Multilayered Halogen-doped Selenium Photoconductive Element

Abstract

A xerographic plate having a two-layer photoconductive segment comprising a highly doped vitreous selenium transport layer of from about 20 to 200 microns in thickness and an overlaying control layer of at least about 5 microns thickness which comprises selenium. The plate is characterized by low residual potential as well as exhibiting a minimum ghosting effect.

Description

BACKGROUND OF THE INVENTION

This invention relates in general to xerography and in particular to an improved xerographic plate and a method for its use as an imaging device.

It is the usual practice in the xerographic art to form an electrostatic image by first evenly distributing electrical charge on the surface of a photoconductive member and then exposing the surface to a pattern of activating radiation corresponding to the desired image. More specifically, 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 non-illuminated areas. The latent electrostatic image may then 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 the field.

The discovery of the photoconductive insulating properties of highly purified vitreous selenium has resulted in this material becoming the standard in reuseable commercial xerographic plates. Other types of xerographic plates are known including, for example, paper coated with a photoconductive layer of zinc oxide particles contained in a film forming insulating resin. However, vitreous selenium xerographic plates still remain the most widely used in that they are capable of holding an electrostatic charge for long periods of time when not exposed to light, because they are relatively sensitive to light compared to other xerographic plates, and because of their durability to be reused hundreds or even thousands of times. The vitreous selenium plate, however, is somewhat limited than its spectral response which is very largely limited to the blue or near ultraviolet portion of the spectrum. An improvement in the light sensitivity in response to longer wavelengths of vitreous selenium is described by Ullrich in the U.S. Pat. No. 2,803,542, which discloses that the addition of arsenic to selenium causes a general increase in the light sensitivity of the xerographic plate and also causes the plate to be sensitive to longer wavelengths of light. Still another contribution was made by Straughan in U.S. Pat. No. 3,312,548 which discloses the addition of halogens to arsenic-selenium alloys resulting in improved spectral sensitivity of the photoreceptor layer.

Although the xerographic processes and devices utilizing the halogen-doped vitreous selenium plates have had highly satisfactory results, it should be recognized that the doped vitreous selenium layers, when used as a single layer, are far from ideal with respect to the electrical properties of dark decay and residual potential. Dark decay is that phenomenon peculiar to xerographic plates whereby there is a loss of apparent surface voltage in the absence of light. When this phenomena is measured per unit of time it is referred to as the dark decay rate and is an effective measure of the retention of the latent electrostatic image by the plate. Hence, a high discharge rate indicates that a plate has become fatigued, that is, the latent charge image will rapidly dissipate. Residual potential is the voltage remaining on the plate after exposure to the erase lamp in a conventional xerographic cycle using a reuseable xerographic plate. More particularly, when a sensitized xerographic plate is exposed to light, the electrical potential undergoes an initial rapid decay, followed by a relatively slow decay. The plate voltage at the point beyond which no further light discharge occurs is called the residual potential. This potential may vary from zero to as much as 20 or 30 percent of the initial potential. A low residual potential is a desirable characteristic of xerographic plates because of the greater voltage contrast obtainable between background and darkened areas of the copy; that is, sufficient voltage contrast is required so as to effectively attract the development toner in a well-contrasted print.

A plate having ideal electrical characteristics would have a low dark discharge as well as low residual potential. A plate having low residual potential is always desirable for satisfactory voltage contrast. In actual practice, however, it has been difficult to produce a photoconductive material having the aforementioned electrical characteristics.

The aforementioned desired electrical characteristics become even more difficult to attain with respect to plates which are to be used commercially in rapid recycling machines. Because of the speed used in rapid cycling machines there is insufficient time for the photogenerated holes in the photoreceptor to be effectively neutralized. As a result, with each cycle there occurs the phenomenon of residual potential buildup, or positive residual buildup, whereby the exposed areas of the plate fail to effectively discharge and the apparent surface voltage in the exposed area increases with each cycle.

It has been found that when a vitreous selenium photoreceptor has been doped with a halogen throughout its bulk at a concentration of about 20 parts per million p.p.m. there is observed an exceptional effectiveness in controlling the residual buildup under moderate speed cycling conditions. Furthermore, when a halogen is used in excess of 20 parts per million the overall residual potential can be reduced to zero. However, it has also been found that if the concentration of the halogen were greater than needed for the reduction of the residual buildup, that is, in excess of 20 parts per million, then the light at the surface of the photoreceptor causes areas of conductivity throughout the vitreous selenium with subsequently higher dark discharge values which after a number of cycles results in a persistently electrically conductive conditions known as "fatigue." Consequently, in the case of long copy runs where the image is in registration with a drum having a halogen doped selenium layer an effect called "ghosting" is observed which is characterized as a positive residual image; that is, the previous image remains on the plate, and upon recharging, will recur in subsequent copy runs for a second or different image. An explanation of this phenomenon lies in the fact that the background areas of the plate have become fatigued to a point where upon recharging, the background areas dissipate the charge leaving a contrast potential with the darkened print areas of the plate.

While it is possible to control residual buildup with halogen doping in an amount of up to about 20 parts per million and still retain a minimum dark discharge rate in machines of moderate cycling speeds, unfortunately the residual buildup of such a doped photoreceptor increases with the speed of cycling. Therefore the sensitivity of a doped plate will decrease with the increased cycling, i.e., the ability of the plate to contrast low-density subjects will be lessened by the increased cycling speed. It therefore becomes imperative in the case of modern fast copying machines when using a halogen-doped selenium plate to use a highly doped plate which has the characteristics of low residual buildup even under conditions of high speed.

OBJECTS OF THE INVENTION

As a result of the aforementioned problems it is an object of the present invention to provide a new halogen doped xerographic plate which has a relatively low residual potential and exhibits a minimum ghosting effect.

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

It is another object of this invention to provide a system utilizing a xerographic plate containing a halogen-doped vitreous selenium layer having improved light fatigue characteristics.

It is yet a further object of this invention to provide an improved xerographic plate which exhibits a minimum residual potential.

SUMMARY OF THE INVENTION

These and other objects are obtained in accordance with the present invention by preparing a xerographic plate having a double-layered photoconductive segment comprising a lower transport layer of vitreous selenium doped with a halogen and a relatively thin overlayer which consists of undoped vitreous selenium. The concentration of halogen in the lower transport layer ranges from about 60 to 10,000 p.p.m.

DESCRIPTION OF THE DRAWINGS

Further objects of the invention, together with additional features contributing thereto will be apparent from the following description of one embodiment of the invention when read in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of one embodiment of a xerographic plate contemplated by the instant invention.

FIGS. 2 and 3 illustrate characteristic discharge curves for doped vitreous selenium at different recycling speeds.

FIG. 4 illustrates discharge curves for a halogen-doped vitreous selenium and an overcoated vitreous selenium plate of the instant invention.

FIG. 1 illustrates one embodiment of an improved xerographic plate 10 according to this invention. Reference character 11 designates a substrate or mechanical support. The substrate may comprise a metal such as brass, aluminum, gold, platinum, steel, or the like. It 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 iodine, or glass coated with a thin layer of chromium or tin oxide. It is usually preferred that the support 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, however, support 11 need not be conductive or may be even dispensed with entirely.

Reference character 12 designates a storage layer which comprises high-purity vitreous selenium heavily doped with a halogen such as chlorine, fluorine, bromine, or iodine. The halogen is present in relatively large amounts which are measured in parts per million. For the purposes of this invention concentrations from about 60 to 10,000 p.p.m. are preferred in order to obtain an effectively doped vitreous selenium layer. As heretofore indicated halogen dopant below 20 p.p.m. results in a residual buildup at high cycling speeds while concentrations above 10,000 are unnecessary to achieve the electrical properties desired in the instant invention.

Storage layer 12 may be in any suitable thickness used for conventional photoconductive layers. Typical thicknesses range from about 20 to 200 microns. A range of from about 40 to 80 microns is preferred since these are the thicknesses that are generally used in conventional xerographic machines.

Overlaying control layer 13 comprises undoped vitreous selenium in a thickness of from about 5 to 20 microns. Thicknesses below about 5 microns fail to effectively overcome the high dark discharge characteristics of the heavily halogenated vitreous selenium transport layer while thicknesses above 20 microns effectively masks the lower transport layer so that the chlorine doped selenium fails to function as a low residual photoreceptor.

It can thus be seen that the photoconductive portion of the plate of FIG. 1 is divided into two functional layers: (1) a highly doped transport layer which functions to prevent positive residual buildup during rapid cycling discharge, thereby ensuring charge contrast, and; (2) an overlaying control layer of more than 5 microns which effectively shields the highly halogenated layer from harmful radiation and thus prevents excessively high dark discharge in the halogen-doped layer.

In FIG. 2 there is a dramatic illustration of the effect of increased speed on the residual potential of a single layer vitreous selenium having moderate amounts of halogen. Here, the residual buildup of a vitreous selenium monolayer photoreceptor containing 20 p.p.m. chlorine dopant was measured by exposure to a cool white fluorescent light source at speeds of 5, 20, and 50 r.p.m. on an oxidized aluminum substrate in the form of a cylindrical drum approximately 4.75 inches in diameter by 10.2 inches long. The residual potential was measured after the plate reached its maximum residual buildup which generally occurred after 30 to 40 cycles. Exposure values ranged from 0.03 to 30 foot-candle seconds at each speed. As is apparent from FIG. 2 a concentration of 20 p.p.m. of chlorine is adequate at a drum rotation speed of 5 r.p.m. in that the selenium layer does not attain a maximum residual potential; that is, there is no apparent voltage after about 1.87 foot-candle seconds of exposure. However, as the speed of the drum is increased a maximum residual potential results and successively increases as the speed is increased from 20 r.p.m. to 50 r.p.m. Consequently at high speeds there will be lessened contrast potential for a given amount of light or, in other words, a decrease in sensitivity which will become apparent in the effectiveness of low density copyability.

By contrast, FIG. 3 graphically demonstrates the advantages of using a highly doped vitreous selenium photoreceptor plate in rapid recycling machines. Here the residual potential of a vitreous selenium monolayer photoreceptor containing 100 p.p.m. chlorine is measured by discharging at speeds of 5, 20, and 50 r.p.m. in the manner as described for FIG. 2. It can be clearly seen that discharge at each speed results in total dissipation of the surface charge effectively eliminating residual potential. The curves of FIG. 3 clearly indicate the utility of the highly doped photoreceptor in rapid recycling machines.

The effect of the overcoating of the instant invention on the discharge characteristics of highly doped vitreous selenium layer is demonstrated in FIG. 4. Here the discharge characteristics of a vitreous selenium monolayer doped with 60 p.p.m. chlorine is graphically compared to the same monolayer overcoated with a 5-micron layer of pure selenium in accordance with the present invention. It can be seen from FIG. 4 that the selenium overcoating has not altered the discharge characteristics to any great extent thus indicating that the selenium overcoating does not adversely affect the sensitivity of highly doped vitreous selenium photoreceptors.

In preparing xerographic plates of the instant invention, selenium may be conveniently purchased to specification with the desired concentration of dopant already present. Canadian Copper Refiners is one source of predoped selenium. If desired, the selenium may be doped by any conventional laboratory technique such as physically mixing the dopant with the selenium and vacuum evaporating the mixture onto the conductive substrate. Bromine may be added in the form of liquid drops to the selenium which has been precooled. Chlorine or fluorine may be added by admitting chlorine or fluorine gas to an evacuated tube containing selenium, which has been precooled, and maintaining the flow of gas until the selenium contains the desired amount of dopant. Suitable doping techniques such as those listed above, which may be used are disclosed in U.S. Pat. No. 3,312,548 to Straughan. It should also be pointed out that the halogen may be added to the selenium in the form of a compound of the selenium or with other compounds such as silver halides.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Two halogen-doped vitreous selenium plates were prepared in order to illustrate the dark discharge characteristics of highly doped plates. Each plates contains a 50-micron-thick photoreceptor layer of halogen doped vitreous selenium and are chlorine doped to a concentration of 66 p.p.m. In one embodiment illustrative of the instant invention, plate 2 is overcoated with a 5-micron layer of undoped selenium.

The doped plates are then tested to measured their dark discharge rate under both rested and fatigued conditions. Both plates were rested overnight and mounted on an aluminum testing fixture in the form of a cylindrical drum approximately 4.75 inches by 10.2 inches long. For dark discharge values in the rested condition the plate was charged to an initial potential of 800 volts by means of a dual corotron and the rate of dark discharge measured at intervals of 4, 10, and 30 seconds. For fatigued values the mounted plates were exposed by means of a cool white fluorescent light source to 1,550 foot-candle seconds for 5 cycles. The fatigued plates were then charged to 800 volts on the sixth cycle and the dark discharge values measured in the same manner described above for the rested plates. The results are presented in table I. --------------------------------------------------------------------------- TABLE

Dark discharge values Rested (sec.) Fatigued (sec.) 4 10 30 4 10 30 __________________________________________________________________________ Plate I 4% 10% 21% 8% 18% 38% (Vitreous selenium con- taining 66 p.p.m. chlorine) Plate II 4% 8.5% 19% 5% 11% 22% (Vitreous selenium con- taining 66 p.p.m. chlorine over- coated with 5 microns on non- halogenated selenium) __________________________________________________________________________

An examination of the dark discharge values in table I indicates the following: First, in accordance with what has hereinbefore mentioned with regard to highly doped vitreous selenium photoreceptors there in a prominent increase of the dark discharge rate of plate 1 going from the rested to the fatigued plates was decreased by the application of the selenium overcoating as indicated by the values of plate 2. The difference in the discharge values between the rested and fatigued plates accounts for the positive residual image, i.e., ghosting, of the uncoated highly doped plate. This can be conceptualized if one realizes that in measuring the rested plate the dark discharge of print or dark areas of a copy is actually being measured while the dark discharge of a fatigued plate effectively measured that of the background of a copy. Hence, if there is enough percentage difference between them then upon recharging a contrast potential will exist such that there is a printout or ghost when the plate is developed.

An illustration of the effectiveness of this selenium overcoating is set forth in the following examples.

EXAMPLE I

An oxidized aluminum drum approximately 4.75 inches in diameter by 10.2 inches long having an arsenic-selenium photoreceptor with 66 p.p.m. chlorine was prepared and placed in a Xerox 813 Office Copier. An off-on switch was placed in series with the white light expose lamp and the preclean corotron and erase lamp were disconnected. With the drum rested overnight three exposures were made and the expose lamp shut off. In cycling without the exposure lamp a ghost image of the original copy appeared thereby indicating that the background areas had become persistently conductive and thereby contrasted with the darkened areas of the copy.

EXAMPLE II

This same drum was then overcoated with a 5-micron layer of pure undoped selenium and the above test repeated. The resultant copy showed virtually no ghosting after the expose lamp has been shut off thereby indicating that the pure selenium overcoating prevented the contrast between the background and darkened areas of the copy.

The plates of the instant invention may be prepared by any of the well-known conventional techniques such as those set forth in U.S. Pat. No. 2,803,542 to Ullrich, U.S. Pat. No. 2,822,300 to Mayer et al., or U.S. Pat. No. 3,312,548 to Straughan. Briefly, such techniques involve forming suitable mixtures of selenium, arsenic and halogen in a container and reacting said elements at elevated temperatures. The resulting alloy is then cooled and applied to a suitable conductive supporting base by vacuum evaporation.

Similarly, the transport layer is also evaporated onto the conductive substrate by any conventional technique such as those 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 transport 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 special components and proportions have been stated in the above description of the preferred embodiments of the selenium overcoated highly chlorinated xerographic plate, other suitable materials, as listed above, may be used with similar results. In addition, other materials, such as sensitizers, may be added to enhance synergize, or otherwise modify the properties of the novel plate contemplated by this invention.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes and equivalence may be substituted for elements thereof without departing from the true spirit and scope of the invention. Such modifications are intended to be included within the scope of this invention.

Claims

What is claimed is:

1. A xerographic plate including a two-layer photoconductive segment, said first layer comprising halogen doped vitreous selenium in a thickness of about 40-80 microns, with the dopant being present in a concentration of from about about 60-10,000 parts per million, and an undoped substantially clear vitreous selenium layer of from about 5-20 microns in thickness overlaying said first layer.

2. The plate of claim 1 wherein the halogen comprises chlorine.

3. The method of imaging comprising

a. providing a xerographic plate having a two-layered photoconductive segment, said segment comprising a first layer 40-80 microns thick comprising vitreous selenium doped with 60-10,000 parts per million of halogen, and an overlaying layer about 5 to 20 microns in thickness comprising undoped selenium,

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

c. developing said image to make it visible.

4. The method of claim 3 wherein the latent image is formed by first uniformly charging the surface followed by exposure to a pattern of activating radiation.

5. The method of claim 3 wherein the halogen dopant comprises chlorine in a concentration of from about 60 to 10,000 parts per million.