US3840368A

US3840368A - Photoconductive layer with overcoat of dye sensitizer

Info

Publication number: US3840368A
Application number: US00185658A
Authority: US
Inventors: M Ikeda; H Sato; Y Hasegawa; E Torii; K Morimoto
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1971-10-01
Filing date: 1971-10-01
Publication date: 1974-10-08
Anticipated expiration: 1991-10-08

Abstract

An electrophotographic element comprising a conductive support, an organic insulating layer attached to said conductive support, and a sensitizing organic layer contiguous to said organic insulating layer, said organic insulating layer having no sensitizers included therein and comprising a poly-Nalkenylcarbazole such as poly-N-vinylcarbazole or a nuclear substituted poly-N-vincylcarbazole, said sensitizing organic layer comprising an organic dye sensitizer. The electrophotographic element has high sensitivity to visible light.

Description

0 United States Patent 1191 1111 3,840,368

lkeda et ai. Oct. 8', 1974 PHOTOCONDUCTIVE LAYER WITH 3,634,079 1/1972 Champ et al. 96/L6 x OVERCOAT 0 E SENSITIZER 3,684,548 8/ 1972 Contois 96/ 1.5 [75] Inventors: Mitsusuke l keda; Hisanao Sato, both FOREIGN PATENTS OR APPLICATIONS of Gsaka; Etsuko Torii, Kyoto; 4,326,710 11/1968 Japan 96/].6 Kazuhisa Morimoto; Yo Hasegawa, both of Osaka an of Japan Primary Examiner-Roland E. Martin, Jr. [73] Assignee: Matsushita Electric Industrial Co., Attorney, Agent, or Firm-Wenderoth, Lind & Ponack Ltd., Kadoma, Osaka, Japan [22] Filed: Oct. 1, 1971 [57] ABSTRACT [21] Appl. N0.: 185,658 An electrophotographic element comprising a conductive support, an organic insulating layer attached to [52] U S Cl 96/1 5 96/1 6 252/501 said conductive support, and a sensitizing organic [51] 5 5/06 layer contiguous to saidhorganic insulating layer], sdaig organic insu atingv ayer aving no sensitize n u e 5 8] Field of Search 96/ 1.6, 1.5, 252/501 therein and Comprising a polrNalkenYlcarbaz-ole such as poly-N-vinylcarbazole 'or a nuclear substituted [56] References Cited poly-N-vincylcarbazole, said sensitizing organic layer UNITED STATES PATENTS comprising an organic dye sensitizer. Th'e electropho- 3,037,86l 1/1962 Hoegl et a1. 96/15 tographic element has high sensitivity to visible light. 3,591,374 7/1971 Sens 252/501 X 3,620,720 11/1971 Yamaguchi et al 96/l.6 X

24 Claims, 2 Drawing Figures PATENTEDHBI :914 3.840.368

sum 2w 2 O l l l Exposwre PHOTOCONDUCTIVE LAYER WITH OVERCOAT OF DYE SENSITIZER This invention relates to an electrophotographic element comprising organic materials, and more particularly to a novel electrophotographic element having a high sensitivity to visible light.

In electrophotography, an electrostatic latent image is formed by exposing a uniformly charged photoconductive layer to a light through or reflected from an original document. Said electrostatic latent image is developed by using a toner directly on the photoconductive layer or after being transferred to a dielectric layer, or is changed to a frost image by heating.

Selenium and zinc oxide are well known as inorganic photoconductive materials useful for electrophotography. Recently, much attention has been paid to organic photoconductive materials because of their excellent characteristics such as transparency, light weight, flexibility, and flat surface. However, the electrophotographic sensitivity or organic photoconductive materials is usually much lower than that of evaporated selenium photoconductive layer. The organic electrophotographic element having sensitizers distributed uniformly therein generally has a considerable residue potential in photo-decay process.

It is an object of this invention to provide an electrophotographic element having a high sensitivity to a visible light.

It is another object of this invention to provide a highly sensitiveelectrophotographic element which is transparent and flexible.

It is still another object of this invention to provide an electrophotographic element which shows little residue potential in the photo-decay process.

Details of this invention becomes apparent upon consideration of the following description taken together with the accompanying drawings in which:

FIG. 1 shows a section through an electrophotographic element according to this invention.

FIG. 2 shows sensitivity curves for a conventional electrophotographic element and a novel electrophotographic element of this invention.

It has been discovered according to this invention that an electrophotographic element comprising a conductive support, an organic insulating layer on said conductive support, and a sensitizing organic layer contiguous to said organic insulating layer, said sensitizing organic layer comprising an organic sensitizer, shows a high sensitivity to a visible light.

Referring to FIG. 1, a conductive support 1 supports an organic insulating layer 2 and a sensitizing organic layer 3.

The conductive support 1 may comprise any suitable conductive support. For example, a flexiblefilm having a cuprous iodide layer or an evaporated conductive metal layer thereon can be used when flexibility and/or transparency is required. And further, a tin oxidecoated conducting glass or metal plate can be used when flexibility and/or transparency is not required.

The organic insulating layer 2 comprises at least one polymer selected from the group consisting of poly-N- vinylcarbazole, poly-N-alkenylcarbazole, and nucleussubstituted poly-N-vinylcarbazole, in accordance with the present invention. Addition of a plasticizer and polycarbonate into theorganic insulating layer 2 is effective to improve mechanical properties of the layer 2. The plasticizer comprises chlorinated diphenyl, epoxy resin, chlorinated fatty acid ester, phthalic acid ester, or mixtures thereof. The amount of the plasticizer is less than weight parts, preferably 20 to 40 weight parts relative to weight parts of the polymer. Polycarbonate has the effect of improving the mechanical properties of the poly-N-vinylcarbazole, or nucleus-substituted poly-N-vinylcarbazole layer without reducing the transparency. The amount of polycarbonate to be added is less than60 weight parts, preferably 10 to 40 weight parts relativeto 100 Weight parts of the polymer.

The organic insulating layer 2 has a thickness of 3 to 30 microns, preferably 5 to 20 microns and is obtained by the following method..A solution including aforesaid polymer in any solvent such as toluene, chlorobenzene, or dioxane is applied to any suitable conductive support by any suitable and available method such as knife, blade, or reverse-roll coating method and is dried at a temperature of 40 to 100C, preferably 6 0 to 80C for a time of more than 5 minutes to form an organic insulating layer in desired thickness. If necessary, the solution may further include aplasticizer and polycarbonate to improve mechanical propertiesof the organic insulating layer. v

The sensitizing organic layer 3 comprises an organic sensitizer to generate photo-carriers upon illumination of visible light. A single use of saidorganic sensitizer is operable but the mechanical properties and the photosensitivity of resultant electrophotographic element is improved by corporating said organic sensitizer with an organic. matrix having an electrical resistivity higher than 10 ohm-cm. A preferably organic matrix is polystyrene or polycarbonate. A thickness of said sensitiz ing organic layer 3 has an effect on the sensitivity of the resultant electrophotographic element and is in a thickness of 0.1 to 1 micron, preferably 0.3. to 0.6 micron. The sensitizing organic layer 3 comprises 0.1 to 10 weight parts, preferably 0.4 to 2 weight parts of an organic sensitizer relative to I00 weight parts of the poly-' mer in said organic insulating layer 2.

The organic sensitizer comprises pyrylium salts described in U.S. Pat. No. 3,250,615, thiapyrylium salts described in U.S. Pat. No. 3,250,615, carbonium salts described in U.S. Pat. No. 3,575,698, benzopyrylium salts described in U.S. Pat. No. 3,526,502, benzopyrylium salt derivatives described in U.S. patent application Ser. No. 4,200 filed Jan. 20, 1970, or mixtures thereof.

Among the many sensitizers described above, the more preferable are a reaction product of 2-(pmethoxystyryl)-3-phenylbenzopyrylium perchlorate and 2-(p-methoxystyryl)-3-phenyl-4-methoxypyrane, 2- 2 '-a-phenyl-p-methoxystyryl-4 benzopyranylidene )methyl ]-3-phenylbenzopyrylium perchlorate, 1 and y 2-(p-methoxystyryl)-3- phenylbenzopyrylium perchlorate. They produce a sensitizing organic layer 3 having spectral response more matched with a tungsten lamp and a high sensitivity.

The reaction product of 2-(p-methoxystyryl)-3- phenylbenzopyrylium perchlorate and 2-(pmethoxystyryl)-3-phenyl-4-methoxypyrane is obtained from the reaction of these in hot l,2-dichloroethane solution described in U.S. Pat. No.,Application 4,200 filed Jan. 20, 1970, and the chemical structure of the reaction product has not been identified.

The organic insulating layer 2 is overcoated with a sensitizing organic layer 3. A solution including the aforesaid sensitizer, if necessary, aforesaid organic matrix having a high electrical resistivity in a solvent inert to the organic insulating layer is applied to the organic insulating layer by any suitable method such as whirler, blade, or kiss coating method and is dried to form a sensitizing organic layer. When the solution includes the organic matrix, the solution is dried at a temperature of 40 to 100C, preferably 60 to 80C for a time of more than 5 minutes. The solution including further polystyrene or polycarbonate is cured at a temperature of 80 to 160C for a time of more than 5 minutes. Suitable solvent is alcohol such as methanol or ethanol, or ketone such as acetone or methyl-ethyl ketone, or butyl-acetate. Further, a sensitizing layer can be formed on the organic insulating layer by vacuum evaporation of aforesaid sensitizer.

In a conventional organic electrophotographic element, an organic photoconductive layer supported on a conductive support has organic sensitizers distributed uniformly therein. Even when the same polymer for-example, poly-N-vinylcarbazole and the same organic sensitizer, for example, the benzopyrylium saltderivative, are used, the novel electro-photographic element has a higher sensitivity to visible light than that of the conventional organic electrophotographic element. The reason for the difference in the sensitivity between the novel electrophotographic element according to the present invention and the conventional electrophotographic element is not certain. A possible explanation is as follows: In the conventional electrophotographic element, the sensitizer distributed uniformlyin the organic photoconductive layer acts as a trap to catch photo-carriers. On the other hand, the sensitizer in contact with the surface of the organic insulating layer as shown in FIG. 1 has no bad effect on the movement of photo-carriers in the organic insulating layer beneath the sensitizing layer. In the element of this invention, photo-carriers generated in the vicinity of the sensitizing layer may drift to the conductive support without being trapped. Thus, when the element of this invention is charged to a positive potential and then is exposed to visible light, the surface potential of the element dissipates rapidly and shows little residual 'potential. A conventional electro-photographic process for forming an electrostatic latent image on the element of this invention comprises a process of charging the element of this invention to a positive potential and exposing it to a light image. Said latent image is developed by using a toner directly on the surface of said element or after being transferred to a suitable dielectric layer, or is changed to a frost image by heating.

Photosensitivity ofthe element is defined as ahalfdecay exposure, said exposure is the exposure to reduce a surface potential of the element to a half ofv the surface potential in the dark. And further, the exposure to reduce a surface potential of the element to 20 percent of the surface potential in the dark is defined as 20 percent-decay exposure which also represents the photosensitivity. The smaller the half-decay or 20 percentdecay exposure, the more photosensitive the element.

Various embodiments of this invention will be shown in the following examples and should not be construed as limitative.

EXAMPLE 1 An electrophotographic element is first prepared. 1.0 gram of poly-N-vinylcarbazole (Luvican M-l 70, available from BASF Company) and 0.5 gram of chlorinated diphenyl (Kanechlor No. 400, available from Kanegafuchi Chemical Industrial Company) are dissolved in 10 milliliters of toluene to form a solution. The solution is applied to an aluminum plate by blade coating and is dried at 60C for 5 minutes to obtain an organic insulating layer of about 10 microns in thickness. A methyl-ethyl ketone solution containing 1.0 gram of polystyrene (Dialex HI -55, available from Mitsubishi Chemical Company), 0.5.gram of chlorinated diphenyl and 0.1 gram of the reaction product of 2-(pmethoxystyryl)-3-phenylbenzopyrylium perchlorate and 2-(p-methoxystyryl)-3-phenyl-4-methoxypyrane is applied to the organic insulating layer and is heated at C for 30 minutes to form a sensitizing organic layer of about 0.5 micron in thickness. The thus obtained electrophotographic element is then charged to a positive or a negative potential of about 800 volts by corona discharge means and exposed to a tungsten lamp (2,800 I(.,) The decay curve of the surface potential of the element is measured by. an ele ctrometer and is shown in FIG. 2. As shown in FIG. 2, the element of this invention has a photoresponse when charged to a positive potential (curve A) but has no photoresponse when charged to a negative potential (curve C). Halfdecay exposureof the element is 6 lux seconds.

For comparison, a conventional electrophotographic element is prepared. A solution containingfl .0 gram of poly-N-vinylcarbazole, 0.5 gram of chlorinated diphenyl, and 5 milligrams of the sensitizer the same as abovein l0 milliliters of toluene is applied to an aluminum plate by blade coating and dried at 60C for 5 minutes to obtain a photo-conductive layer of about 10 microns in thickness. The amount of the sensitizer contained in said photoconductive layer is the same as that in the sensitizing organic layer of the above novel electrophotographic element. The thus obtained conventional electrophotographic element having the sensitizer distributed uniformly throughout the photoconductive layer is then charged to a positive or a negative potential of about 800 volts by corona discharge means and is exposed to a tungsten lamp. The measured decay curve of thesurface potential is shown in FIG. 2. This conventional element has a photo-response when charged to a positive and a negative potential (curve B). The half-decay exposure of the element is 15 lux seconds. Thus, it is evident that the element of this invention is much more photosensitive than the conventional element. I

In FIG. 2, surface potential of the element is indicated on the vertical axis in volt unit, while exposure is indicated along the horizontal axis in lux second unit.

EXAMPLE 2 Electrophotographic elements are prepared as in Example except that here the sensitizer used is 1.. 2,4-diphenyl-6-( a-amyl-3 ,4-dimethoxystyryl pyrylium fluoborate. 2. tris-p-methoxyphenylcarbonium perchlorate, or

. 2-[ 2 '-aphenyl-p-methoxystyryl-4 benzopyranyliand heating of the sensitizing organic layer of the element is performed at 100C for 60 minutes. Half-decay Table Ill-Continued Heating effect of the sensitizing layer at a temperature of 60 to 160C for 30 minutes exposure of these elements is shown in Table l. 5 H For comparison, conventional elements having sensif a i g tizers distributed uniformly throughout the photocon- 70 7 4 ductive layer are prepared as in Example 1 except that go here the sensitizers used is the same as above and halfdecay exposure of these is also shown in Table I. It) H 120 6.2 Table l 130 6.2 140 6.4 Half-decay exposure (in lux seconds) Conventional Novel Sensitizer electrophotographic clectrophotographic element element of this invention Table IV (I 30 [4 Heating effect of the sensitizing layer I 2 so 18 o (3) 6 20 at IZOCforatrme of 5 to 60 minutes Time of heating Halfldecay exposure (in minutes) (in lux seconds) EXAMPLE 3 0 32 Elements are prepared as in Example 1 except that 5 here, the amount of the sensitizer in the sensitizing glayer is varied from 1 to 100 weight parts relative to 100 weight parts of polystyrene (PS). Half-decay expo- 62 sure of these elements is shown in Table Il. 2 1 3'2 Table ll The amount of a sensitizer in the sensitizing EXAMPLE 5 layer versus half-decay exposure g An element is prepared as follows. l.0 gram of bro- The minated poly-N-vinylcarbazole (monobromosub- Half-decay the sensitizing layer (in' weight exposure (in lux seconds) parts relative to I00 weight parts of PS) 1 32 2.5 14 3.5 ll 5 8.2 10 6.0 l5 6.5 20 7.8 30 lo 50 18 I00 30 EXAMPLE 4 Table III Heating effect of the sensitizing layer at a temperature of 60 to 160C for 30 minutes Heating temperature Half-decay exposure (in C) (in lux seconds) stituted compound described in U.S.. Pat. No. 3,42l,89l), 0.3 gram of polycarbonate resin (Panlite L, available from Teijin Company), and 0.3 gram of epoxy resin (Epicote 828, available from Shell Oil Company) are dissolved in mixed solvent of 9 milliliters of chlorobenzene and 3 milliliters of 1,2- dichloroethane to form asolution. The solution is applied to a transparant conductive polyester film having cuprous iodide thereon by blade coating and is dried at C for 20 minutes to obtain an organic insulating layer of about 8 microns in thickness. A sensitizing organic layer containing sensitizers of about 0.6 micron in thickness is formed on the organic insulating layer by applying a methyl-ethyl ketone solution containing 1.0 gram of polystyrene and 0.1 gram of the sensitizer the same as in Example 1 and by heating at C for 10 minutes. This element is transparent and flexible. Halfdecay exposure of the element is 5 lux seconds and 20 percent-decay exposure is 12. lux seconds. Little residue potential is observed in the decay process.

EXAMPLE 6 An element is prepared. An organic insulating layer of about 10 microns in thickness is formed on an aluminum plate as in Example 1 and is then exposed to ultra violet light. By this exposure, the layer becomes insoluble in a solvent such as toluene, chlorobenzene, or dioxane.

A sensitizing organic layer of about 0.5 micron in thickness is formed on the organic insulating layer by applying a l,2-dichloroethane solution containing 1.0

gram of polycarbonate and 0.1 gram of 2-(p-p 7 dimethylaminostyryl)-3-phenylbenzopyrylium perchlorate.

Half-decay exposure of this element is 8 lux seconds and 20 percent-decay exposure is 19 lux seconds.

EXAMPLE 7 An element is prepared as follows. An organic insulating layer of about microns in thickness is formed on an aluminum plate as in Example I. A sensitizing layer of about 0.1 micron in thickness is formed on the layer by the evaporation of tris-p-methoxyphenylcarbonium perchlorate.

The thus obtained element is then charged to a positive potential of about 1,200 volts by corona discharge and then exposed to a visible light. The surface potential of the element is observed to decay to nearly zero volts.

What is claimed is:

1. An electrophotographic element comprising in the following order a conductive support, an organic insu-- lating layer on said conductive support and a sensitizing organic layer on said organic insulating layer, said organic insulating layer having no sensitizers included therein and comprising a poly-N-alkenylcarbazole, said sensitizing organic layer consisting essentially of an organic dye sensitizer.

2. An electrophotographic element according to claim 1, wherein said sensitizing organic layer consists essentially of an organic sensitizer selected from the group consisting of pyrylium salts, thiapyrylium salts, carbonium salts, benzopyrylium salts, benzopyrylium salt derivatives, and mixtures thereof.

3. An electrophotographic element according to claim 1, wherein said poly-N-alkenylcarbazole is brominated poly-N-vinylcarbazole.

4. An electrophotographic element according to claim 1, wherein said poly-N-alkenylcarbazole ispoly- N-vinylcarbazole.

5. An electrophotographic element according to claim 3, wherein said sensitizing organic layer consists essentially of an organic sensitizer selected from the group consisting of pyrylium salts, thiapyrylium salts,

carbonium salts, benzopyrylium salts, benzopyrylium salt derivatives, and mixtures thereof.

6. An electrophotographic element according to claim 5, wherein said organic sensitizer is in an amount of 0.1 to 10 weight parts relative to 100 weight parts of said polymer.

7. An electrophotographic element according to claim 6, wherein said organic insulating layerhas a thickness of 3 to 30 microns.

8. An electrophotographic element according to claim 7, wherein said sensitizing organic layer is in a thickness of 0.1 to 1 micron.

9. An electrophotographic element according to claim 8, wherein said sensitizing organic layer is formed by overcoating of a solution comprising said organic sensitizer combined with an organic matrix having a high electrical resistivity and a solvent onto said organic insulating layer, said solvent comprising a member selected from the group consisting of methanol, ethanol, acetone, methyl-ethylketone and butylacetate.

10. An electrophotographic element according to claim 9, wherein said organic matrix is polystyrene.

11. An electrophotographic element according to claim 9 wherein said organic matrix is polycarbonate.

12. An electrophotographic element according to claim 9, wherein said organic sensitizer is the reaction product of Z-(p-methoxystyryl )-3- phenylbenzopyrylium perchlorate and 2-( pmethoxystyryl)-3-phenyl-4-methoxypyrane.

13. An electrophotographic element according to claim 9, wherein said organic sensitizer is 2-[(2'-aphenyl-p-methoxystyryl-4'- benzopyranylidene)methyl]-3-phenylbenzopyrylium perchlorate.

14. An electrophotographic element according to claim 9, wherein said organic sensitizer is 2-(pmethoxystyryl )-3-phenylbenzopyrylium perchlorate.

15. An electrophotographic element according to claim 4, wherein said sensitizing organic layer consists essentially of an organic sensitizer selected from the group consisting of pyrylium salts, thiapyrylium salts, carbonium salts, benzopyrylium salts, benzopyrylium saltderivatives, and'mixtures thereof.

16. .An electrophotographic element according to claim 15, wherein said organic sensitizer is in an amount of 0.1 to 10 weight parts relative to weight parts of said polymer.

17. An electrophotographic element accordingto claim16, wherein said organic insulating layer has a thickness of 3 to 30microns.

18. An electrophotographic element according to claim 17, wherein said sensitizing organiclayer is in a thickness of 0.1 to 1 micron.

19. An electrophotographic element according to claim 18, wherein said sensitizing organic layer is formed by overcoating of a solution comprising said organic sensitizer combined with an organic matrix having a high electrical resistivity and a solvent onto said organic insulating layer, said solvent comprising a member selectedfrom the group consisting of methanol, ethanol, acetone, methyl-ethylketoneand butylacetate.

20. An electrophotographic element according to claim 19, wherein said organic matrix is polystyrene.

21. An electrophotographic element according to claim 19, wherein said organic matrix is polycarbonate.

22. Anelectrophotographic element according to claim 19, wherein said organic sensitizer is the reaction product of 2-(p-methoxystyryl)-3- phenylbenzopyrylium perchlorate and 2-(pmethoxystyryl)-3-phenyl-4-methoxypyrane.

23. An electrophotographic element according to claim 19, wherein said organic sensitizer is 2-[(2-aphenyl-p-methoxystyryl-4'- I benzopyranylidene)methyl]-3-phenylbenzopyrylium perchlorate.

24. An electrophotographic element according to claim 19, wherein said organic sensitizer is 2-(pmethoxystyryl )-3-phenylbenzopyrylium perchlorate.

Claims

1. AN ELECTROPHOTOGRAPHIC ELEMENT COMPRISING IN THE FOLLOWING ORDER A CONDUCTIVE SUPPORT, AN ORGANIC INSULATING LAYER ON SAID CONDUCTIVE SUPPORT AND A SENSITIZING ORGANIC LAYER ON SAID ORGANIC INSULATING LAYER, SAID ORGANIC INSULATING LAYER HAVING NO SENSITIZERS INCLUDED THEREIN AND COMPRISING A POLYN-ALKENYLCARBAZOL, SAID SENSITIZIG ORGANIC LAYER CONSISTING ESSENTIALLY OF AN ORGANIC DYE SENSITIZER.

4. An electrophotographic element according to claim 1, wherein said poly-N-alkenylcarbazole is poly-N-vinylcarbazole.

5. An electrophotographic element according to claim 3, wherein said sensitizing organic layer consists essentially of an organic sensitizer selected from the group consisting of pyrylium salts, thiapyrylium salts, carbonium salts, benzopyrylium salts, benzopyrylium salt derivatives, and mixtures thereof.

7. An electrophotographic element according to claim 6, wherein said organic insulating layer has a thickness of 3 to 30 microns.

12. An electrophotographic element according to claim 9, wherein said organic sensitizer is the reaction product of 2-(p-methoxystyryl)-3-phenylbenzopyrylium perchlorate and 2-(p-methoxystyryl)-3-phenyl-4-methoxypyrane.

13. An electrophotographic element according to claim 9, wherein said organic sensitizer is 2-((2''- Alpha -phenyl-p-methoxystyryl-4''-benzopyranylidene)methyl)-3-phenylbenzopyrylium perchlorate.

14. An electrophotographic element according to claim 9, wherein said organic sensitizer is 2-(p-methoxystyryl)-3-phenylbenzopyrylium perchlorate.

15. An electrophotographic element according to claim 4, wherein said sensitizing organic layer consisTs essentially of an organic sensitizer selected from the group consisting of pyrylium salts, thiapyrylium salts, carbonium salts, benzopyrylium salts, benzopyrylium salt derivatives, and mixtures thereof.

16. An electrophotographic element according to claim 15, wherein said organic sensitizer is in an amount of 0.1 to 10 weight parts relative to 100 weight parts of said polymer.

17. An electrophotographic element according to claim 16, wherein said organic insulating layer has a thickness of 3 to 30 microns.

18. An electrophotographic element according to claim 17, wherein said sensitizing organic layer is in a thickness of 0.1 to 1 micron.

19. An electrophotographic element according to claim 18, wherein said sensitizing organic layer is formed by overcoating of a solution comprising said organic sensitizer combined with an organic matrix having a high electrical resistivity and a solvent onto said organic insulating layer, said solvent comprising a member selected from the group consisting of methanol, ethanol, acetone, methyl-ethylketone and butylacetate.

22. An electrophotographic element according to claim 19, wherein said organic sensitizer is the reaction product of 2-(p-methoxystyryl)-3-phenylbenzopyrylium perchlorate and 2-(p-methoxystyryl)-3-phenyl-4-methoxypyrane.

23. An electrophotographic element according to claim 19, wherein said organic sensitizer is 2-((2''- Alpha -phenyl-p-methoxystyryl-4''-benzopyranylidene)methyl)-3-phenylbenzopyrylium perchlorate.

24. An electrophotographic element according to claim 19, wherein said organic sensitizer is 2-(p-methoxystyryl)-3-phenylbenzopyrylium perchlorate.