Smith, Michael (Rochester, NY)
Hackett, Charles F. (Williamson, NY)
Radler, Richard W. (Marion, NY)

05/371646

03/11/1975

06/20/1973

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Xerox Corporation (Stamford, CT)

430/31

430/58.250, 430/58.150, 430/58.700, 430/96, 430/58.050

G03G5/06; G03G5/07; G03G5/06

96/1R,1.5 252/501

3598582

August 1971

Herrick et al.

Martin Jr., Roland E.

This application is a continuation-in-part of copending application, Ser. No. 93,994, filed Dec. 1, 1970, now abandoned, which is a continuation-in-part of application, Ser. No. 14,281, filed Feb. 26, 1970, and now abandoned.

What is claimed is

1. A method of imaging which comprises:

2. The method of claim 1 in which the latent electrostatic image is developed to form a visible image.

3. The method of claim 1 in which the activating radiation is within the visible spectrum.

4. The method of claim 1 in which the exposure radiation is in the range of about 4,000 to 8,000 Angstrom Units.

5. The method of claim 1 in which the binder layer is contained on an electrically conductive substrate.

6. The method of claim 1 in which the binder layer is contained on a transparent substrate and exposure to activating radiation is through said substrate.

7. The method of claim 1 in which the binder layer comprises photoconductive particles in an amount from about 0.1 to 1.0 percent by volume of said binder layer.

8. The method of claim 1 in which the active matrix material is selected from the group consisting of poly-N-vinyl carbazole, poly-1-vinyl pyrene, poly-9-vinyl anthracene, polyacenaphthalene, poly-9-(4-pentenyl)-carbazole, poly-9-(5-hexyl)-carbazole, polymethylene pyrene, poly-1-(-αpyrenyl)-butadiene, N-substituted polymeric acrylic acid amides of pyrene, poly-3-amino carbazole, 1,3-dibromo-poly-N-vinyl carbazole, 3,6-dibromo-poly-N-vinyl carbazole, poly-2-vinyl carbazole, poly-3-vinyl carbazole, N-vinyl carbazole/methyl acrylate copolymers, 1-vinyl pyrene/butadiene ABA, and AB block polymers, carbazole, N-ethylcarbazole, N-phenylcarbazole pyrene, tetraphene, 1-acetylpyrene, N-benzochrysene, 6,7-benzopyrene, 1-bromopyrene, 1-ethylpyrene, 1-methylpyrene, perylene, 2-phenylindole, tetracene, picene, 1,3,6,8-tetraphenylpyrene, chrysene, fluorene, fluorenone, phenanthrene triphenylene, 1,2,5,6-dibenzanthracene, 1,2,3,4-dibenzanthracene, 2,3-benzopyrene, 2,3-benzochrysene, anthraquinone, dibenzothiophene, naphthalene and 1-phenylnaphthalene.

9. A method of imaging which comprises:

10. The method of claim 9 which further includes developing the latent image to form a visible image.

11. The method of claim 10 in which the charging, exposing, and developing steps are repeated at least one additional time.

12. The method of claim 9 in which the exposure radiation is in the range of about 4,000 to 8,000 Angstrom Units.

13. The imaging member of claim 11 in which the active matrix comprises a material selected from the group consisting of alkyl, nitro, amino, halogen and hydroxy substituted polymers, said polymer selected from the group consisting of poly-1-vinylpyrene, polymethylenepyrene, N-substituted polymeric acrylic acid amides of pyrene, poly-N-vinylcarbazole, poly-9-vinyl anthracene, polyacenaphthalene, poly-9-(4-pentenyl)-carbazole, poly-9-(5-hexyl)-carbazole, poly-1-(pyrenyl)-butadiene, poly-3-amino carbazole, 1,3-dibromo-poly-N-vinyl carbazole, 3,6-dibromo-poly-N-vinyl carbazole, poly-2-vinyl carbazole, poly-3-vinyl carbazole, N-vinyl carbazole/methyl acrylate copolymer, 1-vinyl pyrene/butadiene ABA block polymer and 1-vinyl pyrene/butadiene AB block polymer.

BACKGROUND OF THE INVENTION

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 nonilluminated areas. This latent electrostatic 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.

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 stable cyclic operation. With the uniform dispersion of photoconductor particles described in Middleton et al., 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 significantly 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 al. 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 80 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 al 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 additive materials which extend its spectral sensitivity.

In addition, certain specialized layer structures particularly designed for reflex 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 layerss 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-to-particle 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 relay 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 a 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.

U.S. 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 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 througout 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 this invention to provide a novel photosensitive device adapted for cyclic imaging which overcomes the above noted disadvantages.

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

It is a further object of this invention to provide a photosensitive member which exhibits facile hole generation and transport.

It is another object of this invention to provide a method of imaging a photosensitive member.

It is a further object of this invention to provide a novel photosensitive binder structure.

It is another object of this invention to provide a novel binder structure characterized by an extremely low ratio of photoconductor to binder.

It is yet another object of this invention to provide a novel photosenstive member which is capable of exhibiting outstanding physical properties.

SUMMARY OF THE INVENTION

The foregoing objects and others are accomplished in accordance with this invention by providing a photosensitive member having a composite photosensitive layer which comprises substantially unoriented photoconductive particles utilized in conjunction with an electrically active organic binder or matrix. The photoconductive particles must be capable of generating and injecting photoexcited holes into the electrically active organic material which comprises a transparent organic polymer or nonpolymeric material which is substantially nonabsorbing to radiation in the spectral region of intended use, but which is active in that it allows the injection of photoexcited holes from the photoconductive particles and allows these holes to be transported through the active matrix. In a preferred form of the invention, the photoconductive particles are dispersed throughout the active matrix material.

It should be understood that the active organic matrix material does not function as a photoconductor in the wavelength region of use. As stated above, hole-electron pairs are photogenerated in the photoconductive particles and the holes are then injected into the active matrix with hole transparent occurring through the active matrix.

One embodiment of a typical application of the instant invention consists of a supporting substrate such as a conductor containing a binder layer thereon. For example, the binder layer may comprise particles of trigonal selenium contained in a transparent polymeric layer which allows for hole injection and transport. The transparent active (polymer) matrix allows one to take advantage of extremely low photoconductor loading not previously available to the art and preferably certain selected matrix materials having high charge injection and transparent efficiency are utilized. In addition, the structure can function effectively for repetitive use or cycling. This structure can be imaged in the conventional xerographic manner which usually includes charging, optical projection exposure, and development.

In general, the advantages of the improved structure and method of imaging will become appparent upon consideration of the following disclosure of the invention; especially when taken in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a plot of photosensitivity versus field dependence for an active material alone, and in conjunction with a photoconductor.

FIG. 2 is a plot similar to FIG. 1 for a second active material.

FIG. 3 represents a plot of the absorption spectrum for polyvinyl carbazole.

FIG. 4 represents a plot of the absorption spectrum for pyrene.

FIG. 5 illustrates the spectral response for three photoconductor materials.

FIG. 6 represents a plot of the absorption spectrum for perylene.

FIG. 7 is a schematic illustration of one embodiment of a device of the instant invention.

FIG. 8 represents a plot of the discharge characteristics for positive and negative corona charging one embodiment of a binder layer of the instant invention.

FIG. 9 represents a plot of the discharge characteristics for positive and negative corona charging a second embodiment of a binder layer of the instant invention.

FIG. 10 illustrates the cycling characteristics at various exposure wavelengths for a device employing an active layer.

DETAILED DESCRIPTION OF THE DRAWINGS

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. Photoconductivity 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 prevalence of photoconductivity, it is therefore necessary that for the instant invention, the term "photoconductor" or "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.

The active material, which is also referred to as the active matrix material when used as a matrix for the binder layer, is a substantially nonphotoconductive material which supports an injection efficiency of photoexcited holes from the photoconductive particles of at least about 10 percent at fields of about 2 × 10 ⁵ volts/cm. This material is further characterized by the ability to transport the carrier at least 10 ^{- 3} cm. at a field of no more than about 10 ⁶ volts/cm. In addition, the active matrix material is substantially transparent in the wavelength region in which the device is to be used.

As can be seen from the above discussion, most materials which are useful active matrices for binder 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 photoconductor is 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 criteria stated above for the active matrix materials implies that these materials do not contribute significantly to the photoresponse of the photoreceptor in the wavelength region of use.

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 binder 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 ¹⁰ ohm-cms.

The reason for the requirement that the active materials must be transparent is based upon the discovery that under all practical conditions, the efficiency of photoinjection from the photoconductor into the active materials, for visible radiation absorbed by the photoconductor, far exceeds the intrinsic photosensitivity of the active material in any wavelength region - visible or otherwise. This situation is illustrated by FIGS. 1 and 2 which shows a comparison of the field dependence of the injection sensitivity of the photoconductor selenium into typical active materials and the intrinsic photosensitivity of two active materials - polyvinyl carbazole and polyvinyl pyrene, each measured at wavelengths of high response. The polyvinyl carbazole and polyvinyl pyrene curves of FIGS. 1 and 2, respectively, are measured on samples 20 microns thick contained on an aluminum substrate and prepared by the method of Example I of Applicants' copending application entitled "Layered Imaging Member and Method" filed concurrently with the instant application. The curves for the layered structures of the same materials having a 0.4 micron layer of vitreous selenium formed between the layer of active material and substrate are similar to the structure illustrated by FIG. 9 and are made by the method set forth in Example III of the above mentioned copending application. The data of FIGS. 1 and 2 is detemined by plotting the initial xerographic gain (G) as a function of the applied field. The xerographic gain was calculated from the initial discharge rate.

G = (dV/dt)t=0/(eId/λ) where I is the incident photon flux, d the thickness of the layer, λ the electric permittivity, and e the electronic charge. A xerographic gain of unity would be observed if one charge carrier per incident photon were excited and moved across the layer. It is clear from FIGS. 1 and 2 that the intrinsic photoconductivity of the active materials at their peak wavelength of absorption (U.V. excitation) leads to gains considerably lower than the two phase structures incorporating efficient photoconductive materials, such as illustrated by the layered structures employing the 0.4 micron thick selenium layers with suitable active materials. These structures can achieve gains of approximately 0.70 at a field of about 10 ⁶ volts/cm, using a excitation wavelength within the visible spectrum (4,000A - 8,000A). It is also clear from FIGS. 3 and 4 that the typical active materials mentioned above will exhibit negligible, if any, discharge when exposed to a wavelength of light useful in xerography, i.e., 4,000A - 8,000A. The obvious improvement in performance which results from the use of the two phase systems can best be realized if the active material is substantially transparent to radiation in a region in which the photoconductor is to be used; for any absorption of desired radiation by the active material will prevent this radiation from reaching the photoconductive particles or pigment where it is much more effectively utiliized. It therefore follows that it is advantageous to use active matrix materials which are transparent in the wavelength in which the photoconductor or pigment has its main response, and more particularly in the wavelength region in which the photoconductor is to be used.

Applications where complete transparency in the visible region is not required for the active material include the selective recording of narrow-band radiation such as that emitted from lasers, spectral pattern recognition, color coded form duplication, and possible functional color xerography.

FIGS. 3, 4, and 6 represent the well known absorption efficiency for active matrix materials polyvinyl carbazole, pyrene, and perylene, respectively. FIG. 5 represents the xerographic response spectra for three typical photoconductor-active matrix material combinations. The amorphous selenium-PVK response is for a 0.4 micron layer of amorphous selenium contained on a 20 micron layer of PVK. The X-form of metal free phthalocyanine and trigonal selenium are contained in a polyvinyl carbazole binder in a concentration of about 30 to 1 (by volume) for the phthalocyanine and about 100 to 1 (by volume) for the trigonal selenium. Both binder layers are about 20 microns in thickness. As can be seen from FIGS. 3, 4, 5, and 6, it may be deduced that certain combinations of active matrix materials and various photoconductors would be of particular use for selective spectral response.

Referring to FIG. 7, reference character 11 illustrates a preferred embodiment of the instant invention which comprises a photosensitive member in the form of a plate having a supporting substrate 11 coated with a binder layer 12. Substrate 11 preferably comprises any suitable conductive material. Typical conductors comprise aluminum, steel, brass, or the like. The substrate may be rigid or flexible and of any convenient thickness. Typical substrates include flexible belts or sleeves, sheets, webs, plates, cylinders, and drums. The substrate or support may also comprise a composite structure such as a thin conductive coating contained on a paper base; a plastic coated with a thin conductive layer such as aluminum or copper iodide; or glass coated with a thin conductive coating of chromium or tin oxide. When using a transparent substrate it should be understood that imagewise exposure may optionally be carried out through the substrate or back of the imaging member.

Binder layer 12 contains photoconductive particles 13 dispersed in an unoriented fashion in an electrically active matrix or binder material 14. The photoconductive particles may consist of any suitable inorganic or organic photoconductor, and mixtures thereof, which are capable of injecting photoexcited holes into the matrix. Typical inorganic materials include inorganic crystalline compounds and inorganic photoconductive glasses. Typical inorganic crystalline compounds include cadmium sulfoselenide, cadmium selenide, cadmium sulfide, and mixtures thereof. Inorganic photoconductive glasses include amorphous selenium, and selenium alloys such as selenium-tellurium and seleniumarsenic. Selenium may also be used as a crystalline form known as trigonal selenium. Typical organic materials include phthalocyanine pigments such as the X-form of metal free phthalocyanine described in U.S. Pat. No. 3,357,989 to Bryne et al, metal phthalocyanines, such as copper phthalocyanine; quinacridones available from DuPont under the tradename Monastral Red, Monastral Violet, and Monastral Red Y; substituted 2,4-diamino-triazines disclosed by Weinberger in U.S. Pat. No. 3,445,227; triphenodioxazines disclosed by Weinberger in U.S. Pat. No. 3,442,781; polynuclear aromatic quinones available from Allied Chemical Corp. under the tradename Indofast Double Scarlet, Indofast Violet Lake B, Indofast Brilliant Scarlet, and Indofast Orange. The above list of photoconductors should in no way be taken as limiting, but is merely illustrative of suitable materials. The size of the photoconductive particles is not critical, but particles in a size range of about 0.01 to 1.0 microns yield particularly satisfactory results.

As previously stated, the photoconductive material of the instant invention is employed in an unoriented manner. By unoriented, it is meant that the pigment or photoconductive material is isotropic with respect to the exciting electromagnetic radiation, in that it is equally sensitive to any polarization of the exciting radiation.

The active matrix material 14 may comprise any suitable transparent organic polymer or nonpolymeric material capable of supporting the injection of photo-excited holes from the photoconductive pigment and allowing the transport of these holes through the active matrix to selectively discharge a surface charge. Polymers having this characteristic have been found to contain repeating units of a polynuclear aromatic hydrocarbon which may also contain heteroatoms such as; for example, nitrogen, oxygen, or sulfur. Typical polymers include poly-N-vinyl carbazole (PVK), poly-1-vinyl pyrene (PVP), poly-9-vinyl anthracene, polyacenaphthalene, poly-9-(4-pentenyl)-carbazole, poly-9-(5-hexyl)-carbazole, polymethylene pyrene, poly-1-(- pyrenyl)-butadiene and N-substituted polymeric acrylic acid amides of pyrene. Also included are derivatives of such polymers including alkyl, nitro, amino, halogen, and hydroxy substituted polymers. Typical examples are poly-3-amino carbazole, 1,3-dibromo-poly-N-vinyl carbazole and 3,6-dibromo-poly-N-vinyl carbazole in particular derivatives of the formula ##SPC1##

where X and Y are substituents and N is an integer. Also included are structural isomers of these polymers, typical examples include poly-N-vinyl carbazole, poly-2-vinyl carbazole, and poly-3-vinyl carbazole. Also included are co-polymers; typical examples are N-vinyl carbazole/methyl acrylate co-polymer and 1-vinyl pyrene/butadiene ABA, and AB block polymers. Typical nonpolymeric materials include carbazole, N-ethylcarbazole, N-phenylcarbazole, pyrene, tetraphene, 1-acetylpyrene, 2,3-benzochrysene, 6,7-benzopyrene, 1-bromopyrene, 1-ethylpyrene, 1-methylpyrene, perylene, 2-phenylindole, tetracene, picene, 1,3,6,8-tetraphenyl-pyrene chrysene, fluorene, fluorenone, phenanthrene, triphenylene, 1,2,5,6-dibenzanthracene, 1,2,3,4-dibenzanthracene, 2,3-benzopyrene, anthraquinone, dibenzothiophene, naphthalene, and 1 -phenylnaphthalene. Due to the poor mechanical properties of the non-polymer materials they are preferably used in conjunction with either an active polymer material or a non-active polymeric binder. Typical examples include suitable mixtures of carbazole in poly-N-vinyl carbazole as an active polymer and carbazole in a non-active binder. Such non-active binder materials include polycarbonates, acrylate polymers, poly amides, polyesters, polyurethanes, and cellulose polymers.

It should be understood that the use of any polymer (a polymer being a large molecule built up by the repetition of small, simple chemical units) whose repeat unit contains the appropriate aromatic hydrocarbon, such as carbazole, and which supports hole injection and transport, may be used. It is therefore not the intent of the invention to restrict the type of polymer which can be employed as the matrix material. Polyesters, polysiloxanes, polyamides, polyurethanes and epoxies as well as block, random, or graft co-polymers (containing the aromatic repeat unit) are exemplary of the various types of polymers which can be employed. In addition suitable mixtures of active polymers with inactive polymers or non-polymeric materials may be employed. One action of certain non-active material is to act as a plasticizer to improve the mechanical properties of the active polymer layer. Typical plasticizers include epoxy resins, polyester resins, polycarbonate resins, 1-phenyl naphthalene and chlorinated diphenyl.

In general, the active layer is substantially transparent or non-absorbing in at least some significant portion of the range from about 4,000-8,000 Angstroms, but will still function to allow injection and transport of holes generated within this wavelength range by the photoconductive pigment particles.

An upper limit on photoconductor volume concentration or occupancy is governed by various factors: Notably (1) the stage at which the physical properties of the polymer are seriously impaired; (2) the stage at which there is significant transport through particle-to-particle contacts; and (3) the stage at which, with conductive pigments such as trigonal selenium, there is excessive hole sweep out during charging. The latter two factors frequently lead to a lack of cycling ability. In general, to attain the best combination of physical and electrical properties, the upper limit for the photoconductive pigment or particles must be no greater than about 5 percent by volume of the binder layer. A lower limit for the photoconductive particles of about 0.1 percent by volume of the binder layer is required to insure that the light absorption coefficient is sufficient to give appreciable carrier generation. In order to achieve a closely equivalent discharge rate under both charging conditions, it is necessary to work in a volume occupancy region where the average depth of light penetration is near the center of the layer. Thus for two examples shown in FIGS. 8 and 9, which represent binder layers of the X-form of metal free phthalocyanine and trigonal selenium contained in a PVK binder, reasonably equivalent discharge is obtained in the volume ranges of above about 1 part in 84 parts by volume for the X-form of metal free phthalocyanine and above about 1 part in 190 by volume for trigonal selenium. It should be noted that these preferred volume ranges are dependent upon the layer thickness. These figures also illustrate that although under positive charging conditions there is a steady increase in discharge rate with increased pigment loading, due to the increased light absorption coefficient, the performance is still very high even at loadings in the 1 percent by volume range.

It can be seen from the above that a critical range of about 0.1 to 5 percent by volume of the photoconductor is required to achieve the advantages of the instant invention. In addition, a preferred range for optimum mechanical properties has been established in the region of about 0.1 to 1.0 percent by volume for the photoconductor material.

The thickness of the binder layer is not particularly critical. Layer thicknesses from about 2 to 100 microns have been found satisfactory, with a preferred thickness of about 5 to 50 yielding particularly good results.

Another variation of the structure described in FIG. 7 consists of the use of a blocking layer at the substrate-binder layer interface. The blocking layer functions to prevent the injection of charge carriers from the substrate into the photoconductive layer. Any suitable blocking material may be used. Typical materials include nylon, epoxy, and aluminum oxide.

Although the active material may comprise any suitable polymer or nonpolymeric material having the required properties, polymeric materials are preferred in that their physical properties such as flexibility, are superior to the physical properties of the nonpolymeric materials.

Although the instant invention has been described above in terms of the preferred embodiment, i.e., the binder configuration, it should be understood that the structure may take other forms. For example, the layered configuration described in Applicants' copending application entitled "Layered Imaging Member and Method" filed concurrently with the instant application, illustrates a second basic embodiment of the instant invention. One embodiment of the layered configuration comprises a substrate having a photoconductive layer thereon which in turn is overcoated with a relatively thick layer of an active organic material. It should be understood that various modifications of the layered and binder configuration are also included within the scope of the instant invention. These alternative embodiments may include structural modifications of either the layered or binder configuration as well as combinations of the two.

In order to demonstrate the improvement provided by the instant invention over the particular binder layer disclosed in the Middleton et al., U.S. Pat. No. 3,121,006 the following tests are carried out. Three typical resin binder materials disclosed by U.S. Pat. No. 3,121,006 to Middleton et al are tested in order to determine the characteristics of these resins in comparison with the active materials of the instant invention. The resins include polystryrene, polyisobutylmethacrylate, and a silicone resin available under the tradename SR-82 from General Electric. The results of the test demonstrate that these resin binder materials cannot support any practically useful level of charge displacement when used with a vitreous selenium photoreceptor. The polyisobutylmethacrylate and silicone resin are tested in a layered plate configuration by first forming a thin nylon blocking layer about 0.1 microns thick over two 4 × 4 inch aluminum substrates from a liquid solution using conventional coating techniques. A 1.0 micron layer of each resin respectively, is then formed over the blocking layers of the two plates. A 0.5 micron layer of vitreous selenium is then formed over the resin layers by vacuum evaporation. A third plate is formed by the above method using polystyrene as the resin layer without a nylon blocking layer.

The three plates are each tested by charging to a known potential, illuminating the charged layer, and measuring the residual potential. If there is no charge displacement across the plastic layer then the residual potential can be calculated from the known properties of the resin, the thickness of the layers, the dielectric constant of the materials, and the initial potential. The calculated residual potential should be the same as the measured residual, within experimental error, until the electrical breakdown point of the plastic layer occurs. If it is assumed that the initial field distribution is capacitive, then the residual potential, V _res , will be defined by the following formula:

V _res = V ₀ / [1 + (k ₁ d ₂ /k ₂ d ₁ )]

If no charge is transported across the resin layer, the plot of the experimental V _res should be proportional to V _o (the initial applied potential) with a slope of

1 / [ 1 + (k ₁ d ₂ )/(k ₂ d ₁ ) ]

In the above formula, the dielectric constant of the resin is k ₁ and the resin thickness d ₁ ; the selenium dielectric constant k ₂ , and the selenium thickness d ₂ . The initial applied voltage is V _o .

The experiments are carried out using a monochromatic light source of 4,000 Angstroms at an intensity of 2 × 10 ¹² photons/cm ² /sec. Each plate is charged to a series of selected voltages between about 0 to 100 volts (about 0 to 65 volts/microns). The residual potential is not limited by the incident light flux since under all conditions of the experiment enough light is used to generate a sufficient number of carriers in the selenium to reduce the field across the selenium layer essentially to zero. The thickness of the layers are kept intentionally small, even though thin samples present some measurement problems, in order to approximate the actual situation in binder structure photoreceptors, where the electrical properties of the thin films of plastic between the pigment particles are controlling. The resules of these calculations and experiments are set forth in Table I below:

TABLE I ____________________________________________________________ ______________ Electrical Properties of Layered Structures Experimental Calculated k d Slope Slope ____________________________________________________________ ______________ Polystyrene 2.4 1.0 0.77 (±) 0.01 0.83 Polyisobutylmethacrylate 2.7 1.0 0.79 (±) 0.02 0.82 Silicone Resin 2.8 1.0 0.70 (±) 0.02 0.81 Selenium 6 0.5 ____________________________________________________________ ______________

The values for the experimental slope are calculated using the Method of Least Squares from the experimental data points. The Method of Least Squares is fully described by J. Topping in the book Errors of Observation And Their Treatment published by Reinhold Publishing Corp. of New York, 1955. The small standard error of the slopes indicate that the data points do not scatter significantly about the best straight line. The comparison between the experimental and calculated or theoretical slope must next be considered. Although the experimental and calculated slopes are not the same, they compare favorably when all the errors are considered. Although the random errors of the measurements are small (i.e., the standard error of the slope), large systematic errors can arise because of the difficulty in making thickness measurements of the layers.

It may, therefore, be concluded from the experimental data in Table I that there is a negligible amount of charge displacement through the three resin layers, even when their thickness is only 1 micron, up to fields of about 45 volts/micron. At fields exceeding about 45 volts/microns, these thin layers exhibit dielectric breakdown. This experimental test does not show whether this lack of charge displacement derives from an inability to support hole injection from vitreous selenium or from a very small hole transport range. When all the error limits are considered, it is safe to say that these plastics act as insulators under the experimental conditions; that is, the charge is either not injected from the selenium into the plastic or, if injected, not transported through the plastic at these fields.

In order to show the advantages of the instant invention with respect to the prior art which uses a combination of at least two or more photosensitive materials, such as U.S. Pat. No. 3,212,007 to Middleton, and U.S. Pat. No. 3,037,861 to Hoegl et al., additional tests are carried out. If during use, the active matrix material of the instant invention absorbs some of the incident exposure illumination, the photoreceptor, whether it be in particulate form in a binder or as a separate photoinjecting layer, becomes less sensitive. In addition to a decrease in discharge sensitivity, the utilization of the photoconductive nature of the active matrix material leads to serious problems in continuous use such as in copy machine cycling. Normally it is desirable that a photoreceptor have stable or consistent electrical properties during cycling to allow for proper design of other components in the system such as, for example, development, exposure, and background control. If these conditions cannot be maintained substantially constant, it becomes difficult, if not impossible, to design a reliable automatic copy machine that does not require constant servicing and adjustments. In order to demonstrate the criticality of imaging structures of the instant invention only within wavelengths in which charge carriers are generated by the photoconductor, and in which the surrounding matrix or active material is substantially transparent, the following test is carried out.

A plate is made for test purposes. The plate comprises a conductive tin oxide coated quartz substrate. A 0.1 micron epoxy blocking layer is formed over the tin oxide, followed by a 0.5 micron layer of amorphous selenium which is formed by vacuum evaporation. A 10 micron testing of PVK is then formed over the selenium layer. In order to illustrate the fact that the active material should be transparent to radiation in order to attain maximum efficiency for the device, the following test is carried out:

The plate is charged to a negative potential of about 200 volts and tested at four different wavelengths by exposure through the top surface of the PVK layer. Upon illumination, through the top, the plate exhibits a characteristic electrical discharge curve. The xerographic speed of the plate can be compared by determining graphically the slope of the discharge curve at the instant of illumination, i.e., (dV/dt) at t=0, normalized to the thickness of the sample and to incident flux of 1 × 10 ¹² photons/cm ² /sec. This calculation is defined as the discharge sensitivity and is shown in Table II below:

TABLE II ______________________________________ Dependence on Discharge Rate on Absorption by PVK ______________________________________ Wavelength V _o (dV/dt) t=o A (Volts) (volts/sec) ______________________________________ 4000 205 157 3550 185 83 3340-3370 200 75 3150-3180 200 45 2720-2740 195 49 ______________________________________

As seen from the data in Table II, at 4,000A, where the PVK is substantially transparent to the exposure illumination, the discharge sensitivity [(dV/dt) t=0] is relatively high. When exposing to wavelengths of 3550 or less, however, some charge carriers are generated in the PVK, and the sensitivity is significantly reduced.

In order to demonstrate the criticality of continuous repetitive use or cycling, and the necessary requirement that the active matrix material be transparent to the illuminating or exposure radiation, additional tests are carried out. A 4 × 4 inch aluminum substrate is first coated with a 0.2 micron layer of epoxy to form a blocking layer, a 0.5 micron layer of vitreous selenium is then formed over the blocking layer by vacuum deposition, the selenium layer is then overcoated with a 12 micron layer of PVK. This plate is then taped to an 8 inch diameter aluminum drum, charged to a negative potential of 900 volts, and exposed to light to obtain 200 volts of contrast potential. The plate is then erased to a negative potential of 40 volts or less by exposure with a quartz iodine lamp, and charged again to 900 volts negative potential. The cycle is then repeated at a peripheral drum speed of about 6 inches per second. For all tests, the starting potential is adjusted to 900 volts by adjusting the corona current at the beginning of the test. The experiments were carried out at exposures of 4,000, 3450, and 2537 Angstroms, respectively. In each case, the intensity is adjusted at the beginning to create 200 volts of contrast potential. The results of the test are illustrated in FIG. 10.

As shown in FIG. 10, at 4,000 Angstroms, where the PVK is transparent to the incident light and not being used as a photoconductor, the structure is stable in its electrical characteristics for more than 1,000 cycles. However, for 3,450 Angstroms, and 2537 Angstroms, where the incident light is strongly absorbed by the PVK layer, and the PVK is being used as a photoconductor, the initial potential decreases upon cycling, and by extrapolation, the photoreceptor would probably not even accept charge at about 10,000 cycles. Over the range of these experiments, the potential after exposure decreased in proportion to the decrease in initial potential, resulting in a constant contrast potential. Although it would be possible to develop such an image, the change in potential with constant contrast would lead to difficulties in development and background control and be unsuitable for automatic cycling in the xerographic mode.

It should be understood that the results of the above tests are readily applicable to structures in which the photoconductor particles are dispersed in an active binder matrix as well as to layered configurations, since the layered case may simply be considered as representative of the events that occur around each pigment particle surrounded by the active matrix.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples further specifically define the present invention with respect to a method of making a photosensitive member containing a binder layer having photoconductive particles dispersed in an active organic matrix. The percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the instant invention.

EXAMPLE I

A photosensitive binder plate similar to that shown in FIG. 7 and containing unoriented photoconductive particles of the X-form of metal free phthalocyanine dispersed in a polyvinyl carbazole (PVK) binder in a ratio of 48 to 1 PVK by weight (60 to 1 by volume) to the photoconductive pigment particles is prepared by the following technique: 31 grams of a 16.7 weight percent PVK stock solution is formed by dissolving the appropriate amount of Luvican M170 grade poly-N-vinyl-carbazole, available from BASF, in 180 grams of toluene and 20 grams of cyclohexanone. This solution is added to 0.108 grams of X-form metal free phthalocyanine and 10 grams of toluene. This mixture is milled with steel milling shot for 15 to 60 minutes until a well dispersed suspension is formed. A coating is then formed on an aluminum substrate utilizing a Gardner Laboratory Bird Applicator. The final thickness after air drying at 110°C for 1 to 24 hours is about 24 microns.

EXAMPLE II

Three plates are made by the method of Example I except that the phthalocyanine concentration is varied to ratios of:

a. 72/1 by weight (90/1 by volume) PVK phthalocyanine with a binder layer thickness of about 20 microns.

b. 24/1 by weight (30/1 by volume) PVK to phthalocyanine with a binder layer thickness of about 20 microns. s

c. 96/1 by weight (120/1 by volume) PVK to phthalocyanine with a binder layer thickness of about 20 microns.

EXAMPLE III

Three plates are made by the method of Example I except that in plate of phthalocyanine, a polynuclear aromatic quinone available frm Allied Chemical Corporation under the tradename Indofast Orange is used as the photoconductive pigment in ratios of:

a. 24/1 by weight (30/1 by volume) PVK to pigment with a binder layer thickness of about 13 microns.

b. 48/1 by weight (60/1 by volume) PVK to pigment with a binder layer thickness of about 15 microns.

c. 72/1 PVK by weight (72/1 by volume) to pigment with a binder layer thickness of about 14 microns.

EXAMPLE IV

Two plates are made by the method of Example I except that trigonal selenium is used as pigment in ratios of:

a. 24/1 by weight (96/1 by volume) PVK to trigonal selenium with a binder layer thickness of about 30 microns.

b. 48/1 by weight (192/1 by volume) PVK to trigonal selenium with a binder layer thickness of about 12 microns.

EXAMPLE VI

EXAMPLE V

A plate is made by the method of Example I except that cadmium sulfoselenide is used as the pigment with the ratio of PVK to cadium sulfoselenide being 24/1 by weight (105/1 by volume). The binder layer has a thickness of about 10 microns.

A plate is made by the method of Example I except that trigonal selenium is used as the pigment with the ratio of PVK to trigonal selenium being 24/1 by weight (96/1 by volume). The binder layer has a thickness of about 10 microns. In addition, a 0.2 nylon micron blocking layer is formed over both the surface of the binder layer, and at the binder layer-substrate interface. The nylon blocking layer is formed by dip coating the plate in a solution of nylon (sold by DuPont under the tradename Zytel) dissolved in methyl alcohol.

EXAMPLE VII

A plate is made by the method of Example I except that trigonal selenium is used as the pigment with the ratio of PVK to trigonal selenium being 24/1 by weight (96/1 by volume). The binder layer has a thickness of about 9 microns.

EXAMPLE VIII

A plate is made by the method of Example I except that trigonal selenium is used as the pigment with the ratio of PVK to trigonal selenium being 6/1 by weight (24/1 by volume). The binder layer has a thickness of about 10 microns.

Each of the plates of Examples I-VIII exhibits excellent electrical properties which are characterized by good charge acceptance and photoresponse upon exposure to light. The gain or maximum efficiency for 7 of the plates of Examples I-VIII is shown in Table III.

The plates of Table III are electrostatically charged to a positive potential to the field values indicated (a field of 50 × 10 ⁴ volts/cm. represents a voltage of 50 × 10 ⁴ volts for each cm. of layer thickness) using a corona charging device. Each sample was then exposed to monochromatic light of wavelength near the peak absorption for the photoconductor pigment being used. The resulting discharge (voltage versus time) are recorded. For this data the xerographic gain was calculated using the formulas previously defined.

TABLE III ____________________________________________________________ ______________ Gain or Max. Efficiency (Charge Exposure Field Carriers Col- Plate Wavelength Photon Flux Range lected Per of In Angstrom (Photons/cm Tested Absorbed Example Units Sec.) (V/Cm × 10 ⁴ ) Photon) ____________________________________________________________ ______________ I 6200 6.5 × 10 ¹² 50 .26 II a 6200 6.5 × 10 ¹² 50 .23 II b 6200 8.0 × 10 ¹² 8-95 .35 IV a 4000 3.8 × 10 ¹² 10-70 .25 IV a 4000 2.0 × 10 ¹² 25 .20 IV b 4000 5.9 × 10 ¹² 30 .22 VIII 4000 5.9 × 10 ¹² 30 .38 ____________________________________________________________ ______________

In addition to the testing described above in Table III, three of the plates are used to reproduce an original image. The plate of Example IVa is electrostatically charged to about 800 volts positive potential using a corona charging device. The plate is then exposed to a pattern of white light, from a quartz iodine source filtered to eliminate all radiation below 4,000 Angstrom Units, which selectively dissipates the charge in the illuminated areas. The latent electrostatic image which is formed is then developed using a liquid development system in which electrostatically negative charged toner particles dispersed in kerosene are allowed to flow over the above latent image. The electrostatically charged areas of the latent image attract the toner particles and form a visible image. The toner image is then transferred to a sheet of paper and fixed to form a permanent copy.

The plate of Example VI is imaged by the method described above for the plate of Example IVa, except that the plate is charged to a potential of about 500 volts.

The plate of Example V is also imaged by the method described for the plate of Example IVa except that the plate is charged to a potential of 500 volts and the latent electrostatic image is developed using cascade development with Xerox 914 toner particles. Each of the above three plates produced an excellent reproduction of an original image.

In order to demonstrate the cycling capability of the device of the instant invention, the plates of Examples I, IIIa, and IVa are cycled electrically by first electrostatically charging the plates in the dark to a field of about 30 volts/micron of binder layer thickness. Plates I, IIIa and IVa are then exposed to wavelengths of 6200, 4,000 and 4,000 Angstrom Units, respectively with a photon flux of about 2 × 10 ² photons/cm ² /sec. to discharge the plate. Following this, the plates were each flood illuminated with white light to remove any residual charge left on the surface of the plate. This entire cycle is repeated 200 times for plate I and IVa and 250 times for the plate IIIa. Each of the plates exhibits excellent charge acceptance and photodischarge at the end of the cyclic testing. The initial potential, contrast potential, and residual potential were essentially the same at the end of cycling as compared with these properties after the first cycle.

EXAMPLE IX

A source of vitreous selenium shot having a purity of 99.999 percent (available from American Smelting and Refining Co.) is sealed in a quartz ampule and placed in a vacuum chamber at a pressure of about 10 ^-5 Torr. The selenium is heat treated at 100°C for 16 hours to convert the vitreous selenium to the crystalline trigonal form.

A mixture of 1 part by volume trigonal to 1 part by volume poly-1-vinylpyrene (PVP) is dispersed in 100 parts of reagent grade chloroform. This mixture is milled on a paint shaker for 1 hour with 1/8 inch diameter steel balls until the selenium particles are ground to a maximum size of no greater than about 1 micron. Enough PVP is then added to achieve a 24/1 ratio of PVP to selenium. This mixture is milled for about 30 minutes and coated onto 3 aluminum alloy substrates to form a dried layer thickness of 25 microns for each plate. Each substrate has a 0.2 micron epoxy barrier formed over the substrate prior to coating with the binder mixture.

Each of the above plates is tested electrically. All 3 plates exhibit good electrical discharge.

Although specific components and proportions have been stated in the above description of the preferred embodiments of the instant invention, other suitable material and procedures such as those listed above may be used with similar results. In addition, other materials and modifications may be utilized which synergize, enhance, or otherwise modify the photosensitive member and method of use. For example, when using a transparent substrate such as a plastic coated with a transparent thin conductive coating of aluminum or tin oxide, the structure may be imaged by exposure through the substrate. In addition, if desired, an electrically insulating substrate may also be used. In this instance, the charge may be placed upon the photosensitive member by simumtaneously double corona charging the surface and insulating substrate with charges of the opposite polarity. Other modifications using an insulating substrate or no substrate at all, involve placing the photosensitive member or place on a conductive backing member and charging the surface of the photosensitive member while in contact with said backing member. Subsequent to imaging, the photoreceptor member may then be stripped from the conductive backing.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are also intended to be within the scope of the invention.