a concentration of about 5 to 5000 parts per million by weight (p.:p.m.). Layer 13 may range in thickness from about 2 to microns. A particularly suitable layer comprises vitreous selenium doped with thallium in a concentration of about 100 ppm. and in a thickness of about 30 microns. Storage layer 13 is overlayed with a charge transport layer 14.

Layer 14 may comprise any photoconductive material which does not have an excessive amount of thermal electron generation of high conductivity. Suitable materials comprise vitreous selenium and vitreous selenium alloyed with arsenic. This layer acts as a charge transport layer and may range in thickness from about 2 to 20 microns. Thicknesses less than about 2 microns will not support enough charge to give useful contrast potential while thicknesses greater than about 20 microns reduce the ultimate resolution at which satisfactory development can be attained.

The device of the instant invention is imaged in the following manner. As shown in FIG. 2, the device is first uniformly electrostatically charged to a positive potential by a corona charging unit 15 while being simultaneously flooded with radiation 16, such as white light. This results in the generation of electron-hole pairs 18 in the transport layer. The holes migrate through transport layer 14 and are trapped in trapping layer 13 at trapping sites 19. Alternatively, the uniform flooding of the surface with light may occur as a separate step after the positive charging step. The device is then uniformly negatively charged as illustrated in FIG. 3. The device is then exposed to imaging radiation 17 as shown on FIG. 4 which results in the generation of additional electronhole pairs in transport layer 14. The generation of these electron-hole pairs in the image area results in the recombination of holes an electrons at the hole trap sites in the areas exposed to the illumination. The remaining negative charge on the surface of the imaging members is then removed by passing a conductive rubber roller 20 over the surface of the imaging device. This results latent electrostatic image 22 which is contained within the photoconductive layer This latent electrostatic image may then be developed with toner particles 23 having a negative polarity as illustrated by FIG. 5. A second image may simpy be formed by cleaning the photoreceptor surface and re-applying toner or transferring the toner to a paper sheet. When a second or additional image is desired the simultaneous positive charging and exposure step is repeated which results in refilling of the empty positive trap sites in layer 13. The additional imaging steps described above are then repeated to form another image. Images formed by this technique are free of any image ghosting of the prior image.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples further specifically define the present invention with respect to making and imaging a multi-layered photoreceptor storage member. The percentages in the disclosure, examples and claims are by weight unless otherwise indicated. The photoconductive vitreous selenium and vitreous selenium alloys of the instant invention may be conveniently formed by any conventional vacuum deposition techniques well known to art. U.S. Pats. 2,753,278 and 3,312,548, incorporated herein by reference, illustrate typical methods of vacuum deposition which are suitable for use in forming the photoconductive layers of the present invention. The following examples and data are intended to illustrate various preferred embodiments of the instant invention.

EXAMPLE I A xerographic drum exemplary of the present invention is prepared as follows: An anodizing bath composed of oz. per gallon of sulfuric acid and 2 oz. per gallon of oxalic acid is used to form an anodized barrier layer on an aluminum cylinder 4.75 inches in diameter and 10.2 inches in length. During the anodizing, the current is maintained at about 18 amps. at 22 volts for about 30 minutes at a bath temperature of C. The resulting anodized barrier layer is about 50 microns thick. A 20 micron photoreceptor trapping layer comprising vitreous selenium containing 0.3 weight percent arsenic and 100 parts per million by weight thallium, is vapor deposited by conventional vacuum techniques over the anodized aluminum oxide surface. A 20 micron top layer of vitreous selenium is then vapor deposited over the thallium doped layer.

The structure of Example I is imaged by simultaneously charging the top surface of the drum with a positive corotron discharge while exposing uniformly to white light. The positive charges are transported through the top photoconductive layer and are trapped within the thallium doped layer. The corotron is adjusted to establish a trapped charging potential of about +800 volts. The top surface of the drum is then uniformly charged to a negative surface potential of about 800 volts. The drum is then exposed to imaging light which effectively discharges the negative surface charge resulting in the recombination of electrons and holes at the hole trap sites in the light struck areas. The remaining negative surface charge is eliminated by passing a conductive rubber roller over the drum surface. A final developable electrostatic image remains trapped within the thallium trapping layer. This image is then developed by cascading toner material over the drum surface.

EXAMPLE H A second drum is fabricated by the method of Example I except that the blocking interface layer comprises a 2.5 mil layer of polycarbonate resin. The blocking layer is made by dip coating in a methylene dichloride solution of polycarbonate. This solution is prepared by dissolving 300 grams of polycarbonate in 1800 ml. of methylene dichloride. Satisfactory visible images are made using the imaging method described in Example I.

Latent electrostatic images formed by the above techniques may be stored within layer 13 indefinitely, and developed at any convenient time. Storage times of several days have been observed. If desired, the latent electrostatic surface image formed by the imaging light 17 (FIG. 4) may also be developed. In this case, the negative surface charge would not be erased by conductive roller 20.

It should be understood that an additional feature of the instant invention includes varying the potential of the charged trapped Within the thallium doped layer. This is accomplished by impressing very high fields across the top storage layer such that the internal field is higher than fields which can be achieved across the same thickness of photoconductor using a conventional interface.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are also intended to be within the scope of this invention.

What is claimed is:

1. A method of imaging which comprises:

(a) providing a xerographic member having a supporting conductive substrate having thereon an overlaying blocking layer comprising an electrically insulating material, said blocking layer having sufficient resistivity and thickness to prevent the flow of electrical charges of either polarity through said layer,

a photoreceptor layer about 2 to microns in thickness comprising vitreous selenium or a seleniumarsenic alloy containing a thallium dopant in a concentration of about 5 to 5,000 parts per million by Weight overlying said blocking layer, and

a layer of photoconductor transport material about 2 to 20 microns in thickness overlaying said thallium doped layer;

(b) uniformly charging the top surface of said member to a positive polarity while simultaneously uniformly exposing to a source of activating radiation, resulting in positive charge being trapped in the thallium doped layer;

(c) charging said layer to a negative polarity;

(d) exposing said member to a source of activating radiation whereby a latent electrostatic image is formed within said thallium doped layer;

(e) removing the remaining negative charge from the surface of said member; and

(f) developing said latent image to form a visible image.

2. The method of claim 1 in which, following the developing step, the member is uniformly charged to a positive polarity while simultaneously exposing to an activating source of radiation, thereby eliminating the stored latent electrostatic image.

3. The method of claim 1 in which the uniform exposure is carried out as a separate step after the uniform positive charging.

4. The method of claim 1 in which the latent image formed on the top surface is developed without the re moval of the negative surface charge.

References Cited UNITED STATES PATENTS 2,901,348 8/1959 Dessauer et al. 961 3,256,089 6/1966 Clark et a1. 961 3,355,289 11/1967 Hall et al. 961.4 3,427,157 2/1969 Cerlon 961.4 3,573,906 4/1971 Goffe 96l.8 3,598,582 8/1971 Herrick et a1. 96l.5

GEORGE F. LESMES, Primary Examiner M. B. W'I'ITENBERG, Assistant Examiner US. Cl. X. R. 96l.3, 1.5

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