EP0138376B1

EP0138376B1 - Method and apparatus of electrophotography

Info

Publication number: EP0138376B1
Application number: EP19840306194
Authority: EP
Inventors: Tutomu C/O Patent Division K.K. Toshiba Saito; Hitoshi C/O Patent Division K.K. Toshiba Yoneda; Tadayoshi C/O Patent Division K.K. Toshiba Ohno
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1983-09-19
Filing date: 1984-09-11
Publication date: 1987-03-04
Also published as: EP0138376A3; DE3462558D1; EP0138376A2

Description

The present invention relates to a method and apparatus of electrophotography utilized for a printer or a copying machine and, more particularly, to a method and apparatus for forming a toner image utilizing a photoreceptor having a photoconductive layer and a toner having no photoconductivity.
Electrophotography generally refers to an image-forming technique combining the photoconductive effect and the electrostatic attraction phenomenon. In each method applied to a copying machine or the like among the image forming techniques utilizing electrophotography, a development step is required in which an electrostatic latent image is formed on a photoreceptor and is converted into a toner image. This has prevented production of a more compact and inexpensive image-forming apparatus. Furthermore, the development step of an electrostatic latent image leads to a degradation in the picture quality due to the edge effect in which the field strength differs between the central and peripheral portions of the electrostatic latent image.
In order to solve this problem, various attempts have been made as in United States Patent No. 2,924,519, Japanese Patent Disclosure (Koukoku) No. 38-22645,- and Japanese Patent Disclosure (Koukai) No. 49-76531 (DE-A-2 357 099). These methods form a toner image in accordance with the following processes. First, a charged photoconductive toner is uniformly applied on a grounded electrically conductive support. The toner layer is exposed in accordance with the image density of the original object (copy) to selectively weaken the electrostatic attractive force acting between the support and toner. The toner in the exposed region with the weakened electrostatic attractive force is transferred to toner-receiving paper. Alternatively, after the toner in such a region is removed, the residual toner is transferred onto the same paper. In this manner, the toner image is formed on the toner-receiving paper.
However, with such a method, the effective sensitivity of the photoconductive toner is considerably lower than that of a photoreceptor used in other electrophotography techniques. This may be attributed to the following. First, during exposure, the light does not reach in a sufficient amount the deep region of the toner layer (i.e., the region near the support of the toner layer). Second, since the contact resistance between the toner particles is great, the charge generated upon exposure has difficulty reaching the support. If the sensitivity of the photoconductive toner is low, the density of the toner image is lowered, and fog occurs around the toner image, thus degrading picture quality. A photoconductive toner having a high sensitivity has not been proposed.
GB-A-876 577 discloses a method of electrophotography using a photoreceptor which comprises a transparent substrate on which a transparent electrically conductive layer is formed. A photoconductive insulating layer is formed on the transparent electrically conductive layer. The method employs the steps of applying toner to the surface of the photoconductive layer and then exposing photoconductive layer from the side of the transparent substrate. Toner particles on an unexposed region of the photoconductive layer are transferred to receiving paper without any prior toner removal and without the use of an opposed electrode.
It is an object of the present invention to provide a method of electrophotography to form an image without forming a latent image and developing the latent image.
It is another object of the present invention to provide a method and apparatus of electrophotography to form an image by using a general non- photoconductive toner.
It is still another object of the present invention to provide a compact electrophotography apparatus having a simple structure.
It is still another object of the present invention to provide an electrophotography apparatus which efficiently uses a toner.
The objects of the present invention are achieved by the methods and apparatus set out in the independent claims.
This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:-

Fig. 1 is a sectional view of a photoreceptor to be used according to the present invention;
Figs. 2A to 2D show image-forming processes according to a first embodiment of a method of electrophotography of the present invention;
Fig. 3 shows the construction of an LED printer utilizing the image-forming processes shown in Figs. 2A to 2D;
Fig. 4 shows the construction of a laser printer utilising the image-forming processes shown in Figs. 2A to 2D;
Figs. 5A to 5D show image-forming processes according to a second embodiment of a method of electrophotography of the present invention;
Fig. 6 shows the construction of an LED printer utilising the image-forming processes shown in Figs. 5A to 5D;
Fig. 7 shows the construction of a copying machine utilising the image-forming processes shown in Figs. 5A to 5D;
Figs. 8A to 8C show image-forming processes according to a third embodiment of a method of electrophotography of the present invention;
Fig. 9 shows the construction of an LED printer utilizing the image-forming processes shown in Figs. 8A to 8C;
Fig. 10 shows the construction of another LED printer utilizing the image-forming processes shown in Figs. 8A to 8C;
Fig. 11 shows the construction of a laser printer utilizing the image-forming processes shown in Figs. 8A to 8C;
Figs. 12A to 12C show image-forming processes according to a fourth embodiment of a method of electrophotography of the present invention;
Fig. 13 shows the construction of an LED printer utilizing the image-forming processes shown in Figs. 12A to 12C; and
Fig. 14 shows the construction of a copying machine utilizing the image-forming processes shown in Figs. 12A to 12C.

The photoreceptor 10 used in the present invention has the structure shown in Fig. 1 wherein a transparent electrically conductive layer 12 and a photoconductive layer 13 are sequentially formed on a transparent substrate 11. The transparent substrate 11 can be a glass plate or an organic material sheet. The transparent conductive layer 12 can comprise a NESA glass film, an indium oxide (ln₂0₃) film, or the like. The photoconductive layer 13 can consist of amorphous-Se, ZnO, OPC, amorphous-Si, CdS or the like. The term "transparent" herein means optical transparency such that light of a specific wavelength used in exposure is transmitted, and does not therefore necessarily mean colorless transparency. The photoconductive layer 13 has a suitable spectral sensitivity for the wavelength of light used for exposure. The thickness of the photoconductive layer 13 is preferably smaller than that of a photoconductive layer which is used in a photoreceptor of a conventional electrophotography apparatus. When the photoconductive layer 13 consists of amorphous-Se, for example, it preferably has a thickness of about 1 to 20 µm as compared to a thickness of 50 to 60 pm of the photoconductive layer of the conventional apparatus. This is attributed to the following. When the photoconductive layer 13 is exposed with light received through the transparent substrate 11, the light must be able to reach near the surface of the photoconductive layer 13 on which the toner is attached.
Image forming processes according to a first embodiment of a method of electrophotography of the present invention will be described with reference to Figs.2A to 2D. First as shown in Fig.2A, the photoconductive layer 13 of the photoreceptor 10 shown in Fig. 1 is uniformly charged to a given polarity (positive in the drawings) by a charger 21 in a dark-environment.
Then, as shown in Fig. 2B, toner 22 charged to the opposite polarity (negative) of the photoconductive layer 13 is applied on the entire surface of the photoconductive layer 13. The toner 22 is an insulating toner generally used in conventional electrophotography and does not have photoconductivity. The toner 22 can be applied by various methods such as the method of applying a charged toner with a blade, the magnetic brush method combining a powder consisting of a magnetic carrier and toner with a magnetic roller, the cascade method using a powder consisting of a toner and a relatively coarse bead-like substance, the fur brush method using a toner and a fur brush, and the powder cloud method of spraying a toner through a metal pipe and atomizing it.
Next, as shown in Fig. 2C, light 23 corresponding to an input image is irradiated onto the photoconductive layer 13 through the transparent conductive layer 12 from the side of the transparent substrate 11, thereby making an exposure. At the same time, a voltage having an opposite polarity (positive) to that of the toner 22 is applied from a power source 25 to an opposite electrode 24 opposite the photoconductive layer 13. Then, the exposed region of the photoconductive layer 13 is rendered electrically conductive. Therefore, the charge contributing to the attractive force of the toner 22 on the exposed region of the photoconductive layer 13 is moved through the transparent conductive layer 12 and disappears. Since the attractive force on the photoconductive layer 13 is weakened, the toner on the exposed region of the photoconductive layer 13 is removed from the surface of the photoconductive layer 13 by an electric field which is generated between the transparent conductive layer 12 and the opposite electrode 24 upon application of a voltage from the power source 25. The toner is moved toward the opposite electrode 24 and is transferred onto the surface of toner-receiving paper 26 held between the photoconductive layer 13 and the opposite electrode 24. The toner-receiving paper 26 can be plain paper.
In order to allow easy removal of the toner from the exposed region of the photoconductive layer 13, the voltage from the power source 25 is preferably a pulsating voltage obtained by superposing an AC voltage on a DC voltage. The timing of the voltage applied between the transparent conductive layer 12 and the opposite electrode 24 from the power source 25 need not be synchronous with the timing of the exposure but may be applied after exposure.
As shown in Fig. 2D, the residual toner on the nonexposed region of the photoconductive layer 13 is removed by a cleaning blade 27 and is reused. In this cleaning process, when the entire lower surface of the transparent substrate 11 is irradiated with light from a charge removal lamp 28, the charge on the photoconductive layer 13 is removed to allow removal of the residual toner on the photoconductive layer 13. The cleaning method may alternatively be a fur brush cleaning method or the like. When the removed toner is recovered in this manner, it is conveyed for reuse in another toner applying process as shown in Fig.2B. Thus, a series of image-forming processes is completed.
Fig. 3 shows an LED printer according to an embodiment of an apparatus of electrophotography utilizing the image-forming processes described above. A photosensitive drum (photoreceptor) 30 is formed in a cylindrical shape such that its photoconductive layer faces outward. A transparent conductive layer of this photosensitive drum 30 comprises, for example, a deposition film of In₂0₃, and the photoconductive layer thereof comprises, for example, a selenium film having a thickness of 15 um. The photosensitive drum 30 is driven to rotate in the direction indicated by arrow 31. A charger 32, a toner applying means 33, an exposing means 36, an opposite electrode 37, and a cleaning station 41 are arranged along the rotating direction of the photosensitive drum 30 as a means which is used in the processes shown in Figs. 2A to 2D.
The charger 32 charges the photoconductive layer of the photosensitive drum 30 to have a surface potential of about +200 V. The toner applying means 33 applies on the surface of the charged photoconductive layer by means of a blade 35 a toner negatively charged by being stirred by a charging roller 34.
The exposing means 36 arranged inside the photosensitive drum 30 comprises an LED (light- emitting diode) array arranged linearly along the direction of the rotating axis of the drum 30, and a rod lens array for guiding light from the LED array. The exposing means 36 exposes the photoconductive layer from the transparent substrate side of the drum 30. In this case, the LED array is driven in accordance with an electrical image signal supplied from an external drive circuit. The exposing means 36 is adjusted such that the photoconductive layer is located within the focal depth of the rod lens array.
The opposite electrode 37 is located opposite the exposing means 36 with the photosensitive drum 30 interposed therebetween. In this embodiment, the opposite electrode 37 comprises an aluminum roller having a diameter of 5 mm and a conductive rubber sheet wound therearound and having a resistivity (or specific resistance) of 1,000 Ω·cm. The opposite electrode 37 is pressed by a spring 38 toward the photosensitive drum 30 through toner-receiving paper 39 at a force of 0.7 kg/cm². In this embodiment, a positive voltage, for example, about +180 V is applied to the opposite electrode 37. This voltage is preferably a voltage obtained by superposing an AC voltage on a DC voltage. Therefore, due to the electric field generated upon application of a voltage on the opposite electrode 37, the toner on the exposed region of the photoconductive layer whose attractive force is weakened upon exposure to the exposing means 36 is transferred to the toner-receiving paper 39. Thus, a toner image corresponding to the electrical image signal supplied to the exposing means 36 is formed on the toner-receiving paper 39. The toner image is fixed on the toner-receiving paper by a fixing station 40 to be an output image. The toner image thus obtained has a uniform density at a solid portion and clear printing elements in a line drawing portion. Thus, an excellent image can be obtained for various types of image patterns.
The residual toner on the photosensitive drum 30 which was not used in image formation is removed by a rotating fur brush at the cleaning station 41. A charge removal lamp 43 for radiating light onto the lower surface of the drum 30 is arranged in the cleaning station 41. The toner removed by the cleaning station 41 is guided to a conveying means 44 to be conveyed to the toner applying means 33 through a chain 45 for reuse.
Fig. 4 shows an embodiment wherein the image-forming processes shown in Figs. 2A to 2D are applied to a laser printer. In this embodiment, a web shape photoreceptor is used. The use of such a photoreceptor provides less limitations on the size of the exposure means or on the length of the optical path from the exposure means to the photoreceptor. A photosensitive web 50 comprises a polyethylene terephthalate film having a thickness of about 50 pm as a transparent substrate, a palladium film deposited thereon as a transparent conductive layer, and an Se-As-Te photosensitive material layer having a thickness of about 15 pm as a photoconductive layer. The web 50 is moved from a supply roller 51 to a take- up roller 55 at a suitable speed. At this time, a suitable tension is applied to the web 50 and is kept flat in each process by rollers 52, 53 and 54. A charger 32, a toner-applying means 56, an exposing means 58, an opposite electrode 59, and a cleaning station 41 are arranged along the moving direction of the photosensitive web 50.
The photoconductive layer of the photosensitive web 50 is charged by the charger 32 to have a surface potential of about +200 V. Then, toner is uniformly applied on the photoconductive layer with the toner-applying means 56. The toner-applying means 56 adopts in this embodiment the magnetic brush method which combines a mixture of a magnetic carrier and toner, and a magnetic roller 57. The means 56 applies the negatively charged toner by friction with the carrier on the photoconductive layer. The amount of toner applied can be controlled by changing the voltage applied from a control power source (not shown) to the magnetic roller 57 within a range of, for example, +50 V to +100 V. Thus, the density of the output image can be changed as needed.
The exposing means 58 exposes the photoconductive layer applied with the toner from the side of the transparent substrate. The exposing means 58 comprises an optical system mainly having a laser diode, a polygon mirror, a scanning lens, and a peripheral circuit including a drive circuit. The exposing means 58 is adjusted such that the photoconductive layer is located within the focal depth of the optical system. The laser beam is modulated in accordance with an electrical modulation signal supplied from an external circuit and linearly scans the photoconductive layer of the web 50 using the polygon mirror and the scanning lens from the side of the transparent substrate along the perpendicular direction toward the sheet of the drawing.
The toner with the weakened attractive force toward the photoconductive layer upon exposure is transferred onto toner-receiving paper 39 by the electric field generated by the voltage applied to the opposite electrode 59 during exposure. A toner image is thus formed. The opposite electrode 59 comprises a corona charger in this embodiment. The distance from the web 50 to a corona wire of the corona charger is set to be 15 mm, and a voltage applied to the corona wire is set to be +5.5 kV. The toner image is fixed by a fixing station 40, as in the case of the embodiment shown in Fig. 3. The toner remaining on the web 50 is removed by a cleaning station 41, and the removed toner is conveyed to a toner-applying means 56 by a toner conveying means 44.
Figs. 5A to 5D show image-forming processes according to a second embodiment of a method of electrophotography of the present invention. The processes shown in Figs. 5A and 5B are the same as those shown in Figs. 2A and 2B. After the process of Fig. 5B, as shown in Fig. 5C, light 23 corresponding to an input image is irradiated onto a photoconductive layer 13 through a transparent conductive layer 12 from the side of a transparent substrate 11, thereby performing exposure. Upon exposure, toner in an exposed region of the photoconductive layer 13 looses some of its attractive force. Utilizing this phenomenon, simultaneously or after the exposure, the toner on this exposed region is removed by a blower 29. In this way, toner remains on the non-exposed region of the photoconductive layer 13. The toner may be removed by other methods such as by using a conductive roller, by combining a conductive roller and a dielectric roller, by using a dielectric film and a corona charger for charging this film, or by other methods.
As shown in Fig. 5D, as in the process shown in Fig. 2C, a voltage having the opposite polarity as that of the toner is applied from a power source 25 to an opposite electrode 24 arranged opposite the photoconductive layer 13. Then, the toner remaining on the non-exposed region of the photoconductive layer 13 is removed from the layer 13 and is transferred to toner-receiving paper 26 held between the photoconductive layer 13 and the opposite electrode 24. When the voltage is applied from the power source 25 to the opposite electrode 24, the photoconductive layer 13 is preferably irradiated with light from a lamp 28 from the side of the transparent substrate 11 through the transparent conductive layer 12. Upon this irradiation with light, the attractive force of toner toward the photoconductive layer 13 is effectively weakened, and the transfer efficiency of the toner image is improved by about 100%. Therefore, the cleaning process after the transfer process can be omitted.
Fig. 6 shows an LED printer as an embodiment of an apparatus of electrophotography utilizing the image-forming processes shown in Figs. 5A to 5D. The primary differences between this printer and the printer shown in Fig. 3 will be described. A toner-removing means 60 is arranged at a position to be opposite to an exposing means 36 with a photosensitive drum 30 interposed therebetween. The toner-removing means 60 comprises a conductive roller 61 similar to that used in the opposite electrode 37 shown in Fig. 3, a spring 62 for pressing the roller 61 toward the drum 30 through the toner-receiving paper 39, and a blade 63. A voltage having a polarity opposite that of the toner, for example, +120 V, is applied to the roller 61. The toner on the exposed region of the photoconductive layer of the photosensitive drum 30 is attracted toward the surface of the roller 61 by the electrostatic attractive force and is removed from the surface of the roller 61 by means of the blade 63. The removed toner is conveyed to a toner-applying means 33 by a toner-conveying means 44. A transferring means 64 comprises an opposite electrode 65 and a lamp 66. In this embodiment, the opposite electrode 65 is a corona charger. The distance from the drum 30 to the corona wire of the corona charger is set to be 15 mm, and the application voltage on the corona wire is set to be +5.5 kV.
Fig. 7 shows an embodiment wherein the image-forming processes shown in Figs. 5A to 5D are applied to a copying machine. In this embodiment, a photosensitive web 50 and a moving means therefor are of the same construction as that shown in Fig. 4, and the remaining structure is the same as that shown in Fig. 6 except an exposing means. An exposing means 70 is an optical system comprising a document table 71 for placing a document (original object) 72 thereon, a light source 73 for illuminating the surface of the document 72 through the document table 71, and a lens 74 for forming an image of the document 72 onto the photoconductive layer of the photosensitive web 50. The document table 71 is moved together with the web 50, so that the image on the entire surface of the document 72 is scanned and formed on the photoconductive layer of the web 50.
Figs. 8A to 8C show image-forming processes according to a third embodiment of a method of electrophotography of the present invention. As shown in Fig. 8A, toner 81 is applied on the entire surface of a photoconductive layer 13 of a photoreceptor 10. The toner 81 has an electrical conductivity and preferably has a resistivity of 10⁶ to 10⁸ Ω·cm. The method of applying the toner can be the same as in the earlier embodiments. Unlike in the processes shown in Figs. 2A to 2D or in Figs. 5A to 5D, the photoconductive layer 13 need not be charged.
Then, as shown in Fig. 8B, light 82 corresponding to an input image is irradiated onto the photoconductive layer 13 through a transparent conductive layer 12 from the side of a transparent substrate 11. At the same time, a voltage of a single polarity is applied from a power source 84 to an opposite electrode 83 opposite the photoconductive layer 13. The voltage is negative in this embodiment but can be positive. In this embodiment, the power source 84 also preferably generates a pulsating voltage. An exposed region of the photoconductive layer 13 is rendered conductive and is injected with positive charges. As a result, the toner on the exposed region of the photoconductive layer 13 is removed from the surface of the photoconductive layer 13 by an electric field generated between the transparent conductive layer 12 and the opposite electrode 83 from the power source 84. The toner is transferred onto the surface of toner-receiving paper 85 held between the photoconductive layer 13 and the opposite electrode 83. A toner image is thus formed.
Subsequently, as shown in Fig. 8C, the toner which was not used in forming the image remaining on the photoconductive layer 13 is removed by a cleaning blade 86 and reused. The removing method may be one of the methods described above. The removed toner is conveyed to be used for another toner application process, as shown in Fig. 8A. In this embodiment, the image can be formed with less processes than is shown in Figs. 2A to 2D or in Figs. 5A to 5D.
Fig. 9 shows an embodiment wherein the image-forming processes shown in Figs.8A to 8C are applied to an LED printer. In a toner-applying means 33, a blade 35 is formed of a conductive rubber, and a voltage is applied to the blade 35 from a power source 91. Then, the toner applied on the photoconductive layer of a photosensitive drum 30 is injected with charges and is thus rendered with an electrostatic attractive force acting in addition to the mechanical attractive force toward the photoconductive layer. Therefore, in the printer as shown in Fig. 9 wherein the toner is subject to the influence of gravity between the toner application process, and the exposure and transfer processes, the inadvertent removal of the toner from the photosensitive drum 30 is prevented. Stable toner application and conveying can therefore be performed. The polarity of the charge injected in the toner in this toner application process is preferably opposite to that of the charges injected in the exposure and transfer processes.
A transferring means 92 comprises a power source 94 and an opposite electrode 93 which is opposite the exposure means 36 comprising LEDs with the drum 30 interposed therebetween. The opposite electrode 93 is an electrically conductive roller as described above and is applied with a voltage of a single polarity, for example, about -600 V from the power source 94, preferably a pulsating voltage. The exposure by the exposing means 36 is performed simultaneously with the transfer of the toner image onto the toner-receiving paper 39. The toner remaining on the drum 30 is removed by a cleaning station 95, and is conveyed to the toner-applying means 33 by a toner-conveying means 44.
In the embodiment shown in Fig. 10, the cleaning station 95 is omitted. In this embodiment, the cleaning station 95 can be omitted for the following reason. Since the toner has an electrical conductivity, the attractive force of the toner remaining on the photosensitive drum 30 is extremely weak, and the toner can be formed into a uniform thin film by the toner applying means 33.
Fig. 11 shows an embodiment wherein the image-forming processes shown in Figs. 8A to 8C are applied to a laser printer. This embodiment is the same as that shown in Fig. 9 except that the exposing means 58 is an optical scanning system using a laser beam.
Figs. 12A to 12C show image-forming processes according to a fourth embodiment of a method of electrophotography of the present invention. In this embodiment, the toner on an exposed region of the photoconductive layer 13 in the process shown in Fig. 12B is removed by an opposite electrode 83. As shown in Fig. 12C, a negative voltage is applied from a power source 88 on another opposite electrode 87 also opposite the photoconductive layer 13. The power source 88 also produces a pulsating voltage. As a result, the toner on a non-exposed region of the photoconductive layer 13 is removed from the surface of the photoconductive layer 13 by an electric field generated between the transparent conductive layer 12 and by the opposite electrode 87 upon application of the voltage from the power source 88 and is transferred to the surface of the toner-receiving paper 85. Thus, a toner image is formed.
Fig. 13 shows an embodiment wherein the image forming processes in Figs. 12A to 12C are applied to an LED printer. The embodiment shown in Fig. 13 is different from that shown in Fig. 9 in that a toner-removing means 100 comprises an electrically conductive roller 101, a power source 102 for applying a voltage of a single polarity to the roller 101, and a toner-removing blade 103. A transfer means 104 comprises a flat opposite electrode 105 and a transfer lamp 106.
Fig. 14 shows an embodiment wherein the image-forming processes shown in Figs. 12A to 12D are applied to a copying machine. This embodiment comprises a combination of the arrangement shown in Fig. 7 with that shown in Fig. 13.

Claims

1. A method of electrophotography using a photoreceptor (10) consisting of a transparent electrically conductive layer (12) and a photoconductive layer (13) sequentially formed on a transparent substrate (11), and a toner (22) having no photoconductivity, comprising the steps of:

applying said toner (22, 81) on a surface of said .photoconductive layer (13);

exposing said photoconductive layer (13) applied with said toner (22, 81) from the side of said transparent substrate (11); characterized by the step of

transferring toner particles on an exposed region of said photoconductive layer to toner-receiving paper (26, 85) opposite said photoconductive layer (13).

2. A method according to claim 1, characterized in that said photoconductive layer (13) is uniformly charged with a polarity, and said toner (22) has an insulating property, is charged with a polarity opposite to that of said photoconductive layer (13) and is applied on the surface of said photoconductive layer (13).

3. A method according to claim 1, characterized in that said toner (81) has conductivity, and said toner particles on said exposed region are selectively charged during exposure.

4. A method of electrophotography using a photoreceptor (10) consisting of a transparent electrically conductive layer (12) and a photoconductive layer (13) sequentially formed on a transparent substrate (11), and a toner (22) having no photoconductivity, comprising the steps of:

applying said toner (22, 81) on a surface of said photoconductive layer (13);

exposing said photoconductive layer (13) applied with said toner (22, 81) from the side of said transparent substrate (11), characterized by the steps of

removing toner particles on an exposed region of said photoconductive layer (13); and

transferring said toner particles on a non-exposed region of said photoconductive layer (13) to toner-receiving paper (26, 85) opposite said photoconductive layer (73).

5. A method according to claim 4, characterized in that said photoconductive layer (13) is uniformly charged with a polarity, and said toner (22) has an insulating property, is charged with a polarity opposite to that of said photoconductive layer (13) and is applied on the surface of said photoconductive layer (13).

6. A method according to claim 4, characterized in that said toner (81) has conductivity, and said toner particles on said exposed portion are selectively charged during exposure.

7. An apparatus of electrophotography for forming a toner image by using a photoreceptor (30, 50) which consists of a transparent electrically conductive layer and a photoconductive layer sequentially formed on a transparent substrate and which is moved along one direction, and a toner having no photoconductivity, comprising:

charging means (32), opposite said photoconductive layer at a proper position, for charging said photoconductive layer;

applying means (33), disposed in front of said charging means (32) along the moving direction of said photoreceptor (30, 50), for applying on a surface of said photoconductive layer a toner charged with a polarity opposite that of said photoconductive layer which is charged by said charging means (32);

exposing means (36, 58), disposed in front of said applying means (33) along the moving direction of said photoreceptor (30, 50), for exposing said photoconductive layer from the side of said transparent substrate, characterized by

an opposite electrode (37, 59) disposed at a position opposite said exposing means (36, 58) in regard of said photoreceptor (30, 50); and

transferring means (64) for applying to said opposite electrode (37, 59) a voltage having a polarity opposite to that of said toner, and for transferring toner particles on an exposed region of said photoconductive layer to toner-receiving . paper (39) held between said opposite electrode (37, 59) and said photoconductive layer.

8. An apparatus according to claim 7, characterized in that said exposing means (36, 58) scans said photoconductive layer with light controlled in response to an electrical image signal and exposes said photoconductive layer.

9. An apparatus according to claim 7, characterized in that said voltage is obtained by superposing an AC voltage on a DC voltage.

10. An apparatus according to claim 7, characterized by further comprising:

residual toner removing means (41) for removing residual toner particles which are not transferred to said toner-receiving paper (39); and conveying means for conveying said residual toner particles removed by said residual toner-removing means (41) to said applying means (33).

11. An apparatus of electrophotography for forming a toner image by using a photoreceptor (30, 50) which consists of a transparent electrically conductive layer and a photoconductive layer sequentially formed on a transparent substrate and which is moved along one direction, and a toner having no photoconductivity, comprising: charging means (32), opposite said photoconductive layer at a proper position, for charging said photoconductive layer;

applying means (33), disposed in front of said charging means (32) along the moving direction of said photoreceptor (30, 50), for applying on a surface of said photoconductive layer a toner charged with a polarity opposite to that of said photoconductive layer which is charged by said charging means (32);

toner-removing means (60), disposed in a vicinity of said exposing means (36, 70) so as to be opposite said photoconductive layer, for removing toner particles on an exposed region of said photoconductive layer;

an opposite electrode (65) disposed in front of said toner-removing means (60) along the moving direction of said photoreceptor (30, 50); and

transferring means (64) for applying to said opposite electrode (37, 59) a voltage having a polarity opposite to that of said toner, and for transferring said toner particles on a non-exposed region of said photoconductive layer to toner-receiving paper (39) held between said opposite electrode (37, 59) and said photoconductive layer.

12. An apparatus according to claim 11, characterized in that said exposing means (70) is an optical system to form an image of an original object (72) onto said photoconductive layer.

13. An apparatus according to claim 11, characterized in that said voltage is obtained by superposing an AC voltage on a DC voltage.

14. An apparatus according to claim 11, characterized by further comprising: conveying means for conveying the toner particles removed by said toner removing means (64) to said applying means (33).

15. An apparatus according to claim 11, characterized in that said transferring means (64) applies the voltage to said opposite electrode (65) and illuminates said photoconductive layer with light from a side of said transparent substrate.

16. An apparatus of electrophotography for forming a toner image by using a photoreceptor (30, 50) which consists of a transparent electrically conductive layer and a photoconductive layer sequentially formed on a transparent substrate and which is moved along one direction, and a toner having no photoconductivity, characterized by comprising:

applying means (33) for applying a toner having conductivity on a surface of said photoconductive layer at a proper position;

exposing means (36, 58), disposed in front of said applying means (33) along the moving direction of said photoreceptor (30, 50) for exposing said photoconductive layer from the side of said transparent substrate;

an opposite electrode (93) disposed at a position opposite said exposing means (36, 58) in regard of said photoreceptor (30, 50); and

transferring means for applying a DC voltage to said opposite electrode (93) to transfer toner particles on an exposed region of said photoconductive layer to toner-receiving paper (39) held between said opposite electrode (93) and said photoconductive layer.

17. An apparatus according to claim 16, characterized in that said exposing means (36, 58) scans said photoconductive layer with light controlled in accordance with an electrical image signal and exposes said photoconductive layer.

18. An apparatus according to claim 16, characterized in that said voltage is obtained by superposing an AC voltage on a DC voltage.

19. An apparatus according to claim 16, characterized by further comprising:

residual-toner removing means (95) for removing residual-toner particles which are not transferred to said toner-receiving paper; and

conveying means (44) for conveying the residual-toner particles removed by said residual-toner removing means (95) to said applying means (33).

20. An apparatus according to claim 16, characterized in that said applying means (33) charges the toner having conductivity and applies the toner on the surface of said photoconductive layer.

21. An apparatus of electrophotography for forming a toner image by using a photoreceptor (30, 50) which consists of a transparent electrically conductive layer and a photoconductive layer sequentially formed on a transparent substrate and which is moved along one direction, and a toner having no photoconductivity, characterized by comprising:

exposing means (36, 70), disposed in front of said applying means (33) along the moving direction of said photoreceptor (30, 50), for exposing said photoconductive layer from the side of said transparent substrate;

toner-removing means (100) opposite to said exposing means (36, 70) in regard of said photoreceptor (30, 50) for removing toner particles on an exposed region of said photoconductive layer;

an opposite electrode (105) disposed away from said toner-removing means (100) along the moving direction of said photoreceptor (30, 50); and

transferring means (104) for applying a single polarity voltage to said opposite electrode (105) to transfer said toner particles on a non-exposed region of said photoconductive layer to toner-receiving paper held between said opposite electrode (105) and said photoconductive layer.

22. An apparatus according to claim 21, characterized in that said exposing means (70) causes an optical system to focus an image of a copy (72) onto said photoconductive layer.

23. An apparatus according to claim 21, characterized in that said voltage is obtained by superposing an AC voltage on a DC voltage.

24. An apparatus according to claim 21, characterized by further comprising conveying means (44) for conveying the toner particles removed by said toner-removing means to said applying means (33).

25. An apparatus according to claim 21, characterized in that said transferring means (104) applies the single polarity voltage to said opposite electrode (105) and illuminates said photoconductive layer with light from the side of said transparent electrode.

26. An apparatus according to claim 21, characterized in that said applying means (33) charges said toner having conductivity and applies the toner on said surface of said photoconductive layer.