CH644959A5 - PHOTOCONDUCTIVE MEMBER FOR ELECTROPHOTOGRAPHIC PROCESS. - Google Patents

Description

The present invention relates to a photoconductive member for an electrophotographic process.

In a conventional electrophotographic process, a photoconductive surface is charged in the dark, then is subjected to light forming an image of a document or photograph which has to be reproduced, with the formation of a latent electrostatic image which corresponds to the original document or photograph. The latent image is then made visible by development using electroscopic particles. The photoconductor most used in electrophotographic machines is a glassy silicon, or more often called amorphous. However, its sensitivity essentially corresponds to the range between blue and yellow, and the gray scale is such that the copies of the photographs are very poor. Furthermore, a selenium-based photoconductor cannot be heated to a temperature above 80 ° C. without losing its electrophotographic properties. The main drawback of a selenium-based photoconductor is that it wears out quickly in an electrophotographic machine and must be replaced after use for the formation of 10,000 to 100,000 copies. Also known is the addition of tellurium, arsenic or other materials of

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doping with amorphous silicon so that these properties are improved to a certain extent, but it is desirable that the improvement be important, especially on the wear characteristics.

Cadmium sulfide has a hardness between 3 and 3.5 on the Moh scale. Its spectral response, when the material has been properly formed and doped, extends over the entire visible range, from blue to red. It has a quantum yield, that is to say an ability to transform light into electrical charges, very high, between 2 and 10 times that of selenium. Its light discharge characteristic is such that it gives an excellent gray scale, making it possible to make excellent reproductions of photographs.

During the implementation of electrophotographic operations, a photoconductor is mounted on a conductive substrate and is charged by a corona discharge. This discharge ionizes the air. The ionized air plays the role of an electrode of a capacitor, the other electrode of which is formed by the conductive substrate. As a photoconductor constitutes a dielectric when it is in the dark, the charge of the corona sticks to the surface of the photoconductor. A charge of opposite polarity is then induced on the conductive substrate. The time required for the development of a latent image on the surface of a photoconductor depends on the voltage to which it can be charged.

The advantages of polycris-tallin cadmium sulfide as a photoconductor have already been recognized. Unfortunately, as described in US Patent No. 3884787, it has not yet been possible to form a photoconductor based on cadmium sulfide having a thickness sufficient for the surface tension to be sufficiently high, with the necessary charge densities. This means that the development of a latent electrostatic image formed on the surface of a photoconductor takes far too long to be usable in practice. When an attempt is made to make a very thick cadmium sulfide photoconductor, it flakes off the conductive substrate or cracks.

U.S. Patent No. 3,151,982 describes an attempt to improve the short life of a glassy selenium photoconductor by using cadmium sulfide particles in a glassy binder. U.S. Patent No. 3,510,298 also describes a photoconductor based on cadmium sulfide present in a vitreous binder. It can be seen that the cadmium sulfide with a glass binder does not form an electrophotographic photoconductor which can be used practically commercially. Latent electrostatic images, when distorted, include spots that soil the images.

Two techniques are also known for forming photoconductors based on cadmium sulfide without the use of a binder. The first is described in US Patent No. 3148084. This patent describes the known techniques for obtaining photoconductive films and the disadvantages of the evaporation processes, chemical deposition and vapor phase reaction . This patent also describes the formation of photoconductive films by spraying reagents on a heated substrate. The films formed contain sulfides of many metals, as well as sulfoselenides of cadmium, cobalt and indium. The photoconductive films are formed on an insulating substrate. One of the inventors of this patent, Chamberlin, further described the process in an article in the "Journal of the Electrochemical Society", volume 113, pp. 86-89, written by J.S. Skarman in 1966. The films are not intended for use in electrophotography, but rather for the manufacture of thin film solar cells. These photovoltaic converters are formed by a thin layer of copper sulfide (0.1 pim) cooperating with a thin layer of cadmium sulfide (1 | tm).

Another method of forming thin film photoconductors is sputtering. This process is described in the aforementioned patent of the United States of America No. 3884787. Films up to 0.5 µm (5000 Å) thick are formed. They are transparent to yellow light and are excellent photoconductors.

A photoconductor charged in the dark is analogous to a charged capacitor, the photoconductor constituting the insulating or dielectric material. A high tension is necessary to attract the particles of turning agent from a great distance, so that the development is rapid. In the case of cadmium sulfide, the charge of the corona is negative, so that the charge of the particles of bleaching agent of the developing material is positive. The speed of movement of the particles loaded with toning agent in the developing liquid towards the latent electrostatic image formed on the photoconductor is a function of the voltage of the latent image. The higher the tension, the faster the development. In the case of a thin film photoconductor, a surface charge density exceeding a certain value cannot be maintained, and the excess charge is transported through the dielectric. The voltage created for the maximum charge is proportional to the thickness of the photoconductive layer and inversely proportional to the dielectric constant. Thus, the use of high voltages necessary for rapid development requires an increase in the charge density accepted by the surface and the use of a film as thick as possible. When an attempt is made to form a thicker film by pyrolytic formation of cadmium sulfide from aqueous solutions of reagents, the film separates from the conductive substrate by flaking.

Cadmium sulfide has several disadvantages when it is an electrophotographic photoconductor. First, it has a memory, that is to say that, after the charge and the exposure to light intended to form a latent image, the development of this image and its transfer onto a support sheet, the latent image still remains on the photoconductor. In other words, the decay time in the presence of light is too long, so that an overprint occurs. Another drawback of an electrophotographic photoconductor based on cadmium sulfide is fatigue, that is to say that, during the repeated use of the photoconductor, the maximum voltage to which it can be charged decreases by more and more. However, cadmium sulfide can maintain a charge density much higher than that retained by selenium doped with tellurium. Cadmium sulfide has a higher photosensitivity than selenium.

The invention relates to an electrophotographic photoconductive member having a spectral sensitivity extending over the entire visible range and having a high resistance to wear.

The documents considered above and which relate more precisely to the field of the invention are the aforementioned United States patents Nos. 3148084, 3151982, 3510298 and 3884787, as well as the article by Chamberlin et al., "Journal of the Electrochemical Society ”, volume 113, pp. 86-89.

None of these documents describes a photoconductor based on cadmium sulfide, for electrophotographic process, having a thickness of at least 3 µm.

U.S. Patent No. 3,754,985 describes a method of manufacturing a sintered photoconductor containing cadmium sulfide doped with copper and chlorine.

The aforementioned article by Chamberlin et al. describes the use of cadmium acetate and thiourea or N, N-dimethylthiourea. This document also describes the doping of a cadmium sulfide film with copper, as well as the fact that, when using cadmium acetate as a raw material, crystallites with a size of less than 400 Å are formed.

The member according to the invention is defined as claimed in claim 1, and a method of manufacturing this member is defined in claim 12. In an advantageous embodiment, a small amount of copper can be used as doping material so that the spectral response is better, that the memory effect which causes an overprint during use is reduced, and that fatigue decreases. The doping copper is codeposited by adding copper acetate to the solution sprayed during the pyrolytic spraying operation. Spraying is carried out on a heated metal surface, for example steel or aluminum, advantageous5

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chrome or cadmium plated, so that adhesion and corrosion are controlled. Spray pyrolysis takes place in three stages. In a first step, a certain amount of lead acetate is added to the spray solution, so that a suitable contact layer is formed and the white spots appearing on the dark parts of the copy are eliminated. During the second stage, the lead acetate is removed and chlorine is added in the form of cadmium chloride. Chlorine constitutes a doping material which facilitates the transport of the photochemically created charge towards the substrate, and which favors the disappearance of the phenomena of memory and fatigue. It also increases the number of carriers. This step of forming the photoconductor is carried out as long as possible so that the thickness of the photoconductor is increased, so that the latter can accept a higher charge than that which could have been used with the photoconductive agents based on sulfide. of cadmium. The first part of the operation is carried out until the formation of a layer whose thickness is approximately 3000 Å. The second layer, constituting a charge transport layer, is produced up to a thickness of approximately 17500 Å. The solution is then changed so that the doping chlorine is removed, so that the solution contains only cadmium acetate, copper acetate and zinc acetate with thiourea. The formation of the third layer, which constitutes the light absorption part of the photoconductor, is carried out to a thickness of approximately 9500 Å which is sufficient for the absorption of almost all of the light reaching the surface. The finished photoconductor can be charged to a very high value and has an excellent response to light, allowing it to form a high contrast image having an excellent gray scale. The photoconductor has a very long useful life during storage, without being affected by variations in temperature or humidity.

The layers can be formed on any suitable substrate, preferably on a cylinder which is to be used in an electrophotographic machine. The cylinder rotates while being heated by a radiant heating element, at a temperature between 125 and 200 C (± 25 C) measured on the surface of the drum. The solutions are sprayed at the rate of approximately 300 cm3 / h.

The spraying takes place in the presence of the atmosphere which contains oxygen. The latter appears adsorbed on the surface of the crystals. It is advantageous because it appears to increase the resistivity of the cadmium sulfide film.

Thus, the invention provides a photoconductive member having excellent wear resistance characteristics and a uniform spectral response throughout the visible spectrum, from blue to red.

It also provides such a photoconductive member having a very high quantum efficiency, and in which the latent electrostatic image can be easily erased by visible light.

The invention also provides a photoconductive member having excellent charge acceptance properties in the dark, as well as a thickness sufficient to give a high voltage when fully charged, and which is moreover practically not not affected by environmental factors such as temperature and humidity, under normal climatic conditions.

The invention will be better understood on reading the description which follows of embodiments, and with reference to the appended drawings in which:

fig. 1 is a very enlarged schematic section of a fragment of a photoconductive member according to the invention;

fig. 2 is a diagram of an apparatus which can be used for the manufacture of the photoconductive member of FIG. 1;

fig. 3 is a section along line 3-3 of FIG. 2;

fig. 4 is a photomicrograph of exposed crystals, taken from the mass of the photoconductive member based on cadmium sulfide according to the invention, photographed with a magnification of 20,000, and FIG. 5 is similar to FIG. 4 and represents another part of the mass of the photoconductive member based on cadmium sulfide according to the invention, photographed with an enlargement of 50,000.

More specifically, during the formation of the photoconductive member for the electrophotographic method according to the invention, a method of pyrolysis of a spray of a type known in the art is used. The photoconductor forming apparatus is shown in fig. 2, on which a metal drum 2, formed of aluminum or mild steel, is coated with chromium or cadmium. It is thoroughly cleaned before carrying out the operations, first with nitric acid, then with water, then with a household detergent, until no oil or grease remains. The presence of oil on the surface of the drum can be detected by a spreading test, a drop of water spreading in the form of a regular film on the surface when the latter is completely free of oil. Then the surface is rinsed with deionized water, then with isopropanol so that it flushes out the water. In the numerous photoconductive members produced, a chrome drum is used which does not exhibit corrosion. However, it is found that a better bond is obtained with cadmium-plated material.

The drum 2 is mounted on two support devices 4, 6 on which it can be held by friction, as clearly indicated in FIG. 2. The mounting devices 4 and 6 have flanges 8 and 10 which cooperate with two pairs of rotary saddles 12 and 14, shown in FIGS. 2 and 3. These saddles are mounted on two shafts 16 and 18 carried by two pairs of uprights 20 and 22. The shaft 16 is driven by a motor, for example an electric motor 24 which receives a voltage by conductors 26 and 28 The shaft 16 carries a pulley 30 which drives a pulley 32 by means of a belt 34. A shaft 36 is mounted in a mounting device 38 so that it rotates with the pulley 32. It carries a screw 40 double helix intended to cause an alternating movement of a head 42 of atomization along the device 18. Two flexible pipes 44 and 46 are fixed to the head 42. The pipes 44 are connected to a reserve of compressed air (not shown) having a pressure of the order of 1.4 bar. The piping 46 communicates with reserves of aqueous solutions of the reagents used successively for obtaining the three different compositions based on cadmium sulfide, forming the electrophotographic photoconductive member according to the invention. The solutions containing the reagents can be transmitted by gravity or by compressed air, or in any other suitable manner known in the art. The flow rate is regulated by a tap, not shown, disposed between the solution reserve and the head 42, so that the head forms a spray, with a flow rate of approximately 300 cm3 / h or less, of reagents intended to come to the contact of the drum 2. A heating element 48 by resistance is housed inside the rotary drum 2. A current, coming from the conductor 28 which is connected to a source of potential, circulates in the armature 50 of a relay, in a conductor 52, in the heating element 48, in a conductor 54, and thus closes the circuit by connection of the conductor 26 to the source of potential. A pyrometer 56 is intended for detecting the temperature on the surface of the drum 2 during coating. It is set at a temperature between 130 and 180 C. When the temperature becomes too high, a winding 58 of the relay opens the circuit by lifting the armature 50. When the temperature decreases out of the desired range, the winding 58 ceases to be supplied and the armature 50 again causes the heating element 48 to be supplied. It should be noted that any suitable pyrometer known in the art can be used, for example a thermistor. The average surface temperature is maintained at around 150 ° C.

The tests for producing photoconductive elements based on cadmium sulphide for the electrophotographic process, carried out over a period of approximately 3 years, covered the coating of approximately 500 test drums, before determining the best method for setting of the invention. It was quickly found that most photoconductive organs based on cadmium sulfide, although having photoconductive properties, were not satisfactory in an electrophotographic process by forming an acceptable image, when used in electrophotographic machines. Cadmium sulfide has a due

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natural retee, and consequently an abrasion resistance, much higher than that of vitreous selenium. The photoconductive member based on cadmium sulfide according to the invention allows the formation of more than a million copies, while vitreous selenium gives no more than 100,000, when used in a conventional photocopier on plain paper. Photoconductive films based on cadmium sulfide having a sufficient thickness cannot be formed by spray pyrolysis. When you try to form a film that is too thick, it will flake off the metal substrate. A thin film gives only low tension. In addition, the decay in the dark is too great, so that it takes several passes under a corona discharge for the charge of the photoconductor to the maximum value allowed by the thin layer of cadmium sulfide. The voltage increase tests cause a breakdown of the cadmium sulfide photoconductor. In addition, cadmium sulfide has a memory property, that is, after exposure to an image, development and printing on the support sheet, the latent image remains on the photoconductor. The light decay time is too long. It is also noted that, after using a photoconductive member based on cadmium sulphide for a certain time, the maximum voltage to which it can be charged decreases more and more. Numerous experiments have been intended to improve this situation and have made it possible to develop the invention, step by step.

Cadmium sulfide is generally not very sensitive to red light. The addition of copper, in a known manner, sensitizes the cadmium sulphide to red light. It is found that the addition of copper also reduces the phenomena of fatigue and memory, and the photoconductive member for electrophotography resulting is made sensitive throughout the spectrum, including the red part.

A good photoconductive member intended for electrophotographic machines must be able to accept a sufficiently high voltage, especially during development by electrophoresis with particles of toning agent suspended in an insulating liquid vehicle, for development to take place quickly. This is a function of both the thickness of the photoconductor and its distance to darkness. It can be seen that the incorporation of zinc, in the form of zinc sulfide, allows the photoconductor to be charged at an increased voltage.

During the production of drums coated with a photoconductor and containing no zinc, it is found that the charge is not high enough for development to be rapid. In addition, the contrast between the most exposed and the least exposed areas suffers. The addition of zinc is a considerable difference. However, this addition reduces the sensitivity of the photoconductor, especially to red light, so that there is a limit to the addition of zinc, easily determined by color sensitivity.

One of the drawbacks of cadmium sulfide, as indicated above, is the rapid decrease in charge in the dark, that is to say that, in the dark, it does not constitute an insulator good enough to constitute a photoconductive member for electrophotography usable in practice. It is found that the manufacture of cadmium sulfide by pyrolysis of an acetate-based spraying greatly improves the dielectric properties of the photoconductor in the dark.

The interface of the conductive substrate and the photoconductor based on cadmium sulfide is important. There must be a suitable rectifier electrical contact at this interface. It is noted that certain additions to the contact layer - that is to say the layer of the photoconductor which is in contact with the conductive substrate -, and in particular of lead, give the suitable electrical contact. When the other parameters are not modified, it is found that photoconductive members based on cadmium sulfide, for electrophotographic process, doped with copper and with chlorine, and containing zinc sulfide, are still not satisfactory, because light spots appear on the dark parts of the copy. After numerous tests, it is determined that this phenomenon may be due to poor electrical contact between the photoconductor and the substrate. Adding a certain amount of lead increases the conductivity of the contact layer, and stains are removed.

The amount of lead is empirically determined by adding an increasing amount until the spots become larger, and the optimum value which removes the spots is determined. The maximum amount of lead that can be used without adverse effects is not determined.

It has conventionally been considered that the presence of oxygen is a drawback. Thus, the aforementioned United States patent No. 3148084, column 4, starting at line 62, indicates that none of the elements present in the photoresist film are taken from the substrate or the surrounding atmosphere. This patent further indicates, column 7, starting from line 65, that, after the film has been formed, the photoconductor is treated at temperatures between approximately 480 and 650 ° C. Obviously, this operation removes all the adsorbed oxygen. The aforementioned United States Patent No. 3,847,787 describes the formation of a film by sputtering in an atmosphere of inert gas, for example argon.

It is found, by analysis of the electrophotographic photoconductive member terminated by Auger spectroscopy, that a small amount of oxygen is present. The nature of the oxygen cannot be determined, but it is considered to be adsorbed at the limits of the crystals which form the photoconductive material. Oxygen increases the resistivity of cadmium sulfide which usually has free electrons inside the crystals. One hypothesis considers that the oxygen, adsorbed on the surface of the crystals, attracts the excess electrons from the interior of the crystals and places them on the surface of these, so that the crystals constitute a better insulator in the dark. Oxygen is present in the water used for the formation of aqueous solutions of the reactants, as well as in the atmosphere.

During the production of a very large number of electrophotographic drums coated with cadmium sulfide, it is determined that the best embodiment of photoconductive members according to the invention is as follows. An aqueous solution is first formed containing a large amount of cadmium acetate and a small amount of copper acetate, lead acetate and zinc acetate. A separate solution of thiourea can be formed. As the solution of the metal acetates and the aqueous solution of thiourea react slowly at room temperature, they can be introduced from separate containers into the atomizing head 42, through the piping 46. The reaction is however so slow that this precaution is not necessary and one can prepare a single solution having the following composition.

Solution I

Thio-urea

0.008M

Cadmium acetate

0.006M

Copper acetate

0.0012 M

Zinc acetate

0.0006 M

Lead acetate

0.00022 M

1 1 of this solution is formed and sprayed for about

ron 300 cm3 / h on the drum 2 which rotates at around 70 rpm. It should be noted, with reference to fig. 2, that the atomizing head 42 moves in translation while the drum 2 rotates. The alternative movement speed corresponds to 4 to 5 cycles / min. When the spray is directed to a part of the drum for too long, it cools it locally below the deposition temperature by pyrolysis. It should be noted that the solution indicated contains an excess of thiourea. It is used so that the reaction is total, since the law of mass action requires an excess of a reagent so that the reaction is displaced in the desired direction. For a spraying rate of 300 cm 3 / h, it takes 3 h and 20 min for the deposition of the first layer, which has a thickness of the order of 0.3 μm. Constant change of spray position relative to the substrate ensures the formation of an extremely uniform layer

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which is very smooth. No binder is used, and the cadmium sulfide region is formed with elongated crystallites having a diameter of about 300 to 400 Å. The mass of the composite layer is then subjected to attack by a beam of argon ions. This operation is known in the art as "spraying argon attack". Fig. 4 is a photomicrograph which represents the crystal structure of the electrophotographic photoconductor based on cadmium sulfide according to the invention, with a magnification of 20000. FIG. 5, similar to FIG. 4, is a photomicrograph representing the crystal structure with a magnification of 50,000 times. This crystal structure is present throughout the composite photoconductor and we now describe its formation.

Since a photoconductive member for electrophotography must have a sufficiently rapid decrease in charge in the presence of light and have a thickness sufficient to accept a sufficiently high charge, a different aqueous solution is formed for the production of a region different from the composite photoconductive member. This solution is as follows.

Solution 2

Thio-urea

0.008M

Cadmium acetate

0.0055 M

Copper acetate

0.0012 M

Zinc acetate

0.0006 M

Cadmium chloride

0.00031 M

It is noted that the metallic salts of cadmium, zinc and copper

vre, used according to the aforementioned patent of the United States of America No. 3148084, are chlorides. It is found that the use of chlorides in place of acetate causes the formation of cadmium sulfide having a low resistance to darkness and not accepting a sufficiently high charge for the development of a latent electrostatic image to be rapid. . A certain amount of cadmium chloride is used as the doping material, so that the residual potential is greatly reduced for a given amount of light. In other words, the time for the charge to decrease in the presence of light in the region formed by solution 2 is greatly reduced. Although the region of the composite photoconductor according to the invention forming the contact layer is thin (0? 3 (xm), the region of the photoconductor formed with doping chlorine, that is to say the charge transport layer , has a thickness of the order of 1.75 μm. For this purpose, a solution having a volume of 61 is used and the pyrolysis of the spraying is continued for an additional 18 h, or more.

The light absorption region of the photoconductor based on cadmium sulfide is then formed. The operation is carried out by forming a following aqueous solution.

Solution 3

Thio-urea 0.008 M

0.006 M cadmium acetate

0.0012 M copper acetate

Zinc acetate 0.0006 M

A quantity of solution is formed of approximately 31, and it takes 10 hours for the constitution of a layer under the same conditions as for the formation of the other layers. The light absorption layer has a thickness of about 0.95 µm.

The composite photoconductive member for electrophotography thus formed according to the invention is schematically represented in FIG. 1. A conductive substrate 100 consists of any suitable metal, such as mild steel or aluminum. A contact layer or region 102 has a thickness of 3000 Å and essentially contains a large amount of cadmium sulfide, a small amount of zinc sulfide and a significant amount of lead sulfide, the layer being doped with copper. It should be noted that all the layers or regions of the composite photoconductor essentially contain a large quantity of cadmium sulphide for a small quantity of zinc sulphide. All the layers are doped with copper. A charge transport layer 104 is doped with copper and chlorine and has a thickness of 17,500 Å. A light absorption layer or region 106 contains a large amount of cadmium sulfide and a small amount of zinc sulfide, the layer being doped with copper.

It should be noted that it is found to be advantageous for the composite photoconductor to have a thickness of at least 3 μm during the manufacture of a photoconductive member for electrophotography which can be used in practice. It should obviously be noted that the contact layer can be thinner since its main role is to form a suitable electrical contact between the conductive substrate and the composite photoconductor. It does not have to be too thick. The charge transport layer greatly facilitates obtaining the desired thickness for the composite photoconductive layer. A thick layer is necessary so that the potential to which the composite photoconductor can be charged is increased. The amount of chlorine contained in the charge transport layer is determined empirically.

In addition, cadmium chloride has a pronounced effect on the decrease in charge in the dark, obviously depending on the charge rate. When charges can leak during charging, the charging speed will decrease accordingly. The effect of cadmium chloride is to increase the dielectric properties of the composite photoconductor in this region. Many attempts have been made to modify the quantity of chlorine in solution 2. It can be seen that a change in concentration of 25%, more or less, does not seem to present too great differences. Too little chlorine increases the charging time, and too much reduces the dielectric properties.

All the regions of the composite photoconductor according to the invention indicate the presence of oxygen which appears to increase the resistivity of cadmium sulfide.

It can be seen that the composite photoconductive member according to the invention accepts a charge of up to 300 V, and that the charge density is higher, in the composite photoconductive member according to the invention, than in selenium doped with tellurium.

No clear demarcation can be detected between the adjacent zones or layers of the composite photoconductor. Its resistance to darkness is between 10 12 and 1016 ß - cm. This is a very clear difference compared to photovoltaic cells which must have a very high conductance.

It is noted that the composite photoconductive member according to the invention, having a photoconductive layer whose thickness is between 3 and 4 μm, has satisfactory operation in an electrophotographic process. In theory, the thicker the photoconductor, the better it is, but we note that cracks appear when a thickness of 6 | im is reached. A thin photoconductor cannot accept a voltage high enough to be usable in practice, i.e. a thin photoconductor requires a longer development time of the latent electrostatic image that it received after a charge .

It should be noted that the invention has been described with reference to the use of thiourea as a reagent containing sulfur and only metal salts of cadmium. Trials have been performed with other sulfur compounds, such as N, N-dimethylthio-urea, but thio-urea is preferable. Dimethylthio-urea and diethylthio-urea do not give as good results as thio-urea, but are usable. A selenourea is less suitable than a thiourea. Although it can be used, its handling is more difficult, its chemical stability is less, and it tends to deposit elemental selenium because it decomposes quickly by itself in ambient light.

Copper is useful in the light absorbing area or layer, because it broadens the spectral response towards short wavelengths, so that the finished electrophotographic photoconductive member is sensitive in the whole visible spectrum, from blue or red.

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The manufacture of a photoconductive member based on cadmium sulphide according to the invention from acetates, and not from chlorides, cadmium and zinc is very important. A small amount of cadmium chloride is only used in the transport layer to ensure the presence of chlorine. It is found that the use of cadmium sulphide, formed by pyrolysis of the spray from chloride, leads to the formation of a photoconductor having a rapid decrease in charge in the dark, that is to say that its resistance or dielectric strength in the dark is not great enough for it to be used in an electrophotographic process. Eliminating excess chlorine using acetates solves this problem.

The introduction of chlorine into the charge transport layer allows the use of a photoconductor high enough to take a charge reaching 250 to 350 V, ensuring rapid development in the case of a toning agent in a liquid vehicle. Satisfactory electrophotographic photoconductive members are prepared with composite zones according to the invention having thicknesses between 3 and 6 μm. When the thickness of 6 (im is reached, cracks appear in the film.

Thus, the invention relates to a photoconductive member for an electrophotographic process based on cadmium sulfide, having excellent wear resistance characteristics. A conducting drum carrying the photoconductor produced according to the invention is tested in mode simulating the use of an office photocopier and is used for a million copies without significant wear. Sensitivity and quantum yield are 2 to 10 times higher than that of selenium. While selenium is only sensitive between blue and yellow, the photoconductor according to the invention is sensitive throughout the visible spectrum, from blue to red. The photoconductor allows the formation of copies having the quality of photographs, that is to say having a grayscale sufficiently graduated so that photographic reproductions are formed from photographic originals. A latent electrostatic image which can be easily and quickly developed with a toning agent suspended in a liquid is formed with the photoconductive member according to the invention. The essential drawback of photoconductive organs containing cadmium sulphide, that is to say the presence of the memory phenomenon, is practically eliminated, and the latent electrostatic image disappears quickly in the light. In this way, copies can be formed at high speed. It is thus possible to produce a high speed photocopying machine using the photoconductive member according to the invention, since a thick layer of photoconductive material can be carried by a conductive substrate, allowing a charge at a potential greater than that thin photoconductors of cadmium sulfide known up to now. Continued use of the photoconductor does not indicate significant fatigue.

A selenium-based photoconductor cannot be heated to more than 80 ° C without harmful effect. The photoconductive member according to the invention is not affected under normal climatic conditions, and it has an apparently infinite storage period. The microcrystalline structure and the presence of oxygen in the photoconductor give the composite photoconductive member sufficiently high properties of insulation in the dark, that is to say a weak decrease of the charge in the dark, for that a very satisfactory latent electrostatic image can be formed with a single exposure.

Of course, various modifications can be made by those skilled in the art to the devices and methods which have just been described only by way of nonlimiting examples, without departing from the scope of the invention.

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Claims (16)

644,959 2 1. Photoconductive member for electrophotography, characterized in that it comprises a conductive substrate (100) on which is formed a coating formed of a homogeneous alloy of metal sulfides having a thickness of at least 3 µm, the coating containing a a large amount of cadmium sulfide and a small amount of zinc sulfide. 2. Photoconductive member according to claim 1, characterized in that the coating is doped with copper. 3. Photoconductive member according to claim 1, characterized in that the coating has a zone (102) adjacent to the conductive substrate (100) and containing lead sulfide. 4. Photoconductive member according to claim 1, characterized in that the coating is doped with copper and has a zone (102) which is in contact with the conductive substrate (100) and which contains lead sulfide. 5. Photoconductive member according to claim 1, characterized in that the coating is doped with copper and is formed of three zones, a first zone (102) placed in contact with the metallic substrate (100) and containing a small quantity of sulphide of lead, a second zone (104) adjacent to the first zone (102) and doped with a small amount of chlorine, and a third zone (106) in contact with the second zone (104) and intended to absorb light, the second zone (104) being intended to transport loads from the light absorption zone (106) to the contact zone (102). 6. Photoconductive member according to claim 1, characterized in that the coating contains a small amount of copper sulfide, the sulfide having a crystal structure, the crystals carrying adsorbed oxygen. 7. Photoconductive member according to claim 1, characterized in that the coating is formed of at least a first and a second zone in mutual contact, the first zone being arranged to absorb light and to retain a large charge. 8. Photoconductive member according to claim 1, characterized in that the coating is a composite coating formed of three zones, the first of which is in contact with the substrate as well as with a second zone arranged for transporting charges, this second zone being in contact with a third zone arranged to absorb light and to retain a large surface charge. 9. Photoconductive member according to claim 1, characterized in that the coating is a composite coating formed of three zones, the first of which is in contact with the substrate as well as with a second zone arranged for transporting charges, this second zone being in contact with a third zone, arranged to absorb light, composed essentially of doped cadmium sulphide having a crystal structure and containing oxygen adsorbed on the surface of the crystals. 10. Photoconductive member according to claim 1, characterized in that the substrate (100) is formed of a conductive metal, in that the coating is a crystalline composite coating containing essentially a large amount of cadmium sulfide and a small amount of zinc sulphide and being formed of three zones, a contact zone (102) adjacent to the conductive metallic substrate (100) containing a small amount of lead sulphide, an external zone (106) containing a small quantity of copper, and a zone intermediate (104), disposed between the outer zone (106) and the contact zone (102), containing a small amount of a chloride. 11. Photoconductive member according to claim 1, characterized in that it comprises a substrate formed of a conductive metal, in that the coating is a crystalline composite coating containing essentially a quand amount of cadmium sulfide and a small amount of sulfide of zinc and being formed by three zones, a contact zone (102) adjacent to the conductive metallic substrate (100) and containing a small amount of lead sulfide, an external zone (106) containing a small amount of copper, and a zone intermediate (104), placed between the external zone (106) and the contact zone (102), containing a small amount of a chloride, the crystals of the crystalline composite photoconductor containing adsorbed oxygen. 12. A method for manufacturing a photoconductive member according to claim 1, characterized in that the coating is formed by pyrolysis from an aqueous solution containing thiourea, cadmium acetate and zinc acetate. 13. The method of claim 12, for the manufacture of a photoconductive member according to claim 2. 14. Method according to claim 12, for the manufacture of a photoconductive member according to claim 7, characterized in that the first zone is formed by pyrolysis from an aqueous solution containing thiourea and acetate of cadmium, as well as smaller amounts of copper acetate and zinc acetate, and in that the second zone is formed by pyrolysis from an aqueous solution containing thiourea and acetate of cadmium, as well as lower amounts of copper acetate, zinc acetate and cadmium chloride. 15. The method of claim 12, for the manufacture of a photoconductive member according to claim 8, characterized in that the first zone is formed by pyrolysis of a spray of an aqueous solution containing thiourea and l cadmium acetate, as well as lower amounts of zinc acetate, lead acetate and copper acetate, in that the second zone is formed by pyrolysis of a spray of an aqueous solution containing thio-urea and cadmium acetate, as well as lower amounts of copper acetate and cadmium chloride, and in that the third zone is formed by pyrolysis of a spray of an aqueous solution containing thio-urea and cadmium acetate, as well as lower amounts of zinc acetate and copper acetate. 16. The method of claim 12, for the manufacture of a photoconductive member according to claim 9, characterized in that the first zone is formed by pyrolysis of an aqueous solution containing thiourea and cadmium acetate , as well as lower amounts of zinc acetate, lead acetate and copper acetate, in that the second zone is formed by pyrolysis of a spray of an aqueous solution containing thiourea and cadmium acetate, as well as lower amounts of copper acetate and cadmium chloride, and in that the third zone is formed by pyrolysis of a spray of an aqueous solution containing thiourea and cadmium acetate, as well as lower amounts of zinc acetate and copper acetate.