Feb. 6, 1940. ` Q_ W, HEWLETT 2,189,580 METHOD OF MAKING A PHOTOELECTRIC CELL Filed May 29, 1937

Inventor : Clarence VV. Hewlett,

is Attorney Patented Feb. 6, 1940 , 2,189,580 UNITED’ STATES PATENT OFFICE _,

g aisance METHOD 0F MAKING A PHUTOELECTRIC

Clarence W. Hewlett, Schenectady, N. Y., assigner to General Electric Company, a corporation of New York Application May 29, 1937, Serial No. 145,539 4 Claims. (CLES-8.9) The present invention relates to light-sensitive posited .on the selenium. The first, which is devices, more particularly to photoelectric cells, designated by reference character 4, is preferably which generate a measurable electromotive force of cadmium, and the uppermost metal layer 6 is when subjected to light of practical intensities. a relatively non-oxidizable metal preferably 5 In my application Serial No. 716,677, filed platinum. A contact ring 6 of any suitable metal March 21, 1934, I have disclosed and claimed a such as cadmium is applied in any convenient photo-voltaic cell which consists of a metal plate manner either to the selenium layer 3 before the or base member coated first with selenium and sputtered layers have­ been deposited, or to the then with cadmium and a relatively non-oxidiz platinum 5 after both metal layers have been de l0 able metal such as platinum in order. The cad posited. In case the ring is applied to the se 10 mium and platinum layers are preferably laid lenium layer, the sputtered layers are deposited down by‘sputtering and are so t-hin as to be semi not only over the selenium layer but also over ` transparent to light. When light shines on Wthe the cadmium ring. The contact ring and the ` double metal layer, an electromotive force is gen manner of forming the same constitute one of 15 erated between this layer and the selenium, and the features of the present invention and will l5 the current produced may be conducted away be described presently. The groove 2 is prefer from the cell by conductors secured in any suit ably positioned directly under the ring 5 and is able manner to this sexnitransparent metal layer slightly wider than the ring as is explained in my and to the base member.' This current is of suiil copending application Serial No. 145,538, entitled 20 cient intensity to be measured by a microammeter “Photoelectric cells.” The purpose of this groove which may be calibrated in foot candles to indi as pointed out in the said application is to pro cate the intensity of the light impinging on the vide an excess of selenium and thereby increase metal. The present invention concerns devices the resistance of the path of the current which of this character, and the objects are to increase iiows from the sputtered layer back to those por the sensitiveness of photo-voltaic cells and to lm tions of the selenium from which it started under 25 prove their manufacture. Another object is to the iniiuence of the light. The selenium con 25 Aprovide a practical method of making a rigid tained in the groove ­also serves to prevent a contact between one of the current-carrying con striking-through of the selenium layer when the ductors and the thin metal layer. In accordance contact ring is deposited in the manner described

with these objects, the sensitiveness of the cell hereinafter. Y ' ` ' 3 is increased by controlling the vapor content of In the manufacture of these-cells, the first step 30 the gaseous chamber in which the sputtering is to provide the metal plate I with the annular takes place. In order to provide the rigid con groove or channel referred to hereinbefore. The tact between the conductor and the thin metal side of the plate which is grooved is then rough 35 layer, a metal ring of vsubstantial thickness is in ened by sandblast in order to provide a good tegrally. joined, for example, by spraying, to the >gripping surface for the selenium which is to be 35 selenium surface and the sputtered ñlm overlays deposited. The plate is then heated and wetted the ring and the selenium surface. Then the with selenium, the latter being thoroughly rubbed conductor is pressed in any suitable manner into the surface. The selenium may be scraped A against this ring. The objects andthe manner off the plate or disk, leaving the channel full and in which they are attained will be more clearly only a thin layer of selenium on the surface. 40 understood when reference is made to'the follow The plate is then chilled. This thin wetting layer ing specification and the accompanying drawing of selenium is then recrystallized by heating the in which Fig. l is an elevational, view of a sputter plate. The plate with the grooved side up is then 45 ing chamber for depositing metal layers on the placed in a heated heavy metal ñxture 8, shown selenium; Fig. 2 is' a sectional view of a cell in in Fig. 2, which has a countersunk or depressed 45 the process of making and the fixture for holding portion of a size snugly to receive the plate. This the same; Fig. 3 shows another step in the proc ,countersunk portion is of a depth greater than ess; Fig.` ‘i shows an improved method of apply . the thickness of the plate i by an amount repre 50 ing a contactring to the cell; and Fig. 5 is a senting the thickness of the selenium layer which sectional view of the completed cell, ready for is to be deposited on the plate. Selenium, in 50 mounting. paste, form, is spread on the disk l and the excess Referring to Fig. 5, numeral I designates a ñat is scraped od by a tool 9 which bridges the metal plate of any configuration, for example, countersunk portion of the ñxture t and leaves a 55 circular, and made preferably of iron or nickel. layer of selenium of the proper thickness depend 55 One side of the plate, the upper as shown, is pro ing upon the depth of the recess in the fixture. vided with an annular groove 2 and on top of the After a smooth surface has been obtained on the plate, filling this groove, is a layer of selenium 3. selenium by drawing the scraper in several di As is pointed out in the said application referred rections'across the upper surface of the fixture 8, ’ 60 to hereinbefore, there are two metal layers de the selenium coated plate l is then removed from 60 2 2,189,580 the fixture and the element is chilled. The next ' carries a rotatable drum I8 and a stationary rod step in the process is to obtain a smooth, glassy member I9. A flat metal plate 20 is secured to appearing, compact layer of selenium of uniform the upper end of the rod I9. 'I'he purpose of this thickness on the plate, and this is conveniently plate will be explained presently. 'I'he drum accomplished by a hot press treatment. member I8 carries a pair of oppositely extending A suitable fixture for this treatment is shown shafts 2|, 22, one of which (element 2I) termi in Fig. 3 and consists of a metal plate P contain nates in a counterweight 23. The other shaft 22 ing heating elements (not shown) let into holes carries a magnetic member 24 of arcuate form, ~ H, and a die I 0 containing a recess of a depth and at a position intermediate the ends of the corresponding to the ñnished thickness of the rod, also carries an upright shaft 25. A carrier 10 selenium coated plate. Before placing the se fixture 26 provided with a central recess 21 is lenium plate on the press, the apparatus is heat secured to the upper end of the rod 25. This ed to a temperature of about 140° C., and the recess has a diametral size and thickness suf selenium coated plate I is then placed, selenium iicient to accommodate the selenium coated disk side down, against the plate P. The die I0 is I. In addition to the upright I1, the base I6 is 15 then placed on top of the selenium coated plate, also provided with a glass upright 28 which ter and pressure applied in any suitable manner, minates at the top in a pair of forked arms 26. for example by means of a clamp. At the tem The arms 29 contain a pair of conductors 3D, 3|, perature mentioned, the selenium softens and one of which is secured to a mesh member 32 flows, the excess being exuded around the edges composed of platinum, and the other carries a of the plate as indicated at B. Although as stat disk 33 of cadmium which lies directly above the ed hereinbefore, the selenium at ñrst softens carrier 26. and flows, in a few minutes it begins to harden, The arrangement is such that by means of a and after 10 to 30 minutes the pressure may be magnet 34, which is adapted to be moved around relieved and the plate removed from fixture. the bell jar, the armature 24 is caused to move The selenium layer is now hard, smooth, and of within the chamber and this movement serves uniform thickness, and the excess exuded sele to rotate the carrier 26 from a position directly nium may be removed from the edges. The sele under the disk 33, as shown in Fig.` 1, to .a po nium coated plate I which has thus been treated sition directly under the mesh member 32. 30 is then baked at a temperature near the melting When it is desired to deposit a metal layer on the 30 point of selenium. selenium-coated disk I, the bell jar is tempo The plate I is then placed in a ñxture similar rarily removed and the disk I­ is placed in the to that shown in Fig. 4 for the deposition of the recess of the carrier 26. A mask (not shown) contact ring. This fixture comprises a heavy is placed about the periphery of the cell, the pur metal ring 36 which has a metal back 31 secured pose of which is to shield the edges of the cell thereto by s'crews 38. The front or upper por from the sputtered deposit. This mask allows tion of the ring 36 is provided with an inwardly the sputtered deposit to cover the central portion extending lip 38 against which the selenium coat of the selenium surface and partly to overlap the ed disk I­ can be pressed by a spring 40 riveted to sprayed-on cadmium ring. After the bell jar is 40 the back plate 31. A conically shaped metal placed in position and properly sealed, the jar is shield 4I provided with a handle 42 is provided evacuated in any suitable manner. Argon or any for protecting the central portion of the metal or other inert gas is introduced into the bell jar at selenium coated disk from the heat effects of a about 240 microns. By means of the leads 30 Schoop gun 45. The shield 4I has an outer di and 3| which pass to the exterior, either of the ameter less than the lip.39 so as to leave an an electrodes 32 and 33 may be made cathode of a nular space. A heavy metal layer of cadmium circuit (not shown), and by means of a lead (not is laid down in this space by the gun 45, in the shown) the plate I6 may be made the anode. form of a ring. As stated hereinbefore, this ring The selenium plate is then sputtered under the is positioned directly above the selenium-filled cadmium cathode in one-second flashes separat groove 2 formed in the plate I­ and is slightly less ed by 20-second intervals until a sputtered layer in width than the width of the channel. This which permits about 30% of the incident light contact ring is for the purpose of readily permit to pass through is obtained. The selenium plate ting a connection between a conductor (not is then adjusted under the platinum cathode by shownl and the metal layer which is to be sput moving the magnet 34 around the bell jar, and tered over the selenium surface, as Will be de sputtered with platinum for about three-fifths 55 scribed presently, and lapping over the contact the length of the time of sputtering the cadmium. ring. After the contact ring has been built up It is apparent that the metal plate 20 serves as to its proper thickness, the cell is removed from a partition to prevent sputtered deposits from the fixture by unscrewing the plate 31, and it one cathode being deposited on the other cath is then ready to be sputtered with the metal ode. layers. While I prefer to apply the contact ring During the sputtering operation, liquid air may _ ,directly to the selenium surface, I may, under be poured into the ñask I4, causing condensible, certain circumstances, deposit the ring on the vapors such as water to condense on the surface sputtered metal layer. of the flask, thereby removing them from the The sputtering apparatus is shown in Fig. 1 gaseous atmosphere. While satisfactory results 65 and consists essentially of a large bell jar I2 have been obtained by causing the condensation which terminates at the top in a hemispherical to take place simultaneously with the start of the portion I3. A reentrant bulb I4 having an open sputtering operation and even before, I have ing I5 is secured to the hemispherical portion I3. found, and in accordance with the present inven 70 The lower end of the bell jar is temporarily closed tion, that outstandingly good cells may be ob 70 in any suitable manner, for example, by being in tained if the introduction of liquid air and the serted within a close-ñtting recessed plate mem condensationare delayed until an appreciable ber I6 which is well known in the art. Secured time after the sputtering of the cadmium layer to the plate member I6 and centrally'positioned, has started. I prefer to delay the introduction 75 there is a metal upright or standard I1 which of- the liquid air until from one to five flashes of 75 2, 189,580 the cadmium sputtering have been carried out. to remain in the argon atmosphere during the After the liquid air has been introduced in this initial stages of the sputtering process, some ac manner and the cadmium sputtering has been tion takes place which causes the selenium sur continued until the proper. thickness of layer has face to be more light-sensitive or causes a more been obtained, the platinum layer is sputtered eifective barrier layer between the selenium sur in the usual manner. as explained above. It has face and the cadmium to be formedso that a cell been found that when the refrigeration of the in which the refrigeration has been delayed‘for a gaseous atmosphere has been delayed for a short predetermined length of time is much more sensi time and is not produced simultaneously with the tive than a cell in which the refrigeration has "i beginning of the cadmium sputtering. the re been provided simultaneously with the initial sulting cell is more satisfactory than those cells sputtering stages. in which the refrigeration is initiated simulta As was pointed out in my application Serial No. neously with the sputtering. The improved cell ' 716,677, a cell of this kind may be inserted in a will delivermore power in an external circuit of casing of hard rubber and when connections have higher resistance, and the temperature coeiiicient been made to the contact ring and the metal ll of output and the fatigue under action of illumi base I, the cell will generate an electromotive nation are both considerably smaller. As a mat force upon exposure to light which is sufiicient ter of fact, a cell so produced will have approxi to operate a microammeter. 'I'his meter may be mately double the open circuit electromotive force calibrated in foot candles to indicate or measure of one in which the refrigeration has not been the intensity of light falling on the metal layers. i0 delayed. ' What I-claim as new and desire to secure by Afterthesputtering operations described above, Letters Patent of the United States, is: . the plate I is removed from the carrier 28 and 1. In the method of fabricating a light-sensi the selenium surface up to the contact ring is tive device, the steps of coating a metal plate with lacquered or varnished as indicated at 48, Fig. 5. selenium, then sputtering la semitransparent lay- 28 The cell is now complete and laid aside to ma er of cadmium onto said selenium in a chamber ture or age. containing argon and water vapor, then >con While I do not wish to be limited to any theory densing out the water vapor by cooling said as to why such a marked improvement is brought chamber, then sputtering an additional semi »- about by simply delaying the application of the transparent layer of cadmium onto the i‘irst layer~` refrigerant, I believe that the action might be__ of cadmium, and finally sputtering a semitrans as follows. It is of course well known that the parent layer of platinum onto the cadmium. so-called "blocking layer” of a cell of this char 2. In the >method of fabricating a selenium cell acter and its ability to generate electromotive the steps of coating a metal plate with a layer of force when exposed to light depend on the char selenium, then sputtering a semitransparent acter of the material at the junction between ­the layer of cadmium onto said selenium from a selenium surface and the semitransparent 'lay dampened cadmium surface and then sputtering . ers of cadmium and platinum. In my improved a semitransparent layer of platinum onto the cell, this intervening layer of material is probably cadmium layer. ‘ composed of selenium dioxide, cadmium oxide, or 3. In the method of fabricating a light-sensi a mixture of these two oxides with cadmium. tive device, the steps of coating a base member There are several methods by which this end can with selenium, positioning the base member in be obtained: ( 1) By exposing the. selenium sur a closed chamber in proximity to a source of face in a glow discharge of oxygen, in which case cadmium. evacuating the chamber, introducing the selenium surface may or may not be made a quantity of an inert gas into the chamber, pro cathode of the discharge, (2) by evaporating ducing an electrical discharge within the cham selenium dioxide and cadmium 'oxide onto the ber to cause sputtering of a thin layer of cad selenium surface (in this case some decomposi mium on the selenium, cooling a portionof the tion of cadmium oxide may occur so that the re chamber to a temperature comparable with sulting deposit includes a mixture of cadmium that of liquid air, thereby to vremove any con- ß and cadmium oxide) , (3) by the preferred method densible constituents contained in the _said inert of dampening the cadmium electrode before ex gas, and thereafter sputtering a further layer of hausting the sputtering apparatus and then sput cadmium on theilrst layer thereof. v tering for a short time before refrigeration (by 4. In the method of fabricating a` light-sensi liquid air or other suitable refrigerant placed in tive device, the steps of coating a base member the reentrant flask I4) in the sputtering space. with selenium, positioning the base member in a This preliminary sputtering using the cadmium closed chamber in proximity to a source of electrode as cathode liberates the residual'water cadmium, evacuating the chamber, introducing from the cadmium cathode, and the water vapor­ a quantity of argon into the chamber, producing becomes ionized in the discharge. The resulting an electrical discharge within the chamber to oxygen ions ioxidize the selenium and the cause sputtering of a thin layer of cadmium on - cadmium, resulting in a layer of selenium dioxide the selenium, cooling a portion of the chamber and sputtered cadmium oxide and cadmium on to a temperature comparable with that of liq ­ the selenium surface. The -sputtering space is uid air, thereby to condense any condcnsible - then refrigerated to remove vapors from the constituents contained in the chamber, there sputtering discharge and the cadmium vsputter after sputtering a further layer of cadmium> on ing proceeds, after which the platinum layer is the first layer thereof, and ñnally depositing produced. Regardless of whether this theory is athin layer of a relatively non-oxidizable metal correct or not, the fact remains that when foreign on the cadmium. 70 substances such as water vapor are permitted CLARENCE W. HEWLETT.