DE1522683C3 - Method and device for charging the photoconductive layer of a recording material and for generating a charge image - Google Patents

Description

The invention relates to a method of uniformly charging the photoconductive layer of a photoconductive recording material in which the photoconductive layer is subjected to total exposure is, as well as an electrophotographic process in which a uniformly charged photoconductive Layer is exposed and on equipment to carry out the procedure.

In one for charging the photoconductive layer of a recording material from US Pat 833 930 known method, this layer is simultaneously exposed to a total exposure while a charge electrode is passed over an insulating layer covering the photoconductive layer. Due to the simultaneous total exposure, the photoconductive layer is between the insulating layer and an electrically conductive carrier plate electrically conductive, so that a charge exchange between the insulating layer and the adjacent surface of the photoconductive layer can occur. Afterward the total exposure is canceled and the charging electrode is reversed with a second voltage Polarity fed, creating an intense charge exchange between the insulating and the photoconductive Layer occurs which applies a sufficiently large electrostatic charge to the photoconductive layer, without the need for particularly high charging voltages.

The object of the invention is to provide a new charging method, an imaging method using this charging method as well as to create devices for carrying out both procedures, which are based on the principle of photo-.

mission work.

In a method of the type mentioned at the outset, this object is achieved according to the invention by that the photoconductive layer is exposed to electromagnetic radiation, the wavelength of which is shorter than the cut-off wavelength for the electron emission of the photoconductor of the photoconductive layer, and that the emerging electrons are sucked off by means of an electrostatic field.

When using light radiation or other electromagnetic radiation with this wavelength Enough energy is transferred to the electrons in the photoconductor to allow them to pass through the wall potential energy escape on the surface of the photoconductor. The impoverishment of the surface of the photoconductor of electrons through photoelectric emission causes a usable positive charge on the plate, which is continuously built up during the exposure, the more electrons are emitted. In order to prevent electrons emitted from the photoconductive layer due to the growing positive Charge is captured again, is an electrostatic field to suck up the escaping electrons provided near the photoconductive surface. The electric field is preferred generated with the help of a suction electrode The potential applied to this electrode can be used to control the voltage to which the photoconductive layer is charged. This tension will do not significantly exceed the electrode voltage, because otherwise the layer is positive with respect to the electrode and traps electrons that have escaped

According to the invention, a radiation source with a relatively broad spectrum is used, which at suitable filtering both for uniform charging and for image-wise exposure of the photoconductor can be used in electrophotographic imaging. This is achieved by filtering out all radiation except for the short-wave, high-energy light from the radiation source for charging the photoconductive layer and the use of longer-wave radiation from the high UV range and / or the visible regions of the spectrum for imagewise exposure. The short-wave light charges the photoconductor by photoelectric emission, but does not expose it because the layer is related to this short-wave light shows at most a low photoconductive sensitivity on the other hand, the discharged visible part of the spectrum and the longer UV radiation the layer imagewise because the layer for this wavelength of radiation shows higher photoconductive sensitivity, but at the same time these wavelengths do not have enough energy to cause photoelectric emission and charge the layer.

The formation of devices for performing the inventive method for charging a photoconductive Further developments relating to the layer and generation of a charge image on this layer of the invention are specified in the further claims.

The invention is explained in more detail with reference to a preferred exemplary embodiment shown in the drawing explained. In detail shows

F i g. 1 is a side view of a simplified charger for a photoconductive recording material in section and

F i g. Figure 2 is a side view of a complete electrophotographic Copier with a radiation source for charging and imagewise exposure of the recording material in section

F i g. 1 shows a photoconductive recording material with a conductive base plate 11 and a photoconductive insulating layer 12. It should be noted that that the electrically conductive base plate 11 does not necessarily have to be present, but can be used can in order to establish an electrical connection with the base of the photoconductive layer 12 to facilitate. Immediately above the photoconductive layer 12 is one made of a transparent quartz layer 13 existing electrode arranged with an extremely thin, optically transparent, electrically conductive layer 14 of tin oxide or other suitable material is coated. The leading part the electrode is connected to the positive pole of a direct current source 16, the negative pole of which with the conductive base plate 11 is connected, which may be grounded. A radiation source is located above the electrode 17 and a reflector 18 arranged so that the radiation through the electrode on the photoconductive Layer is reflected. The radiation source 17 and the reflector 18 do not necessarily have to be arranged in this way be that the radiation penetrates the electrode. But since the electrode is quite close to the surface the photoconductive layer should be arranged in order to -this way as effectively as possible "leaked out of this layer Trapping electrons when the radiation hits the recording material makes it difficult to to arrange the radiation source so that the radiation impinges directly on the layer without the electrode to enforce. But the electrode can also be in many other places, including behind the radiation source or on the surface of the radiation source.

Because electrons that have escaped with the electrode are trapped and also photons of the charging radiation are to be interspersed, in most cases other arrangements can be used instead of those in the drawing shown electrode can be used. Another typical arrangement of this type is a grid or a screen made of stainless steel, copper, brass or other suitable conductive material. Although the charging radiation only penetrates the openings in the grid and not the opaque ones or wire areas themselves, this screen grid has been found to be a preferred electrode shape in the new process because there is a very fine pattern unevenly across the surface of the This type of charge pattern is created by recording material in small, discrete locations of distributed charges is used to create images with extensive, bold dark areas or with more uniform Tint particularly suitable. With ordinary charging, in which the charge pattern was originally is uniform over the entire recording material, even if this is in microscopic magnification is examined, originals with low contrast do not result in high potential gradients Except for their outer edges. As a result, the middle parts of such images are not filled in, it unless special development processes are used The uneven charge pattern calls however, a large number of high potential gradients over the entire surface of the recording material distributed so that even if originals with sufficient

stretched, very dark areas or with a uniform shade are used, the generated charge image does not contain high potential gradients distributed over the entire image area, which results in a greatly improved development even with conventional development processes. The screen used as an electrode can contain rectangular, round, straight, irregular or differently shaped openings, which can vary in size and density within wide limits. Typical constructions contain openings that cover 20 to 80% of the screen area and have a density of 8 to 200 per cm ² . Because of the macroscopic uniformity of the charge pattern, even if this is produced by such a screen, all such patterns in the description or in the claims shall be referred to as uniform.

As explained above, the selection of the photoconductor to be used and the radiation source to be used depend on one another, and each should be considered separately because the radiation source must provide light that is slightly shorter than the wavelength threshold, which is defined as the Axc / work function of the photoconductor can be. Here Λ is Planck's constant and c is the speed of light. For amorphous selenium z. For example, the cut-off wavelength in Angstroms would be less than 12,395 divided by the voltage equivalent of the work function for selenium, which corresponds to a wavelength of about 2,620 Angstrom units. It should be noted that this is only an approximation and that slightly higher wavelengths can be used because the work function for the material is calculated at absolute zero and the materials are usually used at higher temperatures, thereby increasing internal excitation in the photoconductor of electrons. Although photoconductors with a fairly high work function can be used in conjunction with the new method, which operate at fairly short radiation wavelengths, e.g. B. in the range of 2000 Angstroms, the shorter wavelengths may require special care in choosing the atmosphere between the charging radiation and the photoconductor, because some gases absorb the short wavelengths and prevent their transmission to the surface of the photoconductor. So is z. For example, it has been found that oxygen tends to partially absorb wavelengths of 2000 angstroms and less. Of course, this consideration can be ignored in those devices that operate in a vacuum or in a partial vacuum, such as. B. in space, can be used. When the device is used on the ground, it can be inserted into a container, the surface of which extends almost to the surface of the photoconductor, the container containing either a vacuum or a non-absorbing gas, or the area between the radiation source and the Photoconductor constantly from a non-absorbing gas, such as. B. nitrogen, are flushed.

Any suitable photoconductive layer can also be used in the new method. Typical photoconductive layers contain e.g. E.g .: amorphous selenium, alloys of sulfur, arsenic or tellurium with amorphous selenium, with trapping materials such as thallium, cadmium sulfide, cadmium selenide, etc .; doped amorphous Selenium, special photoconductive materials such as zinc sulfide, zinc cadmium sulfide, according to French Process produced zinc oxide, metal-free phthalocyanine, cadmium sulfide, cadmium selenide, zinc silicate, Cadmium sulfoselenide, etc., embedded in an insulating, film-forming inorganic binder, such as glass or in an insulating film-forming organic binder such as an epoxy resin, a silicone resin, an alkyd resin, a styrene-butadiene resin, a wax or the like. Other typical photoconductive insulating materials contain: mixtures, copolymers, terpolymers, etc. of photoconductors and non-photoconductive ones Materials that are either copolymerizable or miscible with one another to form solid solutions. Organic photoconductive materials of this type contain: anthracene, polyvinylanthracene, anthraquinone, Oxydiazole derivatives, such as 2,5-bis (p-amino-phenyl-1), 1,3,4-oxydiazole, 2-phenylbenzoxazole, and charge transfer complexes from complexing resins such as polyvinyl carbazole, phenol-aldehydes, epoxy compounds, Phenoxy compounds, polycarbonates, melamines, etc. with Lewis acids such as naphthalene anhydride; 2,4,7-trinitrofluorenone; Metal chlorides such as aluminum, zinc or iron chloride; 4-bis (dimethylamino) benzophenol; Chloranil; Picric acid; 1,3,5-trinitrobenzene; 1-chloroanthraquinone; Bromal; 4-nitrobenzaldehyde; 4-nitrophenol; Acetic anhydride; Maleic anhydride; Boron trichloride; Maleic acid; Cinnamic acid; Phenylformic acid; Tartaric acid; Malonic acid and mixtures thereof.

Because the new process creates a positive charge and some photoconductors work more efficiently when charged to one polarity or the other, preferably, but not exclusively, those used are those that are most positively charged work. It should be noted, however, that selenium is in its amorphous form with or without doping and alloys of amorphous form of selenium are a preferred material for the photoconductive insulating layer 13 because of their extremely high image reproduction quality and their relatively high quality Photosensitivity with a positive charge. The photoconductor can also be coated with a suitable material or be alloyed to reduce the potential surface energy wall or work function. These Type of coating can be photoconductive or non-photoconductive as long as it is sufficient in the dark Has insulation in order to prevent impairment of the charge image formed. Indeed, the ' Coating also opaque to the light of the wavelength used for exposure be when the photoconductor is applied to a transparent base plate and through this base plate is exposed through.

A preferred form of the new photoemissive charge is with a recording material having trapping sites applied for holes at least in the upper layer of the mass of the photoconductor. the high-energy radiation in the UV range necessary for the photo emission charge is used, penetrates only very slightly into the bulk of the photoconductive layer and here causes an emission of electrons and leaves defect electrons close to the surface, but still in the bulk of the photoconductor. Since the photoconductors, which work most effectively with a positive charge, have a fairly high charge carrier mobility, try the one generated by the charging radiation Defect electrons to some extent through the bulk of the photoconductor to the conductive one Underlay to flow off. As a result, when using such trapping points,

send layer at least in the vicinity of the top of the photoconductor generated by photoemission charge Holes trapped near the surface, causing the formation of a higher charge becomes possible on the shift. Although one containing a small amount of elemental thallium, e.g. B. 0.05 Percentage by weight, doped selenium in its amorphous form excellently causes the trapping point effect, Alternatively, a different material containing hole traps can be used. Typical such materials as selenium doped with thallium include cadmium sulfide and cadmium sulfoselenide. This material can be on the order of magnitude across the entire thickness of the photoconductor or just as the top layer of a few microns thick should be applied as any structure has the effect of making holes trapped in bulk near the surface of the photoconductor. It is pointed out, however, that in each arrangement the exposure from the upper surface of the layer with conventional light sources which a lot of light from the blue end of the visible spectrum and prolonged UV radiation should be avoided because light with these wavelengths is absorbed very close to the surface of most photoconductors and so only holes can form near that surface where the holes are trapped thereby preventing the generation of a charge image.

When using amorphous selenium, that is in its upper layer or through the entire mass is doped with thallium through it, this type of absorption takes place very close to the surface in the UV part of the electromagnetic spectrum. On the other hand, since selenium tends to emit electromagnetic radiation Orange-red end of the visible spectrum and X-rays To transmit, penetrating active radiation of this type can be used for exposure because the charge carriers are formed very close to the conductive base layer. As a result, you can even when using a photoconductive layer made of thallium-doped solid selenium with less Charge carrier mobility in the whole mass most of that generated by the exposing light source Defect electrons migrate through the photoconductor to the conductive base plate for discharge, causing a movement of electrons to trapped holes near the surface of the Layer is made possible in order to discharge this and generate the desired charge image.

Another technique is to form defect electron pairs in the photoconductor near the substrate is the photoconductor on a transparent surface, such as B. with tin oxide coated glass, arranged, the exposure taking place through the transparent substrate. This way can too Light from the UV and the blue parts of the spectrum can be used to create the image. In short, any exposure technique that places the hole pairs in the photoconductor close enough to the substrate so that the holes formed can pass through the Can move photoconductor through to the base. The layer arrangement with a thin layer with having less than about 3 microns thick of a hole trapping material over a photoconductor a wide area for hole electrons is an even better recording material for photo emission charge, because the hole pairs are somewhere in the bulk of the photoconductor under the capture layer can be generated, since this underlying layer has a sufficiently large area for holes possesses, so that this the underlying document also from the position immediately below can reach the trap layer. Another very important advantage of using a recording material With a capture layer is that the photo emission charge also with a radiation source

ίο with broad spectrum including radiation from blue end of the visible spectrum and longer UV rays as well as the shorter high-energy UV rays Can be done without a filter because, although high-energy UV rays emit electrons from the surface, the longer UV rays and the blue light affect the recording material not discharged due to the traps near their surface.

In Fig. 2 is an embodiment of an electrophotographic Copier shown, which is suitable for the application of the new charging technology. The device includes an electrophotographic drum 19 made up of a conductive pad 21 and one placed thereon photoconductive layer 22 consists of selenium.

The entire drum is rotatably mounted on a cylindrical shaft 23. During operation, the drum is with a uniform speed in the direction of the

- in. F i g. 2 rotated so that parts of the The drum circumference is first moved past the charging device, which has an optically transparent Contains electrode 24, as already described in connection with FIG. 1 was described. This electrode 24 is on a supply source 26 connected to a positive potential. A radiation source is located above the electrode 27 arranged with a broad spectrum, the radiation with a relatively short wavelength in the UV range, in the longer UV range and in the blue range of the visible spectrum. Typical Radiation sources of this type work with continuous or pulsating electrical discharge in one Mercury atmosphere, an iodine atmosphere, in noble gases or metal vapors. The radiation source 27 is located in a light-shielding housing 28 which has a slot 29 in a side wall. That The housing contains a filter 31 between the radiation source 27 and the electrode 24, which only absorbs UV radiation with a shorter wavelength and the UV radiation with a longer wavelength and the visible Filters out part of the spectrum. The electrode 24

So and the filter 31 can be combined by placing an optically transparent conductive on the surface of the filter Layer is applied. In this example a photoconductor such as amorphous selenium is used with such a work function that the shorter wavelength UV radiation emits photoelectric electrons causes, and also the photoconductive sensitivity in relation to prolonged UV rays and that When the light source 27 is switched on, the shortwave pass through UV rays with less than about 2650 Angstrom units the filter 31 and that as a breadboard used grid 24 and cause an electron emission from the underlying photoconductive layer 22. The grid 24 catches escaped electrons due to the electrical generated by the voltage source 26 Field by suction until the potential on the photoconductor reaches the potential of the voltage source 26. At the same time, unfiltered radiation also hits

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from the radiation source 27 onto the surface of an original image 32 to be reproduced. This original 32 is held on a cylindrical copying drum 33 by grippers 34 which are inserted in the direction indicated by the arrow direction indicated is rotated at the same peripheral speed as the electrophotographic Drum 19.

The unfiltered light originating from the radiation source 27 illuminates the original to be reproduced 32 and is reflected through the slit 29 and the lens 37 onto the mirror 38 and then exposes the photoconductive layer 22 of the drum 19. The lens prevents the passage of short-wave UV radiation and allows the passage of longer-wave UV rays and visible light because it is made of glass. Since the photoconductive insulating selenium is sensitive to visible light and longer-wave UV radiation, In the exposed areas, charge is diverted from the layer and, accordingly, from the dark areas A charge image is generated in areas of the original. Move after charging and exposure the parts of the drum 19 pass the developing device 41. It is about a A cascade processor which has an outer container or cover 42 with a trough attached comprises its bottom, which contains an inlet for the developing material 43. The development material is from the bottom of the container with a number of cups 44 on a driven endless conveyor belt 46 and cascaded over the drum surface. This developing device, which is described in U.S. Patents 2,618,552 and 2,618,551., uses a two-element mixture of finely divided Toner and carrier grains. The developed image is moved on until it comes into contact with a copy tape 47 arrives, which is pressed by two dummy rollers 48 against the drum surface, so that the The tape moves at the same speed as the drum circumference. The picture can be in usual Way on the tape, e.g. B. electrostatically transferred and then fixed.

1 sheet of drawings

Claims (10)

Patent claims: 1. A method for uniformly charging the photoconductive layer of a photoconductive recording material, in which the photoconductive layer is subjected to total exposure, d a through characterized in that the photoconductive layer emits electromagnetic radiation whose wavelength is shorter than the cut-off wavelength for electron emission the photoconductor of the photoconductive layer, and that the exiting electrons by means of an electrostatic Field to be sucked off. 2. Electrophotographic process for generating a charge image in which a uniform charged photoconductive layer is exposed imagewise, characterized in that a Recording material with a photoconductive layer is used, which is at least in the border area their surface to be charged has sufficient trapping points for holes to prevent them not under the influence of an electrically conductive support for the photoconductive layer through the photoconductive one To let the layer migrate, and characterized in that the photoconductive layer according to Claim 1 uniformly charged and then with a photoconductor generating .über the total exposure with the Auflaifung longer wave electromagnetic radiation is exposed imagewise. 3. The method according to claim 2, characterized in that the photoconductive layer on a transparent electrically conductive support is arranged and the imagewise exposure through takes place through the support 4. Apparatus for performing the method according to claim 1 with a device for total exposure the free surface of a photoconductive layer with electromagnetic radiation, thereby characterized in that the device (17,18) for total exposure an irradiation device (17) with a wavelength which is shorter than that Cutoff wavelength of the electron emission of the photoconductor of the photoconductive layer (12), and that im Area of total exposure opposite the photoconductive layer is an area that sucks off the emitted electrons Electrode (13, 14) is arranged, which is placed on the positive pole of a voltage source (16) is. 5. Apparatus according to claim 4, characterized in that the electrode has a grid of holes 6. Apparatus according to claim 4 or 5, characterized in that the irradiation device (17, 18) consists of a radiation source (17) with a broad spectrum, in front of which an optical filter is arranged is, the radiation of such wavelength filters out that in the photoconductive layer (12) photoconductive generated 7. Electrophotographic copier for performing the method according to claim 2 with a Means for uniformly charging a photoconductive layer and means for the same imagewise exposure, characterized in that the means (24, 27, 31) for uniform Charging consists of an irradiation device (27), the electromagnetic radiation of which has wavelength components that are shorter than the limit world len length for the electron emission of the photoconductor of the photoconductive layer (22) that between the Radiation source of the irradiation device and the photoconductive layer an optical filter (31) is arranged that filters out radiation of such wavelength that photoconductive in the photoconductive layer generated, and that the means (27, 37, 38) for imagewise exposure of the photoconductive Layer works with radiation of such wavelength that photoconductivity in the photoconductive layer generated. 8. Apparatus according to claim 7, characterized in that it is for total exposure and for imagewise Exposing the photoconductive layer (22) has a single source (27) of electromagnetic radiation, with which through the optical filter (31) the photoconductive layer can be totally exposed and at the same time a master copy (32) can be illuminated in an imaging station (29, 33). 9. Apparatus according to claim 7, characterized in that there is one on an electrically conductive transparent support arranged photoconductive layer (22), and that the device for imagewise exposure is arranged so that the photoconductive layer through this support can be exposed. 10. Apparatus according to, claims 7 to 9, characterized characterized in that there are transport devices (19, 23, 33) for the photoconductive recording material and the master (32) has to be synchronized with each other by the exposure or Ablichtstation (29,33) to transport.