US3928034A

US3928034A - Electron transport layer over an inorganic photoconductive layer

Info

Publication number: US3928034A
Application number: US341839A
Authority: US
Inventors: Paul J Regensburger
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1970-12-01
Filing date: 1973-03-16
Publication date: 1975-12-23
Anticipated expiration: 1992-12-23

Abstract

An electrophotographic plate comprising two adjacent layers, one of which is a photoconductive layer capable of photogenerating and injecting electrons into the other contiguous layer, which is an electronically active material capable of supporting electron injection and transport. The electronically active transport material has the additional property of being substantially transparent to radiation in the particular wavelength region of xerographic use thereby rendering it particularly useful as a relatively thick protective overlayer for the photoconductive portion of the plate. The structure may be imaged in the conventional xerographic mode which usually includes charging, exposure to light, and development.

Description

United States Patent [191 Regensburger Dec. 23, 1975 ELECTRON TRANSPORT LAYER OVER AN INORGANIC PHOTOCONDUCTIVE LAYER [75] Inventor: Paul J. Regensburger, Webster,

[21] Appl. No.: 341,839

Related US. Application Data [63] Continuation-impart of Ser. No. 94,071, Dec. 1, 1970, abandoned, which is a continuation-in-part of Ser. No. 14,282, Feb. 26, 1970, abandoned.

[52] US. Cl. .Q 96/15; 96/l.8 [51] Int. Cl. G03G 5/08; GO3G 13/22 [58] Field of Search 96/1.5, 1.8; 252/501 [56] References Cited UNITED STATES PATENTS 2,901,348 8/1959 Dessauer et a1. 96/1.5 3,287,123 11/1966 Hoege 96/1.5 3,408,189 10/1968 Mammino.. 96/1.5

3,573,906 4/1971 Goffe 96/1.8 3,595,771 7/1971 Weigl 96/1.5 X 3,598,582 8/1971 Herrick et al 96/].5

3,634,079 l/l972 Champ et al. 96/l.5 3,725,058 4/1973 Hayashi et al. 96/].5

FOREIGN PATENTS OR APPLICATIONS 4,316,198 7/1968 Japan 96/].5

Primary Examiner-Roland E. Martin, Jr. Attorney, Agent, or Firm-James J. Ralabate; James P. OSullivan; Donald M. MacKay [5 7] ABSTRACT An electrophotographic plate comprising two adjacent layers, one of which is a photoconductive layer capable of photogenerating and injecting electrons into the other contiguous layer, which is an electronically active material capable of supporting electron injection and transport. The electronically active transport material has the additional property of being substantially transparent to radiation in the particular wavelength region of xerographic use thereby rendering it particularly useful as a relatively thick protective overlayer for the photoconductive portion of the plate. The structure may be imaged in the conventional xerographic mode which usually includes charging, exposure to light, and development.

10 Claims, 6 Drawing Figures U.S. Patent Dec.23, 1975 Sheetlof2 3,928,034

FIG ,2

FIG. 2

US. Patent Dec. 23, 1975 Sheet2of2 3,928,034

FIG 4 FIG 6 ELECTRON TRANSPORT LAYER OVER AN INORGANIC PHOTOCONDUCTIVE LAYER BACKGROUND OF THE INVENTION This application is a continuation-in-part of my previous application, Ser. No. 94,071, filed Dec. 1, 1970, now abandoned, which is a continuation-in-part of Ser. No. 14,282, filed Feb. 26, 1970, now abandoned.

This invention relates in general to xerography and more specifically to a novel photosensitive device and method of use.

In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely divided elect'roscopic marking particles on the surface of the photoconductive insulating layer.

A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated by U.S. Pat. No. 3,121,006 to Middleton and Reynolds which describes a number of binder layers comprising finely-divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. In its present commercial form, the binder layer contains particles of zinc oxide uniformly dispersed in a resin binder and is coated on a paper backing.

In the particular examples of binder systems described in Middleton et al., the binder comprises a material which is incapable of transporting injected charge carriers generated by the photoconductor particles for any significant distance. As a result, with the particular materials disclosed in the Middleton et al. patent, the photoconductor particles must be in substantially continuous particle-to-particle contact throughout the layer in order to permit the charge dissipation required for cyclic operation. With the uniform dispersion of photoconductor particles described in Middleton et a1, therefore, a relatively high volume concentration of photoconductor, up to about 50 percent or more by volume, is usually necessary in order to obtain sufficient photoconductor particle-to-particle contact for rapid discharge. It has been found, however, that high photoconductor loadings in the binder layers of the resin type result in the physical continuity of the resin being destroyed, thereby sufficiently reducing the mechanical properties of the binder layer. Layers with high photoconductor loadings are often characterized by a brittle binder layer having little or no flexibility. On the other hand, when the photoconductor concentration is reduced appreciably below about 50 percent by volume, the discharge rate is reduced, making high speed cyclic or repeated imaging difficult or impossible.

U.S. Pat. No. 3,121,007 to Middleton et a1. teaches another type of photoconductor which includes a two phase photoconductive binder layer comprising photoconductive insulating particles dispersed in a homogeneous photoconductive insulating matrix. The photoconductor is in the form of a particulate photoconductive inorganic crystalline pigment broadly disclosed as being present in an amount from about 5 to percent by weight. Photodischarge is'said to be caused by the combination of charge carriers generated in the photoconductive insulating matrix material and charge carriers injected from the photoconductive crystalline pigment into the photoconductive insulating matrix.

U.S. Pat. No. 3,037,861 to Hoegl et al. teaches that polyvinyl carbazole exhibits some long-wave U.V. sensitivity and suggests that its spectral sensitivity be extended into the visible spectrum by the addition of dye sensitizers. Hoegl et a1. further suggests that other additives such as zinc oxide or titanium dioxide may also be used in conjunction with polyvinyl carbazole. In Hoegl et al., it is clear that the polyvinyl carbazole is intended to be used as a photoconductor, with or without additives materials which extend its spectral sensitivity.

In addition, certain specialized layer structures particularly designed for reflux imaging have been proposed. For example, U.S. Pat. No. 3,165,405 to Hoesterey utilizes a two layered zinc oxide binder structure for reflex imaging. The Hoesterey patent utilizes two separate contiguous photoconductive layers having different spectral sensitivities in order to carry out a particular reflex imaging sequence. The Hoesterey device utilizes the properties of multiple photoconductive layers in order to obtain the combined advantages of the separate photoresponse of the respective photoconductive layers.

It can be seen from a review of the conventional composite photoconductive layers cited above, that upon exposure to light, photoconductivity in the layer structure is accomplished by charge transport through the bulk of the photoconductive layer, as in the case of vitreous selenium (and other homogeneous layer modifications). In devices employing photoconductive binder structures, which include inactive electrically insulating resins such as those described in the Middleton et al., U.S. Pat. No. 3,121,006, conductivity or charge transport is accomplished through high loadings of the photoconductive pigment allowing particle-toparticle contact of the photoconductive particles. In the case of photoconductive particles dispersed in a photoconductive matrix, such as illustrated by the Middleton et al., U.S. Pat. No. 3,121,007, photoconductivity occurs through the generation of charge carriers in both the photoconductive matrix and the photoconductor pigment particles.

Although the above patents rely upon distinct mechanisms of discharge throughout the photoconductive layer, they generally suffer from common deficiencies in that the photoconductive surface during operation is exposed to the surrounding environment, and particularly in the case of cycling xerography, susceptible to abrasion, chemical attack, heat, and multiple exposures to light during cycling. These effects are characterized by a gradual deterioration in the electrical characteristics of the photoconductive layer resulting in the printing out of surface defects and scratches, localized areas of persistent conductivity which fail to retain an electrostatic charge, and high dark discharge.

In addition to the problems noted above, these photoconductive layers require that the photoconductor comprise either a hundred percent of the layer, as in the case of the vitreous selenium layer, or that they preferably contain a high proportion of photoconductive material in the binder configuration. The requirements of the photoconductive layer containing all or a major proportion of a photoconductive material further restricts the physical characteristics of the final plate, drum or belt in that the physical characteristics such as flexibility and adhesion of the photoconductor to a supporting substrate are primarily dictated by the physical properties of the photoconductor, and not by the resin or matrix material which is preferably present in a minor amount.

Another form of composite photosensitive layer which has also been considered by the prior art includes a layer of photoconductive material which is covered with a relatively thick plastic layer and coated on a supporting substrate.

US. Pat. No. 3,041,166 to Bardeen describes such a configuration in which a transparent plastic material overlays a layer of vitreous selenium which is contained on a supporting substrate. The plastic material is described as one having a long range for charge carriers of the desired polarity. In operation, the free surface of the transparent plastic is electrostatically charged to a given polarity. The device is then exposed to activating radiation which generates a hole-electron pair in the photoconductive layer. The electron moves through the plastic layer and neutralizes a positive charge on the free surface of the plastic layer thereby creating an electrostatic image. Bardeen, however, does not teach any specific plastic materials which will function in this manner, and confines his examples to structures which use a photoconductor material for the top layer.

French Pat. No. 1,577,855 or U.S. Pat. No. 3,598,582 to Herrick et al. describes a special purpose composite photosensitive device adapted for reflex exposure by polarized light. One embodiment which employs a layer of dichroic organic photoconductive particles arrayed in oriented fashion on a supporting substrate and a layer of polyvinyl carbazole formed over the oriented layer of dichroic material. When charged and exposed to light polarized perpendicularly to the orientation of the dichroic layer, the oriented dichroic layer and polyvinyl carbazole layer are both substantially transparent to the initial exposure light. When the polarized light hits the white background of the document being copied, the light is depolarized, reflected back through the device and absorbed by the dichroic photoconductive material. In another embodiment, the dichroic photoconductor is dispersed in oriented fashion throughout the layer of polyvinyl carbazole.

In view of the state of the art, it can readily be seen that there is a need for a general purpose photoreceptor exhibiting acceptable photoconductive characteristics and which additionally provides the capability of exhibiting outstanding physical strength and flexibility to be reused under rapid cyclic conditions without the progressive deterioration of the xerographic properties due to wear, chemical attack, and light fatigue.

OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide an electrophotographic plate adapted for cyclic imaging devoid of the above noted disadvantages.

Another object of this invention is to provide an electrophotographic plate having excellent abrasion resistance properties.

It is yet another object of this invention to provide a novel imaging system.

It is yet another object of the instant invention to provide an electrophotographic plate having a material which exhibit facile electron transport properties.

It is still another object of this invention to provide a photoconductive insulating layer for an electrophotographic plate which is both relatively easy to make and inexpensive.

SUMMARY OF THE INVENTION The foregoing objects and others are accomplished in accordance with the present invention by providing an electrophotographic plate having a novel two layered structure comprising (a) a photoconductive layer capable of photogenerating hole-electron pairs and injecting the electrons into the adjacent overlayer, and (b) an adjacent electronically active transport material layer, which is substantially transparent and nonabsorbing in the particular wavelength region of xerographic use, said electronically active transport layer comprising an electron transport material in sufficient concentration to be capable of accepting and transporting electrons which have been injected from the photoconductive layer.

As defined herein, a photoconductor is a material which is electrically photoresponsive to light in the wavelength region in which it is to be used. More specifically, it is a material whose electrical conductivity increases significantly in response to the absorption of electromagnetic radiation in a wavelength region in which it is to be used. This definition is necessitated by the fact that a vast number of aromatic organic compounds are known or expected to be photoconductive when irradiated with strongly absorbed ultraviolet, x-ray, or gamma-radiation. Photocon'ductivity in organic materials is a common phenomenon. Practically all highly conjugated organic compounds exhibit some degree of photoconductivity under appropriate conditions. Most of these organic materials have their prime wavelength response in the ultraviolet. However, little commercial utility has been found for ultraviolet responsive materials, and their short wavelength response is not particularly suitable for document copying or color reproduction. In view of the general prevelance of photoconductivity in organic compounds following short wavelength excitation, it is therefore necessary that for the instant invention, the term photoconductor and photoconductive be understood to include only those materials which are in fact substantially photoresponsive in the wavelength region in which they are to be used.

In accordance with the present invention it has been found that a xerographic or electrophotographic sensitive member can be prepared with electronically active transport materials comprising aromatic or heterocyclic electron acceptors which facilitate the transport of photogenerated electrons from a photoconductive layer under the influence of an electric filed. The active transport materials, which are also referred to as active matrix materials when used as matrices for a binder layer, to be described herein, are to be distinguished from those matrix binders or the prior art, described above, in that the present materials have the combined properties of being substantially transparent, hence, non-photoconductive and non-absorbing, in at least some significant portion of a particular wavelength region of xerographic use corresponding to a range of photosensitivity of the photoconductor, and are capable of supporting the injection and transport of electrons which are photogenerated in an adjacent layer of photoconductor. Because of their unique combination of substantial transparency in a wavelength region of particular xerographic use and electron transport capability, the active transport materials of the present invention can be used effectively as a relatively thick electrically insulating overcoating of the photoconductive layer and yet function as both a window and a charge transport means for said photoconductive layer. These particular characteristics of the materials used in the present invention enables use of a relatively small amount of photoconductor in the total photoconductive insulating layer.

It should be understood that the active transport layer does not function as a photoconductor in the wavelength region of use. As stated above, hole-electron pairs are photogenerated in the photoconductive layer and the electrons are then injected across a field modulated barrier into the active layer and electron transport occurs through the active layer.

It is to be further noted that most materials which are useful for active transport layers of the instant invention are incidentally also photoconductive when radiation of wavelengths suitable for electronic excitation is absorbed by them. However, photoresponse in the short wavelength region, which falls outside the spectral region for which the present photoconductors are to be used, is irrelevant to the performance of the device. It is well known that radiation must be absorbed in order to excite photoconductive response, and the transparency criterion, stated above, for the active transport materials implies that these materials do not contribute significantly to the photoresponse of the photoreceptor in the wavelength region of use.

A typical application of this invention includes the use of a sandwich cell or layered configuration which in one embodiment consists of a supporting substrate, such as a conductor, having a photoconductive layer coated thereon. The photoconductive layer may be in the form of a layer of amorphous or vitreous selenium. A layer of active transport material which is substantially transparent in a significant portion of the particular wavelength region in which selenium is photoresponsive and is coated over the selenium photoconductive layer. The use of the active transport material allows one to take advantage of placing a photoconductive layer adjacent to a supporting substrate and protecting said photoconductive layer with a protective overlayer or window which will allow for the transport of photo-excited electrons from the selenium layer and which can be of thickness sufficient to physically protect the photoconductive layer from environmental conditions. This structure can be imaged in the conventional xerographic manner which includes charging, exposure, and development. I

The use of the active transport concept of the present invention enables one to use particular regions of the electromagnetic spectrum for selective xerographic copying. A typical application would be the use of electronically active materials in color xerography to copy particular color sequentially and thereby obtain a complete color print.

DESCRIPTION OF THE DRAWINGS Further objects of the invention, together with additional features contributing thereto will be apparent from the following description of one embodiment of the invention when read in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of one embodiment of a xerographic device contemplated by the instant invention.

FIG. 2 illustrates a second embodiment of a xerographic device of the instant invention.

FIG. 3 illustrates a third embodiment of a xerographic device of the instant invention.

FIG. 4 illustrates a discharge mechanism of photodischarge of the electronically active material layer.

FIG. 5 illustrates the discharge mechanism of one binder system of the prior art.

FIG. 6 illustrates the discharge mechanism of another binder system of the prior art.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates one embodiment in improved xerographic plate 10 according to this invention. Reference character 11 designates a substrate or mechanical support. The substrate may comprise a metal such as brass, aluminum, gold, platinum, steel or the like. It may be of any convenient thickness, rigid or flexible, in the form of j sheet, web, cylinder, or the like, and may be coated with a thin layer of plastic. It may also comprise such other materials as metallized paper, plastic sheets covered with a thin coating of aluminum or copper iodide, or glass coated with a thin layer of chromium or tin oxide. It is usually preferred that the support member be somewhat electrically conductive or have a somewhat conductive surface and that it be strong enough to permit a certain amount of handling. In certain instances, however, support ll need not be conductive or may even be dispensed with entirely.

Reference character 12 designates a photoconductive monolayer or unitary layer, which comprises a photoconductive material which is capable of photogenerating electrons and injecting them into the overlaying active matrix material.

Generally any photoconductive material capable of photogenerating electrons will function with the electron transport materials of the present invention. Typical inorganic crystalline photoconductors include cadmium sulfide, cadmium sulfoselenide, cadmium selenide, zinc sulfide, zinc oxide, and mixtures thereof. The typical inorganic photoconductive glasses include amorphous selenium, and selenium alloys such as seleniumtellurium, and selenium-arsenic. Selenium may also be used in its hexagonal crystalline form, commonly referred to as trigonal selenium. Typical organic photoconductors include phthalocyanine pigments such as the X-form of metal free phthalocyanine described in US. Pat. No. 3,357,989 to Byme et al., and metal phthalocyanine pigments, such as copper phthalocyanine. Other typical organic photoconductors include photoinjecting pigments such as benzimid azole pigments, perylene pigments, quinacridone pigments, indigoid pigments and polynuclear quinones all of which are disclosed in copending applicant Ser. No. 93,974; 94,066; 94,040; 94,067; 94,068; all filed on Dec. 1, 1970 and all abandoned. The above list of photoconductors should in no way be taken as limiting but as merely illustrative of materials having particularly efficient electron injection properties.

Photoconductive monolayer 12 of FIG. 1 may be in any suitable thickness used for carrying out its function in the xerographic insulating member. Typical thicknesses for this purpose range from 0.02 to 20 microns. Thicknessesabove 25 microns tend to produce undesirable positive residual buildup in the photoconductive layer during recycling and excessive dark decay while thicknesses below 0.02 micron become inefficient in absorbing impinging radiation. A range of from about 0.2 to microns is preferred since these thicknesses would ensure maximum functionality of the photoconductor with a minimum amount of said photoconductive substance and completely avoid the above mentioned problems with regard to thickness. As pointed out above, one of the primary advantages of the instant invention is the use of a minimum amount of photoconductor in the photoconductive insulating layer.

Reference character 13 designates the active transport material layer which overlays the photoconductive layer 12. As heretobefore mentioned, the active transport layer comprises an electron transport material which is capable of both supporting electron injection from the photoconductive layer and transporting said photogenerated electrons under the influence of an applied field. In order to function in the manner outlined above, the active transport material should be substantially transparent to the particular wavelength region used for xerographic copying. In particular, the active transport material should be substantially nonabsorbing in at least a significant portion that part of the electromagnetic spectrum which ranges from about 4200 to 8000 Angstroms becuase most xerographically useful photoconductors have photoresponse to wavelengths in this region.

As mentioned above, the active transport layer 12 comprises aromatic or heterocyclic electron acceptor materials which have been found to exhibit negative charge carrier transport properties as well as the requisite transparency characteristics. Typical electron acceptor materials within the purview of the instant invention include phthalic anhydride, tetrachlorophthalic anhydride, benzil, mellitic anhydride, S-tricyanobenzene, picryl chloride, 2,4-dinitrochlorobenzene, 2,4-dinitrobromobenzene, 4-nitrobiphenyl, 4,4-dinitrobiphenyl, 2,4,6-trinitroanisole, trichlorotrinitrobenzene, trinitro-O-toluene, 4,6-dichloro-l, 3-dinitr0benzene, v 4,6-dibromol ,3-dinitrobenzene, P-dinitrobenzene, chloranil, bromanil, and mixtures thereof. It is further intended to include within the scope of those materials suitable for the active transport layer, other reasonable structural or chemical modifications of the above described materials provided that the modified compound exhibits the desired charge carrier transport characteristics.

While any and all aromatic or heterocyclic electron acceptors having the requisite transparency characteristic are within the purview of the instant invention particularly good electron transport properties are found with aromatic or heterocyclic compounds having more than one substituent of the strong electron withdrawing substituents such as nitro-(-NO sulfonate ion (-80 carboxyl-(-COOH) and cyano-(CN) groupings. From this class of materials, 2,4,7-trinitro-9- fluorenone (TNF), 2,4,5,7-tetranitrofluorenone, trinitroanthraccne, dinitroacridine, tetracyanopyrene, and dinitroanthraquinone are preferred materials because of their availability and superior electron transport properties.

It will be obvious to those skilled in the art that the use of any polymer having the described aromatic or heterocyclic electron acceptor moiety as an integral portion of the polymer structure, will function as an active transport material. It is not the intent of the invention to restrict the type of polymer which can be employed as the transport material, provided it has an active electron acceptor moiety to provide the polymer with electron transport characteristics. Polyesters, polysiloxanes, polyamides, polyurethanes, and epoxies, as well as block, random or graft copolymers containing the aromatic moiety are therefore exemplary of the various types of polymers which could be employed. In addition, electronically inactive polymers in which the active electron acceptor material is dispersed at high concentration can be employed as hereinafter described.

The substantial or significant transparency of the active transport material within the context of the instant invention, as exemplified by FIG. 1, means that a sufficient amount of radiation from a source must pass through the active transport layer 13 in order that the photoconductive layer 12 can function in its capacity as a photogenerator and injector of electrons. More specifically, substantial transparency is present in the active transport materials of the present invention when the active transport material is non-photoconductive and non-absorbing in at least some significant portion of the wavelength region of from about 4200 to 8000 Angstrom Units. This property of substantial transparency enables enough activating radiation to impinge the photoconductor layer so as to cause discharge of the charged active transport photoreceptor of the present invention.

It is not the intent of this invention to strictly restrict the choice of active transport materials to those which are transparent in the entire visible region. For example, when used with a transparent substrate, imagewise exposure may be accomplished through the substrate without the light passing through the layer of active transport material. In this case, the active material need not be non-absorbing in the wavelength region of use. This particular application takes advantage of the injection and transport properties of the present active materials and falls within the purview of the instant invention. Other applications where complete transparency is not required for the active material include the selective recording of narrow-band radiation such asthat emitted from lasers, spectral pattern recognition, color coded form duplication, and possibly color xerography.

While the active material layer 13 of FIG. 1 may consist exclusively of charge transport material, for purposes of the present invention, the layer may also comprise the charge transport material at a sufficient concentration in a suitable electronically inert binder material to effect particle-to-particle contact or to effect suflicient proximity thereby permitting effective charge transport from the photoinjecting pigments of the instant invention through the layer. Generally there must be a volume ratio of at least 25 percent active transport material to electronically inert binder material to obtain the desired particle-to-particle contact or proximity. Typical resin binder materials for the practice of the invention are polystyrene; silicone resins such as DC-l, DC-804, and DC-996 all manufactured by the Dow Corning Corporation; and Lexan, a polycarbonate resin, SR-82 manufactured by the General Electric Company; acrylic and methacrylic ester polymers such as Acryloid A10 and Acryloid B72, polymerized ester derivatives of acrylic and alpha-acrylie acids both supplied by Rohm and Haas Company and Lucite 44, Lucite 45 and Lucite 46 polymerized butyl methacrylates supplied by the E. I. duPont de Nemours & Company; chlorinated rubber such as Parlon supplied by the Hercules Powder Company; vinyl polymers and copolymers such as polyvinyl chloride, polyvinyl acetate, etc. including Vinylite VYHH and VMCH manufactured by the Bakelite Corporation; cellulose esters and ethers such as ethyl cellulose, nitrocellulose, etc.; alkyd resins such as Glyptal 2469 manufactured by the General Electric Company; etc. In addition, mixture of such resins with each other or with plasticizers so as to improve adhesion, flexibility, blocking, etc. of the coating may be used. Thus, Rezyl 869 (a linseed oil-glycerol alkyd manufactured by American Cyanamid Company) may be added to chlorinated rubber to improve its adhesion and flexibility. Similarly, Vinylites VYHH and VMCH (polyvinyl chlorideacetate copolymers manufactured by the Bakelite Company) may be blended together. Plasticizers include phthalates, phosphates, adipates, etc. such as tricresyl phosphate, dicotyl phthalate, etc. as is well known to those skilled in the art.

The active transport material which is employed in conjunction with the photoconductive layer in the instant invention is a material which is an insulator to the extent that an electrostatic charge placed on said active transport material is not conducted in the absence of illumination at a rate sufficient to prevent the formation and retention of an electrostatic latent image thereon. In general, this means that the specific resistivity of the active transport material should be at least 10 ohms-cm. and preferably will be several orders higher. For optimum results, however, it is preferred that this specific resistivity of the active transport material be such the the overall resistivity of the active binder layer in the absence of activating illumination or charge injection from an adjacent layer be about 10 ohm-cm.

Because the overlayer functions as an active transport layer thickness is not critical to the function of the xerographic member. However, the thickness of said active transport layer would be dictated by practical needs in terms of the amounts of electrostatic charge necessary to induce an applied field suitable to effect electron injection and transport. Active transport layer thicknesses of from about 5 to 100 microns would be suitable, but thicknesses outside this range may be used. The ratio of the thickness of the active transport layer to the photoconductive layer should be maintained from about 2:1 to 200:1.

Another modification of the layered structure of FIG. 1 is illustrated in FIG. 2 where the photoconductive layer is depicted as being a layer of binder material having crystalline particles of photoconductor dispersed therein. The binder material may be any suitable organic substance used for such purposes including inert binder materials or one of the active matrix materials of the instant invention. The concentration of the photoconductor material will vary according to which type of binder material is used and will range in value from about 5-99 volume percent of the total photoconductive layer. If any electronically inert binder material is used in combination with the photoconductor material a volume fraction of at least 25 percent photoconductor to the electronically inert binder material is necessary to effect particle-to-particle contact or proximity thereby rendering layer l2 photoconductive throughout. The remarks with regard to the thickness of the photoconductive layer of FIG. 1 are generally applicable here; namely, a range of from about 0.05 to 20 microns, with a range of 0.3 to 5 microns being preferred due to the excellent results derived from this thickness range. The size of the photoconductive particles in the binder layer is not particularly critical, but particles in a size range of about 0.01 to 1.0 microns yield particularly satisfactory results.

Another variation of the layered configuration described in FIGS. 1 and 2 consists of the use of a blocking layer 14 at the substrate-photoconductor interface said layer being illustrated in FIG. 3 This blocking layer aids in sustaining an electric field across the photoconductor-active organic layer after the charging step. Any suitable blocking material may be used. Typical materials include nylon, epoxy, aluminum oxide, and insulating resins of various types including polystyrene, butadiene polymers and copolymers, acrylic and methacrylic polymers, vinyl resins, alkyd resins, and cellulose base resin.

It can therefore be seen that the photo-insulating portion of the xerographic members of the instant invention represent in FIGS. l-3 is derived into two functional layers:

1. A photoconductive layer which photogenerates holes and electrons upon excitation by radiation and injects said photogenerated electrons into the overlaying electronically active transport material, and;

2. An overlaying substantially transparent active transport material which allows transmission of radiation to the photoconductive layer, accepts the subsequently photogenerated electrons from the photoconductor material, and actively transports aid conduction electron to its positively charged surface thereby neutralizing said charge.

This is more dramatically illustrated in FIG. 4 where the xerographic member of the present invention has been positively charged by means of corona charging. The light 14 represented by the arrows then passes through the transparent active transport layer and impinges the photoconductive layer thereby creating a hole-electron pair. The electron and hole are then separated by the force of the applied field and the electron injected across the interface into the active transport layer where it is then transported by force of the electrostatic attraction through the active transport layer system' to the surface where it neutralizes the positive charge previously deposited by means of corona charging. Since only photogenerated electrons can move in the electron transport active material layer, large changes in surface potential result only when the electric field in the layer is such as to move the photogenerated electrons from the photoconductor layer where they are generated, through the active matrix layer and then to the charged surface. This means that for maximum utility the active matrix layer must be charged positively.

Referring now to FIG. 5 there is illustrated an electrophotographic plate of the prior art in which sensitizing pigment 12 has been dispersed in a photoconductor binder material 13 for the purpose of increasing the sensitivity of said photoconductor material. The light 14 impinges the electrophotographic member and creates photogenerated holes and electrons in either that photoconductor binder material or the pigment materials depending on which the radiation falls. Since most of the carriers are created at or near the surface of the photoinsulating member charge transport presents no serious problem. Therefore at point (A) light has caused the photogeneration of an electron and a hole in the photoconductor and at point (B) photogeneration takes place in the pigment. As can be seen from the illustration, in order for the pigment to have its effect in increasing the sensitivity of the electrophotographic member it generally has to be present in a relatively large concentration and be at or near the surface of the photoreceptor. This is to be contrasted with FIG. 4 where photogeneration takes place exclusively in the photoconductive layer, the active transport layer being substantially transparent to the incident radiation, and the photoconductor material is well protected by said active layer there being no requirement that the photoconductor be at or near the surface of the photoreceptor member. Furthermore, it can be seen in FIG. 5 that, in order for the pigment to function in the member, a significant amount must be kept on or at the surface where it is prone to inevitable abrasion and exposure to the atmosphere.

FIG. 6 offers by further contrast an illustration of a photoreceptor of the prior art in which pigment 12 is dispersed in an inert resin material 13 in two different concentrations, A and B. Because there is no photogeneration in the resin binder it is generally necessary that the photoconductive pigment or dye be in sufficient concentration or geometric proximity to support charge injection throughout the binder system. Hence, as can be seen, in section (A) where there is a large concentration of pigment impinging light 14 creates a photogenerated hole and electron pair which is then transported through the pigments to the positively charged surface while in section (B), where the concentration of the pigment is insufficient to effect particle-to-particcle proximity impinging light creates the electron and hole pair which remains trapped because of failure of the binder system to transport the photogenerated charges either to other pigment particles or the charged surface. Again this figure is to be contrasted with FIG. 4 where particle-to-particle proximity or contact of the photoconductor is unnecessary in the active matrix structure. In addition, because particle-to-particle contact is necessary in the inert binder structure of FIG. 6 resolution problems occur because the geometry of the particle may not correspond to the direction of the impinging light thereby resulting in irregular dissipation of the charge.

When the two layered configuration of photoconductor and active transport material has sufficient strength to form a self supporting member (termed pellicle), it is possible to eliminate the physical base or support member and substitute therefore any of the various arrangement well known in the art, in place of the ground plane previously supplied by the base layer. A ground plane, in effect, provides a source of image charges of both polarities. The deposition of the insulating two layered structure of the present invention of sensitizing charges of the desired polarity cause those charges in the ground plane of opposite polarity to migrate to the interface at the photoconductive insulating layer. Without this capacity of the insulating member by itself would be such that it could not accept enough charge to sensitize the layer to a xerographically useful potential. It is the electrostatic field between the deposited charges on one side of the xerographic two layered member and the induced charges (from the ground plane) on the other side that stresses the xerographic member so that when an electron is excited (in the photoconductive layer) to the conduction band by a photon thereby creating a hole-electron pair, the charges migrate under the influence of this field thereby creating the latent electrostatic image. Therefore it is obvious that if the physical ground plane is omitted a substitute therefore may be provided by depositing on opposite sides of the two layered xerographic insulating pellicle, simultaneously, electrostatic charges of opposite polarity. Thus if positive electrostatic charges are placed on one side of the pellicle as by corona charging as described in US. Pat. No. 2,777,957 to L. E. Walkup, the simultaneous deposition of negative charges on the other side of the pellicle also by corona charging will created an induced, that is, a virtual, ground plane within the body of the pellicle just as if the charges of opposite polarity has been supplied to the interface by being induced from an actual ground plane. Such an artificial ground plane permits the acceptance of a useable sensitizing charge and at the same time permits migration of charges under the applied field when exposed to activating radiation. As used hereinafter in the specification and claims the term conductive base includes both a physical base and an artificial one as described herein.

The physical shape of the xerographic active transport plate may be in the form whatsoever as desired by the formulator such as a flat, spherical, cylindrical plate, etc. The plate may be flexible or rigid as desired.

DESCRIPTION OF PREFERRED EMBODIMENT For purposes of affording those skilled in the art a better understanding of the invention, the following illustrative examples are given:

EXAMPLE I A photosensitive layered structure plate similar to that shown in FIG. 1 is prepared by the following technique: An aluminum substrate is dip coated with a three percent duPont Zytel nylon-denatured alcohol solution to form a 0.2 micron thick blocking layer. The coated substrate is then dried for approximately 30 minutes. Thereafter a one micron layer of amorphous selenium is vacuum evaporated onto the blocking layer by conventional vacuum techniques such as those disclosed by Bixby in US. Pat. Nos. 2,753,278 and 2,970,906. The selenium coated substrate is then cooled to 0C and a l0micron layer of 2,4,7-trinitro-9- fluorenone (TNF) is vacuum evaporated onto. the amorphous selenium layer.

The TNF overcoated plate is then placed in a Xerox Model D Machine where a copy of an original is made by positive corona charging the plate to a value of 800 volts and exposing the original to radiation in the wavelength region of 4200 to 6500 Angstrom Units whereby an image is formed on the plate. The image is then developed and transferred to paper whereby a reproduction of the original is accomplished. The copy is of excellent quality being comparable to copies made on a conventional amorphous selenium electrophotographic plate. In addition the active transport photoreceptor can be recycled for multiple copies and its organic surface is easily cleaned.

EXAMPLE n A TNF active transport electrophotographic plate is prepared in a manner similar to that outline in Example I except that a 2 micron layer of the [3 form of metal- 13 free phthalocyanine, an organic photoinjecting pigment, is applied on top of the blocking layer to form a 0.5 micron photoconductive layer by dip coating the substrate blocking layer in a solution of the phthalocyanine pigment, dioxane, and dichloromethane and alrowing the coating to dry for several hours.- After drying, a 20 micron layer of dinitroacridine is then vacuum evaporated in the same manner as Example I to form the active transport overlayer.

The resulting electrophotographic plate is then placed in a Model D Xerographic Copy Machine where copy is made in the same manner as Example I by positive corona charging to 800 volts and exposing in a wavelength region of 4200 to 6500 Angstrom Units. The resulting recycled copies have as good a quality of reproduction as that prepared in Example I.

EXAMPLE III the grams of Lexan, a polycarbonate resin, were stirred into a solvent blend of 40 grams dioxane and 40 grams dichloromethane. To this solution ten grams of 2,4,7-trinitro-9-fluorenone (TNF) is added. Stirring is continued until solution is complete.

A layered structure is prepared in the same manner as Example I by dip coating a blocking layer-substrate arrangement in a copper phthalocyanine-solvent composition whereby a 3 micron phthalocyanine layer is formed. The layered phthalocyanine plate is then dip coated in the Lexan-TNF solution to form a 10 micron layer of the resin-TNF composition. The resulting layered structure is dried for a period of 24 hours.

The resin-TNF layered structure is then placed in a Xerox Model D Machine where a copy is made in the same manner as the plate of Example I. The quality of reproduction is equivalent to those in Examples I and II which indicates that charge carriers are transported across the resin-TNF layer. Therefore the electron fi'ansport characteristics are not hindered by placing sufficient quantities of TNF, or any other electron transport material, in an electronically inert binder.

EXAMPLE IV A photosensitive layered plate substantially similar to that shown in FIG. 1 is prepared by the following technique: A glass substrate having a conductive layer of tin oxide was vacuum coated with a one micron layer of amorphous selenium by conventional vacuum techniques such as those disclosed by Bixby in U.S. Pat. Nos. 2,753,278 and 2,970,906. A stock solution of 3.32 g. of duPont 49,000 polyester adhesive and 11.25g. of 2,4,7-trinitro-9-fluorenone was prepared by dissolving these quantities of materials in 58g. of tetrahydrofuran. The selenium coated substrate was overcoated with the described stock solution to form a 23 micron thick charge transport layer having a percentage of 2,4,7- trinitro-9-fluorenone in the film after drying of 75 percent by weight.

To evaluate the sensitivity of the above described plate as compared to certain structures of the prior art in a xerographic operative mode, a plate was prepared generally as described in U.S. Pat. No. 3,598,582 but specifically as follows: A glass plate was vacuum coated with aluminum to allow transmission of 9 percent of the incident light, followed by the sprinkling of a photoconductive pigment over the surface of the plate. The photoconductive pigment employed was 2,6-bis (pN,Ndimethylaminobenzylideneamino )-benzo 1,2- d:5,4-d) bisthiazole and was chosen as the pigment 14 based on test data shown in U.S. Pat. No. 3,489,558 and U.S. Pat. No. 3,501 ,298, these patents being incorporated by reference in U.S. Pat. No. 3,598,582. This data indicated that the described pigment would most likely be the most sensitive pigment for a xerographic operative mode.

After the pigment was applied, it was rubbed unidirectionally until 'a distinct polarization of transmitted light was observed and a transmission density was obtained of about 0.2 to 0.6 as specified in Example I, of U.S. Pat. No. 3,598,582. Following this the plate was overcoated with a layer of poly-N-vinyl carbazole to a thickness of 14 microns and dried.

The plate produced as generally described in U.S. Pat. No. 3,598,582 was then charged negatively while the plate produced pursuant to the instant invention was charged positively. This was necessary for comparative purposes since the poly-N-vinyl carbazole specitied in U.S. Pat. No. 3,598,582 transports holes as opposed to electrons. Following charging, exposure to a substantially unpolarized tungsten light source was carried out while the surface potential was continuously monitored and the exposure times for 50 percent discharge of the surface voltage for both plates were measured. These times are expressed below in Table I as t /2 (50 percent), this being a recognized measure of the xerographic sensitivity of the noted plates.

It may be seen from the listed data and in view of the much faster discharge of the plate of the instant invention that it is much more sensitive than the plate of the prior art for xerography. Comparing the lowest field applied to the prior art plate, the plate of the instant invention is about 16 times faster, while at the highest field applied, it is about 6 times faster.

The present invention has been described with reference to certain specific embodiments which have been presented in illustration of the invention. It is to be understood however that numerous variations of the invention may be made and that it is intended to encompass such variation within the scope and spirit of I the invention as described by the following claims.

15 from the group consisting of phthalic anhydride, tetrachlorophthalic anhydride, benzil, mellitic anhydride, s-tricyanobenzene, picryl chloride, 2,4- dinitrochlorobenzene, 2,4-dinitrobromobenzene, 4-nitrobiphenyl, 4,4-dinitrobiphenyl, 2,4,6-trinitroanisole, trichlorotrinitrobenzene, trinitro-o-toluene 4,6-dichloro- 1 ,3-dinitrobenzene, 4,6- dibromol ,3-dinitrbe nzene, p-dinitrobenzene, chloranil, bromanil, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitrofluorenone, trinitroanthracene, dinitroacridine, tetracyanopyrene, and dinitroanthraquinone; the photoconductive layer having a thickness between 0.02 and 25 microns and dispersed in from 0 to 95 volume percent binder but comprising at least 25 volume percent when an electronically inert binder is employed, the photoconductor binder thickness ranging from 0.05 to 20 microns when a binder is employed; the transport material, if dispersed in an electronically inert binder, is present in a volume ratio of at least 25 percent active transport material to electronically inert binder;

uniformly positive charging said plate, and c. exposing said plate to a source of radiation in the wavelength region of from about 4200 to 6500 Angstroms to which the active layer is substantially transparent and non-absorbing whereby injection and transport of photogenerated electrons from said photoconductive layer occurs through said active transport layer to form a latent electrostatic image on the surface of said plate.

2. The method of claim 1 wherein the photoconductive layer comprises a material selected from the group consisting of vitreous selenium, amorphous selenium, selenium alloys, trigonal selenium, cadmium sulfoselenide, cadmium sulfide, cadmium selenide, zinc oxide, and mixtures thereof.

3. The method of'claim l which further includes developing said latent image to make it visible.

4. The method of claim 1 in which the substrate is substantially transparent and exposure is carried out through said substrate.

5. An electrophotographic plate comprising in successive layers:

a. a conductive substrate,

b. a blocking layer,

c. an inorganic photoconductive layer, and

d. an organic charge transport layer consisting essentially of 2,4,7-trinitor-9-fluorenone; the photoconductive layer having a thickness between 0.02 and 25 microns and dispersed in from 0 to volume percent binder but comprising at least 25 volume percent when an electronically inert binder is employed, the photoconductor binder thickness ranging from 0.05 to 20 microns when a binder is employed; the transport material, if dispersed in an electronically inert binder, is present in a volume ratio of at least 25 percent active transport material to electronically inert binder.

6. An electrophotographic plate as claimed in claim 5 wherein the charge transport layer consists essentially of about 50% by weight of 2,4,7-trinitro-9-fluorenone in a resin binder.

7. An electrophotographic plate as claimed in claim 5 wherein the barrier layer is aluminum oxide.

8. An electrophotographic plate as claimed in claim 5 wherein the inorganic photoconductive layer comprises a material selected from the group consisting of cadmium sulfide, cadmium selenide or zinc sulfide.

9. The method of imaging of claim 1 wherein the transport material is dispersed in an electronicallyv inert binder in a volume ratio of at least 25 percent active transport material to electronically inert binder.

10. An electrophotographic plate as claimed in claim 5 wherein the transport material is dispersed in an electronically inert binder in a volume ratio of at least 25 percent active transport material to electronically inert binder.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3 92 034 DATED i December 23, 1975 INVENTOR(S) Paul J. Regensburger 9 it is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 22, cancel "reflux" and substitute therefore reflex Q Column 4, line 59, cancel "filed" and substitute therefore field Column 7, line 29, cancel "becuase" and substitute therefore because Q Column 9, line 36, cancel" the" and substitute therefore that Column ll, line 36, cancel "particcle" and substitute therefore particle Q Column 12, line 15, cancel created" and substitute therefore create Column 12, line 27, cancel "the and substitute therefore any g Column 12, line 67, cancel "outline" and substitute therefore outlined Engned and Sealed this thirtieth D f March 1976 O [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Anestmg 1 ('mnmissivm'r ofParenIs and Trademarks

Claims

1. A method of imaging which comprises: a. providing a xerographic plate having a supporting substrate, an unoriented inorganic photoconductive layer overlaying said substrate, and an electronically active transport layer overlaying said photoconductive layer, said photoconductive layer comprising a photoconductor having the capability of photogenerating electrons and injecting them into an adjacent active transport layer, said active layer having the capability of supporting the injection of electrons and transporting said electrons through said active material wherein said active material comprises at least one material selected from the group consisting of phthalic anhydride, tetrachlorophthalic anhydride, benzil, mellitic anhydride, s-tricyanobenzene, picryl chloride, 2,4-dinitrochlorobenzene, 2,4-dinitrobromobenzene, 4-nitrobiphenyl, 4,4-dinitrobiphenyl, 2,4, 6-trinitroanisole, trichlorotrinitrobenzene, trinitro-o-toluene 4,6-dichloro-1,3-dinitrobenzene, 4,6-dibromo-1,3-dinitrobenzene, p-dinitrobenzene, chloranil, bromanil, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitrofluorenone, trinitroanthracene, dinitroacridine, tetracyanopyrene, and dinitroanthraquinone; the photoconductive layer having a thickness between 0.02 and 25 microns and dispersed in from 0 to 95 volume percent binder but comprising at least 25 volume percent when an electronically inert binder is employed, the photoconductor binder thickness ranging from 0.05 to 20 microns when a binder is employed; the transport material, if dispersed in an electronically inert binder, is present in a volume ratio of at least 25 percent active transport material to electronically inert biNder; b. uniformly positive charging said plate, and c. exposing said plate to a source of radiation in the wavelength region of from about 4200 to 6500 Angstroms to which the active layer is substantially transparent and non-absorbing whereby injection and transport of photogenerated electrons from said photoconductive layer occurs through said active transport layer to form a latent electrostatic image on the surface of said plate.

3. The method of claim 1 which further includes developing said latent image to make it visible.

5. AN ELECTROPHOTOGRAPHIC PLATE COMPRISING IN SUCCESSIVE LAYERS: A. A CONDUCTIVE SUBSTRATE, B. A BLOCKING LAYER, C. AN INORGANIC PHOTOCONDUCTIVE LAYER, AND D. AN ORGANIC CHARGE TRANSPORT LAYER CONSISTING ESSENTIALLY OF 2,4,7-TRINITOR-9-FLUORENONE; THE PHOTOCONDUCTIVE LAYER HAVING A THICKNESS BETWEEN 0.02 AND 25 MICRONS AND DISPERSED IN FROM 0 TO 95 VOLUME PERCENT BINDER BUT COMPRISING AT LEAST 25 VOLUME PERCENT WHEN AN ELECTRONICALLY INERT BINDER IS EMPLOYED, THE PHOTOCONDUCTOR BINDER THICKNESS RANGING FROM 0.05 TO 20 MICRONS WHEN A BINDER IS EMPLOYED; THE TRANSPORT MATERIAL, IF DISPERSED IN AN ELECTRONICALLY INERT BINDER, IS PRESENT IN A VOLUME RATIO OF AT LEAST 25 PERCENT ACTIVE TRANSPORT MATERIAL TO ELECTRONICALLY INERT BINDER.

9. The method of imaging of claim 1 wherein the transport material is dispersed in an electronically inert binder in a volume ratio of at least 25 percent active transport material to electronically inert binder.