Photoreceptor containing dissimilar charge transporting small molecule and charge transporting polymer

Abstract: An electrophotographic imaging member including a charge generating layer and a charge transport layer, the charge transport layer including a charge transporting small molecule dissolved or molecularly dispersed in a film forming charge transporting polymer including charge transporting moieties in the backbone of the film forming charge transporting polymer, and the charge transporting moieties having a structure unlike the structure of the charge transporting small molecule, the charge transport layer being substantially free of electrically inactive film forming binder. This imaging member may be employed in an electrophotographic imaging process.

Description: This invention relates in general to electrophotographic imaging members and more specifically, to imaging members having an improved charge transport layer and process for using the imaging members. One common type of electrophotographic imaging member is a multilayered device that comprises a conductive layer, a charge generating layer, and a charge transport layer. Either the charge generating layer or the charge transport layer may be located adjacent the conductive layer. The charge transport layer can contain an active aromatic diamine small molecule charge transport compound dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example in US-A 4,265,990. Although excellent toner images may be obtained with such multilayered photoreceptors, it has been found that when high concentrations of active aromatic diamine small molecule charge transport compound are dissolved or molecularly dispersed in a film forming binder the small molecules tend to crystallize with time under conditions such as higher machine operating temperatures, mechanical stress or exposure to chemical vapors. Such crystallization can cause undesirable changes in the electro-optical properties, such as residual potential build-up which can cause cycle-up. Moreover, the range of binders and binder solvent types available for use during coating operations is limited when high concentrations of the small molecules are sought for the charge transport layer. For example, active aromatic diamine small molecules do not disperse in polyurethane binders. Limited selection of binders and binder solvents can affect the life and stability of a photoreceptor under extended cycling conditions. Moreover, such limited selection also affects the choice of binders and solvents used in subsequently applied layers. For example, the solvents employed for subsequently applied layers should not adversely affect any of the underlying layers. This solvent attack problem is particularly acute in dip coating processes. Further, some of the solvents that are commonly utilized, such as methylene chloride, are marginal solvents from the point of view of environmental toxicity. Another type of charge transport layer has been developed which utilizes a charge transporting polymer. This type of charge transport polymer includes materials such as poly N-vinyl carbazole, polysilylenes, and others including those described in US-A 4,806,443, US-A 4,806,444, US-A 4,818,650, US-A 4,935,487, and US-A 4,956,440. Some polymeric charge transporting materials have relatively low charge carrier mobilities. Moreover, the cost of charge transporting polymers having high concentrations of charge transporting moieties in the polymer chain can be very costly. Further, the mechanical properties of charge transporting polymers such as wearability, hardness and craze resistance are reduced when the relative concentration of charge transporting moieties in the chain is increased. Thus, in imaging systems utilizing multilayered photoreceptors containing charge transporting layers, adverse effects may be encountered during extended photoreceptor cycling. This can reduce the practical value of multilayered photoreceptors that are cycled many times in automatic devices such as electrophotographic copiers, duplicators and printers. Various multilayered electrophotographic imaging members which include charge transporting compounds such as polymeric arylamines are described in US-A 4,806,443, US-A 4,818,650, US-A 4,806,444,US-A 4,935,487, US-A 4,956,440, US-A 4,801,517, US-A 4,582,772,US-A 4,265,990 and US-A 4,871,634. EP-A-0,189,991 describes layered photoresponsive imaging members with a polysilylene hole transporting compound in a hole transport layer. FR-A-2,295,462 describes photoconductive polymers which are useful as hole transport materials in layered photoresponsive members. The polymers are low molecular weight condensation products of tertiary aromatic amines and carbonyl-containing compounds. US-A-4,959,288 and US-A-4,937,165 relate to layered photoconductive imaging members with a charge transport layer comprising diaryl biarylylamine copolymers and N,N-bis(biarylyl)aniline polymers respectively. Excellent toner images may be obtained with multilayered photoreceptors in which the charge transport layer contains a charge transporting polymer. However, it has been found that if a charge transporting polymer is mixed with a transporting small molecule in an inactive binder for a transport layer, xerographic performance is very poor as a result of trapping of carriers in the transport layer. This increases the residual potential, thus lowering the useful contrast potential. Furthermore when such a photoreceptor is cycled in a xerographic machine, a condition known as cycle-up results. The residual potential increases and causes the background area densities to increase thereby creating unacceptable images. Thus, there is a continuing need for electrophotographic imaging members having improved electrical performance and resistance to degradation during extended cycling. It is, therefore, an object of the present invention to provide an improved electrophotographic imaging member which meets this need. This object is accomplished in accordance with this invention by providing an electrophotographic imaging member comprising a charge generating layer and a charge transport layer, said charge transport layer comprising a charge transporting small molecule dissolved or molecularly dispersed in a film forming arylamine polyethercarbonate charge transporting polymer comprising charge transporting moieties in the backbone of said film forming charge transporting polymer, said charge transporting moieties having a structure unlike the structure of said charge transporting small molecule, the ionization potential of said charge transporting molecule and said charge transporting moieties differing by less than about 0.05 electron volt, said charge transporting small molecule and said charge transporting polymer being substantially non-absorbing to radiation in the region of intended use, and said charge transport layer being substantially free of electrically inactive film forming binder. The present invention provides an electrophotographic imaging member which avoids crystallization at high concentrations of small molecule charge transport compounds, and which exhibits improved imaging operation during extended image cycling. The imaging member possesses improved integrity of layers underlying the charge transport layer, and exhibits high charge carrier mobilities. It exhibits greater wearability, hardness and craze resistance with high concentrations of charge transporting moieties in a charge transporting polymer, and can be coated employing a variety of solvents. The imaging member may contain either particle contact or dispersed pigment charge generator layers. Electrostatographic imaging members are well known in the art. Electrostatographic imaging members may be prepared by various suitable techniques. Typically, a flexible or rigid substrate is provided having an electrically conductive surface. A charge generating layer is then applied to the electrically conductive surface. A charge blocking layer may be applied to the electrically conductive surface prior to the application of the charge generating layer. If desired, an adhesive layer may be utilized between the charge blocking layer and the charge generating layer. Usually the charge generation layer is applied onto the blocking layer and a charge transport layer is formed on the charge generation layer. However, in some embodiments, the charge transport layer is applied prior to the charge generation layer. The substrate may be opaque or substantially transparent and may comprise numerous suitable materials having the required mechanical properties. Accordingly, the substrate may comprise a layer of an electrically non-conductive or conductive material such as an inorganic or an organic composition. As electrically non-conducting materials there may be employed various resins known for this purpose including polyesters, polycarbonates, polyamides, polyurethanes, and the like which are flexible as thin webs. The electrically insulating or conductive substrate may be in the form of an endless flexible belt, a web, a rigid cylinder, a sheet and the like. After formation of an electrically conductive surface, a hole blocking layer may be applied thereto for photoreceptors. Generally, electron blocking layers for positively charged photoreceptors allow holes from the imaging surface of the photoreceptor to migrate toward the conductive layer. Any suitable blocking layer capable of forming an electronic barrier to holes between the adjacent photoconductive layer and the underlying conductive layer may be utilized. The blocking layer should be continuous and have a thickness of less than about 0.2 micrometer because greater thicknesses may lead to undesirably high residual voltage. An optional adhesive layer may applied to the hole blocking layer. Any suitable adhesive layer well known in the art may be utilized. Satisfactory results may be achieved with adhesive layer thickness between about (50nm) and about (300nm). Any suitable photogenerating layer may be applied to the adhesive blocking layer which can then be overcoated with a contiguous hole transport layer as described hereinafter. Charge generating binder layers comprising particles or layers comprising a photoconductive material such as vanadyl phthalocyanine, metal free phthalocyanine, benzimidazole perylene, amorphous selenium, trigonal selenium, selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium arsenide, and the like and mixtures thereof are especially preferred because of their sensitivity to white light. Vanadyl phthalocyanine, metal free phthalocyanine and tellurium alloys are also preferred because these materials provide the additional benefit of being sensitive to infra-red light. Any suitable polymeric film forming binder material may be employed as the matrix in the photogenerating binder layer. Typical polymeric film forming materials include those described, for example, in US-A 3,121,006. The photogenerating composition or pigment is present in the resinous binder composition in various amounts, generally, however, from about 5 percent by volume to about 90 percent by volume of the photogenerating pigment is dispersed in about 10 percent by volume to about 95 percent by volume of the resinous binder, and preferably from about 20 percent by volume to about 30 percent by volume of the photogenerating pigment is dispersed in about 70 percent by volume to about 80 percent by volume of the resinous binder composition. In one embodiment about 8 percent by volume of the photogenerating pigment is dispersed in about 92 percent by volume of the resinous binder composition. The photogenerating layer containing photoconductive compositions and/or pigments and the resinous binder material generally ranges in thickness of from about 0.1 micrometer to about 5.0 micrometers, and preferably has a thickness of from about 0.3 micrometer to about 3 micrometers. The photogenerating layer thickness is related to binder content. Higher binder content compositions generally require thicker layers for photogeneration. Thicknesses outside these ranges can be selected providing the objectives of the present invention are achieved. The active charge transport layer of this invention comprises a mixture of at least a charge transporting small molecule dissolved or molecularly dispersed in a film forming arylamine polyethercarbonate charge transporting polymer comprising charge transporting moieties in the backbone of the film forming charge transporting polymer, the charge transporting moieties having a structure unlike the structure of the charge transporting small molecule, and the charge transport layer being substantially free of electrically inactive film forming binder. If the ionization potential (I p ) of the charge transporting small molecule is equal to the ionization potential (I p ) of the charge transporting polymer, the proportion of the charge transporting small molecule is between about 20 percent by weight and about 95 percent by weight based on the total weight of the charge transporting layer with the other substantially making up the remainder. Outside of these ranges, the photoreceptor film forming characteristics may not be suitable from mechanical considerations and/or from the point of view of compatibility where the small molecule might crystallize thereby resulting in high residual potentials. Where the ionization potentials of the charge transporting small molecule is less than the ionization potential of the charge transporting polymer; the charge transporting layer should contain between 30 percent and about 80 percent by weight of the small molecule charge transporting compound based on the total weight of the charge transporting layer because, in this case, the lower limit is set by charge carrier mobility requirements and the higher limit is set by considerations of small molecule crystallization. When the ionization potentials are unequal and if the small molecule is present in an amount less than 30 percent, the charge carrier mobilities are severely restricted. If the ionization potential of the charge transporting small molecule is less by a factor of more than about 2kT -3kT (where k is the Boltzmann Constant and T is the absolute temperature) than the ionization potential of the charge transporting polymer, the concentration of the charge transporting small molecule should be much higher because if this requirement is not met, the charge carrier mobility is considerably reduced as a result of the lower ionization material acting as a trap to charge transport through the higher ionization material. The term "much higher" means that the small molecular concentration is higher than 30 wt per cent and the concentration of the active charge transport moiety of the charge transport polymer is less than 10 wt per cent based on the total weight of the transport layer. Where the ionization potentials of the charge transporting small molecule and charge transporting polymer are unequal, a greater choice of solvents are available that are compatible with both the charge transport polymer and charge transporting small molecule. This in turn presents a greater choice of materials having different mechanical and/or surface properties. If the ionization potential of the charge transporting small molecule is greater than the ionization potential (I p ) of the charge transporting polymer, no enhanced benefit other than greater choice of solvents is observed with regard to using the combination of charge transporting small molecule and charge transporting polymer materials. The expression "Ionization potential (I p )" as employed herein is defined as the energy required to raise an electron from the highest occupied state to a free state outside the material. Ionization potential may be determined by photo-emission, photo-electron spectroscopy, and the like. To determine whether the ionization potentials of the materials for a given combination are substantially equal, one can simply measure charge carrier mobility of one of the materials by the time of flight technique, mix the charge transporting polymer and small molecule charge transporting compound together and then measure the time of flight of the mixture. If the I p of each of the components of the mixture are not substantially equal, a drop in the drift mobility of at least about two or more is detected. The time of flight technique consists of applying a known potential on the layered device with a semi-transparent vacuum deposited metal electrode. The device is then exposed to a light flash. Holes photogenerated in the charge generator layer are injected into the charge transport layer. The current due to the drift of the sheet of holes through the transport layer is time resolved. From the transit time (t T in seconds), the drift mobility (µ in CM 2 /Volt sec) is calculated from the expression (µ = L 2 /t T V), where L is the thickness of the transport layer in centimeters and V is the potential in volts. Any suitable charge transporting or electrically active small molecule may be employed in the charge transport layer of this invention. Typical charge transporting small molecules include, for example, pyrazolines such as 1-phenyl -3 (4'-diethylamino styryl)-5-(4"-diethylamino phenyl) pyrazoline, diamines such as N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine, hydrazones such as N-phenyl- N- methyl -3-(9-ethyl) carbazyl hydrazone and 4, diethyl amino benzaldehyde- 1,2 diphenyl hydrazone and oxadiazoles such as 2,5-bis (4-N,N' diethylaminophenyl )-1,2,4 - oxadiazole, triphenyl methanes such as Bis (4,N,N-diethylamino-2-methyl phenyl)-phenyl methane, stilbenes and the like. These electrically active small molecule charge transporting compounds should dissolve or molecularly disperse in electrically active charge transporting polymeric materials. The expression "charge transporting small molecule" as employed herein is defined as a monomeric chemical molecular species capable of supporting charge transport when dispersed in an electrically inactive organic resinous binder matrix. The expression "electrically active", when used to define the charge transport layer, the electrically active small molecule charge transporting compounds and the electrically active charge transporting polymeric materials, means that the material is capable of supporting the injection of photogenerated holes from the generating material and capable of allowing the transport of these holes through the active transport layer in order to discharge a surface charge on the active layer. The expression "electrically inactive", when used to describe the electrically inactive organic resinous binder material which does not contain any electrically active moiety, means that the binder material is not capable of supporting the injection of photogenerated holes from the generating material and is not capable of allowing the transport of these holes through the material. Still other examples of electrically active small molecule charge transporting compounds include aromatic amine compounds represented by the following general formula: wherein X is selected from the group consisting of an alkyl group containing from 1 to 4 carbon atoms and chlorine. Examples of small molecule charge transporting aromatic amines represented by the structural formula above capable of supporting the injection of photogenerated holes and transporting the holes through the overcoating layer include N,N'-diphenyl-N,N'-bis(alkylphenyl)-(1,1'-biphenyl)-4,4'-diamine wherein the alkyl is, for example, methyl, ethyl, propyl, n-butyl, and the like, N,N'-diphenyl-N,N'-bis(chlorophenyl)-[1,1'-biphenyl]-4,4'-diamine, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine, and the like. The specific aromatic diamine charge transport layer compound illustrated in the formula above is described in US-A 4,265,990. Still other examples of aromatic diamine small molecule charge transport layer compounds include those represented by the general formula: wherein wherein R 1 , R 2 and R 3 are selected from the group consisting of hydrogen, CH 3 , C 2 H 5 , OCH 3 , Cl and alkoxycarbonyl. The preferred materials for these diamines are: N,N,N',N'-tetraphenyl-[3,3'-dimethyl-1,1'-biphenyl]-4,4'-diamine; N,N'-diphenyl-N,N'-bis(2-methylphenyl)-[3,3' dimethyl-1,1'-biphenyl]-4,4'- diamine; N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[3,3'-dimethyl-1,1 '-biphenyl]-4,4'- diamine; N,N'-diphenyl-N,N'-bis(4-methylphenyl)-[3,3'dimethyl-1,1'-biphenyl]-4,4'- diamine; N,N,N',N'-tetra(2-methylphenyl)-[3,3'-dimethyl-1,1'-biphenyl]-4,4'-diamine; N,N'-bis(2-methylphenyl)-N,N'-bis(4-methylphenyl)-[3,3'-dimethyl-1,1'-biphenyl]- 4,4'-diamine; N,N'-bis(3-methylphenyl)-N,N'-bis(2-methylphenyl)-[3,3'-dimethyl-1,1'-biphenyl]- 4,4'-diamine; N,N,N',N'-tetra(3-methylphenyl)-[3,3'-dimethyl-1,1'-biphenyl]-4,4'-diamine; N,N'-bis(3-methylphenyl)-N,N'-bis(4-methylphenyl)-[3,3'-dimethyl-1,1'-biphenyl]-4,4'-diamine; and N,N,N',N'-tetra(4-methylphenyl)-[3,3'-dimethyl-1,1'-biphenyl]-4,4'-diamine. The aromatic diamine small molecule charge transport layer compounds illustrated in the formula above are described in US-A 4,299,897. Additional examples of small molecule charge transporting compounds include: N,N,N′,N′-Tetra-(4-methylphenyl)-[3,3′-dimethyl-1,1′-biphenyl]-4,4′-diamine: N,N′-diphenyl-N,N′-bis(4-methylphenyl)-[3,3′-dimethyl-1,1′-biphenyl]-4,4′- diamine; and N,N'-bis(4-methylphenyl)-N,N'-bis(4-ethylphenyl)-[3,3'-dimethyl-1,1'-biphenyl]-4,4'-diamine: The second of these two specific small molecule aromatic diamine charge transport layer compounds is described in US-A 4,299,897. The substituents on both the first and second types of aromatic diamine molecules should be free from electron withdrawing groups such as NO 2 groups, CN groups, and the like. The electrically active charge transporting polymeric materials should be capable of supporting the injection of photogenerated holes from the charge generation material and capable of allowing the transport of these holes therethrough. The expression "charge transporting moieties" of the film forming charge transporting polymer as employed herein is defined as one of the "active" units or segments that support charge transport. The charge transporting moiety of the film forming charge transporting polymer is considered to have a structure "unlike" the structure of the charge transporting small molecule when the basic or core structural units that transport charge are dissimilar. Minor differences such as the presence of substantially inactive groups such as methyl, ethyl, propyl, isopropyl, and butyl groups present on the basic structural unit of the charge transporting moiety of the polymer or the small molecule charge transporting compound and not on the other, do not bring otherwise identical structures within the definition of "unlike". The presence of other groups on either the charge transporting moiety of the polymer or on the small molecule charge transporting compound, but not on the other, which significantly affect the electrical properties of the polymer or small molecule, such as electron withdrawing groups, alkoxy groups, and the like, are included within the definition of unlike chemical structures. Electrical properties that are considered significantly affected include, for example, charge carrier mobilities, trapping characteristics, color and the like. Typical electron withdrawing groups include nitro groups, cyano groups, alkoxy, and the like. Examples of polyethercarbonate charge transporting polymers including arylamine compounds are described in US-A-4,806,443. Further examples are : where n represents a number sufficient to achieve a weight average molecular weight of between about 20,000 and about 500,000. As described above, the active charge transport layer of this invention comprises a mixture of at least a charge transporting small molecule dissolved or molecularly dispersed in a film forming arylamine polyethercarbonate charge transporting polymer comprising charge transporting moieties in the backbone of the film forming charge transporting polymer, the charge transporting moieties having a structure unlike the structure of the charge transporting small molecule, and the charge transport layer being substantially free of electrically inactive film forming binder. The charge transport polymer in the charge transporting layer of this invention should contain charge transporting moieties having a structure unlike the structure of the small molecule charge transport compound dissolved or molecularly dispersed in the charge transport polymer. Significant differences in the core structures themselves render the structure of the charge transporting moiety of the film forming charge transporting polymer "unlike" the structure of the charge transporting small molecule. The combination of charge transport polymer and small molecule charge transport compound in the charge transport layer of this invention should be capable of supporting the injection of photogenerated holes from the generation material and capable of allowing the transport of these holes through the active layer in order to discharge the surface charge on the active layer. The charge transport polymer and the charge transport small molecule should also be miscible in each other. The expression "miscible" is defined as a mixture which forms a solution or molecular dispersion of the small molecule transport compound in the charge transport polymer. Examples of typical combinations of unlike charge arylamine polyethercarbonate transporting polymer and charge transporting small molecule include, polyethercarbonate obtained from the condensation of N,N'-diphenyl-N,N'-bis(3-hydroxy phenyl)-[1,1'-biphenyl]-4,4'-diamine and diethylene glycol bischloroformate and small molecule 1,1-bis-(4-(di-N,N'-methylphenyl)-aminophenyl)cyclohexane; or polyethercarbonate obtained from the condensation of 1,1-bis-(4-(di-N,N'-hydroxy phenyl)-aminophenyl)cyclohexane and diethylene glycol bischloroformate and small molecule, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine; and the like. These combinations involve a charge transporting small molecule having an ionization potential substantially equal to or less than the ionization potential of the charge transporting polymer or combinations. For example, where a pyrazoline small molecule having an I p less than the I p of an aryl diamine small charge transporting moiety of a charge transporting polymer is mixed with a charge transporting polymer containing a high (greater than 10 weight per cent) concentration of diamines, the resulting layer will not perform properly as a charge transport layer because mobility is reduced due to charge trapping. However, a layer formed from a mixture of pyrazoline small molecule and charge transporting polymer containing a relatively small quantity (less than 10 weight per cent) of aryl diamine small charge transporting moiety gives satisfactory charge mobility along with the added advantage of solubility in other solvents, better mechanical property, improved resistance to wear, enhanced bending characteristics without cracking properties and improved surface properties. A charge transporting small molecule is deemed to have an ionization potential "substantially equal" to the ionization potential of the polymer when the difference in ionization potential value is less than about 0.05 electron volt. The concentration of the combined mixture of the charge transporting small molecule and charge transporting polymer in the charge transport layer relative to any other components in the layer should be at least about 90 per cent because any anti oxidants or plasticizers that may be present in a concentration higher than about 10 percent by weight would not contribute to charge transport and would lower the charge carrier mobilities when present in concentrations greater than about 10 percent. The charge transport layer should be substantially free of any electrically inactive film forming resin binder material. The presence of an electrically inactive film forming resin binder material will cause the photoreceptor to have lower mobilities, and might even result in phase separation and this will result in unacceptably high residual potentials. The expression "substantially free" as employed herein is defined as a presence of less than about 5 per cent. Any suitable and conventional technique may be utilized to mix and thereafter apply the charge transport layer coating mixture to the charge generating layer. Typical application techniques include spraying, dip coating, roll coating, wire wound rod coating, and the like. Drying of the deposited coating may be effected by any suitable conventional technique such as oven drying, infra red radiation drying, air drying and the like. Generally, the thickness of the hole transport layer is between about 10 to about 50 micrometers, but thicknesses outside this range can also be used. The hole transport layer should be an insulator to the extent that the electrostatic charge placed on the hole transport layer is not conducted in the absence of illumination at a rate sufficient to prevent formation and retention of an electrostatic latent image thereon. In general, the ratio of the thickness of the hole transport layer to the charge generator layer is preferably maintained from about 2:1 to 200:1 and in some instances as great as 400:1. In other words, the charge transport layer, is substantially non-absorbing to visible light or radiation in the region of intended use but is "active" in that it allows the injection of photogenerated holes from the photoconductive layer, i.e., charge generation layer, and allows these holes to be transported through the active charge transport layer to selectively discharge a surface charge on the surface of the active layer. The photoreceptors of this invention may comprise, for example, a charge generator layer sandwiched between a conductive surface and a charge transport layer as described above or a charge transport layer sandwiched between a conductive surface and a charge generator layer. This structure may be imaged in the conventional xerographic manner which usually includes charging, optical exposure and development. The transport layers of this invention exhibit numerous advantages; in the embodiment where the ionization potential I p of the small molecule is substantially equal to the I p of the charge transporting polymer, charge carrier mobilities are increased beyond that which can be achieved with either charge transporting polymer layers or with layers containing charge transporting small molecules in an electrically inactive transport binder. In embodiment where the ionization potentials are unequal, by using small amounts of the active charge transporting moiety in a polymer molecule, the concept of this invention enables one to use larger amounts of binder material in the same molecule to expand the choice of physical properties, e.g, greater flexibility for use in flexible belts. The transport layers of this invention also overcome the tendency of charge transporting small molecules to crystallize at high concentrations. For, multi active layer photoreceptors employing diamine charge transporting small molecules in inactive polycarbonate binders, at the concentration level of small molecule required to provide adequate transport properties, the system can be thermodynamically unstable during some coating processes. This has been observed with the dip coating process where the maximum concentration of diamine charge transporting small molecules, before the onset of crystallization, is less than 35 percent by weight in the final dried charge transport layer. This concentration is too low to assure charge transport across a 25 micrometer thick film in time periods shorter than the time interval between exposure and development. The charge transporting polymers employed in the charge transport layer of this invention provide a good dispersing medium for the charge transporting small molecule, and exhibit better mechanical properties than conventional electrically inactive film forming binders such as polycarbonates. A shortcoming of many charge transporting small molecules is that there are very few inactive binders in which charge transporting small molecules disperse at high concentrations without crystallization. Polycarbonate is one of the few binders in which charge transporting small molecules disperse to form stable solid solutions. Polycarbonate is soluble in a very limited set of solvents. Methylene chloride is invariably employed to fabricate the current small molecule transport layers. The presence of a relatively small percent of moieties of charge transporting segments (units) in the backbone of the charge transport polymer (e.g., less than about 10 percent by weight based on the total weight of the polymer) enables the use of a variety of different solvents other than the conventional polycarbonate film forming binder to be used to apply the charge transporting layer. The use of different solvents is important to the providing of flexibility in selection of coating techniques such as wire wound rod coating which requires dilute concentrations and dip coating which utilizes high concentrations of film forming binder in the coating solution. Also, since a greater selection of solvents are available, undesirable solvents such as toxic solvents can be avoided. Further, crystallization of small molecule charge transport material can be avoided even when high concentrations of small molecules are utilized. A problem encountered with employing transport layers of charge transporting polymers is the restriction it imposes on the design of the generator layer. Since the transport polymer does not penetrate the charge generation layer, particle contact type generator materials are preferred. The photogenerated charge from the pigment moves from one pigment particle to the next till it is injected into the polymeric transport layer. The combination transport layer of this invention does not so restrict the generator layer geometry. Dispersed pigment generators can readily be employed with the transport layer of this invention. The charge transporting small molecules from the transport layers of this invention penetrate the generator layers and facilitate charge injection from the pigment. A number of examples are set forth hereinbelow and are illustrative of different compositions and conditions that can be utilized in practicing the invention. All proportions are by weight unless otherwise indicated. It will be apparent, however, that the invention can be practiced with many types of compositions and can have many different uses in accordance with the disclosure above and as pointed out hereinafter. EXAMPLE I An aluminum plate bearing a vacuum deposited 0.5 micrometer thick layer of amorphous selenium was coated with a solution containing one gram of polyethercarbonate resin and 0.667 gram of bis(4-diethylamino-2-methylphenyl)-phenylmethane dissolved in 11.5 grams of methylene chloride solvent using a Bird coating applicator. The polyethercarbonate resin was prepared as described in Example III of US-A 4,806,443. This polyethercarbonate resin is an electrically active charge transporting film forming binder and can be represented by the formula: wherein n is about 300 in the above formula so that the molecular weight of the polymer is about 200,000. The coated device was dried at 35°C under vacuum for 12 hours to form a 20 micrometer thick charge transport layer containing 40% by weight of bis(4-diethylamino-2-methylphenyl)-phenylmethane. A vacuum chamber was employed to deposit a semitransparent gold electrode on top of the device. The resulting sandwich device was connected to an electrical circuit containing a power supply and a current measuring resistance. The transit time of the charge carriers was determined by the time of flight technique. This was accomplished by biassing the gold electrode negative and exposing the device to a brief flash of light. Holes photogenerated in amorphous selenium were injected into and transited through the transport layer. The current due to the transit of a sheet of holes was time resolved and displayed on an oscilloscope. The current pulse displayed on the oscilloscope comprised a curve having flat segment followed by a rapid decrease. The flat segment was due to the transit of the sheet of holes through the transport layer. The rapid drop of current signaled the arrival of the holes at the gold electrode. From the transit time, the velocity of the carriers was calculated by the relationship: velocity = transport layer thickness ÷ transit time The hole mobility is related to the velocity by the relationship: velocity = (mobility) × (electric field) The mobility of this dispersion of polyethercarbonate and bis(4-diethylamino-2-methylphenyl)-phenylmethane in the transport layer was determined to be 3 × 10 -4 cm 2 /volt sec at an applied electric field of 2 × 10 5 V/cm. This mobility value suggests very good charge transport. EXAMPLE II An aluminum plate bearing a vacuum deposited 0.5 micrometer layer of amorphous selenium was coated with a solution containing one gram of polyethercarbonate (identical to the polyethercarbonate described in Example I) and 0.667 gram of p-diethylamino-benzaldehyde-diphenyl hydrazone dissolved in 11.5 grams of methylene chloride using a 4 mil Bird coating applicator. The coated device was dried at 24°C under vacuum for 12 hours to form a 20 micrometer thick charge transport layer of polyethercarbonate mixed with 40 percent by weight of p-diethylamino-benzaldehyde-diphenyl hydrazone. A vacuum chamber was employed to deposit a semitransparent gold electrode on top of the device. The resulting sandwich device was connected in an electrical circuit containing a power supply and a current measuring resistance. The transit time of the charge carriers was determined by the time of flight technique. The mobility of this polyethercarbonate and p-diethylamino-benzaldehydediphenyl hydrazone transport layer was determined to be 3 × 10 -6 cm 2 /volt sec at an applied electric field of 2 × 10 5 V/cm. The mobility value suggests that polyether carbonate acts essentially as an inert binder for p-diethylamino-benzaldehyde-diphenyl hydrazone. EXAMPLE III A photoreceptor was prepared by forming coatings using conventional techniques on a substrate comprising a vacuum deposited titanium layer on a polyethylene terephthalate film (Melinex®, available from E.I. duPont de Nemours & Co.). The first deposited coating was a siloxane barrier layer formed from hydrolyzed gamma aminopropyltriethoxysilane having a thickness of 10nm. The second coating was an adhesive layer of polyester resin (49,000, available from E.I. duPont de Nemours & Co.) having a thickness of 5nm. The next coating was a charge generator layer containing 35 percent by weight vanadyl phthalocyanine particles dispersed in a polyester resin (Vitel® PE100, available from Goodyear Tire and Rubber Co.) having a thickness of 1 micrometer. The last coating was a charge transport layer consisting of a 20 micron thick layer of polyethercarbonate (identical to the polyethercarbonate described in Example I) mixed with 40 percent by weight bis(4-diethylamino-2-methylphenyl)-phenylmethane fabricated by the procedure indicated in Example I. The resulting device was heated in a vacuum oven maintained at 80°C. Sensitivity measurements were performed in a scanner. The photoreceptor device was mounted on a cylindrical aluminum drum which was rotated on a shaft. The film was charged by a corotron mounted along the perimeter of the drum. The surface potential of the photoreceptor was measured as a function of time by several capacitively coupled probes placed at different locations around the perimeter of the drum. The probes were calibrated by applying known potentials to the drum substrate. The photoreceptor film on the drum was exposed and erased by light sources located at appropriate positions around the periphery of the drum. The measurement involved charging the photoconductor device in a constant current or voltage mode. As the drum rotated, the initial charging potential was measured by probe 1. Further rotation lead to the exposure station, where the photoconductor device was exposed to monochromatic radiation of a known intensity. The surface potential after exposure was measured by probes 2 and 3. The device was finally exposed to an erase lamp of appropriate intensity and any residual potential was measured by probe 4. The process was repeated with the magnitude of the exposure automatically changed during the next cycle. A photo induced discharge characteristics (PIDC) curve was obtained by plotting the potentials at probes 2 and 3 as a function of exposure. The device was charged to a negative polarity by corotron charging and discharged by monochromatic light in the visible and in the IR portion of the light spectrum. The device initially charged to 850 volts could be discharged to less than 150 Volts when exposed to 775 nm wavelength light with a light energy of 10 ergs/cm 2 . EXAMPLE IV A photoreceptor was prepared by forming coatings using conventional techniques on a substrate comprising a vacuum deposited titanium layer on a polyethylene terephthalate film (Melinex®, available from E. I. duPont de Nemours & Co.). The first coating was a siloxane barrier layer formed from hydrolyzed gamma aminopropyltriethoxysilane having a thickness of 10nm. The second coating was an adhesive layer of polyester resin (49,000, available from E. I. duPont de Nemours & Co.) having a thickness of 5nm. The next coating was a charge generator layer containing 35 percent by weight vanadyl phthalocyanine particles dispersed in a polyester resin (Vitel® PE100, available from Goodyear Tire and Rubber Co.) having a thickness of 1 micrometer. The charge transport layer consisted of a 20 micrometer thick film of polyethercarbonate described in Example I) mixed with 40 percent by weight of p-diethylamino-benzaldehyde-diphenyl hydrazone fabricated by the procedure indicated in Example III. The device was heated in a vacuum oven maintained at 30°C. Sensitivity measurements were performed in a scanner described in Example IV. The photoreceptor device, initially charged to 850 volts by a negative polarity corotron, was discharged to less than 150 volts when exposed to 775 nm wavelength light with a light energy of 10 ergs/cm 2 .

Claims: An electrophotographic imaging member comprising a charge generating layer and a charge transport layer, said charge transport layer comprising a charge transporting small molecule dissolved or molecularly dispersed in a film forming arylamine polyethercarbonate charge transporting polymer comprising charge transporting moieties in the backbone of said film forming charge transporting polymer, said charge transporting moieties having a structure unlike the structure of said charge transporting small molecule, the ionization potential of said charge transporting molecule and said charge transporting moieties differing by less than about 0.05 electron volt, said charge transporting small molecule and said charge transporting polymer being substantially non-absorbing to radiation in the region of intended use, and said charge transport layer being substantially free of electrically inactive film forming binder. An electrophotographic imaging member according to Claim 1 wherein the ionization potential of said charge transporting small molecule is substantially equal to or less that the ionization potential of said charge transporting moieties. An electrophotographic imaging member according to Claim 1 wherein the ionization potential of said charge transporting small molecule is substantially equal to the ionization potential of said charge transporting moieties and the proportion of said charge transporting small molecule is between about 20 percent by weight and about 95 percent by weight based on the total weight of said charge transporting layer with the other substantially making up the remainder. An electrophotographic imaging member according to Claim 1 wherein the ionization potential of said charge transporting small molecule is less than the ionization potential of said charge transporting moieties and the proportion of said charge transporting small molecu!e is between about 80 percent by weight and about 30 percent by weight based on the total weight of said charge transporting layer. An electrophotographic imaging member according to any one of Claims 1 to 4 wherein said film forming charge transporting polymer comprises less than about 10 percent by weight of said charge transporting moieties based on the total weight of said charge transporting polymer. An electrophotographic imaging member according to any one of Claims 1 to 5 wherein said charge transporting small molecule is represented by the general formula: wherein wherein R 1 , R 2 and R 3 are selected from the group consisting of hydrogen, CH 3 , C 2 H 5 , OCH 3 , Cl and alkoxycarbonyl. An imaging process comprising providing an electrophotographic imaging member comprising a charge generating layer and a charge transport layer, said charge transport layer comprising a charge transporting small molecule dissolved or molecularly dispersed in a film forming arylamine polyethercarbonate charge transporting polymer comprising charge transporting moieties in the backbone of said film forming charge transporting polymer, and said charge transporting moieties having a structure unlike the structure of said charge transporting small molecule, the ionization potential of said charge transporting molecule and said charge transporting moieties differing by less than about 0.05 electron volt, said charge transporting small molecule and said charge transporting polymer being substantially non-absorbing to radiation in the region of intended use, and said charge transport layer being substantially free of electrically inactive film forming binder, depositing a uniform electrostatic charge on said imaging member with a corona charging device, exposing said imaging member to activating radiation in image configuration to form an electrostatic latent image on said imaging member, developing said electrostatic latent image with electrostatically attractable marking particles to form a toner image, transferring said toner image to a receiving member and repeating said depositing, exposing, developing and transfering steps.

Inventors: LIMBURG WILLIAM W, PAI DAMODAR M, RENFER DALE S, YANUS JOHN F, LIMBURG, WILLIAM W., PAI, DAMODAR M., RENFER, DALE S., YANUS, JOHN F.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1992
Main classification: G03G5
IPC codes: G03G5