CA1166502A - Photoreceptor construction including an aluminum oxide barrier-change transport layer having porous and non-porous zones - Google Patents
Photoreceptor construction including an aluminum oxide barrier-change transport layer having porous and non-porous zonesInfo
- Publication number
- CA1166502A CA1166502A CA000383874A CA383874A CA1166502A CA 1166502 A CA1166502 A CA 1166502A CA 000383874 A CA000383874 A CA 000383874A CA 383874 A CA383874 A CA 383874A CA 1166502 A CA1166502 A CA 1166502A
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- Prior art keywords
- micrometers
- photoconductive
- layer
- porous
- charge transport
- Prior art date
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Classifications
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
- G03G5/00—Recording members for original recording by exposure, e.g. to light, to heat, to electrons; Manufacture thereof; Selection of materials therefor
- G03G5/02—Charge-receiving layers
- G03G5/04—Photoconductive layers; Charge-generation layers or charge-transporting layers; Additives therefor; Binders therefor
- G03G5/043—Photoconductive layers characterised by having two or more layers or characterised by their composite structure
- G03G5/047—Photoconductive layers characterised by having two or more layers or characterised by their composite structure characterised by the charge-generation layers or charge transport layers
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
- G03G5/00—Recording members for original recording by exposure, e.g. to light, to heat, to electrons; Manufacture thereof; Selection of materials therefor
- G03G5/14—Inert intermediate or cover layers for charge-receiving layers
- G03G5/142—Inert intermediate layers
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Photoreceptors In Electrophotography (AREA)
- Eye Examination Apparatus (AREA)
- Fixing For Electrophotography (AREA)
- Sheets, Magazines, And Separation Thereof (AREA)
Abstract
Abstract Thin photoconductive insulator layers are desirably used because of the reduced amount of materials necessary and their enhanced ability for light transmissi-vity. However, the thin construction tends to reduce the amount of charge that such a photoconductive layer can support. The use of a combined porous and non-porous barrier-charge transport layer below said photoconductive insulator layer enables higher levels of charging in a photoconductive construction.
Description
PHOTORECEPTOR CONSTRUCTION
Field of the Invention .
The present invention relates to novel electro-photographic imaging systems and particularly to novel electrophotographic photoreceptors. These photoreceptors comprise a conductive substrate, an inorganic barrier-charge transport layer, and a photoconductive insulative layer.
Background of the Invention In the art of electrophotography, and particu-larly xerography, it is well known to coat a conductive substrate, such as an electrically conductive aluminum drum or aluminized polymeric sheeting, with a photoconduc-tive insulating layer to form a composite, layered, imag-ing article. The surface of the layered imaging structure is then uniformly electrostatically charged and exposed to a pattern of activating electromagnetic radiation, such as light. m e charge is selectively~d~issipated in the illuminated areas o~ the photoco~nductive insulator, thus leaving an electrostatic charge image in the non-illuminated areas. The electrostatic charge image can then be developed by a number of means to form a visible image. If desired, the developed image may be fixed or made permanent on the photoconductive insulator surface.
Alternatively, the~developed image, in the form of electrostatically adhèred toner powders or liquids, may be transferred to paper or some other material and subsequently affixed by some suitable means. This may be done, for example,~by attracting fusible ~toner particles to the charged areas, then transferring and fusing the imagewise distributed ~particles to another surface.
The conductive substrate utilized in such electrophotographic systems usually comprises a metal such as brass, aluminum, gold, platinum, st~eel or the like and may be of any convenient thickness, rigid or flexible, and :,~,, ~
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Field of the Invention .
The present invention relates to novel electro-photographic imaging systems and particularly to novel electrophotographic photoreceptors. These photoreceptors comprise a conductive substrate, an inorganic barrier-charge transport layer, and a photoconductive insulative layer.
Background of the Invention In the art of electrophotography, and particu-larly xerography, it is well known to coat a conductive substrate, such as an electrically conductive aluminum drum or aluminized polymeric sheeting, with a photoconduc-tive insulating layer to form a composite, layered, imag-ing article. The surface of the layered imaging structure is then uniformly electrostatically charged and exposed to a pattern of activating electromagnetic radiation, such as light. m e charge is selectively~d~issipated in the illuminated areas o~ the photoco~nductive insulator, thus leaving an electrostatic charge image in the non-illuminated areas. The electrostatic charge image can then be developed by a number of means to form a visible image. If desired, the developed image may be fixed or made permanent on the photoconductive insulator surface.
Alternatively, the~developed image, in the form of electrostatically adhèred toner powders or liquids, may be transferred to paper or some other material and subsequently affixed by some suitable means. This may be done, for example,~by attracting fusible ~toner particles to the charged areas, then transferring and fusing the imagewise distributed ~particles to another surface.
The conductive substrate utilized in such electrophotographic systems usually comprises a metal such as brass, aluminum, gold, platinum, st~eel or the like and may be of any convenient thickness, rigid or flexible, and :,~,, ~
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-2-in the form of a sheet, web or cylinder. This substrate may also comprise such materials as metallized paper and plastic sheets, conductive polymers, or glass coated with a thin conductive coating. In all cases, it is usually preferred that the support member be strong enough to permit a certain amount of handling. In some instances, an interfacial blocking layer for at least one type of charge carrier is utilized between the base electrode and the photoconductive insulator.
Typical photoconductive insulating materials useful in electrophotography include: (1) inorganic crystalline photoconductors such as cadmium sulfide, cadmium sulfoselenide, cadmium selenide, zinc sulfide, zinc oxide, and mixtures thereof, (2) inorganic photoconductive glasses such as amorphous selenium, selenium alloys, and selenium-arsenic, and (3) organic photoconductors such as phthalocyanine pigments and polyvinyl carbazole with or without additive materials which extend its spectral sensitivity.
The surface potentiaI is of the utmost importance in the development of an electrostatic charge image. For greatest development latitude, the contrast potential (Vc) resulting from different levels of exposure should be as large as possible. The contrast potential ~5 (Vc) can be expressed by the equation:
::
VC = l~G
where ~a is the change in surface charge density upon exposure to imaging radiation and C is the capacitance per unit area of the photoreceptor.
One prior art method o~ decreasing C and hence increasing Vc has been to simply increase the photo-conductive insulator thickness. However, the low charge ;~ carrier mobility in photoconductive insulators used in electrophotographic devices somewhat limits the useful thickness one can employ to decrease C. If the thickness ., . ..
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Typical photoconductive insulating materials useful in electrophotography include: (1) inorganic crystalline photoconductors such as cadmium sulfide, cadmium sulfoselenide, cadmium selenide, zinc sulfide, zinc oxide, and mixtures thereof, (2) inorganic photoconductive glasses such as amorphous selenium, selenium alloys, and selenium-arsenic, and (3) organic photoconductors such as phthalocyanine pigments and polyvinyl carbazole with or without additive materials which extend its spectral sensitivity.
The surface potentiaI is of the utmost importance in the development of an electrostatic charge image. For greatest development latitude, the contrast potential (Vc) resulting from different levels of exposure should be as large as possible. The contrast potential ~5 (Vc) can be expressed by the equation:
::
VC = l~G
where ~a is the change in surface charge density upon exposure to imaging radiation and C is the capacitance per unit area of the photoreceptor.
One prior art method o~ decreasing C and hence increasing Vc has been to simply increase the photo-conductive insulator thickness. However, the low charge ;~ carrier mobility in photoconductive insulators used in electrophotographic devices somewhat limits the useful thickness one can employ to decrease C. If the thickness ., . ..
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-3-is increased too much, the system will not have a useful discharge speed. In systems where the thickness can be increased somewhat to decrease C, then the increased thickness requirement also restricts the physical characteristics, such as flexibility and adhesion of the photoconductor to the final plate, drum or belt. Thus, to improve potential contrast in such syskems, an electrical-ly active transport overlayer on the photoconductor has been used as, ~or example, in U.S. Patent No. 3,928,034.
For xerographic use, this construction requires that the overlayer be substantially transparent and non-absorbing in the particular imaging radiation wavelength region. In additiont even though the overlayer is substantially transparent, as increasingly thicker layers are required, adsorption and scattering due to included particles and partial crystallization become significant and have a detrimental effect upon the sensitivity of the device and the quality of the copies produced.
me xerographic apparatus disclosed in U.S.
Patent No. 3,684,368 shows the use of photoreceptor constructions which bear some similarities to the constructions of the present invention. The reference shows the use of anodic, porous aluminum oxide layers between the metal layer and photoconductive insulator layer in order to improve the adhesion therebetween. me photoconductive insulative layers tend to be thick to provide decreased capacitance, with the preferred thickness range being 10-15 micrometers. The porous aluminum oxide layer shown in Example 3 is believed to 0 have a thickness o about 0.17 micrometers.
e xerographic photoreceptor shown in Example 3 of U.S. Patent No. 2,901,3~8 discIoses an aluminum sub-strate with a 100 Angstrom ~approximately 0.01 micro-meters) coating of aluminum oxide and a twenty micrometer coating of a vitreous selenium photoconductive insulator layer.
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The present invention is the barrier-charge transport layer is at least 0.15 micrometers thick, the pore diameters of the porous zone are be-tween 0.007 and 0.040 micrometers, the center-to-center spacing of the pores is from 0.010 to 0.400 micrometers, and the photoconductive insulator layer is less than 1.0 micrometers thick.
More particularly, the photoreceptor is a novel two-layered photo-receptor structure comprised of a thin layer of photoconductive insulator depo-sited on an adjacent, relatively thick, porous anodized aluminum barrier layer/
charge transport layer to produce an improved electrophotographic device. It was surprisingly found that the relatively thick porous oxide layer sandwiched between the conductive substrate and the photoconductive insulator also per-formed as a charge transport layer. ~Ioreover, it was found that surface elec-trical potential enhancement was achieved and that this was directly propor-tional to the porous charge transport oxide layer thickness. Because of this novel construction, a low cost electrophotographic device can be produced which has improved imaging contrast, a low background in the developed images, a high recycle rate, long life, and the capability of producing excellent coples.
The invention wlll now be described in greater detail with reference to the accompanying drawing which is a sectional view of the electrophotographic : : :
~device according to this invention.
The novel two-layered photoreceptor structure to provide an improved electrophotographic device can best be understood by reference to the drawing in conjunction with the following discussion. The Figure illustrates a photo-receptor 10 according to this invention. Substrate 12 is an electrically con-ductive substrate which is capable of lending physical support to the structure shown. It ::
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may be comprised of a substantially thick metallic sheet, aluminum drum blanks, metal or conductive polymer coated sheets, conductive particle filled polymeric sheets, or the like or a composite metal coating on a sufficiently rigid dielectric substrate. The metal may be selected from such materials as aluminum, brass, steel, silver, or the like. If it is desired to discharge the device by flooding radiation from the substrate side, then it is understood that a combination of materials must be selected to render substrate 12 sufficiently transparent to the flooding radiation.
Layer 14 is a unique barrier layer/charge transport layer according to this invention which is produced by the anodization of aluminum. Layer 14 has pores 16 in the aluminum oxide layer. An added asset of layer 14 is the barrier layer 18 lying adjacent to the metal surface in which no pores exist. This barrier layer 18 performs as a blocking layer for both positive charges ~holes) and negative charges (electrons).
Layer 20 is a photoconductive insulative film.
Useful photoconductive insulative materials include:
(1) inorganic crystAlline photoconductors such as cadmium sulfide, cadmium sulfoselenide, cadmium selenide, zinc sulfide, zinc oxide, and mixtures thereof, (2) inorganic photoconductive glasses such as amorphous selenium alloys, and (3) organic photoconductors. It is preferable that the photoconductive insulative layer _ be capable of blocking appropriate (i.e., negative or positive) charges at the free surface.
, Detailed Descri~ on of the Invention The conductive substrate used in the practice of the present invention may, as is well known in the art, be any conductive substrate. It may comprise a metal layer, a metal coating on a substrate such as a polymeric resin, a conductive polymer, a coating of a conductive polymer on a non-conductive polymeric resin, or the like. The i5~
substrate may be rigid or flexible, transparent or opaque, and may be in the shape of a cylinder, a sheet, an endless belt, or various other designs.
The photoconductive insulator layer may be any S photoconductive insulator layer as known in the art which is less than 2.0 and preferably less than 1.0 micrometers thick. The composition of the photoconductive insulator layer is not critical to the practice of the present inven-tion and may be selected from amongst any of the known materials in the art such as (1) inorganic crystalline photoconductors such as cadmium sulfide, cadmium sulfosel-enide, cadmium selenide, zinc sulfide, ~inc oxide, and mixtures thereof, (2) inorganic photoconductive glasses such as amorphous selenium, selenium alloys, and selenium-arsenic ~e.g., Ar2Se3), and (3) organic photoconductors such as phthalocyanine pigments and polyvinyl carbazole and its derivatives with or without additive materials which extend its spectral sensitivity.
As long as the layer provides photoconductive and insula-tive properties, it may be as thin as it can be made.Usually it will not be thinner than 0.05 micrometers, preferably it is at least 0.10 micrometers, and more preferably 0.15 micrometers to 0.8 micrometers. The upper limit on thickness is necessary to achieve the charge contrast enhancement of the structure of the present invention.
The barrier-charge transport layer performs uniquely wlthin the structure of the present invention.
The two zones of this single layer performs as both a 30~ blocking or barrier layer or positive charges (holes) and as a`charge transport layer when ~a negative charge (electrons) is photoact~ively released from the photoconductive charge generating layer. The layer is produced by the~anodization of aluminum. Anodization in certain environments generates a porous aluminum oxide layer. This layer preferably may be from about 0.15 to 25 micrometers thick. The pore diameter~ and the .~
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center-to-center spacing bet~een pores :is not critical to the practice of the present invention and varies because of changes in processing conditions during anodization such as temperature, electrolyte concentration, etc. Pore dia-meters on the order of 0.007 to 0.040 micrometers and average center-to-center spacing of from 0.010 to 0.400 are common. It is preferred that the average pore diameters be between 0.008 and 0.030 micrometers and that the center-to-center spacing be between an average of 0.010 and 0.080 micrometers or between 0,020 and 0.060 micrometers. The most preferred ranges are 0.010 to 0.020 ~and specifically 0.012~ micrometers for the pore size and 0.025 to 0.040 ~and specifically 0.033) micrometers for center-to-center spacing of the pores.
The barrier layer portion of the aluminum oxide layer, the non-porous area be tween the conductive substrate and the pores is usually between 0.003 and 0.05 micrometers, and is preferably between 0.006 and 0.03 micrometers. Typical pore-forming electrolytes which are used to anodize aluminum are selected from 15% sulfuric acid, 2% oxalic acid, 4% phosphorlc acid, and 3% chromic acid.
One of the most complete discussions of th0 process of anodization and the effects of parameter changes in the process on the characteristics of the aluminum oxide is to be found m "Anodic Oxide Films on Aluminum", J.W. Diggle, T.C. Downie, and C.W. Goulding, Rutherford College of Technology, Newcastle upon Tyne, England, a paper received July 29, 1968.
The structure of the present invention operates by first receiving an induced charge on the photoconductive insulator surface. The sensitized device is then imaged with imaging radia-.
~ ~ -7-tion. Light is absorbed by the photoconductive layer, creating electron-hole pairs. The holes and electrons are separated under the applied electric field. The electrons are injected into and transported through barrier layer/charge transport layer :~:
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g --and the holes are transported to the surface of photo-conductive insulative layer, thereby imagewise discharging the device where light strikes in proportion to the integrated amount of light which is absorbed. In the regions where radiation does not impinge upon the device, the charge distribution remains substantially the same as before the imaging step. The imaging step is now complete and the electrostatic latent charge image has been formed.
The electrostatic charge is then developed with toner to form a toner image on the electrophotographic drum. Excellent copy quality results when copies are made by transferring the toner image and subsequent toner images to plain paper. Added permanence is introduced in the transferred toner image i~ it i5 heat-fused or pressure-fused to the paper. The photoconductive insulator surface is then easily discharged and cleaned by conventional means. As previously mentioned, if it is desired to discharge by radiation from the underside, then substrate must be sufficiently transparent to the flooding radiation.
Having described in general the embodiment of this invention for electrophotography, some specific examples will now be given.
The photoconductive insulative layer consisted of 0.5 ~m sputter deposited cadmium sulfide (CdS) on commercially available ~lzak~ aluminum (Type 1) which has porous aluminum oxide on one face thereof. A 5 cm by 5 cm substrate was prepared by removing the protective adhesive-backed paper layer and cleaning the exposed al~minum oxide surface by immersing it in successive ultrasonic baths of acetone, trichloroethylene (bath 1) and trichloroethylene (bath 2), followed by rinses of trichloroethylene, methanol and acetone. The substrate was then blow dried in a stream of N2 gas. The aluminum oxide layer on the commerically available Alzak~ aluminum ;t;5~;~
was about 5 ~m thick. The substrate was then Placed into a Randex~ RF sputter deposition vacuum system and coated with about 0.5 ~m of sputter-deposited CdS in the following manner.
me substrate was placed on a 6.3 cm by 8.8 cm aluminum heater block containing a resistive heating element and a calibrated resistive temperature sensitive element. me heater block was separated from the water-cooled J-arm anode platform of a Randex~ sputter module by a 5 cm by 5 cm by l mm thick piece of quartz.
The heater block, quartz and anode table were thermally linked by applying a thin layer of high-vacuum silicone grease to each element. Also, the substrate was joined to the heater block with silicone grease to ensure that the temperature of the substrate was nearly the same as that measured at the heater block. The distance from the substrate to the hot pressed CdS target was about 5 cm.
The heater block was heated to 150C and the temperature was held constant to within 5C throughout the deposition. A premixed gas consisting of 6% H2S and 94%
Ar was admitted to the vacuum chamber at a rate of 20 std ml/min. me pumping speed was adjusted by use of a throttle valve located between the vacuum chamber and the diffuslon pump until the pressure in the vacuum chamber was stabilized at 2.S mT.
The non-functional properties of the novel photoreceptor produced according to this invention were then measured. me surface was charged negatively by passing a single corona wire across the surface several times at a distance of about l cm. The surface voltage was measured with a Monroe electrostatic voltmeter using a transparent probe and recorded on a chart recorder. me photoreceptor described above could be charged to 220 volts. The time required to discharge in the dark to one-half that value (llO volts) was two minutes. When exposed to monochromatic light of 4~0 nm, 1~ ergs/cm2 were required to discharge the surface from 220 V to 110 V.
An anodization cell was fabricated from PVC
plastic to accommodate 15 cm by 8 cm substrates and yielded substrates which were uniformly anodized over a 12.5 cm by 7.5 cm area. The cell was fabricated with three slots at each end which held the anode (aluminum substrate) and two cathodes fixed. The cathodes were 2.5 cm on either side of the anode. The electrolyte consisted of 15% concentrated H2SO4 and 85% deionized distilled water. The electrolyte was continuously circulated through about 6 meters of 1/4 inch plastic tubing which was immersed in a water bath for the purpose of cooling the electrolyte. Current was passed from the anode to both cathodes at a fixed rate which was recorded along lS with the voltage between the cathodes and the anode, the time span of the anodization, and the temperature of the electrolyte. The anodization parameters for this example were:
Substrate 75 ~ thick aluminum foil which was 99.99% pure ~i.e., 1199 alumin~m foil) Current 2.5 amps Voltage 11.5 volts Temperature 19C
Time 4.2 minutes The thickness of the anodized layer is known to be proportional to the product of the current and time for a given substraté material and electrolyte temperature.
Typically, 32 amp-min/ft2 will yield 1 ~m of oxide thickness. Since both~ sides of the substrate are anodized, both sides are counted in the area.
In this Example, therefore, about a 1.5 ~m thick oxide film was produced. Upon removal from the electro-lyte, the substrate was immediately rinsed in running tap water ~ollowed by a rinse in deionized distilled water and in isopropyl alcohol and blown dry with N2 gas.
- ~ . . ~ , A 5 cm by 5 cm piece was cut from this substrate and placed in the Model 3140 Randex~ RF sputter deposition unit of Example 1. A layer about 0.5 ~m thick of CdS was then deposited onto this substrate with the ~ollowing parameters:
RF power 300 W
Gas pressure 2.5 mT
Gas flow 20 std ml/min Gas composition 6% H2S, 94% Ar Substrate temp. 132C
Deposition time 8 minutes The resulting photoreceptor could be charged to -250 V. More than two minutes were required to discharge the surface voltage to ~125 V in the dark. A miximum of 125 V contrast between exposed and unexposed regions was observed. A 'chree second exposure to room light (about 30 ergs/cm2) was required to obtain half of this contrast.
; ~ EXAMPLE 3 A barrier layer/charge transport layer about 5 m thick was prepared on 1199 aluminum as in Example 2. A
photoconductive insulator layer consisting of about 0.24 m thick cadmium sulfide was deposited on layer 18 as in Example 2, however, the sputtering ga~ composition was pure argon.
25 : The resulting photoreceptor could be charged to 240 V, the dark decay to~-120 V required about 12 seconds, and~a voltage contrast of 40 volts was observed.
Again, a~three second exposure to room lights (~30 ergs/cm2) was required to~obtain half of this contrast.
Using resistive heating techniques, a 0.25 ~m thick photoconductive insulative layer comprised of a 94%
Se, 6% Te alloy~, was vacuum deposited on the commercially ~ available Alzak~ substrate prepared as in Example 1.
: ~ 35 ~owever, on~-half of the aluminum oxide barrier layer/
S~
charge transport layer was chemically stripped from ~he substrate prior to the deposition of the photoconductive insulative SeTe layer. The resulting photoreceptor could be charged to -140 V where layer remained, but to only -20 V where the layer was stripped off. The voltage contrast and exposure to one-half contrast were similarly effected by the presence of the layer, i.e., -80 V to -20 V and 70 ergs/cm2 to 20 ergs/cm2, respectively.
To demonstrate that the barrier layer/charge transport layer of this invention produces no advantage and, in fact, is undesirable, for thicker photoconductive insulative layers, a layer 40 ~m thick of 94% Se, 6% Te alloy was deposited as above on the stripped and unstripped commercial Alzak~ suhstrates. When charged negatively, the voltage acceptance was increased from -425 for the stripped portion to -780 V for the anodized portion, however the voltage contrast was decreased from 60 V to zero. When charged positively, the voltage acceptance was reduced slightly from 560 V to 460 V and the voltage contrast was reduced from 560 V for the stripped portion to 380 V for the anodi ed portion.
EXAMPLE S
1 ~m of As2Se3 was deposited using resistive heating techni~ues onto a commercially available Alzak~
substrate, half of which was stripped of the oxide layer.
The voltage acceptance was +113 V when charged positively, and -120 V when charged negatively for the anodized portion and +18 V, -27 for the stripped portion. The corresponding voltage contrast upon exposure was also increased for the anodized portion to +35, -20 from ~18, -15 volts when respectively charged positively and negatively.
In contrast to this when a thick layer (15 ~m) of Ar2Se3 was deposited onto a similar substrate the voltage contrast was reduced to ~ 0 volts for the anodized portion from ~75, -8 volts for the stripped ~ ~ l4 ~ ~ 6~
portion even though voltage acceptance was increased to +305, -365 from +75, -115 volts. This example shows that the voltage contrast is enhanced by the anodized aluminum barrier-charge transport layer of the present invention when used with relatively thin photo-conductive insulator layers. Conversely, it is surprising that the voltage contrast is not enhanced and is in fact reduced with relatively thick (i.e., ~5 micrometers) photoconductive insulator layers.
A photoreceptor ~as prepared by coating a 1 ~m thick coating of Perylene Red onto the aluminum oxide coated substrate of Example 1. This resulted in a 1.5 ~m thick anodized aluminum substrate which was compared to a similar coating on stripped aluminum. The resultlng photoreceptor could be charged to -171 V
compared to -72 V on stripped aluminum. The voltage contrast compared 167 V to 72 ~.
A barrier layer/charge transport layer about 2 micrometers thick was prepared on 1100 aluminum as in Example 2 using 4% phos-phoric acid as the electrolyte. The anodizing conditions were:
Substrate 100 micrometers thick aluminum foil which was 99% pure (i.e., 1100 aluminum foil) Current 0.7 amps Voltage 100 volts Temperature 22C
Time 18 minutes The resulting oxide layer was similar to that in Example 2 except ' ' `~ - 15 - ~ ~6~
that the pore diameter was approximately 0.03 micrometers and the center-to-center spacing was approximately 0.28 micrometers.
The resulting photoreceptor could be charged to -230 volts, the dark decay to -115 volts was greater than two minutes, and a voltage contrast of 127 volts was observed. An exposure of 47 ergs/cm2 was required to obtain half of this contrast.
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For xerographic use, this construction requires that the overlayer be substantially transparent and non-absorbing in the particular imaging radiation wavelength region. In additiont even though the overlayer is substantially transparent, as increasingly thicker layers are required, adsorption and scattering due to included particles and partial crystallization become significant and have a detrimental effect upon the sensitivity of the device and the quality of the copies produced.
me xerographic apparatus disclosed in U.S.
Patent No. 3,684,368 shows the use of photoreceptor constructions which bear some similarities to the constructions of the present invention. The reference shows the use of anodic, porous aluminum oxide layers between the metal layer and photoconductive insulator layer in order to improve the adhesion therebetween. me photoconductive insulative layers tend to be thick to provide decreased capacitance, with the preferred thickness range being 10-15 micrometers. The porous aluminum oxide layer shown in Example 3 is believed to 0 have a thickness o about 0.17 micrometers.
e xerographic photoreceptor shown in Example 3 of U.S. Patent No. 2,901,3~8 discIoses an aluminum sub-strate with a 100 Angstrom ~approximately 0.01 micro-meters) coating of aluminum oxide and a twenty micrometer coating of a vitreous selenium photoconductive insulator layer.
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The present invention is the barrier-charge transport layer is at least 0.15 micrometers thick, the pore diameters of the porous zone are be-tween 0.007 and 0.040 micrometers, the center-to-center spacing of the pores is from 0.010 to 0.400 micrometers, and the photoconductive insulator layer is less than 1.0 micrometers thick.
More particularly, the photoreceptor is a novel two-layered photo-receptor structure comprised of a thin layer of photoconductive insulator depo-sited on an adjacent, relatively thick, porous anodized aluminum barrier layer/
charge transport layer to produce an improved electrophotographic device. It was surprisingly found that the relatively thick porous oxide layer sandwiched between the conductive substrate and the photoconductive insulator also per-formed as a charge transport layer. ~Ioreover, it was found that surface elec-trical potential enhancement was achieved and that this was directly propor-tional to the porous charge transport oxide layer thickness. Because of this novel construction, a low cost electrophotographic device can be produced which has improved imaging contrast, a low background in the developed images, a high recycle rate, long life, and the capability of producing excellent coples.
The invention wlll now be described in greater detail with reference to the accompanying drawing which is a sectional view of the electrophotographic : : :
~device according to this invention.
The novel two-layered photoreceptor structure to provide an improved electrophotographic device can best be understood by reference to the drawing in conjunction with the following discussion. The Figure illustrates a photo-receptor 10 according to this invention. Substrate 12 is an electrically con-ductive substrate which is capable of lending physical support to the structure shown. It ::
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may be comprised of a substantially thick metallic sheet, aluminum drum blanks, metal or conductive polymer coated sheets, conductive particle filled polymeric sheets, or the like or a composite metal coating on a sufficiently rigid dielectric substrate. The metal may be selected from such materials as aluminum, brass, steel, silver, or the like. If it is desired to discharge the device by flooding radiation from the substrate side, then it is understood that a combination of materials must be selected to render substrate 12 sufficiently transparent to the flooding radiation.
Layer 14 is a unique barrier layer/charge transport layer according to this invention which is produced by the anodization of aluminum. Layer 14 has pores 16 in the aluminum oxide layer. An added asset of layer 14 is the barrier layer 18 lying adjacent to the metal surface in which no pores exist. This barrier layer 18 performs as a blocking layer for both positive charges ~holes) and negative charges (electrons).
Layer 20 is a photoconductive insulative film.
Useful photoconductive insulative materials include:
(1) inorganic crystAlline photoconductors such as cadmium sulfide, cadmium sulfoselenide, cadmium selenide, zinc sulfide, zinc oxide, and mixtures thereof, (2) inorganic photoconductive glasses such as amorphous selenium alloys, and (3) organic photoconductors. It is preferable that the photoconductive insulative layer _ be capable of blocking appropriate (i.e., negative or positive) charges at the free surface.
, Detailed Descri~ on of the Invention The conductive substrate used in the practice of the present invention may, as is well known in the art, be any conductive substrate. It may comprise a metal layer, a metal coating on a substrate such as a polymeric resin, a conductive polymer, a coating of a conductive polymer on a non-conductive polymeric resin, or the like. The i5~
substrate may be rigid or flexible, transparent or opaque, and may be in the shape of a cylinder, a sheet, an endless belt, or various other designs.
The photoconductive insulator layer may be any S photoconductive insulator layer as known in the art which is less than 2.0 and preferably less than 1.0 micrometers thick. The composition of the photoconductive insulator layer is not critical to the practice of the present inven-tion and may be selected from amongst any of the known materials in the art such as (1) inorganic crystalline photoconductors such as cadmium sulfide, cadmium sulfosel-enide, cadmium selenide, zinc sulfide, ~inc oxide, and mixtures thereof, (2) inorganic photoconductive glasses such as amorphous selenium, selenium alloys, and selenium-arsenic ~e.g., Ar2Se3), and (3) organic photoconductors such as phthalocyanine pigments and polyvinyl carbazole and its derivatives with or without additive materials which extend its spectral sensitivity.
As long as the layer provides photoconductive and insula-tive properties, it may be as thin as it can be made.Usually it will not be thinner than 0.05 micrometers, preferably it is at least 0.10 micrometers, and more preferably 0.15 micrometers to 0.8 micrometers. The upper limit on thickness is necessary to achieve the charge contrast enhancement of the structure of the present invention.
The barrier-charge transport layer performs uniquely wlthin the structure of the present invention.
The two zones of this single layer performs as both a 30~ blocking or barrier layer or positive charges (holes) and as a`charge transport layer when ~a negative charge (electrons) is photoact~ively released from the photoconductive charge generating layer. The layer is produced by the~anodization of aluminum. Anodization in certain environments generates a porous aluminum oxide layer. This layer preferably may be from about 0.15 to 25 micrometers thick. The pore diameter~ and the .~
~Llti~iS~Z
center-to-center spacing bet~een pores :is not critical to the practice of the present invention and varies because of changes in processing conditions during anodization such as temperature, electrolyte concentration, etc. Pore dia-meters on the order of 0.007 to 0.040 micrometers and average center-to-center spacing of from 0.010 to 0.400 are common. It is preferred that the average pore diameters be between 0.008 and 0.030 micrometers and that the center-to-center spacing be between an average of 0.010 and 0.080 micrometers or between 0,020 and 0.060 micrometers. The most preferred ranges are 0.010 to 0.020 ~and specifically 0.012~ micrometers for the pore size and 0.025 to 0.040 ~and specifically 0.033) micrometers for center-to-center spacing of the pores.
The barrier layer portion of the aluminum oxide layer, the non-porous area be tween the conductive substrate and the pores is usually between 0.003 and 0.05 micrometers, and is preferably between 0.006 and 0.03 micrometers. Typical pore-forming electrolytes which are used to anodize aluminum are selected from 15% sulfuric acid, 2% oxalic acid, 4% phosphorlc acid, and 3% chromic acid.
One of the most complete discussions of th0 process of anodization and the effects of parameter changes in the process on the characteristics of the aluminum oxide is to be found m "Anodic Oxide Films on Aluminum", J.W. Diggle, T.C. Downie, and C.W. Goulding, Rutherford College of Technology, Newcastle upon Tyne, England, a paper received July 29, 1968.
The structure of the present invention operates by first receiving an induced charge on the photoconductive insulator surface. The sensitized device is then imaged with imaging radia-.
~ ~ -7-tion. Light is absorbed by the photoconductive layer, creating electron-hole pairs. The holes and electrons are separated under the applied electric field. The electrons are injected into and transported through barrier layer/charge transport layer :~:
:: :
., S(~Z
g --and the holes are transported to the surface of photo-conductive insulative layer, thereby imagewise discharging the device where light strikes in proportion to the integrated amount of light which is absorbed. In the regions where radiation does not impinge upon the device, the charge distribution remains substantially the same as before the imaging step. The imaging step is now complete and the electrostatic latent charge image has been formed.
The electrostatic charge is then developed with toner to form a toner image on the electrophotographic drum. Excellent copy quality results when copies are made by transferring the toner image and subsequent toner images to plain paper. Added permanence is introduced in the transferred toner image i~ it i5 heat-fused or pressure-fused to the paper. The photoconductive insulator surface is then easily discharged and cleaned by conventional means. As previously mentioned, if it is desired to discharge by radiation from the underside, then substrate must be sufficiently transparent to the flooding radiation.
Having described in general the embodiment of this invention for electrophotography, some specific examples will now be given.
The photoconductive insulative layer consisted of 0.5 ~m sputter deposited cadmium sulfide (CdS) on commercially available ~lzak~ aluminum (Type 1) which has porous aluminum oxide on one face thereof. A 5 cm by 5 cm substrate was prepared by removing the protective adhesive-backed paper layer and cleaning the exposed al~minum oxide surface by immersing it in successive ultrasonic baths of acetone, trichloroethylene (bath 1) and trichloroethylene (bath 2), followed by rinses of trichloroethylene, methanol and acetone. The substrate was then blow dried in a stream of N2 gas. The aluminum oxide layer on the commerically available Alzak~ aluminum ;t;5~;~
was about 5 ~m thick. The substrate was then Placed into a Randex~ RF sputter deposition vacuum system and coated with about 0.5 ~m of sputter-deposited CdS in the following manner.
me substrate was placed on a 6.3 cm by 8.8 cm aluminum heater block containing a resistive heating element and a calibrated resistive temperature sensitive element. me heater block was separated from the water-cooled J-arm anode platform of a Randex~ sputter module by a 5 cm by 5 cm by l mm thick piece of quartz.
The heater block, quartz and anode table were thermally linked by applying a thin layer of high-vacuum silicone grease to each element. Also, the substrate was joined to the heater block with silicone grease to ensure that the temperature of the substrate was nearly the same as that measured at the heater block. The distance from the substrate to the hot pressed CdS target was about 5 cm.
The heater block was heated to 150C and the temperature was held constant to within 5C throughout the deposition. A premixed gas consisting of 6% H2S and 94%
Ar was admitted to the vacuum chamber at a rate of 20 std ml/min. me pumping speed was adjusted by use of a throttle valve located between the vacuum chamber and the diffuslon pump until the pressure in the vacuum chamber was stabilized at 2.S mT.
The non-functional properties of the novel photoreceptor produced according to this invention were then measured. me surface was charged negatively by passing a single corona wire across the surface several times at a distance of about l cm. The surface voltage was measured with a Monroe electrostatic voltmeter using a transparent probe and recorded on a chart recorder. me photoreceptor described above could be charged to 220 volts. The time required to discharge in the dark to one-half that value (llO volts) was two minutes. When exposed to monochromatic light of 4~0 nm, 1~ ergs/cm2 were required to discharge the surface from 220 V to 110 V.
An anodization cell was fabricated from PVC
plastic to accommodate 15 cm by 8 cm substrates and yielded substrates which were uniformly anodized over a 12.5 cm by 7.5 cm area. The cell was fabricated with three slots at each end which held the anode (aluminum substrate) and two cathodes fixed. The cathodes were 2.5 cm on either side of the anode. The electrolyte consisted of 15% concentrated H2SO4 and 85% deionized distilled water. The electrolyte was continuously circulated through about 6 meters of 1/4 inch plastic tubing which was immersed in a water bath for the purpose of cooling the electrolyte. Current was passed from the anode to both cathodes at a fixed rate which was recorded along lS with the voltage between the cathodes and the anode, the time span of the anodization, and the temperature of the electrolyte. The anodization parameters for this example were:
Substrate 75 ~ thick aluminum foil which was 99.99% pure ~i.e., 1199 alumin~m foil) Current 2.5 amps Voltage 11.5 volts Temperature 19C
Time 4.2 minutes The thickness of the anodized layer is known to be proportional to the product of the current and time for a given substraté material and electrolyte temperature.
Typically, 32 amp-min/ft2 will yield 1 ~m of oxide thickness. Since both~ sides of the substrate are anodized, both sides are counted in the area.
In this Example, therefore, about a 1.5 ~m thick oxide film was produced. Upon removal from the electro-lyte, the substrate was immediately rinsed in running tap water ~ollowed by a rinse in deionized distilled water and in isopropyl alcohol and blown dry with N2 gas.
- ~ . . ~ , A 5 cm by 5 cm piece was cut from this substrate and placed in the Model 3140 Randex~ RF sputter deposition unit of Example 1. A layer about 0.5 ~m thick of CdS was then deposited onto this substrate with the ~ollowing parameters:
RF power 300 W
Gas pressure 2.5 mT
Gas flow 20 std ml/min Gas composition 6% H2S, 94% Ar Substrate temp. 132C
Deposition time 8 minutes The resulting photoreceptor could be charged to -250 V. More than two minutes were required to discharge the surface voltage to ~125 V in the dark. A miximum of 125 V contrast between exposed and unexposed regions was observed. A 'chree second exposure to room light (about 30 ergs/cm2) was required to obtain half of this contrast.
; ~ EXAMPLE 3 A barrier layer/charge transport layer about 5 m thick was prepared on 1199 aluminum as in Example 2. A
photoconductive insulator layer consisting of about 0.24 m thick cadmium sulfide was deposited on layer 18 as in Example 2, however, the sputtering ga~ composition was pure argon.
25 : The resulting photoreceptor could be charged to 240 V, the dark decay to~-120 V required about 12 seconds, and~a voltage contrast of 40 volts was observed.
Again, a~three second exposure to room lights (~30 ergs/cm2) was required to~obtain half of this contrast.
Using resistive heating techniques, a 0.25 ~m thick photoconductive insulative layer comprised of a 94%
Se, 6% Te alloy~, was vacuum deposited on the commercially ~ available Alzak~ substrate prepared as in Example 1.
: ~ 35 ~owever, on~-half of the aluminum oxide barrier layer/
S~
charge transport layer was chemically stripped from ~he substrate prior to the deposition of the photoconductive insulative SeTe layer. The resulting photoreceptor could be charged to -140 V where layer remained, but to only -20 V where the layer was stripped off. The voltage contrast and exposure to one-half contrast were similarly effected by the presence of the layer, i.e., -80 V to -20 V and 70 ergs/cm2 to 20 ergs/cm2, respectively.
To demonstrate that the barrier layer/charge transport layer of this invention produces no advantage and, in fact, is undesirable, for thicker photoconductive insulative layers, a layer 40 ~m thick of 94% Se, 6% Te alloy was deposited as above on the stripped and unstripped commercial Alzak~ suhstrates. When charged negatively, the voltage acceptance was increased from -425 for the stripped portion to -780 V for the anodized portion, however the voltage contrast was decreased from 60 V to zero. When charged positively, the voltage acceptance was reduced slightly from 560 V to 460 V and the voltage contrast was reduced from 560 V for the stripped portion to 380 V for the anodi ed portion.
EXAMPLE S
1 ~m of As2Se3 was deposited using resistive heating techni~ues onto a commercially available Alzak~
substrate, half of which was stripped of the oxide layer.
The voltage acceptance was +113 V when charged positively, and -120 V when charged negatively for the anodized portion and +18 V, -27 for the stripped portion. The corresponding voltage contrast upon exposure was also increased for the anodized portion to +35, -20 from ~18, -15 volts when respectively charged positively and negatively.
In contrast to this when a thick layer (15 ~m) of Ar2Se3 was deposited onto a similar substrate the voltage contrast was reduced to ~ 0 volts for the anodized portion from ~75, -8 volts for the stripped ~ ~ l4 ~ ~ 6~
portion even though voltage acceptance was increased to +305, -365 from +75, -115 volts. This example shows that the voltage contrast is enhanced by the anodized aluminum barrier-charge transport layer of the present invention when used with relatively thin photo-conductive insulator layers. Conversely, it is surprising that the voltage contrast is not enhanced and is in fact reduced with relatively thick (i.e., ~5 micrometers) photoconductive insulator layers.
A photoreceptor ~as prepared by coating a 1 ~m thick coating of Perylene Red onto the aluminum oxide coated substrate of Example 1. This resulted in a 1.5 ~m thick anodized aluminum substrate which was compared to a similar coating on stripped aluminum. The resultlng photoreceptor could be charged to -171 V
compared to -72 V on stripped aluminum. The voltage contrast compared 167 V to 72 ~.
A barrier layer/charge transport layer about 2 micrometers thick was prepared on 1100 aluminum as in Example 2 using 4% phos-phoric acid as the electrolyte. The anodizing conditions were:
Substrate 100 micrometers thick aluminum foil which was 99% pure (i.e., 1100 aluminum foil) Current 0.7 amps Voltage 100 volts Temperature 22C
Time 18 minutes The resulting oxide layer was similar to that in Example 2 except ' ' `~ - 15 - ~ ~6~
that the pore diameter was approximately 0.03 micrometers and the center-to-center spacing was approximately 0.28 micrometers.
The resulting photoreceptor could be charged to -230 volts, the dark decay to -115 volts was greater than two minutes, and a voltage contrast of 127 volts was observed. An exposure of 47 ergs/cm2 was required to obtain half of this contrast.
, .
. .
:. ' ' . ' l ' ; .
- '' , :. ': . ' ': , ' ,. ' ~, :
`' ' ' ~ .
Claims (10)
1. An electrophotographic device comprising:
1) an electrically conductive substrate, 2) a barrier charge transport layer comprising aluminum oxide, wherein said layer comprises a non-porous barrier zone adjacent said substrate and a porous charge transport zone, and 3) a photoconductive insulator layer of less than two micrometers adjacent the porous charge transport zone of said barrier-charge transport layer.
1) an electrically conductive substrate, 2) a barrier charge transport layer comprising aluminum oxide, wherein said layer comprises a non-porous barrier zone adjacent said substrate and a porous charge transport zone, and 3) a photoconductive insulator layer of less than two micrometers adjacent the porous charge transport zone of said barrier-charge transport layer.
2. The device of claim 1 wherein the barrier-charge transport layer is at least 0.15 micrometers thick, the pore diameters of the porous zone are between 0.007 and 0.040 micrometers, the center-to-center spacing of the pores is from 0.010 to 0.400 micrometers, and the photo-conductive insulator layer is less than 1.0 micrometers thick.
3. The device of claim 2 wherein said non-porous zone is between 0.003 and 0.05 micrometers thick.
4. The device of claim 1 wherein said photoconductive insulator layer is selected from the class consisting of inorganic crystalline photoconductors, inor-ganic photoconductive glasses and organic photoconductors.
5. The device of claim 3 wherein said photoconductive insulator layer is selected from the class consisting of inorganic crystalline photoconductors, inorganic photoconductive glasses, and organic photoconductors.
6. The device of claim 5 wherein said photoconductive insulator layer is at least 0.05 micrometers and less than 1.0 micrometers in thickness.
7. The device of claim 6 wherein said photoconductive insulator layer comprises cadmium sulfide, cadmium sulfoselenide, cadmium selenide or mixtures thereof.
8. The device of claim 7 wherein said photoconductive insulator layer comprises cadmium sulfide, cadmium sulfoselenide, cadmium selenide, or mixtures thereof.
9. The device of claim 6 wherein said substrate is selected from the group consisting of metal, metal coated polymeric resin, conductive polymeric resin, conductive polymeric resin coated onto a polymeric resin, conductive particle filled polymeric resin, and mixtures thereof.
10. The device of claim 1 wherein said photoconductive insulator layer is between 0.10 and 1.0 micrometers in thickness and comprises a photoconductor selected from the class consisting of inorganic crystalline photoconductors, inorganic photoconductive glasses and organic photoconductors, and wherein said barrier-charge transport layer is between 0.15 and 25 micrometers, the barrier zone of said barrier-charge transport layer is between 0.006 and 0.03 micrometers, the pore diameters of said porous zone are between 0.008 and 0.030 micrometers and the center-to-center spacing of said pores is between 0.020 and 0.060 micrometers.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US19042380A | 1980-09-25 | 1980-09-25 | |
| US190,423 | 1980-09-25 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1166502A true CA1166502A (en) | 1984-05-01 |
Family
ID=22701291
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000383874A Expired CA1166502A (en) | 1980-09-25 | 1981-08-14 | Photoreceptor construction including an aluminum oxide barrier-change transport layer having porous and non-porous zones |
Country Status (7)
| Country | Link |
|---|---|
| EP (1) | EP0049046B1 (en) |
| JP (1) | JPS5788458A (en) |
| AT (1) | ATE12552T1 (en) |
| AU (1) | AU545897B2 (en) |
| BR (1) | BR8106109A (en) |
| CA (1) | CA1166502A (en) |
| DE (1) | DE3169687D1 (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2595591B2 (en) * | 1987-12-15 | 1997-04-02 | 富士ゼロックス株式会社 | Electrophotographic photoreceptor |
| JP2666488B2 (en) * | 1989-09-25 | 1997-10-22 | 富士ゼロックス株式会社 | Electrophotographic photoreceptor and method of manufacturing the same |
| JP2663647B2 (en) * | 1989-09-25 | 1997-10-15 | 富士ゼロックス株式会社 | Electrophotographic photoreceptor and method of manufacturing the same |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3684368A (en) * | 1968-07-10 | 1972-08-15 | Hitachi Ltd | Xerographic apparatus |
| US3928034A (en) * | 1970-12-01 | 1975-12-23 | Xerox Corp | Electron transport layer over an inorganic photoconductive layer |
-
1981
- 1981-08-14 CA CA000383874A patent/CA1166502A/en not_active Expired
- 1981-08-25 EP EP81303874A patent/EP0049046B1/en not_active Expired
- 1981-08-25 DE DE8181303874T patent/DE3169687D1/en not_active Expired
- 1981-08-25 AT AT81303874T patent/ATE12552T1/en not_active IP Right Cessation
- 1981-09-24 AU AU75646/81A patent/AU545897B2/en not_active Ceased
- 1981-09-24 BR BR8106109A patent/BR8106109A/en unknown
- 1981-09-25 JP JP56151908A patent/JPS5788458A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5788458A (en) | 1982-06-02 |
| BR8106109A (en) | 1982-06-15 |
| ATE12552T1 (en) | 1985-04-15 |
| EP0049046A1 (en) | 1982-04-07 |
| AU545897B2 (en) | 1985-08-08 |
| EP0049046B1 (en) | 1985-04-03 |
| DE3169687D1 (en) | 1985-05-09 |
| AU7564681A (en) | 1982-04-01 |
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