Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ D~ N~ N~ENTION
The art of electrostatographic copying, as first
disclosed by Carlson in U.S. Patent 2,297,6~1, involves the
uniEorm electrostatic charging of a plate comprising a photo-
conductive insulating material on a conduc-tive substrate. The
plate is charged in the dark, and -then exposed to a light and
shadow image whereby the exposed areas of the photoconductive
material become conductive and allow the surface charge to
dissipate. The areas of the pla-te which are not illuminated,
i.e., those corresponding to the shadow portions of the light
and shadow image, retain the electrostatic charge in what ls
known as a latent image. This latent image is developed by
contacting it with an electroscopic marking material known as
toner which can either be fused to the photosensitive plate
(as in the case of coated paper xerography1 or transferred to
and fused to a transfer member such as paper (as in the case
of plain paper xerography). ;`
The aforementioned photosensitive plate typically
comprises a layer of amorphous selenium on the order of about
60ju in thickness overlying an aluminum substrate. The
aluminum substrate normally has a thin layer o~ aluminum oxide
on its surface to prevent charge injection from the aluminum
in the dark.
Another type of photosensitive plate is disclosed in
UOS. Patent 3,725,058 assigned to Matsushita Electric
Industrial Company. ~his plate involves a conductive sub-
strate having on its surface a thin (0.05 to 3 ~) layer of
amorphous selenium which is in turn overcoated with a
relatively thick layer of an organic photoconductive insulating
material which is substantially light insensitive to light of
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wavelengths in the range oE ~000 to 7500 ~, e.g. poly(vinyl-
carbazole). The patentees stress the desirability that the
layer of organic material be substantially insensitive to
visible light and state that "the use of [a] sensitizer in said
organic photoconductive material impairs the chemical
stability of the resultant organic photoconductive lnsulating
material under the irradiation of light in the visible ray
region and prevents the resultant organic photoconductive
insulating material from being reused repeatedly." Miki
Hayashi, one of the patentees named in U.S. Patent 3,725,058,
discloses in Japanese Patent Application 73~753 (published as
Publication No. 16198 of 1968) a thin plate comprising a ~
transparent substrate having a thin (1 ~ maximum) layer of ~ -
amorphous selenium on its surface which is in turn overcoated
with a thin ~10 jU maximum) layer of an organic photoconductive
material. This plate must be very thin, since it is intended
to be used as a projection transparency. Thus, by imaging
the plate as previously described and fusing the toner into
the plate after development, a device suitable for use as a
transparency in conventional overhead projectors is prepared.
The extreme thinness of the selenium and organic layers in the
plate tends to reduce its photosensitivity and the author of
said Japanese Application 73-753 discloses the desirability
of adding certain types of chemical sensitizers to the organic ;
layer to increase its photosensitivity. As examples of such
sensiti2ers, he includes Leucomalachite green and Rhodamine B
extract. In both the aforementioned U.SO patent and Japanese
applica-tion, Hayashi is consistent in his belief that the
addition of a sensitizer to the organic overlayer will be
detrimental to the cycling capability of the photosensitive
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d`e~ice under consideration. This is the case because the
device disclosed in the U.S. patent, whlch is intended for
use in a cyclic operation, contains no sensitizer, whereas
the device disclosed in the Japanese application, which contains
a sensitizer, is not intended for cyclic use.
It has been discovered that a device such as that
disclosed by ~ayash~ eit al in the aforementioned U.S. patent,
e.g. a layer of selenium as charge generator overcoated with
a layer of unsensitized poly(vinylcarbazole) as the charge
transport layer is subject to cyclic instability upon continuous
exposure to the charge, image and erase cycle normally encounter-
ed in electrostatographic copying. This cyclic instability is
undesirable due to the need for such a photosensitive device to ;~
remain stable, in terms of electrical properties, over thousands ~ ;
of cycles to be useful in a commercial copying machine. -~
Accordingly, it would be desirable and it is an
obiect of the present invention to provide a novel electro-
statographic photosensitive device of the type previously -
described.
~n additional object is to provide such a device
which retains the desirable features of the prior art devices. ;
A further object is to provide such a device which
is improved over those of the prior art in terms of cyclic
stability.
` SUMMA~ 0~' THE INVENTION
The present invention is an elec~rostatographic
photosensitive device having improved cyclic stability. The
device comprises:
a) a conductive substrate;
b) a uniform layer on said conductive substrate of a
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5361 ~31
hole generating photoconductive material; and
c) a uniform organic layer overcoating said layer
of photoconductive material of poly(vinylcarbazole) having
dispersed therein ~,~,7-trinitro-9-fluorenone in an amount
of ~rom 0.01 to 10 percent by weight of said organic layer.
DETAILED DESCRIPTIO~ AND PREFERRED EMBODIMENTS
The object of the present invention is an improve-
ment in the magnitude o~ the residual voltages and cyclic
stability of an electrostatographic photosensitive device
comprising a conductive substrate having a layer of hole gen-
erating photoconductive material on its surface which is in
turn overcoated with a layer of an active organic material
capable of transporting the holes genera-ted by the photocon-
ductive material and injected into the layer of organic
material. In the state of the art requirements for this type
of photosensitive device, the organic (transport) layer is
substantially transparent in the -visible region of the
spectra beyond wavelengths of 4000 ~; provides no barrier to
hole injection from the generator layer; exhibits high
carrier mobility; is mechanically flexible and exhibits
thermal, environmental and mechanical stability. Of the
polymeric transport materials known to the art, poly~vinyl-
carbazole) most nearly meets these criteria. It is trans-
parent beyond 3500 ~ and exhibits no apparent barrier to hole
injection from hole generating photoconductors such as
trigonal selenium. It does, however, e~hibit a low, field
dependent hole mobility. In some instances with such a
device, the background potential after erase can be signifi-
cant and in fact the background can be observed to increase
several hundred volts with cycling. Eventually such samples
tend -to cycle down. While the cycle up and cycle down of
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image potentials can be controlled somewhat by the magnitude
and composition of the erase energy, control has been diffi-
cult to obtain. The cycle up in image po-tential in the above-
mentioned transport layers has been attributed to the bulk
trapping of holes in the transport layer.
The present invention involves the use of poly
(vinylcarbazole~ as the transpor-t layer which has been doped
with 2,4,7-trinitro-9-fluorenone as a charge -transfer comple~-
ing material to generate electron-hole pairs in sufficient
numbers to neutralize both positive and negative bulk -trapped
charges during exposure and erase with carriers of the opposite ;
charge. For each type of volume charge neutralized, the
charge of the other sign moves through the sample, either as
a free carrier or virtually as in the case of a collapse of
a depletion layer.
It has been found that the lower limit on the con~
centration of TNF is on -the order of about 0.01% by weight of
the organic layer in order to provide the desired cyclic
stability. This is the case because insufficient bulk
absorption will take place to neutralize the trapped bulk
charge and residual voltages will increase with lower con-
centrations. On the other extreme, the addition of an
absorbing but inefficient carrier generating material will
reduce the sensitivity of the device and even effect the cyclic
stability with excess bulk absorption. Consequentlyj it has
been found that about 10% by weight is the upper end of the
desired TNF loading densities. The levels of TNF employed
will not appreciably effect hole mobilities in poly(vinyl-
carbazole).
The present method of using a weakly absorbing
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transport material is also useful in conjunction with
plasticized transport materials such as poly(vinylcarbazole)
plasticized with polyes-tersl polycarbonates, l-phenyl
naphthalene or poly(butadiene) since incorporating plasticizers
into poly(vinylcarbazole) tends to introduce trapping centers.
In summary, the use of a transpor-t layer of poly
(vinylcarbazole) containing small amounts of 2,~,7-trinitro-9-
fluorenone provides the following advantages:
(1) residual voltages can be maintained at low
values;
(2) cyclic stability can be controlled relatively
simply by adjusting the amount of dopant and controlling the
magnitude and composition of the erase lamp eneryy spectrum;
(3) the poly(vinylcarbazole) layer can be treated
with plasticizers without appreciably affecting cyclic
stability.
The PVK, used in the examples set out herein, was
a purified, high molecular weight (Mw~ 1.5 x 106, MWD ~
6.0) commercially available polymer. The purification pro-
cedure involves sequential precipitations from benzene
solution with methanol and a subsequent freeze drying from
benzene solution. More specifically, purification is
carried out by dissolving 75 gms. of BASF bulk polymerized
poly(vinylcarbazole) in 3225 milliliters of Fisher Spectro-
analyzed benzene. To minimize the problems associated with
handling large volumes of solvents in the laboratory, 1100
milliliters (950 gms.) are removed and precipitated into
5375 milliliters of spectroanalyzed methanol (methanol/
benzene ratio of 5:1). The addition of the polymer solution
is carried out drop~ise from a separating funnel. After
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completion, the suspension of poly(vinylcarbazole) is
filtered and the collected polymer washed twice with methanol
(equivalent volume) and partially dried by con-tinued filtra-
tion. The polymer is then dried at 30C under vacuum. The
purifled polymer is finally freeze dried by dissolving it in
spectroanalyzed benzene to give a 9O by weight solution. The
solution is then poured into a stainless steel pan to a maxi-
mum depth of 1/2", covered wi-th aluminum foil, and set in a
bath of dry ice and acetone until thoroughly frozen. The
tray is then removed and placed in a vacuum oven at room
temperature for 24 hours. At the end of this time, the
temperature is raised to 60C and the drying continued for
another 12 hours in order to reduce the retained solvent level
to below 0.01%. The polymer is then removed and stored in a
darkened glass container and the remainder of the original
solution purified, in 1100 ml. batches, in a similar manner.
This procedure insures homogeneity of the starting polymer
solution as the commercial bulk polymer is normally non-
homogeneous. Precautions are taken to avoid exposure of the
poly(vinylcarbazole~ to air and W light.
After purification, the poly(vinylcarbazole) is
combined with the appropriate amount of 2,4,7-trinitro-9-
fluorenone in a suitable solvent and thoroughly mixed to
cause uniform dispersion of the TNF. Suitable solvents are
those compositions which dissolve the poly(vinylcarbazole)
and TNF and do not detrimentally interact with them. The
solvent should be sufficiently volatile so as to be readily
evaporated from the solutes upon film formation. Useful
solvents include tetrahydrofuran (THF), methylene dichloride,
chloroformt chlorobenzene, acetone and methyl ethyl ketone.
The solution is applied to the exposed side of the layer of
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photoconductlve material in the device previously described
by known techniques such as roller coating, knife coating,
mil coating, brushing, etc. Upon casting the film, the
solvent is evaporated, normall.y under vacuum at an eleva-ted
temperature to expedite solvent removal.
The lower limit for the transport layer thickness
is dictated by the required electrostatic contrast po-tentials
and the maximum applied field for operational use of the
device. Breakdown of the layer occurs near 100 V/Ju. Thus,
the minimum thickness of the transport layer w;ll typically
be on the order of 2 ,u. The upper l~mit for the layer thick-
ness is dictated by the transit time limitation of holes in
the PVK layer, in particular for high speed use where the
time between exposure and development is less than 1 sec.
Accordingly, the upper limit would be about 20 ~ when using
PVK having carrier mobilities on the order of 10 7 v/sec. at ~:~
105 v/cm. applied field. An increase in the hole mobility
and/or a decrease in its field dependence in dispersion
would reduce this limitation and allow for thicker transport
layers and the use of the device at lower fields. Alter-
natively thicker transport layers can be used if the device
is operated at still higher fields (50 v/ ~.~ E~ 100 v/,u).
Of course, thicker layers will be satisfactory when using the
device in machines allowing for times greater than 1 sec.
between exposure and development of the image. Therefore,
the thickness of the transport layer is not critical to the
function of the dev.ice when speed of development is not
critical. However, the thickness of the layer would be
dictated by practical needs in terms of the amounts of
electrostatic charge necessary to induce an applied field
suitable to effect carrier injection and transport. ~ctive
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transport layer thicknesses oE from about 2 to 100 microns
~ould be suitable and from 5 to 30 microns are typically
employed.
The ~hickness of the layer of photoconduc-tive
material will typically ranye ~rom 0.02 to 5 microns with a
thickness o~ from 0.1 to 1 micron being preferred. When tri-
gonal selenium is used as the photoconductor, the photo-
conductive layer should be in the range o~ 0.03 to 0.8 micron
in thickness.
Useful photoconductive materials are those organic
or inorganic substances which are capable of photogenerating
and injecting photo-excited holes into the contiguous organic
layer described herein. In addition, the material should be
electrically responsive to light in the wavelength region in
which it is to be used in the sense that its electrical con-
ductivity increases signi~icantly in response to the absorption
of electromagnetic radiation in the wavelength region to which
it is sensitive. Suitable materials for use in fabricating
the photoconductive layer include selenium (either in the
amorphous or trigonal crystalline form), selenium alloys such ;~
as Se/Te or Se/As alloys, phthalocyanine, Se/Bi/I alloys,
Se/Te/As alloys and pyrilium salts such as disclosed in U.S.
Patent 3,615,~14.
The layer of photoconductive material is applied to
the substrate, normally as a homogeneous layer, by conventional
methods such as vapor deposition. Alternatively, -the photo-
conductive material may be dispersed in an organic binder as -
disclosed in Canadian Patent 1,05~,339. The substrate is
typically made of a conductive material such as brass, aluminum,
steel a metallized polymer or a conductively coated dielectric
or insulator. The substrate may be of any convenient thickness,
.
rigid or flexible and in any desired form such as a sheet,
web, belt, plate, cylinder or drum. It may also comprlse
other materials such as aluminum or ~lass coated with a thin
layer of chromium or tin oxide. Unless the substrate is
naturally blocking as in the situation where substantial amounts
of energy are required to promote charge carriers from the
substrate into the photoreceptor body, a distinct blocking
layer between the substrate and layer of photoconductive
material is required to prevent charge injection from the
substrate thereby permitting the device to sustain an electric
field across it after charging. Typical blocking materials may
be employed in thicknesses from about 30 ~ to 1.0 micron and
include nylon, epoxies, aluminum oxide (as in the case of an
aluminum substrate whose surface has oxidized) and insulating
resins of various types including polystyrene, polyesters,
butadiene polymers and copolymers, acrylic and methacrylic
polymers, vinyl resins, alkyl resins and cellulose base resins.
In addition, the blocking layer may act as an adhesive layer
to insure the mechanical stability of the device.
The invention is further illustrated by the follow-
ing examples.
EXAMPLE I
Electrostatographic photosensitive devices accord- ~:
in~ to the present invention are fabricated as follows:
A thin (0.25 ,u thick) layer of amorphous selenium
is vacuum deposited onto a polymeric adhesive interface (a
dual layer of 0.1 ~u poly(vinylcarbazole) /0.2 ,u ~Iytrel
polyester) on a conductive subs~rate (a 75~u - 125 ,u K~pton
polyimide film having on its surface a 200 - 500 A layer
o~ vacuum deposited aluminum with a neutral oxide layer).
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A layer of purifi0d po].y(v:inylcarbazole) containing
the appropriate quantity of 2,4,7-trinitro-9-fluorenone in
solution is coated onto the layer of amorphous selenium to
yield an 18 ~ thick layer of organic material. The device
is thermally treated to a -temperature of from 125 to 210C
for a perlod of from 1 to 24 hours to convert the amorphous
selenium to the trigonal crystalline form and simultaneously
dry the PVK/TNF layer~ This fabrication method, except or
the incorporation of TNF in the organic layer, is more fully -~
described in Canadian Patent 1,042,093.
The solution of organic material which is coated
onto the selenium surface is prepared as follows when a 1% TNF
containing sample is desired~
Purified poly(vinylcarbazole), 6 gms. and 0.06 gm.
of TNF (recrystallized from acetic acid) are dissolved in
44 gms. of tetrahydrofuran to give a solution containing
approximately 12~ by weight of solute. The organic layer is
then draw coatedt in a single pass, onto the layer of
selenium. During the coating procedure for this specific
fabrication, the temperature is controlled at 20 - 22C and
the relative humidity at 15-30% in an atmosphere provided by ; ~
carrying out the operation in a glove box flushed with dry ~-
air at a flow rate of 90-100 cubic feet per hour. -
The fabricated device is held under argon for
several hours at 150C to thereby reduce the retained solvent
level to 200 ~ 100 ppm and simultaneously convert the selenium
to its crystalline trigonal form. Highex drying temperatures
may result in some decomposition of the polymer with a con-
comitant decrease in carrier mobility and increase in
residual potentials. Accordingly, 150 is the preferred
upper temperature limit.
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Various devices are prepared according to the
above-described general pxocedure. In order to examine the
devices in a machine-like environment, they are subjected to
charge, expose and erase operations at l cycle/sec. at a
photoreceptor surface speed of 30 inches/sec~ The charging
corona employs a Model 152 coronatrol set in the constant
current mode~ The corona source is an array of nickel-coated
steel pins ~- l cm. from the photoreceptor surface. The
elect~ometers are Monroe Model 1445-ll electrostatic volt-
meters; the erase lamp is a 40 watt Lumaline lamp, with a
variable intensity control. The potentials are read out by
use of mobile electrometers. The probes are mobile to allow
for readouts to be taken at various times after charging and
exposure and just prior to and after erase.
EXAMPLE II
An electrostatographic photosensitive device accord-
ing to the present invention is prepared by the previously
described general procedure having a 2500 ~ trigonal selenium
layer overcoated with an 18 ,u poly(vinylcarbazole3 layer
containing 1% by weight TNF on its surface.
The device is mounted on the apparatus previously
descrihed, charged negatively and exposed to light from a
filtered quartz-iodine lamp which passes 4000 - 7000 ~ light~
The erase lamp is an unfiltered Lumaline lamp operated such
that a 1.8 x 103 erg/cm2 erase is employed. The image
potentials are taken 0.1 second after exposure and 0.1 second
after erase. The charge, expose and erase cycle is repeated
10,000 times at l sec~/cycle. The cyclic data for various
exposure levels are shown in Figure 1, while the photo-induced
discharge characteristics (PIDCIs) are shown in Figure 2.
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From Figure 1 it can be determined that while there i5 some
increase in residual, cyclic stability is good.
EXAMæLE III
This example incorporates -the use of the same device
as that used in Example II with a decrease in erase energy,
i.e. 5.8 x 10 erg/cm2. The cyclic data and contrasts are
shown in Figures 3 and 4 respectively. There is excellent
cyclic stability over 10,000 cycles at high density image
exposures though decreasing erase does lead to a slight cycle
up in high exposure image potentials after about 4000 - 5000
cycles. A reduction in erase also increases the residual
potentials.
EXA~æLE IV
This example provides data for a device prepared as
previously described wherein the organic transport layer
contains 0.1~ by weight TNF. The erase conditions are modified
to provide 1.5 x 103 erg/cm2 and wavelengths shorter than
4800 ~ are filtered out.
The cyclic and PIDC/contrast data obtained using
this device are presented in Figures 5 and 6 respectively.
These figures reveal extremely stable cyclic characteristics
with essentially no change in contrast potentials for 1Ow
density input for up to 10,000 cycles. Only a slight drop
in contrasts at 1.0 neutral density is observed at 10,000
cycles.
EXAMPLES ~ AND VI
The cyclic data for devices containing 0.05% by
weight TNF and 0.01% TNF are presented in Figures 7 and 8
respectively. The contrast potentials of these devices are
presented in Figures 9 and 30 respectively.
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Several stri~ing ~eatures can be noted:
1) There is a cycle-up in the residual after erase
and in the high exposure image potentials indicating there is
insufficient TNF and thus bulk absorption in the system to
offset deep bulk trapping at these TNF le~els~
2) There is appreciable cycle-down in these two
systems at low exposure image potentials.
3) The background for the 0.01~ TNF device is less
than that for the device containing 0.05% TNF indicating that
loadings in this range are just about marginal in offsetting
the variations in deep trap densities in different samplings
of the bulk polymer.
EXAMPLE VII
Three photosensitive devices are prepared as pre-
viously described except that no TNF is incorporated into the
poly(vinylcarbazole) transport layerO The cyclic data for
these samples which are charged negatively, exposed
sequentially to a fixed lamp exposure of approximately 30 ergs
and given an erase energy of 1.8 x 103, 1.8 x 103 and 1.3 x
103 ergs/cm2 for 10,000 cycles at the rate of 1 cycle/sec.
are set out in Figures ll, 12 a~d 13. Figures 12 and 13
present data for the worst and best samples respectively
whereas Figure ll is between these extremes. The device in
Figure 13 had a somewha-t lower erase energy so that a one-to-
one correla-tion cannot be made. Nevertheless, several trends
can be seen:
l) The samples had appreciable bu-t varying amounts
o~ cycle-up over the first few thousand cycles, which then
yields to a cycle-down.
2) Although all the polymer purification and
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sample preparation steps were performed in identically the
same fashion, there is appreciable sample-to-sample
variation.
3) In all cases, the background for optimum
exposure atf~l000 cycles, is higher than desired, e.g.
VB ~ 500 V. (in Figure 11, it is closer to 400 V.).
EXAMPLE VIII
An electrostatographic photosensitive device is
made according to the procedure outlined in Example I
containing 0.2% by weight TNF in the transport layer. The
generator is an approximately 0.25 ~ thick layer of trigonal
selenium. The device is charged to -1000 v, exposed to a
light and shadow image and developed with a magnetic brush
development system. High quality xerographic prints are
obtained upon transfer of the toner from the device to a
receiving member.
Other electron acceptors such as tetranitro-
fluorenone, quinones, e.g. anthraquinone, naphthaquinone,
benzoquinone and chloranil may be used to sensitize the
poly(vinylcarbazole) transport layer.
