Dual layer switch photoreceptor for digital imaging

Abstract: An electrophotographic imaging member including a charge generating layer (12) and a heterogeneous charge transport layer (14), the charge transport layer having regions of charge transport and inactive regions, with the charge transporting regions being in contact with each other. This imaging member has characteristics that enable high quality and high contrast imaging.

Description: This invention relates to digital electrophtographic imaging members that enable high quality and high contrast imaging. Electrophotographic photoreceptors typically include a photoconductive layer formed on a conductive substrate. The photoconductive layer is a good insulator in the dark so that electric charges can be retained on its surface. But upon exposure to light the charge is dissipated. A latent image is formed on the photoreceptor by first uniformly depositing electric charges over the surface of the photoconductive layer by a conventional means. The photoconductive layer acts as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity (the counter charge) on the conductive substrate. A light image is then projected onto the photoconductive layer. On those portions of the photoconductive layer that are exposed to light, the electric charge is conducted through the layer reducing the surface charge. The portions of the photoconductive surface not exposed to light retain their surface charge. The quantity of electric charge at any particular area of the photoconductive surface is inversely related to the illumination incident thereon, thus forming a latent electrostatic image. The photodischarge of the photoconductive layer requires the layer to photogenerate conductive charge and to transport this charge through the layer thereby neutralizing the charge on the surface Two types of photoreceptor structures have been employed: Multilayer structures wherein separate layers perform the functions of charge generation and charge transport, respectively, and single layer photoconductors which perform both functions. These layers are laminated onto a conducting substrate and may include an optional charge blocking and an adhesive layer between the conducting and the photoconducting layers. Additionally, they may contain protective overcoatings and the substrate may consist of a non-conducting mechanical support with a conductive layer. Other layers to provide special functions such as incoherent reflection of laser light, dot patterns for pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface may be employed. One common type of photoreceptor is a multilayered device that comprises a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer can contain an active aromatic diamine molecule, which enables charge transport, dissolved or molecularly dispersed in a film forming binder This type of charge transport layer is described, for example in U S. Patent No. 4,265,990. Other charge transport molecules disclosed in the prior art include a variety of electron donor, aromatic amines, oxadiazoles, oxazoles, hydrazones and stilbenes for hole transport and electron acceptor molecules for electron transport. Other charge transport layers have been developed that employ a charge transporting polymer wherein the charge transporting moiety is incorporated in the polymer as a pendant or in the chain or may form the backbone of the polymer. This type of charge transport polymer includes materials such as poly ( N-vinylcarbazole), polysilylenes, and others including those described in U. S. Patents 4,618,551, 4,806,443, 4,806,444, 4,818,650, 4,935,487, and 4,956,440. UK-A-1, 488, 266 describes an electrophotographic imaging member comprising a charge transport layer comprising an electrically active material distributed non-uniformly in a resin matrix. Charge generator layers employed include amorphous films of selenium and alloys of selenium and arsenic, tellurium, germanium and the like, hydrogenetated amorphous silicon and compounds of silicon and germanium, carbon, oxygen, nitrogen and the like, fabricated by vacuum evaporation or deposition, inorganic pigments of crystalline selenium and its alloys, III-V and II-VI compounds and organic pigments such as quinacridones, polycyclic pigments such as dibromo anthanthrone pigments, perylene and perinone diamines, polynuclear aromatic quinones, azo pigments including bis-, tris- and tetrakis-azos, and the like dispersed in a film forming polymeric binder and fabricated by solvent coating. Phthalocyanines have been employed as photogenerating materials for use in laser printers with infrared exposures. Infra red sensitivity is required for low cost semiconductor laser diodes used as the light exposure source. The absorption spectrum and photosensitivity depend on the central metal atom. Many metal phthalocyanines have been reported and include, oxyvanadium phthalocyanine, chloroaluminum phthalocyanine, copper phthalocyanine, oxytitanium phthalocyanine, chlorogallium phthalocyanine, magnesium phthalocyanine and metal-free phthalocyanine. The phthalocyanines exist in many crystal forms which have a strong influence on photogeneration. Single layer photoreceptors commonly employed include photoconducting layers laminated onto a conducting substrate and may also include an optional charge blocking and/or an adhesive layer between the conducting and the photoconducting layers. The photoconducting layer materials include amorphous selenium and alloys of selenium and arsenic, tellurium, germanium and the like, hydrogenetated amorphous silicon and compounds of silicon and germanium, carbon, oxygen nitrogen and the like fabricated by vacuum evaporation or deposition, inorganic pigments of crystalline selenium and its alloys, II-VI crystals such as ZnO, CdS, III-V pigments and the like and organic pigments such as quinacridones, polycyclic pigments such as dibromo anthanthrone pigments, perylene and perinone diamines, polynuclear aromatic quinones, azo pigments including bis-, tris- and tetrakis-azos, metal phthalocyanines and the like dispersed in a film forming polymeric binder fabricated by solvent coating. Other organic photoconductor materials are electron donor and acceptor charge transfer systems such as polyvinyl-carbazole (PVK), 2,4,7- trinitro-9-fluorenone (TNF) and the like. The pure pigment photoconducting layers, such as amorphous selenium and silicon, both photogenerate and transport a charge. But when the pigment is in a binder layer, charge transport may take place entirely within the pigment while the binder is substantially insulating, as for example in the ZnO photoreceptor. Alternatively, charge transport may occur in a binder which is either (a) an insulating polymer doped with (i) an electron donor or (ii) acceptor molecules or (b) a charge transporting polymer as described above. Charge generation controls the discharge (both photo and dark) of all the dual layer and nearly all the single layer photoreceptors. Restated, the amount of charge neutralized, as measured by the voltage across the photoconducting layers, is proportional to the light exposure (e g , ergs/cm 2 ). The photodischarge curve is linear with a negative slope from the maximum (dark or zero exposure) voltage to the minimum voltage. The minimum voltage is referred to as the residual voltage. Light exposure beyond that required to reach the residual voltage does not produce any further discharge. In such photogeneration limited discharge, the ideal discharge is a linear discharge down to zero (residual) voltage with the slope being a measure of the photosensitivity. However, because the photogeneration rate in practical materials is electric field dependent, and decreasing with field, the discharge slope decreases and the discharge curve at low voltages increasingly departs from the linear discharge, requiring increasingly more light exposure to the same voltage discharge, as shown in Figure 1. Because dark discharge, which is undesirable, also is generation limited, albeit thermal generation limited, dark discharge has the same electric field dependence, being high at high voltages (electric fields) and low at low voltages (electric fields). Generation limited discharge is undesirable because it contributes to undesirable image quality variation through variations in electricals, that is, the voltages on the photoreceptor. Highest image quality in a xerographic system requires the voltages corresponding to the same image density or white background be constant, both spatially across the entire copy or print and temporally (or cyclically) from print to print. The generation limited discharge contributes to electrical variation in two ways. First, small variations at low light exposure result in large variations in the high (dark) voltage. Secondly, small variations in thermal generation also cause variation in the high (dark) voltage. The previous solutions have been to improve the materials and coating technologies to reduce the electrical variation of photoreceptors and improve the optics and electrical controls in the xerographic imaging machines. Digital imaging provides an improvement in image quality. Digital systems have been used where gray or tone scales are produced by area coverage at constant local image density. Thus it is desirable to have a discharge curve (both photo and dark discharge if possible) that appears as a switch, with negligible voltage discharge until a critical exposure is reached, followed by complete discharge to residual voltage. This type of discharge is called S shaped hereinafter, as shown in Figure 2. Such a binary discharge curve permits variation in both the off (or dark) and on (or fully exposed) light exposure with negligible voltage variation. Additionally, dark charge generation does not cause a dark voltage variation contributing to stable electricals. One approach is to fabricate a single-layer, heterogeneous, particle-contact device in which photoconductor pigments are dispersed in insulating binders. The concentration of the charge generating and transporting pigment particles is high enough to maintain particle contact and thus a conducting path through the layer. The key to an S shaped photodischarge curve is a heterogeneous structure which provides a connected but convoluted path for charge transport or conduction. At high electric fields, after the sample is charged, any charge generated at the surface is directed in a straight line through the layer, encounters a barrier in the insulating region and hence causes negligible voltage discharge. After nearly all the surface charge is injected, the local electric field normal to the surface is negligible and the remaining charge is able to move in other directions and follow the connected path to a depth below where the initial charge was stopped. At this deeper level the charge again sees the full electric field and encounters the insulating barrier But because the motion of the previous charge reduced the electric field in the first level, more charge follows the convoluted path down to the next level. Thus by such a cascade total discharge occurs after a light exposure corresponding to the generation of enough charge required for total discharge, resulting in a step-like or S shaped discharge curve. By a similar argument, the dark discharge also has an S shaped time dependence, enabling very stable dark potentials. The earliest such device with an S shaped photodischarge curve is the single layer ZnO electrophotographic layer. Another single layer device with S shaped photodischarge is described by J. W. Weigl et al. in "Current Problems in Electrophotography", pages 286-300, edited by W. F. Berg and K. Hauffe and published by Walter de Gruyter, Berlin in 1972. The layers consist of microcrystalline dispersions of X-metal free phthalocyanine in suitable binders. The X-metal free phthalocyanine, which are observed as needle like crystals, provides both the photogeneration and the hole transport in this device. Another single layer particle contact device is discussed in articles "An aggregate Organic Photoconductor Part 1 and 2" by Dullmage et al. and Borsenberger et al. and is published in the Journal of Applied Physics, Vol 4, pages 5555-5564, 1978. The device described is a two phase aggregate photoconductor containing a co-crystalline phase of a thiopyrylium dye and a polycarbonate polymer in an amorphous phase of a triphenylmethane derivative in polycarbonate. An S shaped discharge shape is observed when the device is charged negatively and discharged by highly absorbed light. When charged positively, the normal generation limited discharge is observed. The photogeneration is attributed to the thiopyrylium and the discharge proceeds by hole transport through the amorphous phase of the triphenylmethane hole transport molecules in polycarbonate. When charged negatively, the oischarge proceeds by electron transport through the co-crystalline phase, which form a dendritic network. In the prior art, the S shaped discharge is observed in single layer devices which suffer from inflexibility in design. The same material, a pigment, is employed to photogenerate and transport the charge It is an object of the present invention to provide an improved digital electrophotographic imaging member. This object is achieved by a digital electrophotographic imaging member comprising an electrically-conductive substrate (10); a charge generating layer (12); and a charge transport layer (14); said charge transport layer comprising an organic block copolymer consisting of charge transporting blocks that are separated by electrically inactive blocks, said organic block copolymer forming a convoluted charge transport path, characterized in that said charge transporting blocks are selected from the group consisting of polyaryl amines and polysilylenes and said electrically inactive blocks are selected from the group consisting of polymethyl methacrylates, polycarbonates and polystyrene. The device may include optional charge blocking, adhesive and subbing layers. In an imaging member in accordance with the invention, charge generation is separated from charge transport by employing two distinct materials for those purposes. The charge generator layer may have a thickness of between about 0.05 micrometer and about 5 micrometers. The charge transport layer may have a thickness of between 5 micrometers and about 50 micrometers. The charge generating pigment in the charge generating layer may be dispersed in a resinous binder in an amount of between about 5 percent by weight and about 95 percent by weight based on the total weight of said charge generating layer. In an imaging member in accordance with the invention, the charge layer may be vacuum deposited. The substrate may be comprised of a drum. Alternatively, the substrate may be a flexible belt in which case it may have a transparent conductive coating. The substrate may be transparent. By way of example only, embodiments of the invention will be described with reference to the accompanying drawings, in which: Figure 1 shows the relationship between voltage and energy during generation limited photodischarge; Figure 2 is a binary discharge curve; Figure 3 illustrates an electrophotographic imaging member according to the present invention; Figure 4 illustrates an electrophotographic imaging member according to the present invention employing an adhesive and a barrier layer; and Figure 5 illustrates an electrophotographic imaging member according to the present invention employing an inverted structure. Electrophotographic imaging members are well known in the art. Electrophotographic 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. This structure may have the charge generation layer on top or below the charge transport 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. Various resins, including polyesters, polycarbonates, polyamides, polyurethanes, and the like which are flexible as thin webs, may be employed as electrically nonconducting materials. Any metal, for example, aluminum, nickel, steel, copper, and the like or a polymeric material described above, filled with a conducting substance, sucn as carbon, metallic powder, and the like or an organic conducting material may be used as electrically conducting substrate. The electrically insulating, or conductive, substrate may be in the form of an endless flexible belt, a web, a rigid cylinder, a drum, a sheet and the like. The thickness of the substrate layer depends on numerous factors, including strength desired and economical considerations. Thus, a drum layer may be from less than a millimeter to centimeters in thickness. Similarly, a flexible belt may be less than 50 micrometers to about 250 micrometers, provided there are no adverse effects on the final electrophotographic device. The substrate layers surface is preferably cleaned prior to coating to promote greater adhesion of the deposited coating. Cleaning may be effected, for example, by exposing the substrate layer surface to plasma discharge, ion bombardment, solvents, etchents and the like. If a non-conductive substrate layer is used, one must also use a separate electrically conductive layer. The conductive layer may vary in thickness over substantially wide ranges depending on the optical transparency, degree of flexibility desired for the member and economic tactors. Accordingly, for a flexible photoresponsive imaging device, the thickness of the conductive layer may be between about 0,002 µm (20 angstroms) to about 0,075 µm (750 angstroms), and more preferably from about 0,01 µm (100 angstroms) to about (0,02 µm) 200 angstroms for an optimum combination of electrical conouctivity, flexibility and light transmission. The flexible conductive layer may be an electrically conductive metal layer formed, for example, on the substrate by any suitable coating technique, such as a vacuum depositing technique or electrodeposition. Typical metals include aluminum, zirconium, niobium, tantalum, vanadium and hafnium, titanium, nickel, stainless steel, chromium, tungsten, molybdenum, and the like. In general, a continuous metal film can be attained on a suitable substrate, e.g. a polyester web substrate such as Melinex available from E.I du Pont de Nemours & Co. with magnetron sputtering. If desired, an alloy of suitable metals may be deposited. Typical metal alloys may contain two or more metals such as zirconium, niobium, tantalum, vanadium, hafnium, titanium, nickel, stainless steel, chromium, tungsten, molybdenum, and the like, and mixtures thereof. A typical electrical conductivity for conductive layers for electrophotographic imaging members in slow speed copiers is about 102 to 103 ohms/square. 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 U.S. Patent 3,121,006. Thus, typical organic polymeric film forming binders include thermoplastic and thermosetting resins such as polycarbonates, polyesters, polyamides, polyurethanes, polystyrenes, polyarylethers, polyarylsulfones, polybutadienes, polysulfones, polyethersulfones, polyethylenes, polypropylenes, polyimides, polymethylpentenes, polyphenylene sulfides, polyvinyl acetate, polysiloxanes, polyacrylates, polyvinyl acetals, polyamides, polyimides, amino resins, phenylene oxide resins, terephthalic acid resins, phenoxy resins, epoxy resins, phenolic resins, polystyrene and acrylonitrile copolymers, polyvinylchloride, vinylchloride and vinyl acetate copolymers, acrylate copolymers, alkyd resins, cellulosic film formers, poly(amideimide), styrenebutadiene copolymers, vinylidenechloride-vinylchloride copolymers, vinylacetatevinylidenechloride copolymers, styrenealkyd resins, polyvinylcarbazole, and the like. These polymers may be block, random or alternating copolymers. The photogenerating composition or pigment is present in the resinous binder composition in various amounts, generally, however, from about 5 percent to about 90 percent, by volume, of the photogenerating pigment is dispersed in about 10 percent to about 95 percent, by volume, of the resinous binder. Preferably, from about 20 percent to about 30 percent, by volume, of the photogenerating pigment is dispersed in about 70 percent 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 photogenerator layers can also be fabricated by vacuum sublimation in which case there is no binder. Any suitable and conventional technique may be utilized to mix and thereafter apply the photogenerating layer coating mixture. Typical application techniques include spraying, dip coating, roll coating, wire wound rod coating, vacuum sublimation and the like For some applications, the generator layer has to be fabricated in a dot or line pattern. Solvent removal for a solvent coated layer 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 heterogeneous charge 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 layers 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, the charge transport layer is "active" in that it allows the injection of photogenerated holes from the photoconductive layer, i.e., charge generation layer. The charge transport layer also allows the holes to be transported through to selectively discharge any active layer surface charge. Other layers may also be used such as conventional electrically conductive ground strip along one edge of the belt or drum in contact with the conductive layer to facilitate connection of the electrically conductive layer of the photoreceptor to ground or to an electrical bias, blocking layer, adhesive layer. Ground strips are well known and usually comprise conductive particles dispersed in a film forming binder. Optionally, an overcoat layer may also be utilized to improve resistance to abrasion In some cases an anti-curl back coating may be applied to the side opposite the photoreceptor to provide flatness and/or abrasion resistance. These overcoating and anti-curl back coating layers are conventional and may comprise thermoplastic organic polymers or inorganic polymers that are electrically insulating or slightly semiconducting. Overcoatings are continuous and generally have a thickness of less than about 10 micrometers. Figure 3 schematically illustrates an electrophotographic photoreceptor 1 that includes a conductive substrate 10, a charge generator layer 12 that contacts the substrate 10, and a heterogeneous charge transport layer 14 with a structure in which charge transporting regions are intermixed with electrically inactive regions and the charge transporting regions are in contact with each other (hereinafter called charge transporting particle contact type transport layer). Figure 4 schematically illustrates an electrophotographic photoreceptor 1 that includes a conductive substrate 10, a barrier layer 16, an adhesive layer 18, a charge generator layer 12 that contacts the adhesive layer 18, and a heterogeneous charge transporting particle contact type transport layer 14. Figure 5 schematically illustrates an electrophotographic photoreceptor 1 that includes a conductive substrate 10, an adhesive layer 18, a heterogeneous charge transporting particle contact type transport layer 14 and a charge generator layer 12.

Claims: A digital electrophotographic imaging member comprising: an electrically-conductive substrate (10); a charge generating layer (12); and a charge transport layer (14); said charge transport layer comprising an organic block copolymer consisting of charge transporting blocks that are separated by electrically inactive blocks, said organic block copolymer forming a convoluted charge transport path, characterized in that said charge transporting blocks are selected from the group consisting of polyaryl amines and polysilylenes and said electrically inactive blocks are selected from the group consisting of polymethyl methacrylates, polycarbonates and polystyrene. A digital electrophotographic imaging member according to Claim 1, wherein the charge generation layer is interposed between the substrate and the charge transport layer. A digital electrophotographic imaging member according to Claim 1, wherein the charge transport layer is interposed between the substrate and the charge generation layer

Inventors: MELNYK ANDREW R, PAI DAMODAR M, STOLKA MILAN, MELNYK, ANDREW R., PAI, DAMODAR M., STOLKA, MILAN
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: G03G5
IPC codes: G03G5