Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
This invention is directed to an electrophotographic
imaging method and more particularly to a method employing
an overcoated electrophotographic imaging member.
The formation and development of images on the
imaging surfaces of photoconductive materials by electrostatic
means is well known. The best known of the commercial processes,
more commonly known as xerography, involves forming an electro-
static latent image on the imaging surface of an imaging
member by first uniformly electrostatically charging the
surface of the imaging layer in the dark and then exposing
this electrostatically charged surface to an imagewise pattern
, of activating electromagnetic radiation. The light-struck
areas of the imaging layer are thus rendered relatively
conductive and the electrostatic charge is selectively dissi-
pated in these irradiated areas. After the photoconductor
: .
is exposed, the electrostatic latent image on this image
bearing surface is typically rendered visible with a finely
divided colored marking material, known in the art as "tone~".
This toner will be principally attracted to those areas
~-~ on the image bearing surface which retain the electrostatic
charye and thus form a visible powder image. The electrostatic
latent image may also be used in a host of other ways as,
for example, electrosta~ic scanning systems may be employed
to "read" the latent image or the latent image may be transferred
; to other materials by TE~I techniques and stored. A developed
image can then be read or permanently affixed to the photo-
conductor where the imaging layer is not to be reused.
In the commercial "plain paper" copying systems,
the latent image is typically developed on the surface of
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: . .
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a reusable photoreceptor, subsequently transferred to a
sheet of paper and then permanently affixed thereto to form
a permanent reproduction of the original object. The imaging
surface of the photoreceptor is then cleaned oE any residual
toner and additional reproductions of the same or other
original objects can be made thereon.
Various types of photoreceptors are known for
use in electrophotographic copying machines. For example,
there are known in the art photoreceptors wherein the charge
carrier generation and charge carrier transport functions
are performed by discrete contiguous layers. There are
also known in the art photoreceptors which include an overcoat-
ing layer of an electrically insulating polymeric material.
; In conjunction with such so-called "overcoated" photoreceptors
there have been proposed a number of imaging methods. Nevertheless,
as the art of xerography advances and more stringent demands
are imposed upon the copying apparatus because of increased
performance standards there continue to be discovered novel
imaging methods. The present application relates to a novel
electrophotographic imaging method which ut~lizes an overcoated
electrophotographic imaging member.
PRIOR ART STATEMENT
U.S. Patent 3,041,167 discloses an electrophoto-
graphic imaging method which utilizes an overcoated imaging
member comprising a conductive substrate, a photoconductive
insulating layer and an overcoating layer of an electrically
insulating polymeric material.
In operation, the member is utilized in a repetitive
electrophotographic copying method wherein initially the
member is charged with an electrostatic charge o~ a first
~ J~
.
polarity and imagewise exposed to form an electrostatic latent
image whîch is then developed to orm a vIsible image. The
; visible image is transferred to a receiver member and the
surface of the imaging member i~s cleaned to complete the
imaging cycle. Prior to each succeeding cycle, the imaging
member is charged with an electrostatic charge of a second
polarity which is opposite in polarity to said first polarity
Enough additional charges of the second polarity are applied
to create across the member a net electrical field of said
second polarity. At the same time mobile charges of said
first polarity are created in the photoconductive layer such
as by applying an electrical potential to the conductive
substrate. In this method, the imaging potential which is
developed to form the visible image is present across the
photoconductive layer and the overcoating layer.
; An article by Nakamura in IEEE Transactions On
Electron Devices, Vol. EDl9, No. 4, April 1972 discloses
various techniques for forming images on overcoated photo~
receptors comprising a conductive substrate, a photoconductive
insulating layer and an overcoa~ing layer of an electrically
insulating polymeric material. In Technique #l the member is
charged a first time with electrostatic charges of a first
polarity, charged a second time with electrostatic charges of
the opposite polarity to neutralize the previous charges
deposited on the member, imagewise exposed and developed.
SUMMARY OF THE INV:E: NT ION
~ _ . .
It is therefore the object of an aspect of this
invention to provide a novel electrophotographic imaging method.
It is an object of an aspect of the inven~ion to provide
.:
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an imaging method which utilizes an overcoated photoreceptor
which includes a layer of a photoconductive charge carrier
` generator material and a layer of a charge carrier transport
materlal.
It is an object of an aspect of the invention to
provide an electrophotographic imaging method wherein the
imaging potential is created a~ross the charge carrier
generatox and charge carrier transport layers only.
BRIEF 5UMMARY OF THE INVENTION
These and other objects and advantages are accom-
; plished in accordance with an aspect of the invention by
providing an electrophotographic method which utilizes an
imaging member comprising a substrate, a layer of a charge
carrier injecting material, a layer of a charge carrier
transport material, a layer of a photoconductive charge
carrier generating material and an electrically insulating
~ overcoating layer. In operation, the member is charged
`~ a first time with electrostatic charges of a first
,,
polarity, charged a second time with electrostatic
charges of a polarity opposite to said first polarity in
order to substantially nelltralize the charges residing on
the electrically insualting surface of the member and
exposed to an imagewise pattern of activating electro-
;
magnetic radiation whereby an electrostatic latent image
is formed. The electrostatic latent image may be developed
..-
to form a visible image which may be transferred to a
:- receiver member. Subsequently~ the imaging member may be
reused to form additional reprodu~tions after erasure and
cleaning steps are carried out.
-5-
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,
:
. Thus, according to another aspect of the
invention there is provided an electrophotographic imaging
method comprising:
;. (a) providing a photoreceptor comprising in the
stated order
(i) a substrate;
(ii) a laye.r of a charge carrier injecting
material;
(iii) a layer of a charge carrier transport
material capable of transporting at least one species of
; - charge carrier and injecting one species of charge carrier
into the layer of charge carrier generating material des-
.` cribed in (iv) below:
tiv) a layer of charge carrier generating
material which is capable of injecting photogenerated
. charge carriers of one species into said charge carrier
transport material; and
(v) a layer of electrically insulating
; material;
(b) charging said photoreceptor with electro-
static charges of a first polarity
(c) charging said photoreceptor wikh electrostatic
charges opposite in polarity to said first polarity in order
-~ to substantially neutralize the charges residing on the
: 25 surface of said photoreceptor; and
:. (d) exposing said photoreceptor to an imagewise
~;~ pattern of electromagnetic radiation to which said charge
:. carrier generating material is responsive whereby there
is formed an electrostatic latent image within said photo-
receptor.
. -5a- -
.
f~
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention as
well as other objects and further features thereof, reference
;` :
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is made to the following detailed description of various
preferred embodiments thereof taken in conjunction with
the accompanying drawings wherein:
Fig. l is a partially schematic, cross-sectional
view of a photoreceptor which may be utilized in the method
of the invention; and
Figs. ~A-2C illustrate the various method steps
employed.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Fig. l, there is illustrated
a photoreceptor, generally designated lO comprising a
substrate 12, a layer of charge carrier injecting
material 14, a layer of charge carrier transport material
16, a layer of photoconductive charge carrier generating
material 18 and a layer of electrically insulating polymeric
material 20.
Substrate 12 may be opaque or substantially trans-
parent and may comprise any suitable material having the
requisite mechanical properties. The substrate may comprise
a layer of conductive material such as, for example, aluminum,
~; brass or the like; or it may comprise a layer of non-conducting
material such as an inorganic or organic polymeric material;
or it may comprise a layer of an inorganic or organic material
having a conductive surface layer arranged thereon. The
r, 25 substrate may be flexible or rigid and may have any of many
different configurations such as, for example, a plate,
a cylindrical drum, an endless flexible belt, etc. Preferably
~ the substrate is in the form of an endless flexible belt.
; Charge carrier injecting layer 14 must be capable of injecting
~30 charge carriers into charge carrier transport layer 15 under
; : ~
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.
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the influence of an electrical field in the preferred embodi-
ment of the invention. As will be discussed in detail below
herein, the injected charge carriers must be of the same
polarity as the mobile carriers preferentially transported
by layer 16. In one embodiment, the charge carrier injecting
layer may be sufficiently laterally conductive to also serve
as the ground electrode for the photoreceptor in which case
a separate additional conductive layer is not required.
In another embodiment, an additional discrete conductive
layer is required to provide the necessary electrical field
~ when the photoreceptor is charged. In such a configuration,
- the conductive layer is arranged below the charge carrier
injecting layer and conveniently can be provided as an
integral part of the substrate.
Typical suitable materials which are capable of
injecting charge carriers under the influence of an electrical
-;` field and therefore are suitable for use in layer 1~ include
gold, graphite, aluminum, indium and the like. Gold and
` graphite are hole injecting materials; aluminum and indium
are electron injecting materials. Gold, aluminum and indium
;; possess sufficient lateral conductivity so that they may
. also serve as the ground electrode as well as the charge
- carrier injecting material. Graphite typically does not
:,..
` have the requisite lateral conductivity to serve as the
ground electrode and an additional conductive layer is required
; in conjunction therewith. Typically, charge carrier injecting
,
layer 14 has a thickness in the range of from about 0.1
to about 10 microns or more. The maximum thickness in any
specific instance is generally determined by mechanical
considerations; in flexible photoreceptors, for example,
-7
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:
~3~3~
the charge carrier injecting layer is typically very thin.
The charge carrier injecting materials and charge carrier
transport materials require a particular work function relation-
ship in order Eor holes or electrons to be injected from
the former into the latter. Generally hole injecting materials
possess a relatively high work fur~ction whereas electron
injecting materials possess a relatively low work function.
In another embodiment, the charge carrier injecting
layer 14 comprises a material which will inject charge carriers
when irradiated with appropriate electromagnetic radiation.
In this embodiment, layer 14 may comprise any suitable photo-
generating material known for use in electrophotography
such as selenium and the like.
; Charge carrier transport layer 16 may comprise
any suitable material capable of transporting at least one
species of charge carrier. Preferably layer 16 comprises
, a material which is capable of transporting only one speciesof charge carrier. Layer 16 typically has a thickness in
, the range of from about 5 to about 50 microns and preferably
from about 20 to about 4G microns. The layer preferably
. . ~
comprises a material having no photoconductive properties
or which has photoconductive properties in a spectral region
which is completely obscured by the superimposed photogenerator
' material in layer 18 and overcoating material in layer 20
; 25 since it is desired to have layer 18 perform the charge
~; carrier photogeneration function substantially completely.
To this end the materials in layers 18 and 20 and the light
source utilized in the method are generally selected so
- ~ as to preclude any substantial photoexcitation of the charge
~ 30 carrier transport material. It is preferred to utilize
,', '
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, ~
a charge carrier transport material which will transport
only one species of charge carrier because, as will become
apparent from the discussion below, this type of material
eliminates constraints which would have to be placed on
the selec~ion of the charqe carrier injecting material
otherwise.
As mentioned previously, layer 16 must be able
to accept charge carriers injected from injecting layer
14. In addition, layer 16 must be able to accept and transport
charge carriers which are generated by charge carrier generating
layer 18 upon irradiation thereof with appropriate illumination.
Moreover, as will be discussed in detail below herein, charge
carriers must be able to travel across the interface between
,..... .
layers 16 and 18 without hindrance in both directions in
order to have the member function in a repeating cyclic
; mode.
~; The charge carrier transport layer may be a polymeric
. . ,
` film of a charge carrier transporting material or a monomeric
charge carrier transport material incorporated in a solid
solution with an electrically insulating inert polymeric
binder material. Solid solution systems typically have
about 30~-50~ by weight of the monomeric transport material
-` in order to enable rapid and efficient transport of charge
carriers. It is possible to have a lesser amount of the
monomer where the binder material itself is capable of transporting
: charge carriers. By "inert" is meant a material which is
relatively incapable of independently generating charge
carriers in response to the electromagnetic radiation within
.,.
;~-; the spectral range employed in the electrophotographic imaging
method and/or transporting charge carriers which are injected
into its bulk from another source. Typical suitable polymeric
:,
_g_
,:
~3.~
charge carr;er transporting materials include, for example,
poly(N-vinylcarbazole), poly(2-vinylcarbazole~, poly(3-vinyl-
carbazole), poly(3-vinylpyrene~, poly(2-anthrylmethacrylate)
and poly(9-vinylanthracene). Typical suitable monomeric charge
carrier transporting materials are disclosed in U.S. Patents
3,573,906 and 3,870,516. Typical suitable inert polymeric
resins which may be used as solid solution matrices for mono-
meric charge carrier transport materials includ~ for example,
polyolefins, polycarbonates, polysiloxanes, copolymers, blends
and mixtures thereof.
There are known charge carrier ~ransport materials
which will transport both species of charge carriers such as,
; for example, complexes of poly(N-vinylcarbazole) and 2,4,7-
. trinitro-9-fluorenone (TNF). There are also known charge
:~ 15 carrier transport materials which transport primarily only holes
such as, for example, triphenylamine or only electrons such as,
for example, TNF. The selection of a transport material ~or
use in a particular photoreceptor is dependent upon various
,~,
~:~ factors such as the polarity of the electrostatic charges
:-~ 20 deposited in the first charging step, the ability to accept
.~ charge carriers injected from layer 14, the ability to accept
~ charge carriers generated by layer 18 and the ability to form
.~ an interface with layer 18 which will allow charge carriers to
travel across the interface in both directions.
~: 25 Photoconductive charge carrier generating layer 18
.:~ generally may comprise any photoconductive charge carrier
generating material known for use in electrophotography
provided it is electronically compatible with charge carrier
: transport layer 16, that is, it can inject photoexcited
-- 10 --
... . .
;`~
charge carriers into the transport layer and charge carriers
` can travel in both directions across the interface between
the two layers. Particularly preferred photoconductive
charge carrier generating materials include amorphous and
trigonal selenium, selenium-arsenic and selenium-tellurium
alloys and organic charge carrier generating materials such
as phthalocyanine. Layer 18 is typically from about 0.5
to about 10 microns or more in thickness. Generally, it
is desired to provide this layer in a thickness which is
sufficient to absorb at least 90~ (or more) of the incident
radiation which is directed upon it in the imagewise exposure
step. The maximum thickness is dependent primarily on factors
such as mechanical considerations, e.g. whether a flexible
photoreceptor is desired.
Electrically insulating overcoating layer 20 typically
. ~
has a bulk resistivity of from about 1012 to about 5 x 1014
,, ohm-cm and typically is from about 5 to about 25 microns
:, in thickness. Generally, this layer provides a protective
function in that the charge carrier generating layer is
kept from being contacted by toner and ozone which is generated
.,;
during the imaging cycle. The overcoating layer also must
prevent charges from penetrating through it into charge
carrier generating layer 1~ or from being injected into
it by the latter. Preferably, therefore layer 20 comprises
. .. .
materials having higher bulk resistivities. Generally,
the minimum thickness of the layer in any instance is determined
by the functions the layer must provide whereas the maximum
thickness is determined by mechanical considerations and
the resolution capability desired for the photoreceptor.
B30 Typical suitable materials include Mylar (a polyethylene
t r 6 ~
terephthalate film available from E. I. duPont de Nemours),
polyethylenes, polycarbonates, polystyrenes, polyesters,
polyurethanes and the like. The particular material selected
in any instance should not be one which will dissolve or
react with the materials used in layers 16 and 18.
The formation of the electrically insulating layer
20 over the previous layer may be carried out by solution
coating in which case no additional materials are required.
Where layer 20 constitutes a preformed mechanically tough
film, it is typically necessary to provide sufficient adhesive
material in order to provide an integral structure which
is desirable for use in a repetitive imaging method. The
electrical properties of any such adhesive interlayer should
be similar to those of the overcoating. Alternatively,
they may be similar to the binder ma-terial of the charge
carrier generating layer 18 where a binder material is present
, in that layer. Mechanically, the adhesive interlayer should
provide an adhesive state that firmly binds the layers together
without any air gaps or the like which could disturb image
definition.
' .
The operation o~ the member is illustrated with
respect to Figs. 2A-2C. In this illustrative explanation
the charge carrier injecting material which comprises layer
14 is a hole injecting material and the initial charging
step is carried out with negative polarity. As noted previously,
the method is not limited to this embodiment. Moreover,
the description of the method will be given in conjunction
with the proposed theoretical mechanism by which the method
is thought to be operative in order to better aid those
skilled in the art to understand and practice the invention.
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It should be noted however that the method has been proved
to be operable and highly effective through actual experimentation
and any inaccuracy in the proposed theoretical mechanism
, of operation is not to be construed as being limiting of
the invention.
ReEerring now to Fig. 2A, there is seen the condition
of the photoreceptor aEter it has been electrically charged
`: negatively a first time in the absence of illumination by
any suitable electrostatic charging apparatus such as a
corotron. The negative charges reside on the surface of
electrically insulating layer 20. As a consequence of the
', charging an electrical field is established across the photo-
~; receptor and as a consequence of the electrical field holes
are injected from the charge carrier injecting layer into
~ 15 the charge carrier transport layer. The holes injected
;' into the charge carrier transport layer are transported
through the layer, enter into the charge carrier generating
layer 18, and travel through the latter until they reach
the interface between the charge carrier generating layer
. . -
18 and the electrically insulating layer where they become
trapped. The charges thus trapped at the interface establish
an electrical field across the electrically insulating layer
.:,
20. Thus, it is seen that in the embodiment where negative
charging is carried out in the first charging step charge
carrier injecting layer 14 and charge carrier transport
layer 16 must comprise materials which will allow injection
of holes from the former into the latter and charge transport
layer 16 preferably comprises material which will predominantly
transport holes. Also, it can be seen that the charge carrier
transport layer 16 and the charge carrier generating layer
-13~
:
18 must comprise materials which will allow injection of
holes from the former into the latter and allow the holes
to reach the interface between layer 18 and electrically
insulating layer 20. Generally, the charging step is carried
out with a voltage in the range of from about 10 volts/micron
to about 100 volts/micron.
. ,
!' Subse~uently, the member is charged a second time,
~; again in the ahsence of illumination, with a polarity opposite
to that used in the first charging step in order to substantially
neutralize the charges residing on the surface of the member.
In this illustrative instance, the second charging of the
member is with posi~ive polarity. After the second charging
step the surface of the photoreceptor should be substantially
free of electrical charges. The substantially neutralized
~` 15 surface is created by selecting a charging voltage based
,~, . ;,
on the dielectric thickness ratio of the overcoating layer
20 to the total of the charge carrier transport and charge
carrier generating layers, 16 and 18 respectively. By "sub-
stantially neutralized" within the context of this invention
is meant that the voltage across the photoreceptor member,
upon illumination of the photoreceptor, may be brought to
substantially zero.
Fig. 2B illustrates the condition of the photoreceptor
after the second charging step. In this ill~stration no
charges are shown on the surface of the member. The positive
charges residing at the interface of layers 18 and 20 as
; a result of the first charging step remain trapped at that
interface at the end oE the second charging step. However,
there is now a uniform layer of negative charges located
at the interface between layers 14 and 16.
- -14-
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Therefore, it can be seen that the net result
of the second charging step is to establish a uniform electrical
field across the charge carrier transport and charge carrier
generating layers. To achieve this result it is critical
that the negative charges be located at the interface between
charge carrier injecting layer 14 and charge carrier transport
layer 16 and prevented from entering into the transport
layer. For this reason it is preferred to utilize a charge
carrier transport material which will transport only one
species of charge carrier~ holes in this illustrative instance.
Where a charge carrier transport material capable of transporting
both species of charge carriers is employed in layer 16
it is apparent that the charge carrier injecting material
.~ri
would have to be selec-ted so that the latter would be unable
to inject electrons in layer 16 thus placing constraints
on the selection of materialsO
Subsequently, the member is exposed to an imagewise
` pattern of electromagnetic radiation to which the charge
carrier generating material comprising layer 18 is responsive.
The exposure of the member may be effected through the substrate
or the electrically insulating overcoating. As a result
of the imagewise exposure an electrostatic latent image
is formed in the photoreceptor~ This is ~ecause hole-electron
pairs are generated in the light-struck areas of the charge
carrier generating layer. The light-generated holes are
injected into the charge carrier transport layer and travel
through it to ~e neutralized by the negative charges located
at the interface between layers 1~ and 16 whereas the light-
generated electrons neutralize the positive charges trapped
at the interface between layers 18 and 20. In the areas
-15-
.,'~;
3~3~3~
of the member which did not receive any illumination, the
positive charges remain in their original position. Thus,
there continues to be an electrical field across the charge
carrier transport and charge carrier generating layers in
areas which do not receive any illumination whereas the
electrical field across the same layers in the areas which
did recei~e illumination is discharged to some low level.
~ It is readily apparent from the oregoing that
h` charge carriers of one species must be able to pass in both
- 10 directions across the interface of between charge carrier
transport layer 16 and charge carrier generating layer 18
in order to form an electrostatic latent image in the member.
Laboratory experiments have shown that this condition may
not always be satisfied. For example, it has been found
; 15 that an amorphous selenium generator layer would inject
holes into a charge carrier transport layer co~prising N-
isopropyl carbazole in a polycarbonate binder mateLial but
that holes did not inject from the transport layer into
the generator layer.
A convenien~ technique has been devised to test
combinations of charge carrier generator and charge carrier
transport materials in order to determine if the re~uisite
bi-directional charge carrier travel occurs. Initially,
a sample is prepared including a charge carrier injecting
layer, a charge carrier transport layer and a charge carrier
generating layer, The charge carrier generating layer is
provided in a thickness su~ficient so that the capacitance
division of an applied voltage across the sample can be
calculated. For example, a sample can be prepared with
a gold charge carrier injecting layer, a 32.2 micron thick
.
-16-
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` charge carrier transport layer comprising N,N'-diphenyl- N,N'-bis(3-methylphenyl)-[l,l'-biphenyl]-4,4'-diamine in
a polycarbonate binder and a 23.~ micron thick charge carrier
generatiny layer comprising selenium. In such a sample,
approximately three quarters of the voltage drop would be
across the transport layer. Initially, the sample is charged
to a potential of +2400 volts. The sample is then illuminated
from above with activating electromagnetic radiation. The
field across the sample is discharged to substantially zero.
Therefore, it can be concluded that holes inject from the
.
; selenium layer into the charge carrier transport material
and travel through the entire thickness of the layer to
discharge the field. If the holes were not injected from
the generating layer into the transport layer a measurable
voltage across the transport layer would result. Subsequently,
the sample is charged in the dark with negative polarity
using the same corotron voltage. After this charging step,
no substantial voltage can be measured. Therefore, it is
concluded that holes are injected from the gold layer into
the transport layer, travel across the latter and enter
into the charge carrier generating layer. Accordingly,
the requisite bi-directional charge carrier travel occurs.
If after the second charginy step it is possible to measure
" a voltage which would be predicted by the voltage division
between the charge carrier carrier generating and transport
layer then it can be concluded that holes did not travel
across the interface between these layers and the combination
would not be appropriate for use in a photoreceptor according
to the method of the invention. Any combination o charge
carrier generating and charge carrier transport materials
17
3~.~
.
can be tested according to this general procedure. The
procedure thus represents a convenient way to determine
suitable combinations of materials which will exhibit the
requisite properties.
Another significant factor in selectlon of materials
concerns the rate at which charge carriers are injected
from the injecting material into the txansport material
with respect to the rate at which the charge carriers are
; transported through the latter. It is preferred to select
a combination of materials such that the rate at which charge
carriers are injected into the charge carrier transport
layer exceeds the rate at which the charge carriers are
~; transported through the transport layer and it is particularly
preferred that the injection rate greatly exceed the transport
rate.
When this condition is met, the electrical field
which is responsible for causing charge carrier injection
to occur will not diminish as rapidly as would otherwise
be the case. Accordingly, charge carrier injection will
be complete and bulk trapping of charge carriers can be
avoided thus providing a more efficient method ~ith desirable
cycling characteristics. In the preferred embodiments,
charge carrier injection is typically complete in less than
a millisecond whereas charge carrier transport takes place
between the order of a millisecond and tens of milliseconds.
Where the injection/transport rates are reversed, a much
less desirable condition can occur. In thls case, injection
would take place over a relatively long ~ime period with
each injected charge rapidly being transported away and
diminishing the existing electrical field. Eventually,
,~
-18-
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the electrical field would be so weak that only relatively
; sporadic and incomplete injection would take place and trans-
port of the lattermost injected charges would be slowed.
~ This could result in irreversible trapping of charges due
`~ 5 to relatively long residence of the charges on the transport
material molecules Where this condition occurs the cycling
; characteristics of the method are unfavorably affected.
; The electrostatic latent image formed in the member
may be developed to form a visible image by any of the well
known xerographic developmen~ techniques, for example, cascade,
magnetic brush, liquid development, etc. The visible image
is typically trans~erred to a receiver member by any conventional
transfer technique and affixed thereto. While it is preferable
to develop the electrostatic latent image with marking material
the image may be used in a host of other ways such as, for
example, "reading" the latent image with an electrostatic
scanning system.
~ When the photoreceptor is to be reused to make
- additional reproductions as is the case in a recyclible
;20 xerographic apparatus any residual charge remaining on the
photoreceptor after the visible image has been transferred
to a receiver member typically is removed therefrom prior
"
to each repetition of the cycle as is any residual toner
; material remai~ing after the transfer step. ~enerally,
the residual charge can be removed from the photoreceptor
by ionizing the air above the electrically insulating over-
coating of the photoreceptor while the photoconductive carrier
generating layer is uniformly illuminated and grounded.
For example, charge removal can be effected by A.C. corona
discharge in the presence of illumlnation from a light source
~: -19-
"''
or preferably a grounded conductive brush could be brought
into contact with the surEace of the photoreceptor in the
presence of such illumination. This latter mode also will
remove any residual toner particles remaining on the surface
of the photoreceptor.
In another embodiment the charge carrier injecting
; layer 14 comprises a material which will generate charge
carriers when irradiated with appropriate electromagnetic
radiation such as a layer of selenium~ In this embodiment
the substrate must include a conductive layer to serve as
a ground electrode for the photoreceptor. In operation
the photoreceptor is charged a first time and then uniormly
illuminated through the substrate with suitable activating
radiation. The illumination step may be simultaneously
with or subsequent to the initial charging step. The remainder
of the method is the same as described above.
The invention will now be described in detail
with respect to specific preferred embodiments thereof by
way of examples it being understood that these are intended
to be illustrative only and the in~ention is not intended
to be limited to the materials, conditions, process parameters,
etc. recited therein. All parts and percentages are by
weight unless otherwise indicated.
EX~MPLE I
A photoreceptor was ~abricated by initially providing
an aluminum sheet substrate approximately ~ mils in thickness.
Subse~uently, an approximately 25 micron thick layer of
a charge carrier transport material comprising a 4~1 by
weight mixture of PE-200 polyester resin (available from
Goodyear Chemical) and X-form phthalocyanine was deposited
-20-
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~ 3~S~
'X
on the aluminum layer by solvent coating from a methyl ethyl
ketone and toluene solution. The member was then dried
overnight in a vacuum oven at a temperature o~ about 50C.
An approximately l micron thick amorphous selenium layer
was vacuum deposited over the transport layer followed by
the formation of an approximately 15 micron thick layer
of phenoxy insulating material by solvent coating from a
solution of methyl ethyl ketone. The photoreceptor was
then dried to remove any residual solvent.
The photoreceptor was charged a first time to
a potential of -1200 volts and then charged a second time
to a potential of +400 volts. The photoreceptor was then
uniformly illuminated with 454 nm radiation obtained by
passing the output from a tungsten lamp through an interference
bandpass filter. Electrical measurements showed that the
field across the photoreceptor was discharged to about zero
potential thus indicating that the photoreceptor is suitable
for use according to the method of the invention.
A 4" x 4" sample of the photoreceptor was prepared
in the same manner described above. A xerographic reproduction
was made with a Xerox Model D Processor using this sample
;:~
;~ as the photoreceptor. A readable reproduction was obtained.
EXAMPLE II
A photoreceptor was fabricated by initially vacuum
depositing an approximately 200~ thick gold layer on an
aluminum sheet substrate such as was described in Example 1.
An approximately 25 micron thick layer of N,N'-diphenyl-
N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine in
a polycarbonate binder (l:l ratio) was formed on the gold
layer by solvent coating from a methylene chloride solution
-21-
::`
3~
using a draw bar coating technique. The member was then
dried in a vacuum oven at a temperature of about 70~C for
about 24 hours. ~n approximately 0.6 micron thick amorphous
arsenic triselenide layer was vacuum deposited over the
transport layer and an approximately l/2 mil thick acrylic
resin overcoating layer (Futura Floor Wax, available from
Johnson & Johnson) was then placed over the arsenic triselenide
layer and air dried for 24 hours.
The photoreceptor was charged a first time with
a potential of -500 volts and then charged a second time
- with a potential of +1500 voltsO The photoreceptor was then
uniformly illuminated with white light. Electrical measurements
showed that the field across the photoreceptor was discharged
to substantially zero potential thus indicating that the
- 15 photoreceptor is suitable for use according to the method
of the invention.
EXAMPLE III
A photoreceptor similar to that described in Example
II was made with the exception that the charge carrier generating
layer was selenium instead of arsenic triselenide and the
~ charge carxier transport layer was about 20 microns thick.
;~ The photoreceptor was charged a first time with a potential
of -1300 volts, a second time with a potential of ~1800
volts and then uniformly illuminated with white light.
Electrical measurements showed that the photoreceptor was
discharged to substantially zero potential thus indicating
the photoreceptor is suitable for use according to the method
of the invention.
EX~MPLE IV
A photoreceptor was fabricated by initially vacuum
~ J f ~2~-
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, .,
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~ ~ ~3~
.
depositing an approximately 0.2 micron thick gold layer
on an aluminum substrate. An approximately 20 micron thick
charge carrier transport layer comprising N,N'-diphenyl-
N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine in
5 ~ Makrolon~ta polycarbonate available from Bayer Gesel) in
a 1:1 ratio was then deposited over the gold layer by solvent
coating from a methylene chloride solution. The member
was then dried in a vacuum oven at a temperature of about
70C for about 24 hours. An approximately 1 micron thick
selenium layer was vacuum deposited over the transport layer
followed by the formation of an approximately 15 micron
thick phenoxy overcoating layer by solvent coating from
a solution of meth~l ethyl ketone. The photoreceptor was
then dried to remove any residual solvent.
The photoreceptor was charged a first time with
a potential of -800 volts, a second time with a potential
`: of ~2000 volts and then uniformly illuminated with white
light. Electrical measurements showed that the field across
the photoreceptor was dischargec~ to substantially zero thus
2Q indicating that the photoreceptor is suitable for use in
the method of the invention.
A ~" x 4" sample of the photoreceptor was prepared
in the same manner described above. A xerographic reproduction
was made with a Xerox Model D Processor using this sample
as the photoreceptor. An excellent quality reproduction
~ was obtained.
; EX~MPLE V
A photoreceptor was fabricated using an approxi-
mately 5 mil thick Mylar substrate. A charge injecting
composition was formed by preparing an 11~ solution of Flex-
23-
clad polyester resin (available from Goodyear) in chloroform,
adding to it 11% by weight of graphite and ball milling
the mixture for about ~4 hours with steel shot. An approxi-
mately 3-5 micron thick layer of the composition was deposited
on the Mylar substrate and the sample was then dried to
remove residual solvent. An approximately 20 micron thick
charge carrier transport layer made up of the composition
used in Example IV was deposited over the charge carrier
injecting layer by solvent coating from a methylene chloride
solution. The sample was dried to remove residual solvent
by placing it in a vacuum oven at a temperature of about
70C for about 24 hours. An approximately 0.5 micron thick
layer of amorphous selenium was vacuum deposited over the
transport layer. Finally, an approximately 1.5 mil thick
layer of Mylar having a polyester adhesive preapplied thereto
was laminated to the selenium layer with the polyester adhesive
in contact with the selenium employing a Model 275 LM Laminator
(available from General Binding Corpoxation, Northbrook,
Illinois). The charge carrier injecting layer had sufficient
lateral conductivity to also serve as the ground electrode
for the photoreceptor.
Testing of this photoreceptor according to the
method of the invention showed that it is suitable for use
in such method.
EXAMPLE VI
~; A photoreceptor was fabricated by depositing on
an approximately 6 mil thick aluminum substrate, an approxi-
mately 6 micron thick layer of the charge carrier injecting
composition described in Example V by the same technique
; 30 described in that example~ An approximately 28 micron thick
,:
. ~ f ~ 2 ~ S -24-
....
:
charge carrier transport layer of the same composition used
in Example IV was deposited over the charge carrier injecting
layer by solvent coating from a methylene chloride solution.
The sample was then dried in a vacuum oven at a temperature
of about 70C for about 24 hours.
A charge carrier generating composition was prepared
by placing 0.7 gm of alpha-phthalocyanine and 1.5 gms of
49000 polyester resin (available from E. I. duPont de Nemours)
in methylene chloride and ball milling for about 24 hours.
An approximately 3-4 micron thick layer of this composition
was deposited over the transport layer by solvent coating
~ using a draw bar coating technique. The sample was dried
`~' to remove residual solvent. Finally, an approximately 10
micron thlck layer of Flexclad polyester resin was deposited
over the charge carrier generating layer by solvent coating
;~ from methylene chloride solution using a draw bar coating
technique. The sample was again dried to remove residual
solvent.
The photoreceptor was charyed a first time with
a potential of -1200 volts, charged a second time with a
potential of ~2400 volts and then illuminated with white
light. Electrical measurements showed that the field across
the photoreceptor was discharged to substantially zero thus
indicating that the photoreceptor is suitable for use according
to the method of the invention.
A reproduction was made with a Xerox Model D Processor
employing the photoreceptor described above. A good quality
reproduction was obtained.
Although the invention has been described with
respect to specific preferred embodiments, i~ is not intended
to be limited thereto but rather those skilled in the art
-25-
3~
will recognize that variations and modifications may be
made therein which are with the spirit of the invention
and the scope of the claims.
~,'
'
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