U.S. patent number 3,639,120 [Application Number 04/557,930] was granted by the patent office on 1972-02-01 for two-layered photoconductive element containing a halogen-doped storage layer and a selenium alloy control layer.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Christopher Snelling.
United States Patent |
3,639,120 |
Snelling |
February 1, 1972 |
TWO-LAYERED PHOTOCONDUCTIVE ELEMENT CONTAINING A HALOGEN-DOPED
STORAGE LAYER AND A SELENIUM ALLOY CONTROL LAYER
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.
Inventors: |
Snelling; Christopher
(Penfield, NY) |
Assignee: |
Xerox Corporation (Rochester,
NY)
|
Family
ID: |
24227441 |
Appl.
No.: |
04/557,930 |
Filed: |
June 16, 1966 |
Current U.S.
Class: |
430/58.1;
399/166; 430/85; 430/95 |
Current CPC
Class: |
G03G
5/0433 (20130101) |
Current International
Class: |
G03G
5/043 (20060101); G03g 005/04 () |
Field of
Search: |
;96/1.5,1 ;252/501 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lesmes; George F.
Assistant Examiner: Cooper, III; John C.
Claims
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 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.
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.
Description
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 the field.
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 U.S. 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. U.S. 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. U.S. Pat. No. 2,901,349, 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
double-layered photoconductive portion comprising a lower storage
layer which comprises vitreous selenium doped with a halogen and 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 11 designates an electrically
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 U.S. Pat. No. 2,803,542, to Ullrich
or U.S. 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 0.1 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 10 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
selenium-tellurium 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 I 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 I. Two different alloys of the same composition were
used in each set with the data being tabulated in Table I below.
Each plate contains a 25 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
Vi(4500 angstroms) Vi(5750 angstroms)
__________________________________________________________________________
Plate 1 20 volts 15-20 volts Control layer 0.1 microns (90%
selenium-10% tellurium) Storage layer 25 micron layer of selenium
doped with 60 parts per million chlorine on an aluminum substrate
Plate 2 70 volts 60 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 Plate 3 40-45 volts
30 volts Control layer 0.1 micron alloy of 90% selenium- 10%
tellurium Storage layer 25 micron layer of 60 parts per million
chlorine on an aluminum substrate Plate 4 100 volts 85 volts
Control layer 0.1 micron layer of alloy 90% selenium- 10% tellurium
Storage layer 25 micron layer of selenium (no doping) on an
aluminum substrate
__________________________________________________________________________
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.
In 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.degree.-900.degree. 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.
* * * * *