U.S. patent number 3,928,034 [Application Number 05/341,839] was granted by the patent office on 1975-12-23 for electron transport layer over an inorganic photoconductive layer.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Paul J. Regensburger.
United States Patent |
3,928,034 |
Regensburger |
December 23, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Electron transport layer over an inorganic photoconductive
layer
Abstract
An electrophotographic plate comprising two adjacent layers, one
of which is a photoconductive layer capable of photogenerating and
injecting electrons into the other contiguous layer, which is an
electronically active material capable of supporting electron
injection and transport. The electronically active transport
material has the additional property of being substantially
transparent to radiation in the particular wavelength region of
xerographic use thereby rendering it particularly useful as a
relatively thick protective overlayer for the photoconductive
portion of the plate. The structure may be imaged in the
conventional xerographic mode which usually includes charging,
exposure to light, and development.
Inventors: |
Regensburger; Paul J. (Webster,
NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
26788310 |
Appl.
No.: |
05/341,839 |
Filed: |
March 16, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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94071 |
Dec 1, 1970 |
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14282 |
Feb 26, 1970 |
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Current U.S.
Class: |
430/65;
430/58.05; 430/58.25; 430/58.35; 430/58.5 |
Current CPC
Class: |
G03G
5/0436 (20130101) |
Current International
Class: |
G03G
5/043 (20060101); G03G 005/08 (); G03G
013/22 () |
Field of
Search: |
;96/1.5,1.8
;252/501 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Martin, Jr.; Roland E.
Attorney, Agent or Firm: Ralabate; James J. O'Sullivan;
James P. MacKay; Donald M.
Parent Case Text
BACKGROUND OF THE INVENTION
This application is a continuation-in-part of my previous
application, Ser. No. 94,071, filed Dec. 1, l970, now abandoned,
which is a continuation-in-part of Ser. No. 14,282, filed Feb. 26,
1970, now abandoned.
Claims
What is claimed is:
1. A method of imaging which comprises:
a. providing a xerographic plate having a supporting substrate, an
unoriented inorganic photoconductive layer overlaying said
substrate, and an electronically active transport layer overlaying
said photoconductive layer, said photoconductive layer comprising a
photoconductor having the capability of photogenerating electrons
and injecting them into an adjacent active transport layer, said
active layer having the capability of supporting the injection of
electrons and transporting said electrons through said active
material wherein said active material comprises at least one
material selected from the group consisting of phthalic anhydride,
tetrachlorophthalic anhydride, benzil, mellitic anhydride,
s-tricyanobenzene, picryl chloride, 2,4-dinitrochlorobenzene,
2,4-dinitrobromobenzene, 4-nitrobiphenyl, 4,4-dinitrobiphenyl,
2,4,6-trinitroanisole, trichlorotrinitrobenzene, trinitro-o-toluene
4,6-dichloro-1,3-dinitrobenzene, 4,6-dibromo-1,3-dinitrobenzene,
p-dinitrobenzene, chloranil, bromanil, 2,4,7-trinitro-9-fluorenone,
2,4,5,7-tetranitrofluorenone, trinitroanthracene, dinitroacridine,
tetracyanopyrene, and dinitroanthraquinone; the photoconductive
layer having a thickness between 0.02 and 25 microns and dispersed
in from 0 to 95 volume percent binder but comprising at least 25
volume percent when an electronically inert binder is employed, the
photoconductor binder thickness ranging from 0.05 to 20 microns
when a binder is employed; the transport material, if dispersed in
an electronically inert binder, is present in a volume ratio of at
least 25 percent active transport material to electronically inert
binder;
b. uniformly positive charging said plate, and
c. exposing said plate to a source of radiation in the wavelength
region of from about 4200 to 6500 Angstroms to which the active
layer is substantially transparent and non-absorbing whereby
injection and transport of photogenerated electrons from said
photoconductive layer occurs through said active transport layer to
form a latent electrostatic image on the surface of said plate.
2. The method of claim 1 wherein the photoconductive layer
comprises a material selected from the group consisting of vitreous
selenium, amorphous selenium, selenium alloys, trigonal selenium,
cadmium sulfoselenide, cadmium sulfide, cadmium selenide, zinc
oxide, and mixtures thereof.
3. The method of claim 1 which further includes developing said
latent image to make it visible.
4. The method of claim 1 in which the substrate is substantially
transparent and exposure is carried out through said substrate.
5. An electrophotographic plate comprising in successive
layers:
a. a conductive substrate,
b. a blocking layer,
c. an inorganic photoconductive layer, and
d. an organic charge transport layer consisting essentially of
2,4,7-trinitor-9-fluorenone; the photoconductive layer having a
thickness between 0.02 and 25 microns and dispersed in from 0 to 95
volume percent binder but comprising at least 25 volume percent
when an electronically inert binder is employed, the photoconductor
binder thickness ranging from 0.05 to 20 microns when a binder is
employed; the transport material, if dispersed in an electronically
inert binder, is present in a volume ratio of at least 25 percent
active transport material to electronically inert binder.
6. An electrophotographic plate as claimed in claim 5 wherein the
charge transport layer consists essentially of about 50% by weight
of 2,4,7-trinitro-9-fluorenone in a resin binder.
7. An electrophotographic plate as claimed in claim 5 wherein the
barrier layer is aluminum oxide.
8. An electrophotographic plate as claimed in claim 5 wherein the
inorganic photoconductive layer comprises a material selected from
the group consisting of cadmium sulfide, cadmium selenide or zinc
sulfide.
9. The method of imaging of claim 1 wherein the transport material
is dispersed in an electronically inert binder in a volume ratio of
at least 25 percent active transport material to electronically
inert binder.
10. An electrophotographic plate as claimed in claim 5 wherein the
transport material is dispersed in an electronically inert binder
in a volume ratio of at least 25 percent active transport material
to electronically inert binder.
Description
This invention relates in general to xerography and more
specifically to a novel photosensitive device and method of
use.
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 in the non-illuminated areas. This latent
electrostatic image may then be developed to form a visible image
by depositing finely divided electroscopic marking particles on the
surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous
layer of a single material such as vitreous selenium or it may be a
composite layer containing a photoconductor and another material.
One type of composite photoconductive layer used in xerography is
illustrated by U.S. Pat. No. 3,121,006 to Middleton and Reynolds
which describes a number of binder layers comprising finely-divided
particles of a photoconductive inorganic compound dispersed in an
electrically insulating organic resin binder. In its present
commercial form, the binder layer contains particles of zinc oxide
uniformly dispersed in a resin binder and is coated on a paper
backing.
In the particular examples of binder systems described in Middleton
et al., the binder comprises a material which is incapable of
transporting injected charge carriers generated by the
photoconductor particles for any significant distance. As a result,
with the particular materials disclosed in the Middleton et al.
patent, the photoconductor particles must be in substantially
continuous particle-to-particle contact throughout the layer in
order to permit the charge dissipation required for cyclic
operation. With the uniform dispersion of photoconductor particles
described in Middleton et al, therefore, a relatively high volume
concentration of photoconductor, up to about 50 percent or more by
volume, is usually necessary in order to obtain sufficient
photoconductor particle-to-particle contact for rapid discharge. It
has been found, however, that high photoconductor loadings in the
binder layers of the resin type result in the physical continuity
of the resin being destroyed, thereby sufficiently reducing the
mechanical properties of the binder layer. Layers with high
photoconductor loadings are often characterized by a brittle binder
layer having little or no flexibility. On the other hand, when the
photoconductor concentration is reduced appreciably below about 50
percent by volume, the discharge rate is reduced, making high speed
cyclic or repeated imaging difficult or impossible.
U.S. Pat. No. 3,121,007 to Middleton et al. teaches another type of
photoconductor which includes a two phase photoconductive binder
layer comprising photoconductive insulating particles dispersed in
a homogeneous photoconductive insulating matrix. The photoconductor
is in the form of a particulate photoconductive inorganic
crystalline pigment broadly disclosed as being present in an amount
from about 5 to 80 percent by weight. Photodischarge is said to be
caused by the combination of charge carriers generated in the
photoconductive insulating matrix material and charge carriers
injected from the photoconductive crystalline pigment into the
photoconductive insulating matrix.
U.S. Pat. No. 3,037,861 to Hoegl et al. teaches that polyvinyl
carbazole exhibits some long-wave U.V. sensitivity and suggests
that its spectral sensitivity be extended into the visible spectrum
by the addition of dye sensitizers. Hoegl et al. further suggests
that other additives such as zinc oxide or titanium dioxide may
also be used in conjunction with polyvinyl carbazole. In Hoegl et
al., it is clear that the polyvinyl carbazole is intended to be
used as a photoconductor, with or without additives materials which
extend its spectral sensitivity.
In addition, certain specialized layer structures particularly
designed for reflux imaging have been proposed. For example, U.S.
Pat. No. 3,165,405 to Hoesterey utilizes a two layered zinc oxide
binder structure for reflex imaging. The Hoesterey patent utilizes
two separate contiguous photoconductive layers having different
spectral sensitivities in order to carry out a particular reflex
imaging sequence. The Hoesterey device utilizes the properties of
multiple photoconductive layers in order to obtain the combined
advantages of the separate photoresponse of the respective
photoconductive layers.
It can be seen from a review of the conventional composite
photoconductive layers cited above, that upon exposure to light,
photoconductivity in the layer structure is accomplished by charge
transport through the bulk of the photoconductive layer, as in the
case of vitreous selenium (and other homogeneous layer
modifications). In devices employing photoconductive binder
structures, which include inactive electrically insulating resins
such as those described in the Middleton et al., U.S. Pat. No.
3,121,006, conductivity or charge transport is accomplished through
high loadings of the photoconductive pigment allowing
particle-to-particle contact of the photoconductive particles. In
the case of photoconductive particles dispersed in a
photoconductive matrix, such as illustrated by the Middleton et
al., U.S. Pat. No. 3,121,007, photoconductivity occurs through the
generation of charge carriers in both the photoconductive matrix
and the photoconductor pigment particles.
Although the above patents rely upon distinct mechanisms of
discharge throughout the photoconductive layer, they generally
suffer from common deficiencies in that the photoconductive surface
during operation is exposed to the surrounding environment, and
particularly in the case of cycling xerography, susceptible to
abrasion, chemical attack, heat, and multiple exposures to light
during cycling. These effects are characterized by a gradual
deterioration in the electrical characteristics of the
photoconductive layer resulting in the printing out of surface
defects and scratches, localized areas of persistent conductivity
which fail to retain an electrostatic charge, and high dark
discharge.
In addition to the problems noted above, these photoconductive
layers require that the photoconductor comprise either a hundred
percent of the layer, as in the case of the vitreous selenium
layer, or that they preferably contain a high proportion of
photoconductive material in the binder configuration. The
requirements of the photoconductive layer containing all or a major
proportion of a photoconductive material further restricts the
physical characteristics of the final plate, drum or belt in that
the physical characteristics such as flexibility and adhesion of
the photoconductor to a supporting substrate are primarily dictated
by the physical properties of the photoconductor, and not by the
resin or matrix material which is preferably present in a minor
amount.
Another form of composite photosensitive layer which has also been
considered by the prior art includes a layer of photoconductive
material which is covered with a relatively thick plastic layer and
coated on a supporting substrate.
U.S. Pat. No. 3,041,166 to Bardeen describes such a configuration
in which a transparent plastic material overlays a layer of
vitreous selenium which is contained on a supporting substrate. The
plastic material is described as one having a long range for charge
carriers of the desired polarity. In operation, the free surface of
the transparent plastic is electrostatically charged to a given
polarity. The device is then exposed to activating radiation which
generates a hole-electron pair in the photoconductive layer. The
electron moves through the plastic layer and neutralizes a positive
charge on the free surface of the plastic layer thereby creating an
electrostatic image. Bardeen, however, does not teach any specific
plastic materials which will function in this manner, and confines
his examples to structures which use a photoconductor material for
the top layer.
French Pat. No. 1,577,855 or U.S. Pat. No. 3,598,582 to Herrick et
al. describes a special purpose composite photosensitive device
adapted for reflex exposure by polarized light. One embodiment
which employs a layer of dichroic organic photoconductive particles
arrayed in oriented fashion on a supporting substrate and a layer
of polyvinyl carbazole formed over the oriented layer of dichroic
material. When charged and exposed to light polarized
perpendicularly to the orientation of the dichroic layer, the
oriented dichroic layer and polyvinyl carbazole layer are both
substantially transparent to the initial exposure light. When the
polarized light hits the white background of the document being
copied, the light is depolarized, reflected back through the device
and absorbed by the dichroic photoconductive material. In another
embodiment, the dichroic photoconductor is dispersed in oriented
fashion throughout the layer of polyvinyl carbazole.
In view of the state of the art, it can readily be seen that there
is a need for a general purpose photoreceptor exhibiting acceptable
photoconductive characteristics and which additionally provides the
capability of exhibiting outstanding physical strength and
flexibility to be reused under rapid cyclic conditions without the
progressive deterioration of the xerographic properties due to
wear, chemical attack, and light fatigue.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide an
electrophotographic plate adapted for cyclic imaging devoid of the
above noted disadvantages.
Another object of this invention is to provide an
electrophotographic plate having excellent abrasion resistance
properties.
It is yet another object of this invention to provide a novel
imaging system.
It is yet another object of the instant invention to provide an
electrophotographic plate having a material which exhibit facile
electron transport properties.
It is still another object of this invention to provide a
photoconductive insulating layer for an electrophotographic plate
which is both relatively easy to make and inexpensive.
SUMMARY OF THE INVENTION
The foregoing objects and others are accomplished in accordance
with the present invention by providing an electrophotographic
plate having a novel two layered structure comprising (a) a
photoconductive layer capable of photogenerating hole-electron
pairs and injecting the electrons into the adjacent overlayer, and
(b) an adjacent electronically active transport material layer,
which is substantially transparent and non-absorbing in the
particular wavelength region of xerographic use, said
electronically active transport layer comprising an electron
transport material in sufficient concentration to be capable of
accepting and transporting electrons which have been injected from
the photoconductive layer.
As defined herein, a photoconductor is a material which is
electrically photoresponsive to light in the wavelength region in
which it is to be used. More specifically, it is a material whose
electrical conductivity increases significantly in response to the
absorption of electromagnetic radiation in a wavelength region in
which it is to be used. This definition is necessitated by the fact
that a vast number of aromatic organic compounds are known or
expected to be photoconductive when irradiated with strongly
absorbed ultraviolet, x-ray, or gamma-radiation. Photoconductivity
in organic materials is a common phenomenon. Practically all highly
conjugated organic compounds exhibit some degree of
photoconductivity under appropriate conditions. Most of these
organic materials have their prime wavelength response in the
ultraviolet. However, little commercial utility has been found for
ultraviolet responsive materials, and their short wavelength
response is not particularly suitable for document copying or color
reproduction. In view of the general prevelance of
photoconductivity in organic compounds following short wavelength
excitation, it is therefore necessary that for the instant
invention, the term "photoconductor" and "photoconductive" be
understood to include only those materials which are in fact
substantially photoresponsive in the wavelength region in which
they are to be used.
In accordance with the present invention it has been found that a
xerographic or electrophotographic sensitive member can be prepared
with electronically active transport materials comprising aromatic
or heterocyclic electron acceptors which facilitate the transport
of photogenerated electrons from a photoconductive layer under the
influence of an electric filed. The active transport materials,
which are also referred to as active matrix materials when used as
matrices for a binder layer, to be described herein, are to be
distinguished from those matrix binders or the prior art, described
above, in that the present materials have the combined properties
of being substantially transparent, hence, non-photoconductive and
non-absorbing, in at least some significant portion of a particular
wavelength region of xerographic use corresponding to a range of
photosensitivity of the photoconductor, and are capable of
supporting the injection and transport of electrons which are
photogenerated in an adjacent layer of photoconductor. Because of
their unique combination of substantial transparency in a
wavelength region of particular xerographic use and electron
transport capability, the active transport materials of the present
invention can be used effectively as a relatively thick
electrically insulating overcoating of the photoconductive layer
and yet function as both a "window" and a charge transport means
for said photoconductive layer. These particular characteristics of
the materials used in the present invention enables use of a
relatively small amount of photoconductor in the total
photoconductive insulating layer.
It should be understood that the active transport layer does not
function as a photoconductor in the wavelength region of use. As
stated above, hole-electron pairs are photogenerated in the
photoconductive layer and the electrons are then injected across a
field modulated barrier into the active layer and electron
transport occurs through the active layer.
It is to be further noted that most materials which are useful for
active transport layers of the instant invention are incidentally
also photoconductive when radiation of wavelengths suitable for
electronic excitation is absorbed by them. However, photoresponse
in the short wavelength region, which falls outside the spectral
region for which the present photoconductors are to be used, is
irrelevant to the performance of the device. It is well known that
radiation must be absorbed in order to excite photoconductive
response, and the transparency criterion, stated above, for the
active transport materials implies that these materials do not
contribute significantly to the photoresponse of the photoreceptor
in the wavelength region of use.
A typical application of this invention includes the use of a
sandwich cell or layered configuration which in one embodiment
consists of a supporting substrate, such as a conductor, having a
photoconductive layer coated thereon. The photoconductive layer may
be in the form of a layer of amorphous or vitreous selenium. A
layer of active transport material which is substantially
transparent in a significant portion of the particular wavelength
region in which selenium is photoresponsive and is coated over the
selenium photoconductive layer. The use of the active transport
material allows one to take advantage of placing a photoconductive
layer adjacent to a supporting substrate and protecting said
photoconductive layer with a protective overlayer or "window" which
will allow for the transport of photo-excited electrons from the
selenium layer and which can be of thickness sufficient to
physically protect the photoconductive layer from environmental
conditions. This structure can be imaged in the conventional
xerographic manner which includes charging, exposure, and
development.
The use of the active transport concept of the present invention
enables one to use particular regions of the electromagnetic
spectrum for selective xerographic copying. A typical application
would be the use of electronically active materials in color
xerography to copy particular color sequentially and thereby obtain
a complete color print.
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 device contemplated by the instant invention.
FIG. 2 illustrates a second embodiment of a xerographic device of
the instant invention.
FIG. 3 illustrates a third embodiment of a xerographic device of
the instant invention.
FIG. 4 illustrates a discharge mechanism of photo-discharge of the
electronically active material layer.
FIG. 5 illustrates the discharge mechanism of one binder system of
the prior art.
FIG. 6 illustrates the discharge mechanism of another binder system
of the prior art.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates one embodiment in 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 j
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. 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 even be dispensed with entirely.
Reference character 12 designates a photoconductive monolayer or
unitary layer, which comprises a photoconductive material which is
capable of photogenerating electrons and injecting them into the
overlaying active matrix material.
Generally any photoconductive material capable of photogenerating
electrons will function with the electron transport materials of
the present invention. Typical inorganic crystalline
photoconductors include cadmium sulfide, cadmium sulfoselenide,
cadmium selenide, zinc sulfide, zinc oxide, and mixtures thereof.
The typical inorganic photoconductive glasses include amorphous
selenium, and selenium alloys such as seleniumtellurium, and
selenium-arsenic. Selenium may also be used in its hexagonal
crystalline form, commonly referred to as trigonal selenium.
Typical organic photoconductors include phthalocyanine pigments
such as the X-form of metal free phthalocyanine described in U.S.
Pat. No. 3,357,989 to Byrne et al., and metal phthalocyanine
pigments, such as copper phthalocyanine. Other typical organic
photoconductors include photoinjecting pigments such as
benzimidazole pigments, perylene pigments, quinacridone pigments,
indigoid pigments and polynuclear quinones all of which are
disclosed in copending applicant Ser. No. 93,974; 94,066; 94,040;
94,067; 94,068; all filed on Dec. 1, 1970 and all abandoned. The
above list of photoconductors should in no way be taken as limiting
but as merely illustrative of materials having particularly
efficient electron injection properties.
Photoconductive monolayer 12 of FIG. 1 may be in any suitable
thickness used for carrying out its function in the xerographic
insulating member. Typical thicknesses for this purpose range from
0.02 to 20 microns. Thicknesses above 25 microns tend to produce
undesirable positive residual buildup in the photoconductive layer
during recycling and excessive dark decay while thicknesses below
0.02 micron become inefficient in absorbing impinging radiation. A
range of from about 0.2 to 5 microns is preferred since these
thicknesses would ensure maximum functionality of the
photoconductor with a minimum amount of said photoconductive
substance and completely avoid the above mentioned problems with
regard to thickness. As pointed out above, one of the primary
advantages of the instant invention is the use of a minimum amount
of photoconductor in the photoconductive insulating layer.
Reference character 13 designates the active transport material
layer which overlays the photoconductive layer 12. As heretobefore
mentioned, the active transport layer comprises an electron
transport material which is capable of both supporting electron
injection from the photoconductive layer and transporting said
photogenerated electrons under the influence of an applied field.
In order to function in the manner outlined above, the active
transport material should be substantially transparent to the
particular wavelength region used for xerographic copying. In
particular, the active transport material should be substantially
non-absorbing in at least a significant portion that part of the
electromagnetic spectrum which ranges from about 4200 to 8000
Angstroms becuase most xerographically useful photoconductors have
photoresponse to wavelengths in this region.
As mentioned above, the active transport layer 12 comprises
aromatic or heterocyclic electron acceptor materials which have
been found to exhibit negative charge carrier transport properties
as well as the requisite transparency characteristics. Typical
electron acceptor materials within the purview of the instant
invention include phthalic anhydride, tetrachlorophthalic
anhydride, benzil, mellitic anhydride, S-tricyanobenzene, picryl
chloride, 2,4-dinitrochlorobenzene, 2,4-dinitrobromobenzene,
4-nitrobiphenyl, 4,4-dinitrobiphenyl, 2,4,6-trinitroanisole,
trichlorotrinitrobenzene, trinitro-0-toluene, 4,6-dichloro-1,
3-dinitrobenzene, 4,6-dibromo-1,3-dinitrobenzene, P-dinitrobenzene,
chloranil, bromanil, and mixtures thereof. It is further intended
to include within the scope of those materials suitable for the
active transport layer, other reasonable structural or chemical
modifications of the above described materials provided that the
modified compound exhibits the desired charge carrier transport
characteristics.
While any and all aromatic or heterocyclic electron acceptors
having the requisite transparency characteristic are within the
purview of the instant invention particularly good electron
transport properties are found with aromatic or heterocyclic
compounds having more than one substituent of the strong electron
withdrawing substituents such as nitro-(-NO.sub.2), sulfonate ion
(-SO.sub.3), carboxyl-(-COOH) and cyano-(CN) groupings. From this
class of materials, 2,4,7-trinitro-9-fluorenone (TNF),
2,4,5,7-tetranitrofluorenone, trinitroanthracene, dinitroacridine,
tetracyanopyrene, and dinitroanthraquinone are preferred materials
because of their availability and superior electron transport
properties.
It will be obvious to those skilled in the art that the use of any
polymer having the described aromatic or heterocyclic electron
acceptor moiety as an integral portion of the polymer structure,
will function as an active transport material. It is not the intent
of the invention to restrict the type of polymer which can be
employed as the transport material, provided it has an active
electron acceptor moiety to provide the polymer with electron
transport characteristics. Polyesters, polysiloxanes, polyamides,
polyurethanes, and epoxies, as well as block, random or graft
copolymers containing the aromatic moiety are therefore exemplary
of the various types of polymers which could be employed. In
addition, electronically inactive polymers in which the active
electron acceptor material is dispersed at high concentration can
be employed as hereinafter described.
The substantial or significant transparency of the active transport
material within the context of the instant invention, as
exemplified by FIG. 1, means that a sufficient amount of radiation
from a source must pass through the active transport layer 13 in
order that the photoconductive layer 12 can function in its
capacity as a photogenerator and injector of electrons. More
specifically, substantial transparency is present in the active
transport materials of the present invention when the active
transport material is non-photoconductive and non-absorbing in at
least some significant portion of the wavelength region of from
about 4200 to 8000 Angstrom Units. This property of substantial
transparency enables enough activating radiation to impinge the
photoconductor layer so as to cause discharge of the charged active
transport photoreceptor of the present invention.
It is not the intent of this invention to strictly restrict the
choice of active transport materials to those which are transparent
in the entire visible region. For example, when used with a
transparent substrate, imagewise exposure may be accomplished
through the substrate without the light passing through the layer
of active transport material. In this case, the active material
need not be non-absorbing in the wavelength region of use. This
particular application takes advantage of the injection and
transport properties of the present active materials and falls
within the purview of the instant invention. Other applications
where complete transparency is not required for the active material
include the selective recording of narrow-band radiation such as
that emitted from lasers, spectral pattern recognition, color coded
form duplication, and possibly color xerography.
While the active material layer 13 of FIG. 1 may consist
exclusively of charge transport material, for purposes of the
present invention, the layer may also comprise the charge transport
material at a sufficient concentration in a suitable electronically
inert binder material to effect particle-to-particle contact or to
effect sufficient proximity thereby permitting effective charge
transport from the photoinjecting pigments of the instant invention
through the layer. Generally there must be a volume ratio of at
least 25 percent active transport material to electronically inert
binder material to obtain the desired particle-to-particle contact
or proximity. Typical resin binder materials for the practice of
the invention are polystyrene; silicone resins such as DC-801,
DC-804, and DC-996 all manufactured by the Dow Corning Corporation;
and Lexan, a polycarbonate resin, SR-82 manufactured by the General
Electric Company; acrylic and methacrylic ester polymers such as
Acryloid A10 and Acryloid B72, polymerized ester derivatives of
acrylic and alpha-acrylic acids both supplied by Rohm and Haas
Company and Lucite 44, Lucite 45 and Lucite 46 polymerized butyl
methacrylates supplied by the E. I. duPont de Nemours &
Company; chlorinated rubber such as Parlon supplied by the Hercules
Powder Company; vinyl polymers and copolymers such as polyvinyl
chloride, polyvinyl acetate, etc. including Vinylite VYHH and VMCH
manufactured by the Bakelite Corporation; cellulose esters and
ethers such as ethyl cellulose, nitrocellulose, etc.; alkyd resins
such as Glyptal 2469 manufactured by the General Electric Company;
etc. In addition, mixture of such resins with each other or with
plasticizers so as to improve adhesion, flexibility, blocking, etc.
of the coating may be used. Thus, Rezyl 869 (a linseed oil-glycerol
alkyd manufactured by American Cyanamid Company) may be added to
chlorinated rubber to improve its adhesion and flexibility.
Similarly, Vinylites VYHH and VMCH (polyvinyl chlorideacetate
copolymers manufactured by the Bakelite Company) may be blended
together. Plasticizers include phthalates, phosphates, adipates,
etc. such as tricresyl phosphate, dicotyl phthalate, etc. as is
well known to those skilled in the art.
The active transport material which is employed in conjunction with
the photoconductive layer in the instant invention is a material
which is an insulator to the extent that an electrostatic charge
placed on said active transport material is not conducted in the
absence of illumination at a rate sufficient to prevent the
formation and retention of an electrostatic latent image thereon.
In general, this means that the specific resistivity of the active
transport material should be at least 10.sup.10 ohms-cm. and
preferably will be several orders higher. For optimum results,
however, it is preferred that this specific resistivity of the
active transport material be such the the overall resistivity of
the active binder layer in the absence of activating illumination
or charge injection from an adjacent layer be about 10.sup.12
ohm-cm.
Because the overlayer functions as an active transport layer
thickness is not critical to the function of the xerographic
member. However, the thickness of said active transport layer would
be dictated by practical needs in terms of the amounts of
electrostatic charge necessary to induce an applied field suitable
to effect electron injection and transport. Active transport layer
thicknesses of from about 5 to 100 microns would be suitable, but
thicknesses outside this range may be used. The ratio of the
thickness of the active transport layer to the photoconductive
layer should be maintained from about 2:1 to 200:1.
Another modification of the layered structure of FIG. 1 is
illustrated in FIG. 2 where the photoconductive layer is depicted
as being a layer of binder material having crystalline particles of
photoconductor dispersed therein. The binder material may be any
suitable organic substance used for such purposes including inert
binder materials or one of the active matrix materials of the
instant invention. The concentration of the photoconductor material
will vary according to which type of binder material is used and
will range in value from about 5-99 volume percent of the total
photoconductive layer. If any electronically inert binder material
is used in combination with the photoconductor material a volume
fraction of at least 25 percent photoconductor to the
electronically inert binder material is necessary to effect
particle-to-particle contact or proximity thereby rendering layer
12 photoconductive throughout. The remarks with regard to the
thickness of the photoconductive layer of FIG. 1 are generally
applicable here; namely, a range of from about 0.05 to 20 microns,
with a range of 0.3 to 5 microns being preferred due to the
excellent results derived from this thickness range. The size of
the photoconductive particles in the binder layer is not
particularly critical, but particles in a size range of about 0.01
to 1.0 microns yield particularly satisfactory results.
Another variation of the layered configuration described in FIGS. 1
and 2 consists of the use of a blocking layer 14 at the
substrate-photoconductor interface said layer being illustrated in
FIG. 3 This blocking layer aids in sustaining an electric field
across the photoconductor-active organic layer after the charging
step. Any suitable blocking material may be used. Typical materials
include nylon, epoxy, aluminum oxide, and insulating resins of
various types including polystyrene, butadiene polymers and
copolymers, acrylic and methacrylic polymers, vinyl resins, alkyd
resins, and cellulose base resin.
It can therefore be seen that the photo-insulating portion of the
xerographic members of the instant invention represent in FIGS. 1-3
is derived into two functional layers:
1. A photoconductive layer which photogenerates holes and electrons
upon excitation by radiation and injects said photogenerated
electrons into the overlaying electronically active transport
material, and;
2. An overlaying substantially transparent active transport
material which allows transmission of radiation to the
photoconductive layer, accepts the subsequently photogenerated
electrons from the photoconductor material, and actively transports
aid conduction electron to its positively charged surface thereby
neutralizing said charge.
This is more dramatically illustrated in FIG. 4 where the
xerographic member of the present invention has been positively
charged by means of corona charging. The light 14 represented by
the arrows then passes through the transparent active transport
layer and impinges the photoconductive layer thereby creating a
hole-electron pair. The electron and hole are then separated by the
force of the applied field and the electron injected across the
interface into the active transport layer where it is then
transported by force of the electrostatic attraction through the
active transport layer system to the surface where it neutralizes
the positive charge previously deposited by means of corona
charging. Since only photogenerated electrons can move in the
electron transport active material layer, large changes in surface
potential result only when the electric field in the layer is such
as to move the photogenerated electrons from the photoconductor
layer where they are generated, through the active matrix layer and
then to the charged surface. This means that for maximum utility
the active matrix layer must be charged positively.
Referring now to FIG. 5 there is illustrated an electrophotographic
plate of the prior art in which sensitizing pigment 12 has been
dispersed in a photoconductor binder material 13 for the purpose of
increasing the sensitivity of said photoconductor material. The
light 14 impinges the electrophotographic member and creates
photogenerated holes and electrons in either that photoconductor
binder material or the pigment materials depending on which the
radiation falls. Since most of the carriers are created at or near
the surface of the photoinsulating member charge transport presents
no serious problem. Therefore at point (A) light has caused the
photogeneration of an electron and a hole in the photoconductor and
at point (B) photogeneration takes place in the pigment. As can be
seen from the illustration, in order for the pigment to have its
effect in increasing the sensitivity of the electrophotographic
member it generally has to be present in a relatively large
concentration and be at or near the surface of the photoreceptor.
This is to be contrasted with FIG. 4 where photogeneration takes
place exclusively in the photoconductive layer, the active
transport layer being substantially transparent to the incident
radiation, and the photoconductor material is well protected by
said active layer there being no requirement that the
photoconductor be at or near the surface of the photoreceptor
member. Furthermore, it can be seen in FIG. 5 that, in order for
the pigment to function in the member, a significant amount must be
kept on or at the surface where it is prone to inevitable abrasion
and exposure to the atmosphere.
FIG. 6 offers by further contrast an illustration of a
photoreceptor of the prior art in which pigment 12 is dispersed in
an inert resin material 13 in two different concentrations, A and
B. Because there is no photogeneration in the resin binder it is
generally necessary that the photoconductive pigment or dye be in
sufficient concentration or geometric proximity to support charge
injection throughout the binder system. Hence, as can be seen, in
section (A) where there is a large concentration of pigment
impinging light 14 creates a photogenerated hole and electron pair
which is then transported through the pigments to the positively
charged surface while in section (B), where the concentration of
the pigment is insufficient to effect particle-to-particcle
proximity impinging light creates the electron and hole pair which
remains trapped because of failure of the binder system to
transport the photogenerated charges either to other pigment
particles or the charged surface. Again this figure is to be
contrasted with FIG. 4 where particle-to-particle proximity or
contact of the photoconductor is unnecessary in the active matrix
structure. In addition, because particle-to-particle contact is
necessary in the inert binder structure of FIG. 6 resolution
problems occur because the geometry of the particle may not
correspond to the direction of the impinging light thereby
resulting in irregular dissipation of the charge.
When the two layered configuration of photoconductor and active
transport material has sufficient strength to form a self
supporting member (termed "pellicle"), it is possible to eliminate
the physical base or support member and substitute therefore any of
the various arrangement well known in the art, in place of the
ground plane previously supplied by the base layer. A ground plane,
in effect, provides a source of image charges of both polarities.
The deposition of the insulating two layered structure of the
present invention of sensitizing charges of the desired polarity
cause those charges in the ground plane of opposite polarity to
migrate to the interface at the photoconductive insulating layer.
Without this capacity of the insulating member by itself would be
such that it could not accept enough charge to sensitize the layer
to a xerographically useful potential. It is the electrostatic
field between the deposited charges on one side of the xerographic
two layered member and the induced charges (from the ground plane)
on the other side that stresses the xerographic member so that when
an electron is excited (in the photoconductive layer) to the
conduction band by a photon thereby creating a hole-electron pair,
the charges migrate under the influence of this field thereby
creating the latent electrostatic image. Therefore it is obvious
that if the physical ground plane is omitted a substitute therefore
may be provided by depositing on opposite sides of the two layered
xerographic insulating pellicle, simultaneously, electrostatic
charges of opposite polarity. Thus if positive electrostatic
charges are placed on one side of the pellicle as by corona
charging as described in U.S. Pat. No. 2,777,957 to L. E. Walkup,
the simultaneous deposition of negative charges on the other side
of the pellicle also by corona charging will created an induced,
that is, a virtual, ground plane within the body of the pellicle
just as if the charges of opposite polarity has been supplied to
the interface by being induced from an actual ground plane. Such an
artificial ground plane permits the acceptance of a useable
sensitizing charge and at the same time permits migration of
charges under the applied field when exposed to activating
radiation. As used hereinafter in the specification and claims the
term "conductive base" includes both a physical base and an
"artificial" one as described herein.
The physical shape of the xerographic active transport plate may be
in the form whatsoever as desired by the formulator such as a flat,
spherical, cylindrical plate, etc. The plate may be flexible or
rigid as desired.
DESCRIPTION OF PREFERRED EMBODIMENT
For purposes of affording those skilled in the art a better
understanding of the invention, the following illustrative examples
are given:
EXAMPLE I
A photosensitive layered structure plate similar to that shown in
FIG. 1 is prepared by the following technique: An aluminum
substrate is dip coated with a three percent duPont Zytel
nylon-denatured alcohol solution to form a 0.2 micron thick
blocking layer. The coated substrate is then dried for
approximately 30 minutes. Thereafter a one micron layer of
amorphous selenium is vacuum evaporated onto the blocking layer by
conventional vacuum techniques such as those disclosed by Bixby in
U.S. Pat. Nos. 2,753,278 and 2,970,906. The selenium coated
substrate is then cooled to 0.degree.C and a 10micron layer of
2,4,7-trinitro-9-fluorenone (TNF) is vacuum evaporated onto the
amorphous selenium layer.
The TNF overcoated plate is then placed in a Xerox Model D Machine
where a copy of an original is made by positive corona charging the
plate to a value of 800 volts and exposing the original to
radiation in the wavelength region of 4200 to 6500 Angstrom Units
whereby an image is formed on the plate. The image is then
developed and transferred to paper whereby a reproduction of the
original is accomplished. The copy is of excellent quality being
comparable to copies made on a conventional amorphous selenium
electrophotographic plate. In addition the active transport
photoreceptor can be recycled for multiple copies and its organic
surface is easily cleaned.
EXAMPLE II
A TNF active transport electrophotographic plate is prepared in a
manner similar to that outline in Example I except that a 2 micron
layer of the .beta. form of metal-free phthalocyanine, an organic
photoinjecting pigment, is applied on top of the blocking layer to
form a 0.5 micron photoconductive layer by dip coating the
substrate blocking layer in a solution of the phthalocyanine
pigment, dioxane, and dichloromethane and allowing the coating to
dry for several hours. After drying, a 20 micron layer of
dinitroacridine is then vacuum evaporated in the same manner as
Example I to form the active transport overlayer.
The resulting electrophotographic plate is then placed in a Model D
Xerographic Copy Machine where copy is made in the same manner as
Example I by positive corona charging to 800 volts and exposing in
a wavelength region of 4200 to 6500 Angstrom Units. The resulting
recycled copies have as good a quality of reproduction as that
prepared in Example I.
EXAMPLE III
the grams of Lexan, a polycarbonate resin, were stirred into a
solvent blend of 40 grams dioxane and 40 grams dichloromethane. To
this solution ten grams of 2,4,7-trinitro-9-fluorenone (TNF) is
added. Stirring is continued until solution is complete.
A layered structure is prepared in the same manner as Example I by
dip coating a blocking layer-substrate arrangement in a copper
phthalocyanine-solvent composition whereby a 3 micron
phthalocyanine layer is formed. The layered phthalocyanine plate is
then dip coated in the Lexan-TNF solution to form a 10 micron layer
of the resin-TNF composition. The resulting layered structure is
dried for a period of 24 hours.
The resin-TNF layered structure is then placed in a Xerox Model D
Machine where a copy is made in the same manner as the plate of
Example I. The quality of reproduction is equivalent to those in
Examples I and II which indicates that charge carriers are
transported across the resin-TNF layer. Therefore the electron
transport characteristics are not hindered by placing sufficient
quantities of TNF, or any other electron transport material, in an
electronically inert binder.
EXAMPLE IV
A photosensitive layered plate substantially similar to that shown
in FIG. 1 is prepared by the following technique: A glass substrate
having a conductive layer of tin oxide was vacuum coated with a one
micron layer of amorphous selenium by conventional vacuum
techniques such as those disclosed by Bixby in U.S. Pat. Nos.
2,753,278 and 2,970,906. A stock solution of 3.32 g. of duPont
49,000 polyester adhesive and 11.25 g. of
2,4,7-trinitro-9-fluorenone was prepared by dissolving these
quantities of materials in 58g. of tetrahydrofuran. The selenium
coated substrate was overcoated with the described stock solution
to form a 23 micron thick charge transport layer having a
percentage of 2,4,7-trinitro-9-fluorenone in the film after drying
of 75 percent by weight.
To evaluate the sensitivity of the above described plate as
compared to certain structures of the prior art in a xerographic
operative mode, a plate was prepared generally as described in U.S.
Pat. No. 3,598,582 but specifically as follows: A glass plate was
vacuum coated with aluminum to allow transmission of 9 percent of
the incident light, followed by the sprinkling of a photoconductive
pigment over the surface of the plate. The photoconductive pigment
employed was 2,6-bis (pN,N'dimethylaminobenzylideneamino)-benzo
(1,2-d:5,4-d') bisthiazole and was chosen as the pigment based on
test data shown in U.S. Pat. No. 3,489,558 and U.S. Pat. No.
3,501,298, these patents being incorporated by reference in U.S.
Pat. No. 3,598,582. This data indicated that the described pigment
would most likely be the most sensitive pigment for a xerographic
operative mode.
After the pigment was applied, it was rubbed unidirectionally until
a distinct polarization of transmitted light was observed and a
transmission density was obtained of about 0.2 to 0.6 as specified
in Example I, of U.S. Pat. No. 3,598,582. Following this the plate
was overcoated with a layer of poly-N-vinyl carbazole to a
thickness of 14 microns and dried.
The plate produced as generally described in U.S. Pat. No.
3,598,582 was then charged negatively while the plate produced
pursuant to the instant invention was charged positively. This was
necessary for comparative purposes since the poly-N-vinyl carbazole
specified in U.S. Pat. No. 3,598,582 transports holes as opposed to
electrons. Following charging, exposure to a substantially
unpolarized tungsten light source was carried out while the surface
potential was continuously monitored and the exposure times for 50
percent discharge of the surface voltage for both plates were
measured. These times are expressed below in Table I as t 1/2 (50
percent), this being a recognized measure of the xerographic
sensitivity of the noted plates.
TABLE I ______________________________________ Average Field
-(volts/ Average micron) t 1/2 (sec)
______________________________________ 75% TNF charge transport
layer over selenium 22 .068 Plate pursuant to U.S. 3,598,582 30 1.1
38 0.58 43 0.53 64 0.37 ______________________________________
It may be seen from the listed data and in view of the much faster
discharge of the plate of the instant invention that it is much
more sensitive than the plate of the prior art for xerography.
Comparing the lowest field applied to the prior art plate, the
plate of the instant invention is about 16 times faster, while at
the highest field applied, it is about 6 times faster.
The present invention has been described with reference to certain
specific embodiments which have been presented in illustration of
the invention. It is to be understood however that numerous
variations of the invention may be made and that it is intended to
encompass such variation within the scope and spirit of the
invention as described by the following claims.
* * * * *