EP0173620B1 - Procédé électrophotographique à champ bas - Google Patents

Procédé électrophotographique à champ bas Download PDF

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Publication number
EP0173620B1
EP0173620B1 EP19850401622 EP85401622A EP0173620B1 EP 0173620 B1 EP0173620 B1 EP 0173620B1 EP 19850401622 EP19850401622 EP 19850401622 EP 85401622 A EP85401622 A EP 85401622A EP 0173620 B1 EP0173620 B1 EP 0173620B1
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EP
European Patent Office
Prior art keywords
layer
doped
intrinsic
range
thickness
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Expired
Application number
EP19850401622
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German (de)
English (en)
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EP0173620A1 (fr
Inventor
Paul M. Borsenberger
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Eastman Kodak Co
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Eastman Kodak Co
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    • 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
    • G03G5/08—Photoconductive layers; Charge-generation layers or charge-transporting layers; Additives therefor; Binders therefor characterised by the photoconductive material being inorganic
    • G03G5/082—Photoconductive layers; Charge-generation layers or charge-transporting layers; Additives therefor; Binders therefor characterised by the photoconductive material being inorganic and not being incorporated in a bonding material, e.g. vacuum deposited
    • G03G5/08214—Silicon-based
    • G03G5/08221—Silicon-based comprising one or two silicon based layers

Definitions

  • This invention relates in general to electrophotography and in particular to a novel low field electrophotographic process. More specifically, this invention relates to a low field electrophotographic process employing a photoconductive insulating element which exhibits high quantum efficiency at low voltage.
  • Photoconductive elements comprise a conducting support bearing a layer of a photoconductive material which is insulating in the dark but which becomes conductive upon exposure to radiation.
  • a common technique for forming images with such elements is to uniformly electrostatically charge the surface of the element and then imagewise expose it to radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface of the element and there dissipate the surface charge. This leaves behind a charge pattern in nonirradiated areas, referred to as a latent electrostatic image.
  • This latent electrostatic image can then be developed, either on the surface on which it is formed, or on another surface to which it has been transferred, by application of a liquid or dry developer composition which contains electroscopic marking particles.
  • These particles are selectively attracted to and deposit in the charged areas or are repelled by the charged areas and selectively deposited in the uncharged areas.
  • the pattern of marking particles can be fixed to the surface on which they are deposited or they can be transferred to another surface and fixed there.
  • Photoconductive elements can comprise a single active layer, containing the photoconductive material, or they can comprise multiple active layers. Elements with multiple active layers (sometimes referred to as multi-active elements) have at least one charge-generating layer and at least one charge-transport layer.
  • the charge-generating layer responds to radiation by generating mobile charge carriers and the charge-transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge to light- struck areas and form the latent electrostatic image.
  • the photoreceptor properties that determine the radiation necessary to form the latent image are the quantum efficiency, the thickness, the dielectric constant, and the existance of trapping.
  • the exposure can be expressed as: where E is the exposure in ergs/cm 2 , s the relative dielectric constant, L the thickness in cm, e the electronic charge in esu, ⁇ the wavelength in nm, 0 th quantum efficiency, k a constant equal to 3.2x10 -13 , and AV the voltage difference between the image and background area, V,-V b .
  • the quantum efficiency which cannot exceed unity, represents the fraction of incident photons that are absorbed and result in free electron-hole pairs.
  • the exposure required to form a latent image in conventional silver halide photography is in the range of 10- 2 to 10- 1 ergs/cm 2 , or less, and, accordingly, the radiation sensitivity of electrophotography is less than that of convenitonal silver halide photography by a factor of at least 1 03 .
  • V b the magnitude of V b must also be reduced, since a reduction in AV without a corresponding reduction in V b results in a very low signal to noise (S/N) ratio.
  • the problem of this invention is to provide a high speed electrophotographic process which exhibits minimal electrical noise.
  • the solution to this problem is a novel electrophotographic process in which a photoconductive insulating element is uniformly electrostatically charged and image-wise exposed to activating radiation to thereby form a latent electrostatic image on the surface thereof, said element (1) comprising:
  • activating radiation as used herein is defined as electromagnetic radiation which is capable of generating electron-hole pairs in the photoconductive insulating element upon exposure thereof.
  • a doped a-Si(H) layer refers to a layer of hydrogenated amorphous silicon that has been doped with one or more elements to a degree sufficient to render it either n-type or p-type).
  • a doped a-Si(H) layer refers to a layer of hydrogenated amorphous silicon that has been doped with one or more elements to a degree sufficient to render it either n-type or p-type.
  • the present invention makes use of a particular type of photoconductive insulating element, characterized by the presence of both doped and intrinsic layers of a-Si(H), in an electrophotographic process in which the element is electrostatically charged to a low surface voltage, that is a voltage in the range of from 5 to 50 volts.
  • the photoconductive insulating element utilized in the electrophotographic process of this invention comprises:
  • the photoconductive stratum comprise both an intrinsic a-Si(H) layer and a doped a-(H) layer, since use of an intrinsic a-Si(H) layer alone would not be an effective means of generating the necessary charge carriers when employing a low surface voltage; while use of a doped a-Si(H) layer alone would result in too high a dark conductivity for the element to be useful in the iow field process of this invention. It is also very important that the doped layer be very much thinner than the intrinsic layer, since, if this were not the case, the dark conductivity would be excessively high for use in the low field process of this invention.
  • the element be electrostatically charged to a very low surface voltage, that is a voltage in the range of from 5 to 50 volts. Only by the use of such a low voltage is it possible to achieve very high electrophotographic sensitivity-a sensitivity which is so high that the element can be reasonably characterized as a camera-speed material-without the generation of excessive electrical noise. It is this use of very low voltage which specifically distinguishes the process of this invention from conventional electrophotographic processes which utilise much higher voltages.
  • Photoconductive insulating elements whether of the single-active-layer or multiple-active-layer types, typically exhibit a quantum efficiency at low voltage which is much less than they exhibit at high voltage.
  • the photoconductive insulating elements described herein exhibit a quantum efficiency at low voltage which is substantially the same as that at high voltage. It is this characteristic which renders them especially suitable for use in the novel low field electrophotographic process of this invention.
  • the elements employed in the process of this invention utilize an electrically-conductive support, and such support can be either an electrically-conductive material or a composite material comprised of an electrically-insulating substrate coated with one or more conductive layers.
  • the electrically-conductive support should be a relatively rigid material and preferably one that has a thermal expansion coefficient that is fairly close so that of a layer of a-Si(H).
  • Particularly useful materials include aluminum, steel, and glass that has been coated with a suitable conductive coating.
  • the support is fabricated in a drum or tube configuration, since such configurations are most appropriate for use with a relatively brittle and fragile material such as a-Si(H).
  • a particularly important feature of the photoconductive insulating element employed in the process of this invention is the barrier layer. It serves to prevent the injection of charge carriers from the substrate into the photoconductive stratum. Specifically, it prevents the injection of holes from the substrate when the photoreceptor is charged to a negative potential, and it prevents the injection of electrons from the substrate when the photoreceptor is charged to a positive potential. Either positive or negative charging can, of course, be used in the process of this invention, as desired. Inclusion of a barrier layer in the element is necessary in order for the element to provide adequate charge acceptance.
  • barrier layer is a layer of a-Si(H) which has been heavily doped with a suitable doping agent.
  • suitable doping agent is intended to mean a concentration of doping agent of at least 100 ppm.
  • a photoconductive stratum is used herein to refer to the combination of an intrinsic a-Si(H) layer and a doped a-Si(H) layer in electrical contact therewith. Since the essential requirement is merely that the activating radiation be incident upon the doped layer, the particular order of these layers in the photoconductive stratum is not ordinarily critical.
  • the doped layer can be the outermost layer and the exposure can be from the front side of the element, or the order of the doped and intrinsic layers can be reversed and the exposure can be from the rear side.
  • the layer of intrinsic a-Si(H) can be formed by processes which are well known in the art. Most commonly, the process employed is a gas phase reaction, known as plasma-induced dissociation, using a silane (for example SiH 4 ) as the starting material.
  • the hydrogen content of the intrinsic a-Si(H) layer can be varied over a broad range to provide particular characteristics as desired. Generally, the hydrogen content is in the range of 1 to 50 percent and preferably in the range of 5 to 25 percent (the content of hydrogen being defined in atomic percentage).
  • the layer of doped a-Si(H) can be formed in the same manner as the layer of intrinsic a-Si(H), except that one or more doping elements are utilized in the layer-forming process in an amount sufficient to render the layer n-type or p-type.
  • doping elements can also be used in the formation of the intrinsic layer since a layer of hydrogenated amorpho.us silicon, as typically prepared by the plasma-induced dissociation of SiH 4 , is slightly n-type and a slight degree of p-doping is typically employed to render it intrinsic).
  • the hydrogen concentration in the doped layer can be in the same general range as in the intrinsic layer.
  • doping agents are known in the art to be of utility in advantageously modifying the characteristics of a layer of a-Si(H). Included among such doping agents are the elements of Group VA of the Periodic Table, namely N, P, As, Sb and Bi, which provide an n-type layer-that is, one which exhibits a preference for conduction of negative charge carriers (electrons)-and the elements of Group IIIA of the Periodic Table, namely B, Al, Ga, In and TI, which provide a p-type layer-that is one which exhibits a preference for conduction of positive charge carriers (holes).
  • Group VA of the Periodic Table namely N, P, As, Sb and Bi
  • the elements of Group IIIA of the Periodic Table namely B, Al, Ga, In and TI, which provide a p-type layer-that is one which exhibits a preference for conduction of positive charge carriers (holes).
  • the preferred doping agent for forming an n-type layer is phosphorus, and it is conveniently utilized in the plasma-induced dissociation in the form of phosphine gas (PH 3 ).
  • the preferred doping agent for forming a p-type layer is boron, and it is conveniently utilized in the plasma-induced dissocation in the form of diborane gas (B 2 H 6 ). '
  • the concentration of doping agent employed in forming the doped a-Si(H) layer can be varied over a very broad range.
  • the doping agent is employed in an amount of up to 1,000 ppm in the gaseous composition used to form the doped layer, and preferably in an amount of 15 to 150 ppm.
  • a doped a-Si(H) layer is utilized as the barrier layer in the element, it is typically a heavily doped layer, for example, a layer formed from a composition containing 500 to 5,000 ppm of the doping agent.
  • the thickness of the various layers making up the photoconductive insulating elements employed in the process of this invention can be varied widely.
  • the barrier layer will typically have a thickness in the range of from 0.01 to 5 microns (micrometer), and preferably in the range of from 0.05 to 1 micron.
  • the intrinsic a-Si(H) layer will typically have a thickness in the range of from 1 to 50 microns, and preferably in the range of from 3 to 30 microns.
  • the doped a-Si(H) layer will typically have a thickness in the range of from 0.01 to 0.2 microns, and preferably in the range of from 0.02 to 0.1 microns.
  • the doped a-Si(H) layer must be sufficiently thin to provide the element with a high degree of dark resistivity, generally a dark resistivity of at least 1011 ohm-cm, and most typically in the range of 10 11 to 10 14 ohm-cm. While the exact ratio of the thickness of the doped layer to the thickness of the intrinsic layer is not critical, the doped layer is typically very thin in relation to the thickness of the intrinsic layer. It is preferred that the ratio of the thickness of the doped a-Si(H) layer to the thickness of the intrinsic a-Si(H) layer be less than 0.01 and particularly preferred that it be in the range of from 0.001 to 0.005.
  • the preferred doping agent for forming an n-type layer is phosphorus
  • the preferred doping agent for forming a p-type layer is boron. These agents are preferably utilized in the doped layer at a concentration of 15 to 150 ppm.
  • the amount of doping agent utilized needs to be carefully controlled to achieve optimum results. For example, an amount of doping agent which is too low will result in an undesirably low quantum efficiency, while an amount of doping agent that is too great will result in an excessively high dark conductivity.
  • the photoconductive insulating elements employed in the process of this invention can contain certain optional layers.
  • they can contain anti-reflection layers to reduce reflection and thereby increase efficiency.
  • Silicon nitride is a particularly useful material for forming an anti-reflection layer, and is advantageously employed at a thickness of 0.1 to 0.5 microns.
  • the photoconductive insulating element is electrostatically charged to a surface voltage of 5 to 50 volts, and most preferably of 10 to 20 volts. Charging to this low voltage provides the basis for a very high speed electrophotographic process.
  • the process is also advantageous in that the element has an extremely fast response time, exhibits sensitometry which is essentially temperature independent, and can be readily adapted to provide panchromatic sensitivity through appropriate control of the hydrogen content.
  • a photoconductive insulating element was prepared with the following layers arranged in the indicated order:
  • the quantum efficiency was determined in relation to the magnitude of the surface potential.
  • the quantum efficiency is defined as the ratio of the decrease in the surface charge density to the absorbed photon flux, assuming the charge density is related to the surface voltage by the geometrical capacitance).
  • Figure 1 also provides the results for an otherwise identical control element which did not have the doped a-Si(H) layer.
  • the results for the test element of the invention are shown by open circles, while those for the control element are shown by solid circles.
  • the quantum efficiency of the control element decreased substantially with decreasing surface voltage, while the quantum efficiency of the test element was substantially independent of surface voltage over a wide range of voltages. With both the control and test elements, the quantum efficiency at high voltage was unity.
  • the thin layer of doped a-Si(H) is a critical component of the photoconductive insulating elements which are useful in the process of this invention, as this layer strongly reduces the field dependence of the photogeneration efficiency and thereby gives rise to the high sensitivity that is observed at low fields.
  • the exposure wavelength was 400 nm
  • the exposure duration was 160 microseconds
  • the voltage was sampled 0.5 seconds after the cessation of exposure.
  • the control element exhibited discharge from V. to VJ2 with an exposoure of 0.29 ergs/cm 2 , corresponding to an ASA rating of about 12, while the test element required only 0.11 ergs/cm 2 , corresponding to an ASA rating of about 30.

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  • Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Photoreceptors In Electrophotography (AREA)
  • Electrophotography Using Other Than Carlson'S Method (AREA)

Claims (7)

1. Procédé électrophotographique dans lequel on charge électrostatiquement un élément isolant photoconducteur et on l'expose à un rayonnement activant afin de former une image latente électrostatique sur la surface dudit élément, ledit élément comprenant:
(a) un support conducteur vis-à-vis de l'électricité
(b) une couche barrière recouvrant ledit support, et
(c) un ensemble de couches photoconducteur recouvrant ladite couche barrière, caractérisé en ce que,
(1) ledit ensemble de couches photoconducteur comprend une couche d'a-Si(H) non dopé en contact électrique avec une couche d'a-Si(H) dopé, ladite couche d'a-Si(H) dopé étant très mince par rapport à ladite couche d'a-Si(H) non dopé, et
(2) ledit élément est chargé électrostatiquement uniformément à un potentiel de surface compris entre 5 et 50 volts, et
(3) ladite couche d'a-Si(H) dopé est exposée à un rayonnement activant.
2. Procédé selon la revendication 1, dans lequel le potentiel de surface est compris entre 10 et 20 volts.
3. Procédé selon la revendication 1 ou 2, dans lequel ladite couche d'a-Si(H) dopé est dopée avec un élément du groupe III A ou V A de la classification périodique.
4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel ladite couche d'a-Si(H) dopé est dopée avec du phosphore à une concentration de 15 à 150 ppm.
5. Procédé selon l'une quelconque des revendications 1 à 4, dans lequel la concentration en hydrogène dans chacune desdites couches d'a-Si(H) non dopé et d'a-Si(H) dopé est comprise entre 5 et 25 pour cent.
6. Procédé selon l'une quelconque des revendications 1 à 5, dans lequel l'épaisseur de ladite couche d'a-Si(H) non dopé est comprise entre 3 et 30 µm et l'épaisseur de ladite couche d'a-Si(H) dopé est comprise entre 0,02 et 0,1 µm.
7. Procédé selon l'une quelconque des revendications 1 à 6, dans lequel le rapport de l'épaisseur de ladite couche d'a-Si(H) dopé à l'épaisseur de ladite couche d'a-Si(H) non dopé est compris entre 0,001 et 0,005.
EP19850401622 1984-08-20 1985-08-09 Procédé électrophotographique à champ bas Expired EP0173620B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US64260384A 1984-08-20 1984-08-20
US642603 1984-08-20

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EP0173620A1 EP0173620A1 (fr) 1986-03-05
EP0173620B1 true EP0173620B1 (fr) 1989-04-26

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Country Status (4)

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EP (1) EP0173620B1 (fr)
JP (1) JPS6159353A (fr)
CA (1) CA1249476A (fr)
DE (1) DE3569843D1 (fr)

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4265991A (en) * 1977-12-22 1981-05-05 Canon Kabushiki Kaisha Electrophotographic photosensitive member and process for production thereof
DE2954551C2 (fr) * 1978-03-03 1989-02-09 Canon K.K., Tokio/Tokyo, Jp
US4409308A (en) * 1980-10-03 1983-10-11 Canon Kabuskiki Kaisha Photoconductive member with two amorphous silicon layers
JPS57177156A (en) * 1981-04-24 1982-10-30 Canon Inc Photoconductive material

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Publication number Publication date
DE3569843D1 (en) 1989-06-01
JPS6159353A (ja) 1986-03-26
EP0173620A1 (fr) 1986-03-05
CA1249476A (fr) 1989-01-31

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