GB2069165A - Electrophotogaphic record carrier - Google Patents

Abstract

An electrophotographic record carrier, e.g., a drum for a photocopying machine, comprises a metal substrate, e.g. aluminum, covered first with a thin layer of a material such as tellurium or gold which is capable of injecting holes into a second thick layer of selenium-tellurium. The outer layer may also be doped with a halogen, such as chlorine.

Description

SPECIFICATION Electrophotographic record carrier The present invention relates to electrophotographic record carriers and to methods of manufacturing the same. Record carriers using electrophotographic effects are used mainly in photocopying machines. The record carriers are usually aluminum drums coated with a semiconductor material whose electrical resistance is considerably reduced when the material is exposed to an optical image.

Commonly used materials are, for example, selenium or cadmium sulfide. Selenium is particularly sensitive to the blue components of visible light. To improve the red sensitivity of selenium, various methods are known. It is possible, for example, to add tellurium as a dopant, which is described in detail in a book by R. M. Schaffert, "Elektrophotography", 1975, page 300 and Figure 129. To reduce the residual potential of record carriers comprising selenium or selenium-tellurium layers, halogens, such as chlorine, are commonly added to these semiconductor materials.

Electrophotographic images are usually formed by first charging a record carrier; if selenium or selenium-tellurium is used, the charge is positive.

The record carrier is then exposed to the image to be copied, thereby discharging the exposed areas.

The charge image thus formed on the record carrier corresponds to the image to be copied. This charge image is developed by applying coloured, charged particles, frequently plastic-coated soot particles, called the toner, to the record carrier.

Toner particles with a charge opposite to that of the carrier adhere to the charged areas of the carrier. Finally, the black areas of the carrier are transferred to a sheet of paper by an electrostatic process or by pressure. Excess toner particles are washed away from the record carrier, and the remaining charge pattern is erased.

To erase the charge pattern, the substrate of the record carrier, i.e., usually the aluminum drum, is commonly connected to ground, and the surface of the semiconductor layer is grounded through a corona. The charge pattern is thus erased, but it turns out that the record carrier has a residual potential which is usually more positive than -1 00V in the case of a selenium layer but even more negative than -200V in the case of a selenium-tellurium layer.

There are copying machines in which the toner is pumped between the record carrier and an anode sheet, the negative particles of the toner adhering not only to the positively charged areas of the record carrier but also to the anode sheet.

To free the anode sheet from these toner particles before making a copy, the anode sheet is charged to about -400V for a short time. It has turned out that if a record carrier with a selenium-tellurium layer and hence a residual potential more negative than -200V is used, a voltage across the anode sheet of -400V no longer suffices to sufficiently repel the negatively charged toner particles from the anode sheet. Voltages more negative than -400V across the anode sheet have proved disadvantageous because the toner segregates at such high negative voltages.

The object of the invention is to provide an electrophotographic record carrier consisting essentially of a metal substrate coated with a layer of a halogen-doped selenium-tellurium alloy containing more than 5% tellurium with which a residual potential more positive than -1 50V can be achieved after a corona discharge.

According to the invention there is provided an electrophotographic record carrier, comprising a metal substrate coated with a layer of a halogendoped selenium-tellurium alloy containing more than 5% tellurium, provided with a layer of a material which injects holes into the seleniumtellurium alloy, said layer being between the selenium-tellurium layer and the metal substrate.

A special advantage of this invention is that conventional record carriers with selenium layers can be easily replaced by record carriers according to the invention without having to make any electrical adjustments in the copying machine.

Compared with a record carrier having a selenium layer, a record carrier with a selenium-tellurium layer according to the invention has a considerably better red sensitivity but essentially the same residual potential after a corona discharge.

A particularly inexpensive record carrier is obtained if the metal substrate is an aluminum drum. Particularly advantageously, the holeinjecting material is tellurium, because the conditions for depositing such a layer are almost identical to those required for depositing the selenium-tellurium layer. The layers are advantageously evaporated onto the substrate under vacuum. A substrate temperature of about 50 70 C gives record carriers with high red sensitivity and low residual potential. By using gold instead of tellurium for the hole-injecting layer, the residual potential can be set practically to zero.

Embodiments of the invention will now be explained in more detail with reference to the accompanying drawing, in which: Fig. 1 shows a part section of a conventional record carrier and a basic circuit for the corona discharge, and Fig. 2 shows a part section of a record carrier according to the invention.

In Fig. 1 the metal substrate 1 of a record carrier is coated with a light-sensitive layer, e.g., a selenium-tellurium alloy. To remove residual charges from the surface 3 of the layer 2, the metal substrate 1 is connected directly to ground 5, while the surface of the layer 2 is grounded through a corona 4. The corona ionizes the surrounding air, thus providing a conductive connection to ground. It has turned out that with this kind of discharge, an over-compensation of the initially positive charge of the record carrier takes place, and that the record carrier has a negative charge after the discharge. This negative charge is about -100 to -1 50V is a selenium layer is used, and about -200 to -300V if the layer is an alloy of selenium and more than 5% of tellurium.

Fig. 2 shows the structure of a record carrier according to the invention, which also comprises a selenium-tellurium layer but has a residual potential of only about -1 00V after the corona discharge. The change of the residual potential from the -200 to -300V range to about -1 00V is achieved by inserting a tellurium layer 6 between the substrate 1 and the seleniumtellurium layer 2. Fig. 2 is not drawn to scale, however. The tellurium layer is usually about 1 Mm thick, while the thickness of the seleniumtellurium layer usually ranges between 50 and 60 Mm. The change in the residual potential of the record carrier provided with the intermediate tellurium layer 6 is probably due to the fact that the tellurium injects holes into the seleniumtellurium layer.This explanation is suggested, for example, by J. L. Hartke in an article published in "Physical Review", Vol. 125, No. 4, 1962, page 11 77, which describes that hole-injecting tellurium contacts are used to measure drift mobilities of electrons and holes in amorphous selenium films.

A record carrier according to the invention can be manufactured as follows. An aluminum drum, which is typically about 120 mm in diameter and about 300 mm in length, is degreased and cleaned in a trichloroethylene bath under the action of ultrasonic waves. The aluminum drum is then rotatably mounted in a vacuum vessel which is evacuated to about 10-4 torr. The drum is heated to 500--700C, typically 550C, by infrared heating, and held at this temperature throughout the evaporation of the semiconductor layers.

About 40-50 mm below the aluminum drum is a molybdenum boat which is approximately equal in length to the aluminum drum and in which tellurium is heated to about 2500C. During a time of 2 minutes, about 1 gram of tellurium evaporates under these conditions and produces an about 1 ym thick layer on the drum.

After the deposition of the tellurium layer, the tellurium-containing molybdenum boat is covered, and from a likewise 2500C hot molybdenum boat an approximately 50-60 m thick seleniumtellurium layer is evaporated onto the tellurium layer during 2 hours. The selenium tellurium, like the tellurium used in the first step, is present in the form of fine granular material containing about 1 5-30 ppm of chlorine. The substances are commercially available in this form. The tellurium content of the selenium-tellurium alloy is typically between 5 and 20%, preferably about 15%.

Finally, air is slowly admitted to the vacuum vessel, and the finished record carrier is removed.

The record carriers described in the foregoing have a metal substrate in the form of an aluminum drum. For the invention, however, it is, of course, completely irrelevant whether a metal substrate in the form of a drum or, for example, a flat substrate or any other form of substrate is used. The substrate may, of course, be made of materials other than aluminum. As the material for the holeinjecting layer, only tellurium was mentioned in the above embodiments, but it is possible, of course, to use all materials which inject holes into selenium-tellurium; a gold layer has proved particularly advantageous.

Claims (9)

1. An electrophotographic record carrier, comprising a metal substrate coated with a layer.

of a halogen-doped selenium-tellurium alloy containing more than 5% tellurium, provided with a layer of a material which injects holes into the selenium-tellurium alloy, said layer being between the selenium-tellurium layer and the metal substrate.

2. A record carrier as claimed'in claim 1, wherein the hole-injecting material is tellurium.

3. A record carrier as claimed in claim 1, wherein the hole-injecting material is gold.

4. A record carrier as claimed in any one of the preceding claims, wherein the metal substrate is an aluminum drum.

5. A record carrier as claimed in any one of claims 1 to 4, wherein the selenium-tellurium alloy contains 520% tellurium.

6. An electrophotographic record carrier substantially as described with reference to Fig. 2 of the accompanying drawings.

7. Method of manufacturing a record carrier as claimed in any one of claims 2, 4, 5 or 6 wherein first the tellurium layer and then the seleniumtellurium layer are evaporated onto the substrate under vacuum.

8. A method as claimed in claim 7, wherein in both evaporation steps the substrate temperature is about 500--700C.

9. A method of manufacturing an electrophotographic record carrier substantially as hereinbefore described.