US3639120A

US3639120A - Two-layered photoconductive element containing a halogen-doped storage layer and a selenium alloy control layer

Info

Publication number: US3639120A
Application number: US557930A
Authority: US
Inventors: Christopher Snelling
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1966-06-16
Filing date: 1966-06-16
Publication date: 1972-02-01
Anticipated expiration: 1989-02-01
Also published as: DE1572375A1; GB1193876A; DE1572375B2

Abstract

A xerographic plate having a double layered photoconductive portion, the lower layer being designated as a storage layer and the upper layer a control layer. The storage layer consists of halogen doped selenium in a thickness from about 20 to 200 microns. The control layer consists of undoped selenium alloys in a thickness of about 0.1 to 5 microns. The plate utilizes the optimum photoconductive properties of each layer.

Description

United States Patent [151 3,639,120

shelling Feb. 1, 1972 [s41 TWO-LAYERED PHOTOCONDUCTIVE 2,803,541 8/1957 Paris ..96/l

ELEMENT CONTAINING A HALOGEN- g DOPED STORAGE LAYER AND A my 3,312,547 4/1967 Levy .96/l.5

SELENIUM ALLOY CONTRGL LAYER 3,312,548 4/1967 Straubhan ..96/l.5

Primary Examiner-George F. Lesmes Assistant Examiner-John C. Cooper, lll

Attorney-Frank A. Steinhilper, Stanley 2. Cole and James J. Ralabate [5 7] ABSTRACT A xerographic plate having a double layered photoconductive portion, the lower layer being designated as a storage layer and the upper layer a control layer. The storage layer consists of halogen doped selenium in a thickness from about 20 to 200 microns. The control layer consists of undoped selenium alloys in a thickness of about 0.1 to 5 microns. The plate utilizes the optimum photoconcluctive properties of each layer.

9 Claims, 3 Drawing Figures PATENTEDFEB H97? $639,120

POTENTIAL (VOLTS) I I l o 0.4 0.8 U2 U6 TIME (secouos) soo- /INIT|AL DISCHARGE POTENTIAL (VOLTS) loo /RESlDUAL DISCHARGE O l I 1 TIME (SECONDS)*** INVILN'IUR.

CHRISTOPHER SNELLING ATTORNEY TWO-LAYERED PHOTOCONDUCTIVE ELEMENT CONTAINING A HALOGEN-DOPED STORAGE LAYER AND A SELENIUM ALLOY CONTROL LAYER This invention relates to xerography, and more specifically, to a system employing a xerographic plate having a novel storage layer.

In the art of xerography, a xerographic plate containing a photoconductive insulating layer is first given a uniform electrostatic charge in order to sensitize its surface. The plate is then exposed to an image of activating electromagnetic radiation such as light, X-ray, or the like which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the nonilluminated areas. The latent electrostatic image may be developed and made visible by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. This concept was originally disclosed by Carlson in U.S. Pat. No. 2,297,691, and is further amplified and described by many related patents in thefield.

The discovery of the photoconductive insulating properties of highly purified vitreous selenium has resulted in this material becoming standard in commercial xerography. Vitreous selenium, however, is somewhat limited in its spectral response which is very largely limited to the blue or near ultraviolet portion of the spectrum. Selenium photoconductive coatings are also subject to abrasive wear over long range cyclic use, and under conditions of high humidity with or without abrasive wear, the selenium exhibits poor printing properties due to lateral surface conductivity.

It is also well known that the light discharge characteristics of selenium, and plates employing a thin layer of a more sensitive photoconductor over selenium, both exhibit a pronounced knee" beyond which the rate of discharge (loss of voltage per unit time under a given exposure to a source of activating radiation) is drastically reduced.

With the above problems in mind, the art has looked for ways to improve both the photosensitive and physical properties of vitreous selenium. In US. Pat. No. 2,860,048, to Deubner, a photoconductive insulating layer such as selenium, is protected by a thin coating of about 1 micron in thickness of an organic material which improves both humidity stability and abrasion resistance. US. Pat. No. 2,901,348, to Dessauer et al. discloses a plate structure wherein conventional photoconductive layers such as vitreous selenium is sandwiched between two barrier layers to prevent any charge leakage from the plate prior to exposure to activating radiation. US. Pat. No. 2,90l ,349, to Schaffert et al. contemplates a conventional selenium layer which overlays a layer of arsenic trisulfide the purpose of. which is to render the plate capable of being charged both positively and negatively without loss of spectral sensitivity and also to improve fatigue resistance. It is also contemplated that the photoconductive part of the plate may consist of a plurality of discrete layers each containing a different type of photoconductive insulating material. One suggested combination is a vitreous selenium layer on top of which is placed a mixture of tellurium and selenium. It can thus be seen that the art of commercial xerography has structurally modified the basic conventional selenium plate so as to improve both its photosensitive and physical properties.

it is, therefore, an object of this invention to provide a xerographic system which overcomes the above-noted disadvantages.

It is another object of this invention to provide a xerographic plate having improved physical properties.

It is a further object of this invention to provide a system utilizing a xerographic plate having improved discharge characteristics.

it is yet a further object of this invention to provide a xerographic plate having maximum photosensitivity and charge discharge characteristics and yet which can be made by conventional techniques.

The foregoing objects and others are accomplished in accordance with this invention by preparing a xerographic plate having a doublelayered photoconductive portion comprising a lower storage layer which comprises vitreous selenium doped with a halogenand a relatively thin control layer overlaying such storage layer comprising an undoped vitreous selenium alloy having greater photosensitivity than said storage layer.

The advantages of this invention will become apparent upon consideration of the following disclosure of this invention; especially when taken in conjunction with the following drawing wherein:

FIG. 1 is a schematic illustration of one embodiment of a xerographic plate as contemplated by this invention.

FIG. 2 graphically illustrates the discharge properties of three different xerographic plates.

FIG. 3 graphically illustrates the discharge characteristics of the novel plate of this invention.

FIG. 1 shows an improved xerographic plate 10 according to this invention. Reference character lll designates an elec' trically conductive substrate or mechanical support. This is conventionally a metal such as brass, aluminum, gold, platinum, steel or the like. The support member may be of any convenient thickness, rigid or flexible, in the form of a sheet, web, cylinder, or the like, and may be coated with a thin layer of plastic. It may also comprise such other materials as metallized paper, plastic sheets covered with a thin coating of aluminum or copper iodide, or glass coated with a thin layer of chromium or tin oxide. An important consideration is that the member be somewhat electrically conductive or have a somewhat conductive surface and that it be strong enough to permit a certain amount of handling. In certain instances support 11 may even be dispensed with entirely. Reference character 12 designates a storage layer which comprises conventional high-purity vitreous selenium doped with a halogen such as chlorine, fluorine, bromine, or iodine. The halogen is present in relatively small amounts which are measured in parts per million. For the purposes of this invention concentrations of from about 10 to 10,000 parts per million of a halogen have been found effective in doping the selenium layer.

The selenium is conveniently purchased to specification with the desired concentration of dopant already present. Canadian Copper Refiners is a source of predoped selenium. if desired, the selenium may be doped by any conventional laboratory techniques such as physically mixing the dopant (such as iodine) with the selenium and vacuum evaporating the mixture onto the conductive substrate. Bromine could be added in. the form of liquid drops to the selenium which is precooled. Chlorine or fluorine may be added by admitting chlorine or fluorine gas to an evacuated tube containing selenium (which is precooled) and maintaining the flow of gas until the selenium contains the desired amount of dopant. It should also be pointed out that the halogen may be added to the selenium as a compound of the selenium.

Storage layer 12 may be in any suitable thickness used for conventional photoconductive layers. Typical thicknesses conveniently range from about 20 to 200 microns. A relatively thinner control layer 13 overlays photoconductive storage layer 12. Control layer 13 contains a more sensitive photoconductive material than said storage layer 12. The control layer may consist of a selenium-arsenic alloy containing up to about 50 percent by weight arsenic, such as that shown by US. Pat. No. 2,803,542, to Ullrich or US. Pat. No. 2,822,300, to Mayer et al., or a selenium-tellurium alloy containing up to about 30 percent by weight tellurium. This overlaying control layer should be in a thickness of about: 0.l to 5.0 microns in thickness. Thicknesses above about 5 microns result in an undesirable high dark discharge and light fatigue conditions while thicknesses below about 0.1 microns fail to give a substantial increase in photosensitivity.

Halogen doping a photosensitive member having a single photoconductive layer throughout the layer would appear to be undesirable due to a resulting high dark discharge rate.

It can thus be seen that the plate of FIG. 1 divides the photoconductive portion of the plate into two layers: a storage layer which functions to control the field and thus control the discharge rate of the plate, and an overlaying control layer having a greater sensitivity than the lower storage layer. By thus combining the two separate layers, each layer functions to give a particular optimum property resulting in a novel xerographic plate having optimum optical properties for light sensitivity together with a desirable high discharge rate.

FIG; 2 illustrates the substantial improvement in the electrical characteristics of a two-layered plate as contemplated by this invention. The light discharge characteristics of a conventional selenium plate (curve A) having a 25 micron selenium layer overlaying an aluminum substrate; a selenium-tellurium two-layered plate (curve B) having a 0.1 micron selenium-tellur layer overlaying a 25 micron selenium layer on an aluminum substrate; and a 25 micron chlorine doped selenium storage layer, with a 0.1 micron selenium-tellurium layer overlaying the storage layer, on an aluminum substrate (Curve C), are each compared in regard to their discharge rate as shown in FIG. 2.

The plates of Curves B and C contain about percent tellurium in the control layer, and the plate of Curve C is doped with 60 parts per million of chlorine in the selenium storage layer.

The potential or voltage is plotted on the ordinant while exposure time is plotted on the abscissa. It can be seen that the rate of discharge of Curve A for the single layer selenium plate is further to the right than Curve B or C. Curve B has a pronounced knee below which the rate of discharge is drastically reduced, which is above that for Curve C for the selenium-tellurium overcoated plate using a chlorine doped selenium storage layer. This plate (Curve C) shows the most favorable discharge rate of the three plates and has a knee which is lower than both that of the selenium plate and the seleniumtellurium two layered plate of Curve B which is not doped with chlorine.

Although the theory is not completely understood, it is suggested that the knee" might be related to an unfavorable electric field reduction occurring in the region of light absorption. For example, the establishment of a space charge of trapped holes at a distance below the surface might be occurring. As a remedial measure, the use of halogen doped selenium as a lower layer was used based upon experiments which have indicated a capability of controlling the local electrical field in this material by virtue of electron trapping in thermal equilibrium. It was proposed that the decrease in electric field at the surface due to hole trapping might be offset by the increase due to the space charge of electrons trapped in the lower layer. The results as shown as illustrated by the discharge curve in FIG. 2 have been consistent with this theory.

Two series of plates such as those illustrated by B and C of FIG. 2 were prepared to demonstrate the utility of the chlorine addition to the lower layer. The plates are charged to a uniform positive surface potential of 300 volts and exposed to radiation bands of 4,500 and 5,775 angstroms.

The term Vi in Table l is the voltage at which straight line extensions of the initial discharge and residual discharge segments intersect which is an approximation to knee voltage. This is illustrated by FIG. 3 of the drawings wherein Vi is shown for plate 1 of Table l. Two different alloys of the same composition were used in each set with the data being tabulated in Table l below. Each plate contains a micron thick storage layer of selenium on an aluminum substrate, with a 0.1 micron control layer of selenium-tellurium overlaying the storage layer. The storage layer of plates 1 and 3 are doped with chlorine, while the storage layer of plates 2 and 4 are undoped.

TABLE I V|'(4500 angstroms) Vi(5750 angstroms) 20 volts Control layer 0.] microns selenium-10% tellurium) Storage layer 25 micron layer of selenium doped with 60 parts per million chlorine on an aluminum substrate 70 volts Control layer 0.1 microns of 90% selenium-10% tellurium Storage layer 25 micron layer of high purity selenium (no doping) on an aluminum substrate 40-45 volts Control layer 0.l micron alloy of 90% seleniuml0% tellurium Storage layer 25 micron layer of 60 parts per million chlorine on an aluminum substrate volts Control layer 0.l micron layer of alloy 90% seleniuml0% tellurium Storage layer 25 micron layer of selenium (no doping) on an aluminum substrate Plate l 15-20 volts 60 volts Plate 2 Plate 3 30 volts Plate 4 85 volts It can be seen from Table I that chlorine doped plates 1 and 3 both shown approximately three times the discharge (Vi) as undoped plates 2 and 4, at both wavelengths.

ln addition to reducing Vi by a factor of approximately 3x for halogen doped plates as opposed to undoped plates, an increase in measured response is indicated by these data due to the inclusion of chlorine in the storage layer. This effect is interpreted as a general straightening of the discharge curve even above the knee.

The plates of this invention may be prepared by any of the well-known conventional techniques such as those set forth in the above mentioned Ullrich and Mayer et al., patents. Such techniques briefly involve forming an alloy such as selenium and arsenic by melting the appropriate amount of arsenic and selenium together in a temperature range of approximately 750-900 F. The resulting alloy is then evaporated under vacuum conditions onto the overlaying storage layer.

The storage layer is also evaporated onto the conductive substrate by any conventional techniques such as that shown by U.S. Pat. No. 2,753,278, to Bixby et al., and U.S. Pat. No. 2,970,906, to Bixby. If desired, both the storage layer and control layer may be evaporated sequentially without breaking the vacuum. This avoids the possible danger of contaminating the surface of the plate.

Although specific components and proportions have been stated in the above description of the preferred embodiments of this invention, other suitable materials and procedures such as those listed above may be used with similar results. in addition, other materials may be added to the plates which synergize, enhance, or otherwise modify their properties.

Various additions, such as sensitizers, may be added to enhance the properties of the novel plate contemplated by this invention.

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

What is claimed is:

1. A xerographic plate comprising a two-layered photoconductive portion, said first portion comprising a storage layer consisting of vitreous selenium doped with a halogen in a concentration of from about l0 to 30,000 parts per million, and an overlaying photoconductive control layer having a thickness of from about 0.1 to 5 microns comprising a photoconductive material selected from the group consisting of selenium-tellurium and selenium-arsenic alloys.

2. The plate of claim 1 wherein the storage layer ranges in thickness from about 20 to 200 microns.

3. The plate of claim 1 wherein the halogen is chlorine.

4. A xerographic plate comprising:

a. a conductive substrate,

b. a vitreous selenium layer doped with a halogen in a concentration of from about to 10,000 parts per million overlaying said substrate, and

c. a control layer having a thickness of from about 0.1 to 5 microns comprising a photoconductive material selected from the group consisting of selenium-tellurium and selenium-arsenic alloys.

5. The plate of claim 8 wherein the halogen dopant is chlorine.

6. A xerographic plate comprising:

a. a conductive substrate having thereon b. an overlaying photoconductive storage layer from about 20 to 200 microns thick, comprising vitreous selenium doped with chlorine in a concentration of about 10 to 10,000 parts per million,

0. a control layer in a thickness of from 0.1 to 5 microns in thickness overlaying said storage layer, said control layer comprising a more sensitive photoconductor than said storage layer, and said photoconductor consisting of a member selected from the group of arsenic-selenium and selenium-tellurium.

7. An imaging method comprising:

a. providing a xerographic plate having a two-layered photoconductive portion, said first portion comprising a storage layer consisting of vitreous selenium doped with a halogen in a concentration from about 10 to 10,000 parts per million, and an overlaying photoconductive control layer, having a thickness of from about 0.1 to 5 microns comprising a photoconductive material selected from the group consisting of selenium-tellurium and selenium-arsenic alloys.

b. forming an electrostatic latent image of said plate, and

c. developing said image to make it visible.

8. A method of forming an electrostatic latent image which 10 comprises:

developed to make it visible.

Claims

2. The plate of claim 1 wherein the storage layer ranges in thickness from about 20 to 200 microns.
3. The plate of claim 1 wherein the halogen is chlorine.
4. A xerographic plate comprising: a. a conductive substrate, b. a vitreous selenium layer doped with a halogen in a concentration of from about 10 to 10,000 parts per million overlaying said substrate, and c. a control layer having a thickness of from about 0.1 to 5 microns comprising a photoconductive material selected from the group consisting of selenium-tellurium and selenium-arsenic alloys.
5. The plate of claim 8 wherein the halogen dopant is chlorine.
6. A xerographic plate comprising: a. a conductive substrate having thereon b. an overlaying photoconductive storage layer from about 20 to 200 microns thick, comprising vitreous selenium doped with chlorine in a concentration of about 10 to 10,000 parts per million, c. a control layer in a thickness of from 0.1 to 5 microns in thickness overlaying said storage layer, said control layer comprising a more sensitive photoconductor than said storage layer, and said photoconductor consisting of a member selected from the group of arsenic-selenium and selenium-tellurium.
7. An imaging method comprising: a. providing a xerographic plate having a two-layered photoconductive portion, said first portion comprising a storage layer consisting of vitreous selenium doped with a halogen in a concentration from about 10 to 10,000 parts per million, and an overlaying photoconductive control layer, having a thickness of from about 0.1 to 5 microns comprising a photoconductive material selected from the group consisting of selenium-tellurium and selenium-arsenic alloys. b. forming an electrostatic latent image of said plate, and c. developing said image to make it visible.
8. A method of forming an electrostatic latent image which comprises: a. providing a xerographic plate having a two-layered photoconductive portion, said first portion comprising a storage layer consisting of vitreous selenium doped with a halogen in a concentration of from about 10 to 10,000 parts per million, and an overlaying photoconductive control layer having a thickness of about 0.1 to 5 microns comprising a photoconductive material selected from the group consisting of selenium-tellurium and selenium-arsenic alloys, b. substantially uniformly electrostatically charging said plate, and c. exposing said plate to a pattern of activating electromagnetic radiation.
9. The method of claim 8 wherein the latent image is developed to make it visible.