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Publication numberUS3679405 A
Publication typeGrant
Publication dateJul 25, 1972
Filing dateAug 26, 1968
Priority dateAug 26, 1967
Also published asDE1797176A1, DE1797176B2, DE1797176C3
Publication numberUS 3679405 A, US 3679405A, US-A-3679405, US3679405 A, US3679405A
InventorsMakino Katsuo, Sawato Iwao
Original AssigneeFuji Photo Film Co Ltd
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Electrophotographic element having a series of alternate photoconductive and insulating layers
US 3679405 A
Abstract  available in
Images(2)
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Claims  available in
Description  (OCR text may contain errors)

July 25, 1972 KATSUQ MAKINO ETAL 3,679,405

ELECTROPHOTOGRAPHIC ELEMENT HAVING A SERIES OF ALTERNATE PHOTOCONDUCTIVE AND INSULATING LAYERS Filed Aug. 26. 1968 2 Sheets-Sheet 1 l q; 2; 2 4 2 FIG. IA FIG. I B FIG. IG

SURFACE POTENTIAL TIME AFTER EXPOSURE OPTICAL DENSITY INVENTORS LOGARITHMIC EXPOSURE ILLUMINATION KATSUO MAKINO IWAO SAWATO FIG. 3

July 25, 1972 KATSUQ MAKINQ EI'AL 3,679,405

ELECTROPHOTOGRAPHIC ELEMENT HAVING A SERIES OF ALTERNATE PHOTOCONDUCTIVE AND INSULATING LAYERS 2 Sheets-Sheet 2 Filed Aug. 26. 1968 0 TIME AFTER EXPOSURE FIG 6 LOGARITHMIC EXPOSURE QUANTITY I IflIII r Ewzma 2250 FIG] LOGARITHMIC EXPOSURE QUANTITY 1N VENT 0R5 KATSUO IIAKINO IWAO SAWATO Emzma EEEQ v I ATTORN 3Y5 1U nited States Patent ()fli 3,679,405 Patented July 25, 1972 US. Cl. 96-1-5 14 Claims ABSTRACT OF THE DISCLOSURE An electrophotographic element is made having n photoconductive insulating layers and n-1 insulating layers respectively interposed between said n photoconductive insulating layers, where n=2, 3 il, i, IN. The photoelectric current generated in the ith photoconductive layer is greater than that generated in the (i'l)th photoconductive layer, the (il)th layer being on the exposed side of the element.

BACKGROUND OF THE INVENTION Field of invention The present invention relates to electrophotography, and more particularly to a photosensitive material used in electrophotography.

[Description of prior art Electrophotography is defined as a technique wherein copies of photographs or prints are made by charging an electrophotosensitive material and image exposing the photosensitive material to form an electrostatic latent image on the surface of a photoconductive insulating layer. The electrostatic latent image is developed with the electrostatic forces of line colored charged particles.

In electrophotography a photoconductive insulating material which consists of photoconductive fine powders containing mainly amorphous selenium or cadmium sulfide dispersed in electric insulating binder is used. Besides, zinc oxide, titanium oxide and the like is normally used as the fine photoconductive powder. The photoconductive powder is dispersed in the electric insulating binder and coated on a conductive base such as a metal plate, metal sheet, paper sheet or plastic sheet treated with conductive material to form an electrophotosensitive material. This material is uniformly charged on the photoconductive insulating layer by corona discharge or the like. The charge is maintained according to its dielectricity in the dark portion of the photoconductive insulating layer. When the photoconductive insulating layer is exposed with a picture image, the charged insulating layer is discharged to an extent proportional to the exposed light intensity so as to form a charge pattern on the surface of the photoconductive insulating layer. The electrostatic latent image formed on the surface of the photoconductive insulating layer as described above is then developed for instance by being cascaded with charged colored resin powder. Since some of the charge remains on the unexposed areas of the photoconductive insulating layer, a large amount of the colored resin powder will adhere to the unexposed areas and a high density powder image will be created on the photoconductive insulating layer. When the colored resin powder is a thermoplastic, the powder image is fixed by heat fusing.

There is no established theory explaining how the static charge on the charged photoconductive insulating layer is discharged by light exposure but it is believed to occur as follows: Aetinic light is absorbed near the surface of the photoconductive layer. The free charge carriers carry photoelectric current exclusively in the vicinity of the surface of the photoconductive insulating layer. It is therefore necessary in order to increase the sensitivity, to transfer these free charged carriers as close as possible to the base, that is to lengthen the range of the free charge carrier as much as possible. Alternatively, the sensitivity can be indirectly increased by making the thickness of the photoconductive insulating layer as thin as possible, or by making it possible for the actinic light to penetrate the photoconductive insulating layer. In any event, it is most important that the range of the free charged carriers of the photoconductive insulating layer of high sensitivity be as long as possible.

The electrophotographic gradation of the electrophotographic photosensitive material with the photosensitive layer of photoconductive insulating material having a long range free charged carrier, as described above, is generally sharp. That is, the density varies from fog density to maximum density in the range of 0.8 to 1.0 on a log exposure value scale graded. This sharpness is very favorable for copying a line drawing or stipple but is unsuitable for copying picture photographs having a continuous gradation.

SUMMARY OF THE \INVEN'TION The present invention provides an electrophotographic photosensitive material having a gradual photographic gradation using photoconductive insulating materials and having long range free charged carriers. The present invention further provides a method of manufacturing the electrophotographic photosensitive material which can change the gradation of the photograph from sharp to gradual by use of a highly sensitive photoconductive insulating material having long range free charge carriers.

The electrophotographic photosensitive material provided by the present invention comprises a base and. a photosensitive layer. The base is not essential and may be omitted. The base may be conductive or not, and may be transparent or opaque to light or radiation. The photosensitive layer consists of at least two photoconductive insulating layers and a thin insulating layer sandwiched between said photoconductive insulating layers. These photoconductive insulating layers should be able to meet the following conditions:

The unexposed side of the photoconductive insulating layer has a higher sensitivity in the spectroscopic region than in the exposed side of the photoconductive insulating layer. Any photoconductive insulating layer between the opposite sides of the photoconductive insulating layers has a lower sensitivity than the unexposed side of the photoconductive insulating layer and higher sensitivities than the photoconductive insulating layer on the exposed side in the spectroscopic region or in another spectroscopic region. Each photoconductive insulating layer is transparent to light within at least a portion of the spectroscopic range where the unexposed side of the photoconductive insulating layers is sensitive.

The exposed side of the photoconductive layer is the photoconductive insulating layer, on the top of the photosenstitive material, formed by providing a photosensitive layer on the opaque base, and the unexposed side photoconductive insulating layer is the photoconductive insulating layer at the bottom of said photosensitive layer. In both cases, the photosensitive material is exposed from the photosensitive layer side. When the photosensitive material having the photosensitive layer is provided on a transparent base and is exposed from the base side, the photoconductive insulating layer on the top of the photosensitive material becomes the unexposed side of the photoconductive insulating layer and the photoconductive insulating layer on the bottom of the photosensitive layer becomes the exposed side of the photoconductive insulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. l-a, l-b and 1-c are cross-sectional views of photosensitive material showing the structure of the conventional single layer and semisingle layer photosensitive material;

FIG. 2 is a diagram showing the variation with exposure time of the electric surface potential of the electrophotosensitive material shown in FIGS. l-a, l-b and 1-0;

FIG. 3 is a diagram showing the relation between the log exposure value and the optical density of the photosensitive layer consisting of a singlp photoconductive insulating layer;

FIG. 4 is a cross-sectional view of an embodiment of the electrophotosensitive material in accordance with the present invention consisting of two photoconductive insulating layers;

FIGS. 5 and 6 are diagrams showing variations with exposure time of the surface electric potential of the electrophotosensitive material shown in FIG. 4;

FIG. 7 is a diagram showing the characteristic curve representing the variation of the optical density with log exposure value of the photoconductive insulating layer consisting of two layers shown in FIG. 4;

. FIG. 8 is a cross-sectional view of the electrophotosensitive material consisting of three photoconductive insulating layers; and

FIG. 9 is a diagram showing the similar characteristic curve, as one shown in FIG. 7, of the electrophotosensitive material shown in FIG. 8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The photosensitive layer, in accordance with the present invention, preferably is applied to a conductive base. Each photoconductive insulating layer has long range free charge carriers. Desirably, the range of the free charge carrier is longer than Lu. The free charge carries may be electrons or positive holes and both may be characterized -by a long range. Each photoconductive insulating layer may be constructed with quite different types of material so long as the layer meets certain necessary conditions. For instance, a photosensitive material prepared by forming a layer of red sensitive photoconductive powder dispersed in an insulating binder on a conductive base, an insulating thin layer is formed thereon, and a uniform layer of bluegreen sensitive amorphous selenium is deposited thereon. The photosensitive material is then exposed from the selenium side. The purpose of the insulating layer is to prevent the free charge carriers generated in each photoconductive insulating layer from moving to the adjacent photoconductive insulating layer and drifting therein. Therefore the thin insulating layer should be able to prevent the drift of the free charge carriers. If the requirements are met, the insulating layer is as thin as possible. It is not advantageous for the insulatinglayer to be completely insulating.

Basically two types of photosensitive material are provided by the present invention; one in which the spectroscopic range of each photoconductive insulating layer is different from that of the others, and one in which each photoconductive insulating layer has the same spectroscopic range but a difierent sensitivity from the others. .Mixed types of the two are also contemplated within the scope of this invention.

Powders containing large proportions of cadmium sulfide or cadmium carbonate are used as a photoconductive powder. Superior photoconductivity is obtained with photoconductive powders having cadmium iodide adsorbed thereon. The photoconductive powder is dye-sensitized with a dye and the sensitivity or spectral-sensitivity within non-coloring photoconductive insulating layer has a sensitivity of between 400 m and 600m The photoconductive powder is sensitized by various types of coloring matter in various spectral-sensitive regions. It is possible to increase the spectral-sensitive range by adding other material other than a coloring material. For instance, the spectral-sensitive region can be broadened by substituting part of the sulfur in the cadmium sulfide with selenium. Variation of the spectral-sensitivity is not however a basic problem of this invention.

The photoconductive powder containing mostlycadmium sulfide and cadmium carbonate with adsorbed cadmium iodide, can be sensitized to. various sensitivities by dye-sensitization. Another type of photosensitive material having a photosensitive layer is obtained by applying a layer dispersed with a highly sensitized photoconductive powder, applying a layer dispersed with a poorly sensitized photoconductive powder, and interposing therebetween a thin insulating layer. The highly sensitized powder is exposed to light which is not completely absorbed into the poorly sensitized photoconductive powder where the absorbed light generates photoelectric current. Photoelectric current is also generated in the unexposed layer. Since the actinic light intensity in the unexposed side is normally low, the desired sensitivity should be higher than at the exposed side.

A photosensitive layer consisting of two photoconductive insulating layers has so far been described, but a photosensitive layer having three or more layers can be prepared in basically the same way.

The desired property of the photoconductive insulating layer or insulating thin layer for making a complex photosensitive layer in accordance with present invention is summarized as follows: i

(l) The range of the free positive holes and/or free electrons of the photoconductive insulating layer is long.

(2) The thin insulating layer is able to prevent the free charge carried generated in the photoconductive insulating layer from moving into or drifting into a photoconductive insulating layer on the opposite side of the thin insulating layer.

(3) The photoconductive insulating layer and the insulating layer are light transparent in at least a part of the spectral-sensitive region of the photoconductive layers at a position nearer to the unexposed side than the photoconductive insulating or the insulating thin layers.

(4) Each photoconductive insulating layer has a different spectral-sensitive region from either that of others, or, when two or more photoconductive insulating layers have the same spectral sensitivity region, the one which is closer to the unexposed side has the higher sensitivity in the spectral region.

FIGS. l-a, 1-b and 1-0 are cross-sectional views of photosensitive materials showing the structure of the conventional single layer and semisingle layer photosensitive material. In FIG. l-a a single photoconductive insulating layer is formed on a conductive base 1. A description of the invention will be set forth according to a case in which the photoconductive insulating layer consists of photoconductive powder containing cadmium sulfide and cadmium carbonate with adsorbed cadmium iodide dispersed in a resinous binder. When the surface of the photosensitive material is charged into negative polarity and exposed to blue light, the surface electric potential is attenuated as shown in FIG. 2 with the curve 1. The blue light generates a pair of free electrons and a positive hole in the vicinity of the surface of the photoconductive insulating layer. As a result of free electron drift toward base 1, the surface electric potential is attenuated as shown in FIG. 2 with the curve 1. If the photoconductive material is charged into a positive polarity and exposed to blue light, the surface electric potential is represented by the curve 2 in FIG. 2, which shows that it is dilficult for the positive holes to drift from close to the surface toward base 1. It is apparent that the photoconductive insulating layer has a sufliciently long range of electrons and insuflicient range of positive holes. The thickness of the photoconductive insulating layer employed here is about 80 u. The surface electric potential in case that the photosensitive material is charged negatively and exposed to red light is shown in FIG. 2 with the curve 3, which shows that the photosensitive material has little sensitivity for red. The photosensitive material shown in FIG. 1-b is made by applying the non-sensitized photoconductive insulating layer 3 on a base 1 and applying thereon a coloring matter sensitized photoconductive insulating layer 4. The surface electric potential of the photosensitive material shown in FIG. l-b is represented by the curve 4 in FIG. 2 when charged on the surface thereof into negative polarity and exposed to red light from the photoconductive insulating layer 4 side. This shows that free charge carriers are generated only in the photoconductive insulating layer 4 since the photoconductive insulating layer 3 is not sensitive for red light, and that the generated free charge carriers move from the layer 4 to the layer 3 and drift in layer 3 till the free charge carriers reach the base 1 to attenuate the surface electric potential. That is, it is apparent from the above behavior of free charge carriers that the carriers are able to move to the adjacent photoconductive insulating layer. The photosensitive material shown in FIG. l-c is made by applying the coloring matter sensitized photoconductive insulating layer 5 on a base 1 and applying thereon a non-sensitized photoconductive insulating layer 6. The surface electric potential of the photosensitive material shown in FIG. l-c is represented by the curve 5 and 6 in FIG. 2 when charged on the surface thereof into negative polarity and exposed to red light. The curve 5 is one in which the thickness of the photoconductive insulating layers 5 and 6 is the same, and the curve 6 is one in which the ratio of the thickness of layer 5 to 6 is 1/ 9. In both cases, the free charge carriers are generated only in the coloring matter sensitized photoconductive insullating layer 5 since the exposure light is red light. The free positive holes do not drift substantially, therefore the positive holes never move into the layer 6 and drift therein. The free electrons drift only in the layer 5 toward the base 1. Therefore, the surface potential is attenuated to the extent corresponding to the moved amount of the free charge carriers in the photoconductive insulating layer 5. The surface potential of this photosensitive material in the case that the surface of the photosensitive material on the exposed side is charged into positive polarity and exposed to red light is represented by the curve 7 in FIG. 2. In this case, the free electrons generated in the layer 5 is moved into the layer 6 and drift up to the surface thereof. In the layer 5 on the other hand, even down in the layer near the base 1 some amount of free electrons are generated and are moved toward the layer 6. Therefore, the surface potential as a whole is attenuated as represented by the curve 7 in FIG. 2. As described hereinabove, in the case that two photoconductive insulating layers are superposed in direct contact with each other without interposing any insulating layer, the free charge carriers generated in one of the layers are easily movable to the other photoconductive insulating layer. Therfore even the photosensitive layer consisting of two photoconductive insulating layers behaves as if it consisted of single layer except as to the spectroscopic sensitivity. As described hereinabove and as shown in FIG. 3, the semisingle layer and single layer have almost the same photographic characteristic curve.

The ordinate in FIG. 3 represents the optical density when developed with colored (black) charged resinous powder, and the abscissa represents the log of the exposure light value. The part 8 of the curve in the figure represents the developed density of the image of unexposed or low exposure part. In the photosensitive layer described in the above description, the density varies from the fog (almost zero) density to the maximum density in the range of 0.8 to 1.0 in the log scale of the expousure light value.

FIG. 4 shows an embodiment of the electrophotographic photosensitive material in accordance with the present invention. The dye-sensitized photoconductive insulating layer 9 is applied on a conductive base 8 and non-sensitized photoconductive insulating layer 11 is formed thereon interposing an insulating thin layer 10 to make an electrophotographic photosensitive material. This photosensitive material is exposed from the photoconductive insulating layer 11 side. The photoconductive insulating layer 9 on the non-exposed side of this photosensitive material is panchromatic and the opposite photoconductive insulating layer 11 on the exposed side is sensitive to blue-green. That is, this photosensitive material has similar characteristics to that shown in FIG. l-c. But it is different therefrom in that an insulating layer is interposed between the photoconductive insulating layers. When this photosensitive material is charged into negative polarity and exposed to blue-green light free electrons and positive holes are generated in the vicinity of the surface of the photoconductive insulating layer 11 and the free electrons drift toward the base 8. But the free charge carriers are prevented from moving to the other layers by the insulating thin layer 10. Therefore when exposed to blue-green light, only in the photoconductive insulating layer 11 the charges are discharged, and the surface potential is attentuated as represented by the curve 8 in FIG. 5. When this photosensitive material is charged into negative polarity and exposed to red light, the surface potential is represented by the curve 9 in FIG. 5. In this case, only in the photoconductive insulating layer 9 are the free charge carriers generated and only in this layer are the charges discharged. The curves in FIG. 5 represent the variation of the surface potential when the photoconductive insulating layers 9 and 11 have almost the same thickness. If the thickness of the layers are different from each other, the residual surface potential after exposure is different from that in said case wherein the thickness of two layers are the same. As apparent from the above description, in the photosensitive material in accordance with the present invention shown in FIG. 4, the photoconductive insulating layers 9 and 11 are discharged independently, and the two layers never interfere with each other in discharging as the photosensitive material as shown in FIG. 1-c wherein the photoconductive insulating layers 5 and 6 interfere each other. Accordingly, when the photosensitive material shown in FIG. 4 is negatively charged, and exposed to red and then blue-green light or vice versa, the surface potential is attenuated as represented by the curve 10 in FIG. 6. The part 12 of the curve 10 is a part attenuated in case of exposed to red (or blue-green) light and the part 13 thereof is a part in case of exposed to blue-green (or red) light. The part 14 of the curve 10 represents the potential when not exposed and disappears if the red (or blue-green) light then blue-green (or red) light are exposed successively thereon, then the curve changes into the curve as represented by the curve 11 in FIG. 6. Since there is no failure of the reciprocity law in these photosensitive material, a curve of the relation of density to exposure as the curve 12 in FIG. 7 is obtained by exposing through a light intensity scale and developing the exposed photosensitive material. The density varies from the fog (almost zero) density to the maximum density in the range of 1.6 to 2.0 on the log scale of the exposure value. This is almost twice as large as the variable range of the photosensitive material consisting of single photoconductive insulating layer.

A photosensitive material having three photoconductive insulating layers is shown -with its cross-section in FIG. 8. This photosensitive material is prepared by applying to a base 15 a photoconductive insulating layer 16 highly sensitized for red light, and applying thereto a photoconductive insulating layer 18 lowly sensitized for red light and interposing an insulating thin layer therebetween. Further, onto the layer is applied a non-sensitized photoconductiveinsulating layer 20 and interposing an insulating thin layer 19. When this photosensitive material is negatively charged and exposed to blue-green light, the free charge carriers are generated mainly in the photoconductive insulating layer 20 and only this layer 20 is discharged. When exposed to red light, free charge carriers are generated in the photoconductive insulating layers 16 and 18, and the charges are discharged independently in respective photoconductive insulating layer, that is, the free charge carriers generated in the photoconductive insulating layer 18 do not move into the photoconductive insulating layer 16. Since the photoconductive insulating layers 16 and 18 are sensitive for red light and the layer 16 on the unexposed side is sensitized higher than the layer 18 on the exposed side, even the attenuated red light reaching the layer 16 through the layer 18, where it is absorbed to some extent, causes discharging in the layer 16 as much as in the layer 18. In the photosensitive material shown in FIG. 8, the layers are independently discharged since the insulating thin layer is interposed between the photoconductive insulating layers. Therefore in this case, as described above, the density varies from the fog density to the maximum density in the range of the sum of the ranges of the exposure values on log scale of the two photoconductive insulating layers when exposed through a light intensity scale and developed. This result is shown in FIG. 9.

As the three layers photoconductive insulating layer having a structure shown in FIG. 8, a photoconductive insulating layer consisting of three layers having different spectroscopic sensitivity from one another, and as the photoconductive insulating layer consisting of two or three layers having the structure as shown in FIGS. 4 or 8, a photoconductive insulating layer consisting of layers having the same spectroscopic sensitivity.

It should be readily understood that there may also be used a photosensitive material having more than three layers as well as the photosensitive material having two or three photoconductive insulating layers.

When the photosensitive material consisting of a plurality of photoconductive insulating layers is used, it is desirable that the characteristic curves are continued in series as shown in FIG. 9. This is controllable by regulating the spectral sensitivity, absolute sensitivity (e.g., the degree of dye sensitization), and thickness of the photoconductive insulating layers and the light property used.

The insulating thin layer prevents the free charge carriers generated in each photoconductive insulating layer from drifting into the other layers as described above. If

the thickness of the insulating layer to the total thickness of the photosensitive layeris too great, the non-sensitive terval of the electrophotographic process such as charging, exposing, developing and the like, and generally preferred to be 1 to 10 seconds. The purpose of the insulating thin layer is in eflfect to prevent the free positive holes generated in the photoconductive insulating layer on the I unexposed side from drifting and being captured and to prevent the free charges generated in the photoconductive insulating layer on the exposed side from drifting and being captured or neutralize them. For that reason, the term insulating thin layer may be changed to drift preventing layer of the free charge carriers in a more strict sense. In this specification and in the appended claims, the term insulating thin layer. is used and will be used as a term havingsuch meaning as described hereinabove.

Therefore, the insulating thin layer is preferred to be extremely thin and to be a material having a low insulation resistance. The material is preferred to a specific resistance of 10 -45 9 cm. and to have high resistance in the lateral direction.

Practically, various types of thin layer forming high molecular materials can be used, and a compound containing pigment powders which are transparent for some light added to the high molecular material can also be used as the insulating thin layer. The pigment powder may itself have some degree of photoconductivity. It is necessary, however, that these materials should have a short range free charge carrier, that is, the range of the free charge carries should not be longer than the thickness of the think layer. The thickness of the insulating thin layer is preferred to be less than 1,u.

Now the method of preparing and using the photographic photosensitive material in accordance with the present invention will be described in detail as to some examples thereof.

EXAMPLE 1 An electrophotosensitive material consisting of two photoconductive insulating layers one of which is red sensitive on the unexposed side and the other of which is bluegreen sensitive was prepared as follows:

Fine photoconductive particles were obtained by preparing a slurry which was made by adding 160 parts by weight of yellow-orange pigment cadmium-yellow-orange #4700 (made by Mitsubishi Metal Co., Ltd.) and 40 parts by weight of cadmium iodide into ethanol allowing it to ticles of sensitized photosensitive material were obtained by adding to parts by weight of photoconductive fine particles. 0.1 weight part of Malachite Green, a sensitizing dye, dissolved in ethanol to make a slurry. The slurry was allowed to stand for 24 hours and the ethanol was removed by evaporation at 80 C. Using this powder, a photosensitive coating material B was obtained by the same process as employed in preparing the non-sensitized powder. The photosensitive coating material B was applied on an aluminum plate which had been treated to remove grease by spray coating. The Magicrone #200 clear was applied thereon as the insulating thin layer, and the photosensitive coating material A was applied on the exposed side as the photoconductive insulating layer. After the aluminum plate with the layers thus coated was sufficiently dried at 70 C., the plate was heated for 30 minutes at C., whereby an electrophotosensitive material consisting of two photoconductive insulating layers having strong coating layers was obtained. The thickness of each layer after heating was 20p. at the base side, 15p. at the exposed side, and about 1p. at the thin insulating layer.

The photosensitive material consisting of two photoconductive insulating layers obtained as described above has a blue-green and red light sensitivity. It was sensitive for the green light of a tungsten lamp light at 2700 K. through the Fuji Filter K# 17, and for the red light of the same light source through the Fuji Filter K#7. When exposed to a mixed light of these two frequencies through an optical density wedge for 0.5 second, and developed with a magnetic brush, this photosensitive material was developed with an image having a variable density from fog to the maximum density in a range on a log scale of the exposure of twice as large as that of the photosensitive material consisting of a single photoconductive insulating layer. That is, the photographic gradation is half of that of the photosensitive material consisting of a single layer.

When preparing the photosensitive material in accordance with this embodiment, a photosensitive material was made without applying the intermediate insulating thin layer. This photosensitive material was also sensitive to both blue-green and red light, but the photographic gradation was almost the same as that of the photosensitive material consisting of a single photoconductive insulating layer.

EXAMPLE 2 An electrophotographic photosensitive material consisting of two photoconductive insulating layers which is panchromatic at the opposite surfaces thereof and wherein the photoconductive insulating layer on the unexposed side has a higher sensitivity than the photoconductive insulating layer on the exposed side, was prepared as follows:

Dried sensitized photoconductive fine particles were prepared by adding an ethanol solution with a resolved sensitizing brilliant green coloring matter of 0.1 part by weight to 100 parts by weight of non-sensitized photoconductive powder which was made by adding cadmium iodide to cadmium yellow orange #4700 obtained in the Example 1. The composition was formed into a slurry, left to stand for 24 hours and heated to 80 C. to vaporize the ethanol. A photosensitive coating material C was prepared by adding 50 parts by weight of said Magicrone #200 clear to 100 parts by weight of said photoconductive powder, and dispersing it for 18 hours. By the same process, sensitized photoconductive powder which was made by adding 0.02 part by weight of sensitized coloring matter brilliant green to 100 parts by weight of said nonsensitized photoconductive powder was prepared. Using the photoconductive powder photosensitive coating, material D was prepared.

On a degreased aluminum sheet, said photosensitive coating material was applied, and another coating material made 'by the following process was coated thereon to form an insulating thin layer. That is, a coating material made by adding 80 parts by weight of cadmium carbonate powder of less than 0.1a diameter to 50 parts by weight of said Magicrone #200 clear. The composition as formed was dispersed for 24 hours in a ball mill and was thereafter used as the insulating thin layer. Onto this layer, a photosensitive coating material -D was coated. The aluminum sheet with these layers coated was dried by heating for 45 minutes at 70 C. and heated for 30 minutes at 150 C. (Thickness after heating was 15 1. at the low sensitized layer, 20 at high sensitized layer, and I at the insulating thin layer.)

The electrophotosensitive material obtained by the process as described above had a strongly sensitized layer of panchromatic. That is, the layer was sensitive in the range of 400 m to 700 m of wavelength, although the sensitivity became somewhat attenuated around 550 m The electrophotosensitive material thus obtained was charged into a negative polarity by corona discharge, exposed to white light through an optical density wedge, and developed by magnetic brush methods. Thus photographic gradation of about half of that of the photosensitive material consisting of single layer was obtained.

But when the photosensitive material was removed of its thin insulating layer, prepared at the same time, photographic gradation was approximately the same as that of the photosensitive material consisting of single layer.

EXAMPLE 3 Onto a conductive glass, the photosensitive coating material A, prepared in Example 1, was coated with the thickness of 15 after heating. A coating material containing Aerozyl (silica powder made by Degussa, West Germany) instead of the coating material containing cadmium carbonate, as in Example 2, was applied as the insulating thin layer thereon. The thickness of the insulating thin layer after heating was less than 1.0 Onto this layer, the photosensitive coating material D, as prepared in Example 2, was coated with a thickness of 15a. Said coating material for the insulating thin layer was applied at a thickness of less than 111., and the photosensitive material C in Example 2 was applied thereon at a thickness of 30].! And this was dried for 3 hours at 70 C. and heated for 30 minutes at C.

The electrophotographic photosensitive material prepared as described above was charged by corona discharge exposed from the conductive glass, that is, base side, and developed from the opposite side by a magnetic brush. The photographic gradation of the developed image was about /3 of that of the electrophotosensitive material consisting of a single layer.

The invention has been described in detail with reference to some embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

What is claimed is:

1. An electrophotographic element comprising n photoconductive insulating layers where n=2, 3, i-l, i, N the first layer being on the exposed side of the element and the nth being on the unexposed side and n1 thin, continuous insulating layers respectively interposed and non-removably connected between said n photoconductive insulating layers, the thickness of each said thin insulating layer being less than 1 micron where the range of any free charge carriers generated in either of the photoconductive insulating layers on the respective opposite sides of said thin insulating layer is less than the thickness of said thin insulating layer and the specific resistance of each of said thin insulating layers being in the range from 10 to 10 ohm-centimeters, and wherein the photoelectric current generated by radiation employed during the exposure step of the imaging cycle in the ith photoconductive insulating layer is greater than that generated in the (il)th photoconductive insulating layer and where the (il)th photoconductive insulating layer and insulating layer between the (il)th and ith photoconductive insulating layers transmit light in at least a portion of the spectral wavelength region to which the ith photoconductive insulating layer is sensitive.

2. An element as in claim 1 wherein n=2.

3. An element as in claim 2 where the second photoconductive insulating layer is sensitive to light over a [first wavelength range and where said first photoconductive insulating layer is sensitive to light over a second wavelength range smaller than said first wavelength range.

4. An element as in claim 3 where said first wavelentgh range approximately extends from 400 to 700 millimicrons.

5. An element as in claim 4 where said second wavelength ranged approximately extended from 400 to 600 millimicrons.

6. An element as in claim 2 where the second photoconductive insulating element is responsive to light over a first wavelength range and the first photoconductive layer is responsive to light over approximately the same first wavelength range, said first photoconductive insulating layer being less sensitive to light over said first waver 11 length range than said second photoconducti-ve insulating layer.

7. An element as in claim 6 where said first Wavelength range approximately extends from 400 to 700 millimicrons and said portion thereof approximately extends from' 600 to 700 millimicrons.

8. An element as in claim 1 where n-=3.

9. An element as in claim 8 where the third photoconductive insulating layer is sensitive to light over a first wavelength range and the second photoconductive insulating layer is also sensitive to light over said first wavelength range, said second photoconductive insulating layer being less sensitive to light over said first wavelength range than said third photoconductive insulating layer.

10. An element as in claim 9 where said portion of said first wavelength range approximately extends from 600 to 700 millimicrons.

11. An element as in claim 9 where the first photoconductive insulating layer is sensitive to light over a second wavelength range smaller than said first wavelength range.

12. An element as in claim 11 where said first wavelength range approximately extends from 400 to 7 00 millimicrons, said portion of said first wavelength range approximately extends from 600 to 700 millimicrons, said second wavelength range extends from 400 to 600 millimicrons.

13. An element as in claim 1 where said thininsulating layer is selected from the group consisting of acrylic resin, cadmium carbon-ate and acrylic resin, and silica and acrylic resin.

14. An element as in claim 13 where said photoconductive insulating layers are composed of inorganic materials.

References Cited UNITED STATES PATENTS 2,476,800 7/ 1949 Blackburn 96l.5 X 2,962,374 5/1956 Dessauer 961.2 X 2,962,375 5/1956 Schaffert 96 1.2 X 2,803,541 8/1957 Paris 961.5 X 2,901,348 8/1959 Dassaner et al. 96--1.5 X 2,962,376 11/1960 Schaffert 11720l X 3,170,790 2/1965 Clark 961.5 3,312,548 4/1967 Straughan l17201 X 3,394,001 7/ 1968 Makino 96-1.5 3,468,660 9/1969 Davenport et al. 96-1.5 3,508,918 4/1970 Levy 961.5 V

GEORGE F. LESMES, Primary Examiner I. R. MILLER, Assistant Examiner U.S. Cl. X.R. 96-1 .6

Referenced by
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Classifications
U.S. Classification430/57.2, 430/65, 359/241, 430/64, 430/67
International ClassificationG03G5/00, G03G5/043
Cooperative ClassificationG03G5/043
European ClassificationG03G5/043