EP0475644A2

EP0475644A2 - A process for forming an electrophotographic imaging member

Info

Publication number: EP0475644A2
Application number: EP91307956A
Authority: EP
Inventors: Andrew R. Melnyk
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1990-08-31
Filing date: 1991-08-30
Publication date: 1992-03-18
Also published as: EP0475644A3; JPH04234763A

Abstract

A fabrication process for a photoreceptor, in which a conductive layer (3), a blocking layer (4) and a charge generation layer (6) are formed in a continuously maintained reduced-pressure environment.

Description

This invention relates to electrophotography, and in particular, to an electrophotographic imaging member and a process for forming the imaging member.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer, leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Patent No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogeneration layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. Layered photoreceptors consisting of a thin charge generation layer and a thick charge transport layer have many advantages over single layer structures.
A key to the performance of a layered photoreceptor is a very thin charge generation layer to reduce the distance the photogenerated charge must move and to reduce the amount of photogenerated charge that can be trapped in this layer. However, to function as a good generator, the charge generation layer must also be optically thick to absorb most of the light during imagewise exposure. These two parameters require the material of the charge generation layer to be highly absorbent, which favors pigments with crystalline forms.
One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generation layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor may also comprise additional layers such as an anti-curl layer and an optional overcoating layer. To fabricate such a multilayered photoreceptor, it is necessary to form the various layers on one another. Depositing the material in the various layers may require various methods of forming the layers.
For example, one method of forming the charge generation layer from organic materials is solution casting. However, photogenerator pigments, for example phthalocyanines and perylene amides, are predominantly insoluble and require dispersion in a solvent soluble binder.
U.S. Patent No. 4,587,189 to Hor et al discloses a layered photoresponsive imaging member comprising a conducting supporting substrate, an adhesive layer of a polymeric material, a vacuum evaporated photogenerator layer comprised of a perylene diimide pigment, and an aryl amine hole transport layer. The photoconductive layers are formed by vacuum depositing perylene pigment followed by a solvent coated layer of the polyaryl diimide material in a resinous binder.
U.S. Patents Nos. 4,578,334 and 4,618,560 to Borsenberger et al discloses a multilayer photoconducting element which exhibits a very high electrophotographic speed, and its method of manufacture. A charge generation layer of N,N′-bis(2-phenethyl)perylene-3,4:9,10 bis(dicarboximide) and a charge transport layer consisting of an aryl amine substance in a polymeric binder are provided in the layered structure. The layer of perylene pigment may be obtained by vacuum depositing the pigment and overcoating it with a solution of an organic solvent, a polymer binder and the aryl amine, and curing the layers to remove the solvent. U.S. Patent No. 4,578,333 to Staudenmaier et al further discloses the above multilayer photoreceptor with an acrylonitrile copolymer adhesive interlayer between the conductive support and the charge generation layer.
U.S. Patent No. 4,780,386 to Hordon et al discloses a selenium alloy treatment which controls tellurium fractionation when vacuum depositing a charge generation layer. The treatment involves abrading the surfaces of an alloy prior to vacuum deposition. A charge transport layer comprising selenium or a selenium alloy may be provided positioned between a charge generation layer comprising selenium-tellurium-arsenic and a substrate. The charge generation layer and charge transport layer may be vacuum deposited.
U.S. Patent No. 4,770,965 to Fender et al discloses a process for preparing a photoreceptor wherein a selenium alloy is vacuum deposited on a substrate to form a vitreous photoconductive insulating layer. This insulating layer is then heated until the selenium in the layer adjacent the substrate crystallizes. An aluminum oxide layer may be formed on the substrate by glow discharge treatment of the substrate in a vacuum coater.
U.S. Patent No. 4,842,973 to Badesha et al discloses a process for vacuum depositing a plurality of selenium alloy layers by providing in a vacuum chamber at least one first layer crucible containing particles of selenium or a selenium alloy and at least a second layer crucible containing an alloy comprising selenium and another alloying component. The particles in the first layer crucible are heated to deposit a first layer on the substrate while maintaining the particles in the second layer crucible below a depositing temperature. The particles in the second layer crucible are then rapidly heated to deposit a second layer on the substrate.
Other examples of multilayered photoreceptors include vacuum deposited phthalocyanines as charge generation layers with solvent coated charge transport layers. U.S. Patents Nos. 4,471,039 to Borsenberger et al, 4,555,463 to Loutfy et al and 4,731,312 to Kato et al disclose a number of vacuum deposited phthalocyanines. In particular, these patents disclose a photoconductor of indium phthalocyanine vacuum deposited on a substrate.
The manufacturing processes as described above provide imaging members which suffer from a number of disadvantages. In particular, in the above methods, other coating techniques, e.g., solvent coating, are utilized to coat adhesive and blocking layers in the photoreceptor. Since the conductive metal layers on polymeric substrates are vacuum deposited, such additional coating techniques require the transfer of the formed layer(s) from a vacuum deposition chamber to another coating facility and back to the vacuum deposition chamber. These transfers not only increase production time, but also increase the likelihood of damage or defects in the photoreceptor due to increased handling. Therefore, it is desirable to decrease the time required to produce an electrophotographic imaging member while also producing an imaging member having fewer defects. It is an object of the invention to alleviate the shortcomings of the prior art by providing a process for preparing a layered photoreceptor which produces fewer print-out defects.
Accordingly, the present invention provides a process for preparing an electrophotographic imaging member, characterised by vacuum depositing a conductive layer on a substrate, vacuum depositing a charge blocking layer on said conductive layer, and vacuum depositing a charge generation layer on said charge blocking layer.
The invention provides an efficient coating process for fabricating an electrophotographic imaging member, and in one embodiment provides a process for coating a conductive metal layer, an oxide layer and a photogenerating layer on a surface of an electrophotographic imaging member.
Embodiments of the invention also improve adhesion of layers in a multilayered electrophotographic imaging member.
In a preferred embodiment there is provided a coating process for a layered photoreceptor comprising the steps of providing a substrate in a reduced pressure environment, forming a conductive layer on the substrate, forming a blocking layer on the conductive layer, and forming a charge generation layer on the blocking layer, while continuously maintaining the reduced pressure environment during the coating processes. The layers deposited in the reduced pressure environment can be formed on a single pass, eliminating many defects. Furthermore, material which cannot be formed via solvent solution, e.g., photogeneration materials, can be used in accordance with embodiments of the present invention.
In one embodiment the charge generation layer comprises an organic pigment or an organic dyestuff. The charge blocking layer may be formed by evaporating a metal oxide, by reactively evaporating metal in said conductive layer in a partial pressure of oxygen or by oxidizing a surface of the conductive layer through glow discharge.
The present invention will be described further with reference to the accompanying Figure 1 which is a cross-sectional view of a multilayer photoreceptor according to one embodiment of the invention.
The electrophotographic imaging member according to an embodiment of the present invention contains at least a conductive layer, a blocking layer and a charge generation layer all formed within a continuously maintained reduced pressure environment. The layers are formed by providing a substrate in a reduced-pressure environment, and then successively vacuum depositing each layer.
The first layer coated on the substrate in the reduced-pressure environment is the conductive layer. The deposition is carried out to provide a substrate/conductive layer web or drum which has a suitable conductive layer thickness. This conductive layer may be formed from, for example, metal which is vacuum deposited onto the substrate.
After deposition of the conductive layer and while the substrate/conductive layer web or drum remains in the reduced-pressure environment, a blocking layer is formed on the conductive layer. The blocking layer may be a metal oxide layer which is deposited by evaporation directly, deposited by reactively evaporating metal in a partial pressure of oxygen, or deposited by oxidation of the previously applied conductive layer. A substrate/conductive layer/blocking layer web is obtained.
A charge generation layer is then applied to the substrate/conductive layer/blocking layer web while still in the reduced pressure environment. The charge generation layer may be vacuum deposited onto the blocking layer to obtain a substrate/ conductive layer/blocking layer/charge generation layer web. Deposition is carried out until the desired thickness is obtained.
Subsequently, other layers, if desired, may be applied to the substrate/conductive layer/blocking layer/photogeneration layer within the reduced pressure environment or at other coating facilities. For example, a charge transport layer may be applied, for example, by melt extrusion in the reduced pressure environment, or it may be applied by other coating methods elsewhere.
A description of suitable materials for forming the layers in the electrophotographic imaging member will be made hereinafter with reference to the representative structure of an electrophotographic imaging member of the invention shown in Figure 1. This imaging member is provided with an anti- curl layer 1, a supporting substrate 2, an electrically conductive layer 3, a blocking layer 4, an optional adhesive layer 5, a charge generation layer 6, a charge transport layer 7 and an overcoating layer 8.
The supporting substrate 2 may be opaque or substantially transparent and may comprise any of numerous suitable materials having adequate mechanical properties. Accordingly, the substrate may comprise a layer of an electrically non-conductive or conductive material such as an inorganic or organic composition. As electrically non-conducting materials, there may be employed any of various resins, including polyesters, polycarbonates, polyamides, polyurethanes, and the like. The electrically insulating or conductive substrate can be flexible and may be of any of a number of different configurations such as, for example, a sheet, a scroll, an endless flexible belt, a cylinder, and the like. Preferably, the substrate is in the form of an endless flexible belt and comprises a commercially available biaxially oriented polyester known as Mylar, available from E.I. du Pont de Nemours & Co., or Melinex, available from ICI Americas Inc. Alternatively, the preferred substrate may be in the form of a rigid cylinder formed by a mold or extrusion.
The selection of thickness for the substrate depends on numerous factors, including mechanical performance and economic considerations and whether the final form is a flexible belt or rigid drum. The thickness of a flexible layer is preferably within the range of from about 25 micrometers to about 150 micrometers, preferably from about 75 micrometers to about 125 micrometers for good flexibility and minimum induced surface bending stress when cycled around small diameter rollers, e.g., 19 millimeter diameter rollers. The substrate for a flexible belt may be of substantial thickness, for example, over 200 micrometers, or of minimum thickness, for example, less than 50 micrometers, provided there are no adverse effects on the photoconductive device. The thickness of the rigid drum substrate may range from about one millimeter to several centimeters depending on the diameter of the drum and its composition. The surface of the substrate layer is preferably cleaned prior to coating to promote better adhesion of the deposited coating. Cleaning may be effected by exposing the surface of the substrate layer to plasma discharge, ion bombardment or the like.
The electrically conductive layer 3 is provided adjacent the substrate 2. The electrically conductive layer 3 preferably comprises an electrically conductive metal layer which is formed on the substrate 2 in a reduced-pressure environment. The conductive layer can be applied by any vacuum depositing technique. Vacuum depositing techniques include thermal evaporation, electron-beam bombardment and RF and DC sputtering. Typical metals which may be vacuum deposited include aluminum, zirconium, niobium, tantalum, vanadium, hafnium, titanium, nickel, stainless steel, chromium, tungsten, molybdenum, brass, gold, and the like, and mixtures thereof. Semiconductors, for example silicon, or certain metal oxides, for example, antimony-tin-oxide, which are conductive may also be vacuum deposited.
The conductive layer is preferably of a thickness within a wide range, depending on the optical transparency and flexibility desired for the electrophotoconductive member. Accordingly, for a flexible photoresponsive imaging device, the thickness of the conductive layer may be between about 2 nanometers and about 100 nanometers, more preferably from about 5 nanometers to about 30 nanometers to optimize electrical conductivity, flexibility and light transmission.
After deposition of the electrically conductive layer 3, the blocking layer 4 is applied thereto. Electron blocking layers for positively charged photoreceptors allow holes from the imaging surface of the photoreceptor to migrate toward the conductive layer. For negatively charged photoreceptors, any suitable hole blocking layer capable of forming a barrier to prevent hole injection from the conductive layer to the opposite photoconductive layer may be utilized. The hole blocking layer may include any such material which can be vacuum deposited. For example, the blocking layer is preferably a metal oxide or nitride. Aluminum oxide makes a good blocking layer, and other oxides are suitable and may provide better surfaces for charge generation layer adhesion. Other oxides which can be used to form the blocking layer 4 include, for example, oxides of silicon, both stoichiometric SiO₂ and non-stoichiometric SiO_x, stoichiometric and non-stoichiometric oxides of titanium, zirconium, and the like.
The blocking layer of this invention is formed by any of a number of methods while the substrate/conductive layer web or drum remains in the reduced pressure environment. According to one method, after a conductive layer of metal is deposited in a previous step, a metal oxide layer is formed by exposing the previously deposited metal layer to oxygen, such that a thin layer of metal oxide forms on the outer surface of the metal layer upon exposure to oxygen. Exposure to oxygen can be effected by introducing a partial pressure of oxygen into the reduced-pressure environment.
Alternatively, the blocking layer of the invention may be evaporated directly from, for example, a metal oxide, onto the conductive layer by electron beam evaporation or sputtering. Sputtering may involve direct sputtering of the preferred oxide or nitride, or reactive sputtering of a metal in an oxygen or nitrogen partial pressure resulting in the deposition of the compound. Alternatively, reactive sputtering may be combined with direct sputtering of the metal so that first the metal conductive layer is deposited followed by the oxide or nitride of the metal.
The blocking layer is preferably continuous and has a thickness of less than about 0.5 micrometer. Greater thicknesses tend to lead to undesirably high residual voltage. A hole blocking layer of thickness between about 0.005 micrometer and about 0.3 micrometer is preferred to achieve optimum electrical performance. A thickness of between about 0.03 micrometer and about 0.06 micrometer is most preferred for optimum electrical behavior. The choice of the blocking layer material, deposition technique and thickness is also affected by the desire to have good adhesion between the conductive layer, the blocking layer and the photogenerating layer.
In some cases, intermediate layers, such as an adhesive layer 5, between the blocking layer 4 and the charge generation layer 6 may be desired for greater adhesion. The optional adhesive layer is not necessary, provided that an appropriate selection of materials for the blocking layer and charge generation layer is made. It is believed that the vacuum deposition process provides greater adhesion between adjacent layers than is provided when layers are, for example, solvent coated. If an adhesive layer is utilized, it preferably has a thickness between about 0.001 micrometer and about 0.2 micrometer and preferably is applied while in the reduced pressure environment, i.e. without breaking the vacuum from the coating of the conductive layer to the coating of the charge generating layer. To apply such a layer, an adhesive material must be used which can be vacuum deposited. Alternatively, the adhesive layer may be solvent coated. In such a case, the web material comprising the substrate, conductive layer and blocking layer is isolated to prevent the evaporation of solvent from interacting with the web. Adhesives include, for example, film-forming polymers such as polyester (e.g., du Pont 49,000 resin, available from E.I. du Pont de Nemours & Co.; Vitel PE-100 and Vitel PE-200 resins available from Goodyear Rubber & Tire Co.), polyvinylbutyral, polyvinylpyrrolidone, polyurethane, polymethyl methacrylate, 2-vinylpyridene, 4-vinylpryidene, polyvinyl alcohol, polyvinyl chloride, and the like.
For a web coating, the electrically conductive layer 3 and the blocking layer 4 can generally easily be coated in a single evaporation run. It is also possible but not necessary that the generating layer 5 is deposited in the same evaporation run.
The charge generation layer 6 is deposited on the substrate/conductive layer/blocking layer while the reduced pressure environment is maintained. Any charge generation (photogeneration) layer 6 which may be applied by vacuum deposition can be utilized in accordance with this invention. Examples of materials for photogeneration layers include inorganic and organic photoconductive pigment materials and dyestuffs which are stable at temperatures required for vacuum deposition, including inorganic and organic pigments, for example, chalcogens, II-VI and III-V compounds, phthalocyanines, diamines of perylene and perinone anhydride and other polycyclic pigment molecules. Thus photogeneration layers may comprise phthalocyanine pigments such as metal free phthalocyanine described in U.S. Patents No. 3,357,989 and No. 3,816,118; metal phthalocyanines such as magnesium, zinc and copper phthalocyanine; metal oxide phthalocyanines such as vanadyl, titanyl, zirconyl phthalocyanines; metal halide phthalocyanines such as chloro-indium and bromo- indium phthalocyanines; and other variants and substitutional variants of phthalocyanines as described, for example, in U.S. Patent No. 3,816,118. "Phthalocyanine Compounds" by F. H. Moser and A. L. Thomas, published by Reinhold Co. (1963) includes a detailed description of phthalocyanines and their synthesis. Other materials include polycyclic molecules such as dibromoanthanthrone, quinacridones such as those available from du Pont under the tradename Monastral Red, Monastral Violet and Monastral Red Y, dibromo anthanthrone pigments such as those available under the trade names Vat orange 1 and Vat orange 3, benzimidazole perylene, polynuclear aromatic quinones such as those available from Allied Chemical Corporation under the tradenames Indofast Double Scarlet, Indofast Violet Lake B, Indofast Brilliant Scarlet and Indofast Orange, and amorphous selenium, trigonal selenium, and selenium alloys such as, for example, selenium-tellurium, selenium-tellurium-arsenic, selenium arsenide, and the like. Multi-photogenerating layer compositions may be utilized where a photoconductive layer enhances or reduces the properties of the photogeneration layer. Examples of this type of configuration are described in U.S. Patent No. 4,415,639. Other suitable photogeneration materials known in the art may also be utilized, if desired. Charge generation layers comprising a photoconductive material such as vanadyl phthalocyanine, chloro-indium phthalocyanine, titanyl phthalocyanine, metal free phthalocyanine, benzimidazole perylene, phenethyl diimide perylene, N-butyl and N- propyl diimide perylene, trigonal selenium, selenium alloys such as selenium-tellurium, selenium-tellurium- arsenic, selenium arsenide, and the like and mixtures thereof are especially preferred because of their sensitivity to white light. Vanadyl phthalocyanine, chloro- indium phthalocyanine, titanyl phthalocyanine, bromo- indium phthalocyanine, zirconyl phthalocyanine, magnesium phthalocyanine, metal free phthalocyanine and tellurium alloys are also preferred because these materials provide the additional benefit of being sensitive to infrared light.
The photogeneration layer of the invention generally is of a thickness in the range of from about 0.1 to about 2 micrometers, preferably from about 0.2 to about 0.7 micrometers. Thicknesses outside these ranges can be selected, providing the objectives of the present invention are achieved.
After completion of the coating of the conductive layer, blocking layer, optional adhesive layer, and photogeneration layer within the maintained-reduced pressure environment, the charge transport layer is applied. The charge transport layer is relatively thick, and formation of this layer may be accomplished outside the vacuum deposition chamber by a coating technique other than vacuum deposition, e.g., with a solvent.
The charge transport layer 7 may comprise any suitable transparent organic polymeric or non-polymeric material capable of supporting the injection of photo- generated holes or electrons from the charge generation layer 6 and allowing the transport of these holes or electrons through the layer to selectively discharge the surface charge. The charge transport layer not only serves to transport holes or electrons, but also protects the photoconductive layer from abrasion or chemical attack, and therefore extends the operating life of the photoreceptor imaging member. The charge transport layer should exhibit negligible, if any, discharge when exposed to a wavelength of light useful in xerography, e.g. 400 nanometers to 900 nanometers. The charge transport layer is preferably substantially transparent to radiation in a region in which the photoconductor is to be used. It is comprised of a material which supports the injection of photogenerated holes or electrons from the charge generation layer. The charge transport layer is normally transparent when exposure is effected therethrough to ensure that most of the incident radiation is utilized by the underlying charge generation layer. When used with a transparent substrate, imagewise exposure or erasure may be accomplished through the substrate with all light passing through the substrate. In this case, the charge transport material need not transmit light in the wavelength region of use. The charge transport layer in conjunction with the charge generation layer is an insulator to the extent that an electrostatic charge placed on the charge transport layer is not conducted in the absence of illumination.
The charge transport layer may comprise activating compounds or charge transport molecules dispersed in normally electrically inactive film forming polymeric materials for making these materials electrically active. These charge transport molecules may be added to polymeric materials which are incapable of supporting the injection of photogenerated holes or electrons and incapable of allowing the transport of these holes or electrons. An especially preferred transport layer employed in multilayer photoconductors comprises from about 25 percent to about 75 percent by weight of at least one charge transporting compounds, and about 75 percent to about 25 percent by weight of a polymeric film forming resin in which the charge transporting compound is soluble. Alternatively, the charge transporting moiety may be incorporated either in the backbone in a polymer or as a pendant group in a polymer.
The hole charge transport layer is preferably formed from a mixture comprising at least one aromatic amine compound of the formula:

wherein R₁ and R₂ are each an aromatic group selected from the group consisting of a substituted or unsubstituted phenyl group, naphthyl group, and polyphenyl group and R₃ is selected from the group consisting of a substituted or unsubstituted aryl group, an alkyl group having from 1 to 18 carbon atoms and a cycloaliphatic group having from 3 to 18 carbon atoms. The substituents should be free from electron withdrawing groups such as NO₂ groups, CN groups, and the like. Typical aromatic amine compounds that are represented by this structural formula include:
I. Triphenyl amines such as:
II. Bis and poly triarylamines such as:
III. Bis arylamine ethers such as:
IV. Bis alkyl-arylamines such as:

where R is hydrogen or an alkyl group such as CH₃, C₂H₅, etc.
A preferred aromatic amine compound has the general formula V :

wherein R₁ and R₂ are as defined above, and R₄ is selected from the group consisting of a substituted or unsubstituted biphenyl group, a diphenyl ether group, an alkyl group having from 1 to 18 carbon atoms, and a cycloaliphatic group having from 3 to 12 carbon atoms. The substituents should be free from electron withdrawing groups such as NO₂ groups, CN groups, and the like. Examples of charge transporting aromatic amines represented by the structural formulae above include triphenylmethane, bis(4-diethylamine-2-methylphenyl) phenylmethane; 4′-4˝-bis(diethylamino)-2′,2˝ -dimethyltriphenylmethane; N,N′-bis(alkylphenyl)-(1,1′-biphenyl)-4,4′-diamine wherein the alkyl is, for example, methyl, ethyl, propyl, n-butyl, etc.; N,N′-diphenyl-N-N′-bis(3˝-methylphenyl)-(1,1′biphenyl)-4,4′-diamine; and the like, dispersed in an inactive resin binder.
Any suitable inactive resin binder soluble in methylene chloride or other suitable solvents may be employed. Typical inactive resin binders soluble in methylene chloride include polycarbonate resin, polystyrene, polyphenylene, polyester, polyarylate, polyacrylate, polyether, polysulfone, and the like. Molecular weight can be from about 20,000 to about 1,500,000. Other solvents that may dissolve these binders include tetrahydrofuran, toluene, trichloroethylene, 1,1,2-trichloroethane, 1,1,1-trichloroethane, and the like.
The preferred electrically inactive resin materials are polycarbonate resins having molecular weight from about 20,000 to about 120,000, more preferably from about 50,000 to about 100,000. The materials most preferred as the electrically inactive resin material are poly(4,4′-dipropylidene-diphenylene carbonate) with a molecular weight of from about 35,000 to about 40,000, available as Lexan 145 from General Electric Company; poly(4,4′-isopropylidenediphenylene carbonate) with a molecular weight of from about 40,000 to about 45,000, available as Lexan 141 from General Electric Company; a polycarbonate resin having a molecular weight of from about 50,000 to about 100,000, available as Makrolon from Farbenfabricken Bayer A.G.; a polycarbonate resin having a molecular weight of from about 20,000 to about 50,000, available as Merlon from Mobay Chemical Company; polyether carbonates; and 4,4′-cyclohexylidene diphenyl polycarbonate. Methylene chloride solvent is a desirable component of the charge transport layer coating mixture for adequate dissolving of all the components and for its low boiling point.
An especially preferred multilayered photoconductor comprises a charge generation layer comprising a binder layer of photoconductive material and a hole transport layer of a polycarbonate resin material having a molecular weight of from about 20,000 to about 120,000 having dispersed therein from about 25 to about 75 percent by weight of one or more compounds having the formula:
wherein X and Y are selected from the group consisting of an alkyl group, having from 0 to about 4 carbon atoms, and chlorine, the photoconductive layer exhibiting the capability of photogeneration of holes and injection of the holes, the hole transport layer being substantially non-absorbing in the spectral region at which the photoconductive layer generates and injects photogenerated holes but being capable of supporting the injection of photogenerated holes from the photoconductive layer and transporting the holes through the hole transport layer.
The thickness of the charge transport layer is preferably within the range of from about 5 micrometers to about 100 micrometers, preferably from about 20 micrometers to about 50 micrometers. Optimum thicknesses depend on specific application requirements of electrical stability, life, and electrophotographic speed.
Optional layers can be applied to the above- described electrophotographic device, such as a ground strip, an anti-curl layer, or an overcoating layer.
The invention will further be illustrated in the following non-limiting examples, it being understood that these examples are illustrative only and that the invention is not limited to the materials, conditions, process parameters and the like recited herein.

EXAMPLE 1

A multilayer imaging member is prepared utilizing benzimidazole perylene (BZP), formed by the condensation of perylene dianhydride with O-phenylene diamine as described in U.S. Patent No. 4,587,189, as the charge generation material and a triaryl diamine of the formula:

as the hole transport layer.
The substrate consists of a poly(ethylene terephthalate) film (available from ICI as Melinex) and is placed in a vacuum chamber at a pressure of 1 x 10⁻⁵ Torr. Using an electron beam source, a conductive layer of titanium about 10 nanometers thick is deposited. This is followed by a silicon dioxide layer approximately 10 nanometers thick. Finally, without breaking vacuum, an approximately 0.2 micrometer thick layer of BZP is deposited from a resistively heated tantalum boat heated to 500°C.
The vacuum deposited layers are next overcoated with a solution containing the triaryl diamine hole transport material in a polycarbonate polymer. The ratio of the triaryl diamine to polymer is 1:1 and the solvent is a mixture of 65% dichloromethane and 35% of 1,1,2-trichloroethane. The solids to solvent ratio is adjusted to give a dried thickness of 15 micrometers. The solution is cast using a draw bar applicator and the resulting coating isdried at 60°C for 1 hr.
The preparation of the metal-oxide-pigment multilayer described above results in a high adhesion between the metal and the pigment layer of about 400 g/cm and higher. The adhesion is measured by attaching an adhesive coated tape to the pigment layer and using an Instron Model #6022 adhesion tester. The electrophotographic properties are measured by corona charging to a negative voltage and observing the dark discharge and photodischarge with 650 nm light. The charging characteristics are obtained by plotting the initial voltage, measured 190 milliseconds after charging, against the applied negative charge which is varied from 10 to 250 nanocoulombs/cm². The multilayer imaging member prepared in the above manner charges capacitively over 2000 volts and shows less than 10% dark discharge in two seconds even at electric fields in excess of 70 volts/micrometer. The photosensitivity is excellent with a discharge slope of 150 volts/erg/cm² and requires less than 7 erg/cm² to discharge from 1000 volts to 100 volts with a residual voltage of less than 20 volts.

EXAMPLE 2

Other Oxides

Multilayer imaging members are prepared in the same manner as Example 1 except that in place of silicon dioxide, oxides of zirconium, aluminum and titanium are used. Additionally, oxide thicknesses of nominally 30 nm and 100 nm are made in addition to 10 nm. The resulting imaging members are substantially identical in performance to that of Example 1.

EXAMPLE 3

Other Substrates

Multilayer imaging members are prepared in the same manner as Example 1 except that in place of the poly(ethylene terephthalate) substrate, substrates of poly(vinylidene chloride) and poly(ether sulfone) are used. The resulting imaging members are substantially identical in performance to that of Example 1.

EXAMPLE 4

A long web 18 inches in width is placed in a large roll vacuum coater and a 10 nanometer titanium layer followed by a 10 nanometer zirconium oxide layer are applied by sputtering in one pass. The oxide layer is formed by reactive sputtering in a partial pressure of oxygen. The web is rolled up and transferred to a second vacuum roll coater where a BZP pigment layer of a thickness corresponding to an optical density of 1.0 at a wavelength of 650 nanometers is thermally evaporated out of a stainless steel crucible. The crucible temperature is controlled at 550°C and the web speed is adjusted to give the desired optical density uniformly over the width and length of the web. After the pigment charge generation layer is deposited, the rolled up web is transferred to a web solvent coater where a charge transport layer of the same composition as in Example 1 is applied using an extrusion die. The resulting dry thickness is 25 micrometers. The electrical properties of the resulting multilayer imaging member are identical to those in Example 1 if scaled by the capacitive differences in charge transport layer thicknesses.
A Xerox 4050 laser printer is specially modified so that the discharged areas on the imaging member corresponding to -100 volts develop black toner while the charged areas corresponding to -500 volts develop no toner. Thus the resulting paper prints consist of black images where the laser discharges the imaging member and white elsewhere. This imaging scheme is very sensitive to defects in the imaging member that cause local poor charge acceptance or high dark discharge. Even microscopic defects print as black spots and are easily detected. Sections from the imaging member produced in the manner described above are cut and welded into belts of the size required by the Xerox 4050 printer. Prints made on the modified 4050 printer reveal that the imaging members produced by the above process are free of defects.

EXAMPLE 5

Other Pigments

Multilayer imaging members are prepared as in Example 1 except that the silicon oxide is replaced by zirconium oxide and the BZP pigment is replaced by chloro- indium phthalocyanine, bis N-propyl imido perylene and bis N-methyl imido perylene. The chloro-indium phthalocyanine is prepared as described in U.S. Patent No. 4,555,465 while the propyl and methyl perylene diamines are prepared by a condensation of perylene 3,4,9,10-tetracarboxylic acid or the corresponding anhydrides with an appropriate amine in quinoline, as discussed in U.S. Patent No. 4,587,189.
The multilayer imaging members prepared by the process described above show excellent capacitive charging characteristics with low dark discharge. The chloro- indium phthocyanine imaging member exhibits high sensitivity from 550 nm to 800 nm; the n-propyl perylene diamine exhibits high sensitivity from 420 nm to 660 nm and the N-methyl perylene diamine exhibits somewhat lower sensitivity from 450 nm to 580 nm.

EXAMPLE 6

A multilayer imaging member is prepared on a molded polypropylene cylinder substrate 84 mm in diameter. The cylinder is placed in a vacuum chamber and rotated on its axis while a 20 nm thick layer of aluminum and a 10 nm thick layer of aluminum oxide are deposited by an electron beam source. This is followed by a thermally sublimed, 0.2 micrometer thick layer of BZP pigment as described in Example 1. The cylinder is removed from the vacuum chamber and a solution of a 40/60 ratio of bis-triaryl amine to polycarbonate in a 40/60 mixture of 1,1,2 trichloroethane and dichloromethane is sprayed on the cylinder while it is rotated. The solids-solvent ratio is adjusted to give a smooth coating and the number of passes is selected to give a dry thickness of 20 micrometers. After spray coating, the charge transport layer is dried at 60°C overnight.
The resulting imaging member is installed in a Xerox 1012 copier and excellent copies are obtained after the light exposure is decreased using the "copy darker" slide switch of the copier.
Although the invention has been described with reference to specific preferred embodiments, it is not intended to be limited thereto; rather, those skilled in the art will recognize that variations and modifications can be made thereto.

Claims

A process for preparing an electrophotographic imaging member, characterised by vacuum depositing a conductive layer (3) on a substrate (2), vacuum depositing a charge blocking layer (4) on said conductive layer (3), and vacuum depositing a charge generation layer (6) on said charge blocking layer (4).
A process as claimed in claim 1, characterised in that said vacuum depositing steps are performed while maintaining a reduced pressure environment.
A process as claimed in claim 2, characterised by applying a charge transport layer (7) to said charge generation layer (6), said charge transport layer (7) being applied while maintained in said reduced pressure environment.
A process as claimed in claim 2, characterised by applying a charge transport layer (7) to said charge generation layer (6), said charge transport layer (7) being solvent coated.
A process for preparing an electrophotographic imaging member, characterised by forming a conductive layer (3) on a substrate (2), forming a charge blocking layer (4) by evaporating metal oxide on said conductive layer (3), and forming a charge generation layer (6) on said charge blocking layer (4), said forming steps being performed under vacuum and without breaking vacuum.
An electrophotographic imaging member, comprising a substrate (2), a conductive layer (3), a charge blocking layer (4) and a charge generation layer (6), characterised in that said conductive layer (3), charge blocking layer (4) and charge generation layer (6) are produced by a process as claimed in any one of claims 1 to 5.
A member as claimed in claim 6, characterised in that said vacuum deposited charge generation layer (6) comprises a material selected from the group consisting of phthalocyanines, amides of perylene and perinone, and fused polyaromatic molecules.
An imaging member as claimed in claim 6 or claim 7, characterised by said charge blocking layer (4) being formed by vacuum depositing an oxide layer.
A member as claimed in claim 8, characterised by said oxide charge blocking layer (4) being formed by evaporating a metal oxide onto said conductive layer (3).
An electrophotographic imaging member, comprising a substrate (2), an oxide charge blocking layer (4) and a charge generation layer (6), characterised in that said charge blocking layer (6) is formed by evaporating a metal oxide.