CA1095309A - Photosensitive device with organic topcoat containing 2,4,7-trinitro-9-fluorenone dispersed in poly (vinylcarbazole) - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Disclosed is an electrostatographic photosensitive device which exhibits improved cycling characteristics. The device comprises:
a) a conductive substrate;
b) a uniform layer on the conductive substrate of a hole generating photoconductive material; and c) an organic layer overcoating the layer of photoconductive material of poly (vinylcarbazole) having dispersed therein 2,4,7-trinitro-9-fluorenone in an amount of from 0.01 to 10 percent by weight of the organic layer.

Description

~ 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 . , ~

S3~9 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 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 ;3~

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 ~53~

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 ~0~3~i3a~9 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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~9S3~9 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 ~5~9 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.

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrostatographic photosensitive device having improved cyclic stability which comprises:
a) a conductive substrate;
b) a uniform layer on said conductive substrate of a hole generating photoconductive material selected from the group consisting of selenium, selenium alloys, metal phthalocyanine, metal free phthalocyanines and pyrilium salts; and c) a uniform organic layer overcoating said layer of photoconductive material of poly(vinylcarbazole) having dispersed therein 2,4,7-trinitro-9-fluorenone in an amount of from 0.01 to 10 percent by weight of said organic layer.

2. The photosensitive device of Claim 1 in which a plasticizer is incorporated into the organic layer.

3. The photosensitive device of Claim 2 wherein the plasticizer is l-phenyl naphthalene or poly(butadiene).

4. The photosensitive device of Claim 1 wherein the organic layer is from 2 to 100 microns in thickness.

5. The photosensitive device of Claim 4 wherein the organic layer is from 5 to 30 microns in thickness.

6. The photosensitive device of Claim 1 wherein the layer of photoconductive material is from 0.02 to 5 microns in thickness.

7. The photosensitive device of Claim 6 wherein the layer of photoconductive material is from 0.1 to 1 micron in thickness.

8. The photoconductive device of Claim 1 wherein the layer of photoconductive material is trigonal selenium and its thickness is in the range of from 0.03 to 0.8 micron.

9. The photosensitive device of Claim 1 containing a distinct blocking layer between the layer of photo-conductive material and the conductive substrate.

10. The photosensitive device of Claim 9 wherein the blocking layer is a dual layer of poly(vinylcarbazole) and a polyester.