IT9067184A1 - ELECTRONIC SWITCH INCLUDING A PHOTOSENSITIVE SEMICONDUCTOR - Google Patents

Description

DESCRIPTION of the industrial invention entitled: "ELECTRONIC SWITCH INCLUDING A PHOTOSENSITIVE SEMICON-DUCTOR"

SUMMARY

Electronic switch comprising a photosensitive semiconductor (1, 11, 12) and a source (14, 24) which, when activated, illuminates the semiconductor making it conductive, the photosensitive semiconductor being a sintered mixture comprising, by weight, 63-74% of cadmium , 16-24% selenium, 8-14% sulfur, 0.1-1% chlorine and 0.005-0.1% copper.

The present invention refers to an electronic switch comprising a photosensitive semiconductor and a light source which, when activated, illuminates the semiconductor making it become conductive.

Photosensitive semiconductors are well known and need not be described in detail. The main feature of such semiconductors is their ability to pass from an electrically non-conductive, high-resistance layer to an electrically conductive, low-resistance layer when illuminated by light.

The materials currently used such as or in industrial photosensitive semiconductors are for example: silicon, carbon, germanium, gallium arsenide, silicon carbide, cadmium selenide, cadmium sulfide, indium phosphide, and potassium phosphide. These prior art photosensitive semiconductors can be used in various electronic devices, for example, in electronic switches, and are generally doped with "impurities" such as, for example: cobalt, copper, chromium, gold, iron, oxygen, silver and zinc.

Electronic switches comprising a photosensitive semiconductor are disclosed, for example, in the following US patents: 4,301,362; 4,347,437; 4,438,331; 4,490,709; 4,577,114 and 4,695,733. Various switch configurations are disclosed in these patents. They all comprise a light source and a photosensitive semiconductor equipped with two spaced electrodes. Semiconductors are made up of various materials and the light source is usually a laser.

When, for a given semiconductor, a certain bias voltage is applied across the electrodes of a photosensitive switch, the switch will not allow electric current to pass through the electrodes and the switches will be in the "off" position, but when the light source projects light energy onto the area of the semiconductor located between the electrodes, the semiconductor becomes electrically conductive and the switch will be in the "on" position. The main advantage of switches of this type is that they can very quickly switch an electric current into the on and off state.

A disadvantage of the known photosensitive switches is, in particular, of the switches described in the aforementioned six US patents, consisting of the fact that they are only capable of switching power supplies that have voltage below a few kV, for example up to 3 -5 kV, and thus cannot be used in electrical devices where a higher voltage is required.

It is therefore a first object of the invention to provide a light-activated electronic switch that is capable of withstanding / switching low-value, high-voltage electrical currents, i.e. power supplies having a voltage of at least 30 kV and having a current lower than 0. , 2A. It is a further object of the invention to provide a very compact and very economical low current and high voltage photosensitive switch, and it is also an object of the invention to provide a photosensitive switch capable of operating at high frequency.

It has now been found that some cadmium-doped sulfo-selenide semiconductor compositions exhibit a desirable combination of properties such that when used as a semiconductor in a photosensitive switch, the latter is capable of switching low current and high voltage supplies, and which can be built compactly and economically and operated at high frequency.

According to the present invention, an electronic switch is provided comprising a photosensitive semiconductor and a light source which, when operated, illuminates the semiconductor and causes the latter to become conductive, wherein the photosensitive semiconductor is a sintered mixture comprising, by weight , 63 to 74% cadmium, 16 to 24% selenium, 8 to 14% sulfur, 0.1 to 1% chlorine and 0.005 to 0.1% copper.

The semiconductor preferably has the form of an adherent layer on an electrically insulating substrate. The semiconductor is equipped with spaced electrodes to which, in use, the circuit that the switch must control is connected. The electrodes can be two layers of adherent, spaced-apart electrodes on the sintered semiconductor layer or the substrate can be provided with two adherent, spaced-apart electrode layers and the adherent sintered semiconductor layer can be applied to the electrode layers and substrate therebetween; in the latter case the electrode layers are located under the semiconductor layer.

The electrode layers can be formed of any suitable electroconductive material, preferred materials being, for example, silver, indium and aluminum and resins, for example epoxy resins, loaded with one or more of these metals.

According to another aspect of the present invention, a method is provided for constructing an electronic switch according to the invention which comprises forming a finely divided powder mixture paste comprising, by weight, 35 to 55% cadmium selenide, 35 to 55% cadmium sulfide, 5 to 15% cadmium chloride and 0.01 to 0.1% copper chloride, a binder and a volatile liquid, forming a paste coating on an electrically insulating substrate and drying the coating by sintering the dried coating at a temperature of 540 ° C to 800 ° C to form an adherent layer of photosensitive semiconductor on the substrate, providing spaced adherent electronic layers either on or under the semiconductor layer, and mounting the coated substrate with a source of light.

The formation of the sintered semiconductor layer can be obtained in accordance with conventional sintering techniques which are well known to those skilled in the art.

To achieve the desired photosensitivity and electrical properties in the sintered semiconductor, the finely divided powder materials must be of the highest purity, for example 99.999% or more, and the size of the powder particles preferably less than 3µm.

Convenient binders and volatile liquids for forming the paste are known to those skilled in the art. In general it is preferred to use ethyl cellulose as the binder, however other suitable binders include, for example, linseed oil and cellulose acetate. Preferred volatile liquids are organic liquids; Suitable organic liquids include, for example, terpineol, acetone and ethanol, the former of which is generally preferred. The binder is preferably present as a 10% by weight solution in the volatile liquid. The proportion of binder / volatile liquid solution in the paste will of course depend on the desired viscosity of the paste which, in turn, will depend on the process of coating the substrate with the paste being used. For many purposes, a paste containing about 15% by weight of the volatile binder / liquid solution is convenient.

Convenient processes for forming a layer of the paste on the substrate include, for example, screen printing, spraying, rotary motion and settling. Screen printing is a very precise process which produces a layer having a smooth, regular surface and a thickness of 10 to 50 µm depending on the mesh of the sieve; this method is generally preferred.

Suitable electrically insulating substrate materials are, for example, alumina ceramics, fused silica and "pyrex" glass (trade name). The coated substrate is then dried to remove the volatile liquid, preferably by heating it in an oven at a temperature of about 100'C for about 10 minutes.

The dried coating is then sintered in a substantially inert atmosphere at a temperature from 540 "C to 800" C. To this end, the coated substrate is preferably placed in a container through which a stream of nitrogen (or other inert gas) containing a small proportion of oxygen, preferably added as air, can be passed and the container is placed in a convenient oven. or furnace, preferably an electric furnace. The proportion of air in the inert gas / air mixture is convenient? 3⁄4.tead |% by volume. The inert gas / air mixture is passed through the container at a relatively slow rate, for example 2 to 6 liters per hour.

The oven is gradually heated to the sintering temperature, for example for a period of 30 minutes, and is held at the sintering temperature for 45 to 150 minutes, preferably 45 to 120 minutes. The sintering temperature does not appear critical; if the temperature is much lower than 540 * C, the layer fails to sinter and if it is above 800 * C, the layer decomposes. Better results are obtained at sintering temperatures ranging from 540 ° C to 700 ° C.

The sintered layers thus obtained are firm, adherent and chemically stable. A reduction in thickness occurs during the sintering operation and the final thicknesses are generally in the range of 5 to 25 µm, depending on the initial thickness of the layer. There is also always a weight loss during sintering and the final chemical composition of the semiconductor can vary somewhat as a function of the sintering temperature and as a function of the sintering time.

The sintered semiconductor layer is then provided with two spaced electrodes which may be made of, for example, steel, indium or aluminum or an epoxy resin loaded with one of these metals. The electrodes can be formed on the semiconductor either (a) by evaporating the electrode metal onto the surface of the semiconductor, either (b) by screen printing the epoxy resin loaded onto the surface, or (c) by pressing a sheet of the metal onto the surface of the semiconductor.

Of these three processes, processes (a) and (b) are generally preferred. In both methods, the area between the electrodes is masked prior to applying the electrode material to the semiconductor surface. Process (a) is normally followed by a short heat treatment which, when silver or aluminum is used, consists, for example, of heating the electrode layer in nitrogen at 300 ° C for 20 minutes. Procedure (b) requires that the epoxy resin is heated in air, for example, at 150 C for 35 minutes, in order to harden the layer.

Another method of providing the semiconductor with two spaced electrodes is to form the electrodes as layers on the electrically insulating substrate, by any of the above-described methods, while masking the area between the electrodes and then forming the semiconductor layer over the electrodes. and the substrate area therebetween.

The electronic switch according to the invention can take various forms. For reasons of compactness, it is preferable that the semiconductor layer and the substrate have the shape of a hollow cylinder. It is also preferred that in this arrangement the semiconductor layer is located on the inside of the hollow cylinder and the light source is located on the longitudinal axis of the cylinder.

A variety of light sources can be used. It is preferable to use one or more light emitting diodes (LEOs) or a glow discharge lamp or tube providing light having a wavelength of 500 to 900 nm. Affordable LEDs include high-efficiency red LEDs with a peak output of approximately 660 nm of visible red light and a viewing angle of 140 °, and infrared LEDs that emit a peak output of 830 nm and have an angle of of vision greater than 30 *.

Convenient discharge lamps are, for example, neon discharge lamps and tubes.

For a better understanding of the invention, preferred embodiments of semiconductor / substrate and switch assemblies will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a plan view of a semiconductor / substrate embodiment,

Figure 2 is a section on the line II-II of Figure Ir-Figure 3 is a section on the line III-III of Figure 1;

Figure 4 is a cross section of a first embodiment of an electronic switch assembly;

Figure 5 is a sectional elevation on the line V-V of Figure 4;

figure 6 is a cross section of a second embodiment of a cylindrical electronic switch assembly, e

Figure 7 is a sectional elevation on the line VII-VII of Figure 6.

With reference to Figures 1 to 3, a semiconductor / substrate assembly comprises a sintered photosensitive semiconductor layer 1 coated on an electrically insulating flat substrate 2 and spaced-apart electroconductive electrodes 3A and 3B. The semiconductor layer 1 has the composition Cd / Se / S / Cl / Cu mentioned above.

The electronic switch shown in Figures 4 and 5 comprises an adherent sintered photosensitive semiconductor layer 11 and an insulating substrate 12 having the shape of a hollow cylinder, and two annular electrodes 13A and 13B at each end of the cylinder. The switch further comprises a light source 14 which is a discharge tube which gives light having a wavelength of 500 to 900 nanometers; the light source 14 is arranged along the axis of the cylinder 11/12. The semiconductor layer 11 has the composition Cd / Se / S / Cl / Cu mentioned above.

The electronic switch shown in Figures 6 to 7 comprises a hollow cylindrical semiconductor layer 21 and an insulating substrate 22 similar to the semiconductor layer 11 and the substrate 12 of the previous embodiment, but in this embodiment the electrodes take the form of two elongated strip electrodes 23A and 23B arranged along diametrically opposite generatrices of the cylinder 21. The switch unit further comprises six light-emitting diodes 24 arranged along the axis of the cylinder 21/22. The semiconductor layer 21 has the composition Cd / Se / S / Cl / Cu mentioned above.

The operation of the electronic switches shown in figures 4 and 5 and in figures 6 and 7 is as follows. A bias voltage is applied between the electrodes (13A-13B, 23A-23B); no current flows until the light source (14, 24) is illuminated and as soon as it is illuminated the current passes.

The electronic switches as illustrated can be of very compact construction. The properties of the described semiconductors are such that the switches can safely handle voltage pulses of up to 30 kV or more with an electrode spacing of only 10 to 20 mm.

In order for the invention to be understood more fully, only the following examples are given for illustrative purposes, in which all percentages are by weight, unless otherwise indicated.

Example 1

A powder mixture of the following composition ("the first composition") was formed:

Cadmium selenide 45% Cadmium sulfide 45% Cadmium chloride 9.91% Copper chloride (cupric), 0.09%

and it was dry ground in a mechanical grinder, such as a microlyser, to obtain a homogeneous mixture in which all the particles were less than 3 µm in size.

A 10% solution of ethyl cellulose in terpineol was mixed with the powder mixture, the amount of the solution being 15% of the combined weights of the solution and the powder mixture. The mixing was continued until a uniform paste was obtained.

The paste was applied to the surface of a high density 96% alumina ceramic substrate of the type used in the production of thick film circuits, using screen printing. The substrate was 5cm x 3.5cm x 0.05cm in size and the paste was printed to cover a 5cm x 3cm rectangular area. A 305 mesh sieve material was used resulting in a paste layer approximately 15 µm thick.

The coated substrate was placed in an oven and heated at 100 "C for 10 minutes to evaporate the terpineol.

A paste similar to the one described above, but having the following composition of inorganic components ("the second composition"):

Cadmium selenide 35% Cadmium sulfide 55% Cadmium chloride 9.99% Copper chloride 0.01%

it was screen printed to form two strips, 3 mm wide and 4.5 cm long, which overlapped the major edges of the previously printed layer by 1.5 mm.

The purpose of these strips was to provide better electrical contact between the sintered semiconductor layer and the indium electrodes (see below); It was found that compositions containing more than 45% cadmium sulfide and a relatively low proportion of copper gave better electrical contact than compositions containing 45% or more of cadmium selenide and a higher proportion of copper.

The coated substrate was returned to an oven and heated at 100 "C for 10 minutes in order to evaporate the terpineol from the printed strips.

The coated substrate was then placed in a pyrex glass container having a top lid of the same material. Gas was introduced into one end of the container through a pyrex glass tube and evacuated from opposite ends through an interval between the container and its top lid. This complex was placed in an electrically heated oven.

Before heating, pure nitrogen was first passed through the container at a flow rate of 4.5 liters per hour for a period of 20 minutes in order to purge away the mass of air that was initially present. The oven was then heated to 580 ° C for a period of 50 minutes while the passage of pure nitrogen continued. Once a temperature of 580'C was obtained, a small proportion of air / '■' Tsiavl% by volume was added to the nitrogen and the flow of this mixture was continued at 4.5 liters per hour. The temperature of 580eC was maintained for 60 minutes and at the end of this period, the oven was switched off and allowed to cool for a period of 60 to 90 minutes. The flow of the nitrogen / air mixture continued until the furnace was cooled below 150 "C. After which air could be blown through the furnace to accelerate cooling.

The obtained sintered layer was firm, adherent and chemically stable. An approximate 40% reduction in layer thickness was achieved. Electron microscopic studies of the surface of the sintered material showed that the latter consisted essentially of large grains of about 9um fused together at the grain boundaries.

The chemical composition of the first sintered composition was as follows:

Cadmium 68.4% Selenium 20.2% Sulfur 11.06% Chlorine 0.3% Copper 0.04%.

Indium was then evaporated on the marginal portions of the major sides of the substrate to form the electrodes. To this end, a metal mask was placed so as to cover the entire first sintered composition and the 1 mm wide marginal areas of the second composition on both sides of the first composition. Indium was then evaporated using a conventional metal evaporation apparatus to coat the masked substrate. The mask was then carefully removed; the thickness of the indium coating was 0.5 µm. The coated substrate was placed in an oven and heated at 160 ° C for 15 minutes causing the indium to melt to the surface of the second sintered composition.

The final product then had a band of the first composition of about 27mm wide (between the longer sides of the substrate), a strip of the second composition of about Imm wide on both its sides, and an indium electrode strip of about 3mm. wide on the other side of the second composition and extending to the major edges of the substrate.

To form the switch, six light emitting diodes (LEDs) of the visible light type were mounted on a piece of printed circuit board measuring 5cm x 3.5cm in two rows of three arranged across the width of the postcard. Each row was arranged with its center at 1.75cm from each end and the LEDs were arranged to be equidistant along the length of the row. Each row of 3 LEDs was electrically connected in series and the two rows were connected in parallel to form a common control connection. The LED array was mounted on the semiconductor surface on the substrate with a spacing of 3mm from the surface to the tip of each LED. The LED plate assembly and coated substrate were mounted in a rigid plastic frame that electrically isolated the coated substrate from the LEDs. Electrical connections were made with the indium electrodes on each edge of the substrate. The whole complex was then immersed in transformer oil in order to prevent the occurrence of a discharge across the surface of the semiconductor layer.

The LEDs used were high-efficiency red diodes with a peak output of 660 nm in visible red light and a viewing angle of 140 * C. Their light output for a current of 20 ma was 200 med (0.2 candelas); for pulsed current operation, the light output per 100 ma pulses was typically a spark plug at 25 ° C.

The electrical properties of the switch were as follows:

Conditions in the dark.

The following properties were recorded with the circuit breaker immersed in transformer oil at a temperature of 25 ° C and in total darkness.

For a DC voltage of 30 kV applied between the semiconductor contacts, was a current of less than 2 uA recorded? this corresponds to a dark resistance of the switch of more than 15,000 mega ohms.

Electrical piercing of the semiconductor occurred at a direct current voltage of 35-40 kV from tests performed on various samples.

The switch was able to withstand high voltage pulses of the type generated by the ignition winding of a gasoline engine up to an amplitude of at least 35 kV and a pulse repetition rate of 200 per second.

Illuminated conditions.

The following properties were recorded with the circuit breaker immersed in transformer oil at a temperature of 25'C and under the following lighting conditions.

Forward diode current, pulse current of 3ms in duration and amplitudes greater than 0.15, at a pulse repetition rate of 50 per second.

Response tests over time.

A 500 Volt DC bias was applied between the semiconductor layer contacts and the previous forward current conditions applied to the diodes resulting in the generation of current pulses in the 500 Volt bias circuit. The characteristics of these impulses were as follows:

Peak recorded current: 18.5 mA Time from minimum current to

maximum current in which the instant

zero assigns the start of the illumination pulse 1.4 ms Time for the current to fall to 1%

of the peak value once the current pulse of the diode has ended 1.7 ms Minimum resistance evaluated 27,000 ohms

Maximum ignition pulse current capability.

Test conditions.

An H.T. of the ignition winding has been connected in series with the electrodes of the semiconductor layer so that the switch represents the only impedance to the current in the circuit. The ignition coil was arranged to produce an ignition current pulse at a point in time which coincided with the center of the LED forward current pulse time duration, i.e. the ignition current pulse occurs at a point where the pulse operated switch is in a state of least resistance and leads to a pulse.

The fundamental waveform of the current pulse produced by this circuit was that of a rise to the peak current over a period of 75 us and then a progressive decay of the current to 0 over a period of time of 1.5 ms. at a pulse repetition rate of 50 per second.

The peak currents are varied by changing the primary current of the ignition coil and to a greater degree by changing the type of ignition coil.

Pulse peak current which

it lasted for a period of

time greater than two hours

with no signs of damage

for switch 70 ma Estimated energy of the pulse of

ignition 45 mJ Power dissipated by layer a

25 'C 2.2 W Pulsed peak current which

caused damage to the layer

semiconductor after 10 min 100 ma Estimated energy of the pulse of

ignition 93 mJ Power dissipated by the layer

at 25 <e> C 4.75 W

Example 2

A sintered semiconductor layer was formed on an alumina substrate as described in Example 1, but with the following variants.

The composition of the inorganic materials was as follows:

Cadmium selenide 35%

Cadmium sulfide 55%

Cadmium chloride 9.95%

Copper chloride 0.05%

Only this composition was used (i.e. a second inorganic composition was not used in this example).

The substrate was of the same material and the same size as that of Example 1, but the paste was sieve molded to form a 3cm x 2 era rectangle.

In the sintering step, the percentage of air in the nitrogen / air mixture was 2% by volume and the sintering temperature 540 ° C.

Indium was evaporated onto the sintered semiconductor to form contact layers arranged across the width of the substrate and each extending from a short edge of the substrate to overlap the semiconductor layer by 1.5mm.

The finished coated substrate was, therefore, generally of the type shown in Figures 1, 2 and 3 and included a semiconductor layer approximately 2.0cm wide and 2.16cm long with indium contacts arranged across the width of the layer and superimposed on the edge of the latter for a distance of 1.5mm at each end.

This coated substrate was mounted with two neon glow discharge tubes as the light source. These were commercially available neon indicator lamps. The two neon tubes were arranged with the length of each tube passing across the width of the semiconductor and spaced so that the central axis of the tubes was positioned lem from the electrodes and lem above the surface of the semiconductor. A small flat reflector was placed above the neon tubes to increase the illumination of the surface.

Each of the neon tubes was connected in series to the other and the current waveform used to drive the neon tubes was the same as that of Example 1 except that the peak current was limited to 20 mA.

Electrical properties:

Conditions in the dark.

The following properties were recorded with this switch immersed in transformer oil at a temperature of 25'C and in total darkness.

For a DC voltage of 30 kV applied between the contacts of the semiconductor layer, a current of less than luA was recorded, equivalent to a resistance in the dark of the switch greater than 30,000 megohms.

Electrical piercing of the semiconductor material occurred at a DC voltage of 37 to 39 kV from tests conducted on various samples.

The switch could withstand high voltage pulses of the type generated by an ignition coil up to an amplitude of at least 32 kV and a pulse repetition rate of 200 per second.

Lighting conditions.

The properties of the switch were recorded with the switch immersed in transformer oil at a temperature of 25 * C and under the following lighting conditions:

neon current, current pulses of 3ms duration and amplitudes greater than 0.02, at a pulse repetition rate of 50 per second.

Response test over time.

A 500 Volt direct current bias applied between the semiconductor layer contacts and with the previous forward current conditions applied to the neon tubes to cause the generation of current pulses in the 500 volt bias circuit. The characteristics of these impulses were as follows:

Peak recorded current 12.5 mA Time from minimum current to

maximum current in which the instant

Zero indicates the start of the illumination pulse 1.2 ms Time for the current to drop to 1%

of the peak value once ends

to diode current pulses 3.7 ms Minimum estimated resistance 40,000 ohms Maximum pulse capacity of

power on

Test condition.

An H.T. has been connected in series with the electrodes of the semiconductor layer so that the switch represents the only impedance to the current in the circuit. The ignition coil was arranged to produce an ignition current pulse at an instant of time which coincided with the center of the neon current pulse time duration, i.e. the ignition current pulse occurs at a instant in which the pulse operated switch is in a state of minimum resistance and leads to each pulse.

The fundamental waveform of the current pulse produced by this circuit was that of a rise to the peak current over a period of 75 us followed by a progressive decay to zero current over a period of time of 1.5 ms to a pulse repetition rate of 50 per second.

The peak currents are varied by changing the primary current of the ignition coil and in

"

higher degree by changing the type of ignition coil.

Pulsed peak current it has

resisted for a period of time

more than 2 hours without any sign

of damage for the circuit breaker 60 ma <'> Estimated energy of the impulse of

ignition 54 mJ Power dissipated by layer a

25 * C 2.7 W Pulsed peak current which caused damage to the photoconductor layer after 10 min 80 ma

Estimated energy of the pulse of

ignition 96 mJ

Power dissipated by layer a

25 "C 4.80 W

Claims (10)

CLAIMS 1. Electronic switch comprising a photosensitive semiconductor (1, 11, 21) and a light source (14, 24) which, when operated, illuminates the semiconductor and causes the latter to become a conductor, characterized in that the photosensitive semiconductor is a sintered blend comprising, by weight, 63 to 74% cadmium, 16 to 24% selenium, 8 to 14% sulfur, 0.1 to 1% chlorine, and 0.005 to 0.1% copper. 2. Switch according to claim 1, characterized in that the semiconductor (1, 11, 21) has the form of an adherent layer Y of an electrically insulating substrate (2, 12, 22). Switch according to claim 2, characterized in that two adherent, spaced-apart electrode layers (3A, 3B; 13A, 13B; 23A, 23B) are arranged on the sintered semiconductor layer (1, 11, 21). 4. Switch according to claim 1, characterized in that it comprises an electrically insulating substrate, two layers of spaced, adhering electrodes on the substrate and an adherent layer of the sintered semiconductor composition disposed over the electrode layers and the substrate therebetween. . 5. Switch according to claim 3 or 4, characterized in that the electrode layers (3A, 3B; 13A, 13B; 23A, 23B) consist of silver, indium or aluminum or an epoxy resin loaded with silver, indium or aluminum. Switch according to any one of claims 2 to 5, characterized in that the adherent semiconductor layer (11, 21) and the substrate (12, 22) have the shape of a hollow cylinder. 7. Switch according to claim 6, characterized in that the light source (14, 24) is located on the longitudinal axis of the cylinder. 8. Switch according to any one of claims 1 to 7, characterized in that the light source consists of one or more light-emitting diodes (24). Switch according to any one of claims 1 to 7, characterized in that the light source is a glow discharge lamp or tube (14) which gives light having a wavelength of 500 to 900 nanometers. 10. Process for the construction of an electronic switch according to claim 1, characterized in that a paste is formed of a finely divided powder mixture comprising, by weight, 35 to 55% of cadmium selenide, 35 to 55% of sulphide of cadmium, 5 to 15% cadmium chloride and 0.01 to 0.1% copper chloride, a binder and a volatile liquid, a paste coating is applied to an electrically insulating substrate and the coating is dried, the dried coating is sintered at a temperature of 540 to 800'C to form an adherent layer of photosensitive semiconductor on the substrate, adherent spaced electrode layers are arranged either above or below the semiconductor layer, and the coated substrate is mounted with a light source .