US2296670A - Photoelectric cell - Google Patents

Description

Patented Sept. 22, i942 UNITED STATE PHOTOELECTBIC CELL Clarence W. Hewlett, Schenectady, N. Y.,

. or to General Electric Company, a' corporation of New York Application ringen zi,- isse, sei-m Nb. *116,617

ici. recaci 5Claims.

The present invention relates to light-sensitive devices, more particularly to photo-electric cells, which generate a measurable electromotive force when subjected to light of practical intensities.

An object of the invention is to provide a highly sensitive photo-electric cell, which responds emciently to light radiations, giving rise to greater potentials, so that the cell may be employed as a direct-reading photometer. Another object is to provide a light-sensitive clevice of this character in which the response to the light radiations, as measured in terms of the output current, shall follow 'faithfully the undulations of the light which impinges on the n device.

A stm further Objectis to improve the method in number) adapted to receive screws 4 for rigid= ly securing a cover 5, preferably also of a phenolic of .manufacturing a cell oi this sort so as to'v adapt the technique of manufacture to quantity production methods and still obtain devices of standard or uniform performance with a high operating efficiency.

Due to its high degree of sensitivity, the improved cell is particularly applicable to the various electrical systems in which a light-sensitive cell is an integral part thereof and specifically to the transmission of photographs, facsimile and the like, and to television systems wherein it is essential to employ a highly elcient cell with respect to light sensitivene and current output.

In carrying out these objects, my invention contemplates the use of a disk of iron or other suitable metal with a coating oi selenium and layers of certain metalslspeciflcally cadmium and platinum) superposed on the selenium, as will be explained hereinafter. The invention will be better understood when reference is made to the following specification and accompanying drawing in which-Fig. l represents a cross-sectional view of the improved light-sensitive member; Fig. 2 shows an exploded view of the various parts of the device, arranged in the order in which they are assembled, and Fig. 3 is a fragmentary view showing a! preferred method of making contact between'an external conductor and a thin metal layer which is deposited onthe light-sensitive substance.

Referring to Fig. l, inumeral l designates a cylindrical casing or container, preferably made of insulating material/such as a. phenolic condensation product. the bottom with an inwardly extending flange 2 which conveniently serves Ias a ledge or shoulder for the light-sensitive unit, described hereinafter. At the upper edge oflthemember l, as

shown, there are several threaded holes 3 (four The'casing is provided at condensation product, to the casing. There is a pair of contact pins 6, shown more clearly Fig. 2, arranged adjacent one another in y l el arrangement and secured in any suitable u er to the periphery of the casing. For example, these pins may be fitted snugly within opes extending parallelly through 'the casing. The contact pins have a tine central bore. as shown in Fig. 2, which receives a conductor 1 for bring-f ing out, in a convenient manner, electrical conv nections from the active surfaces of the light sensitive unit.

y The light-sensitive unit itself is shown more clearly in Fig. \2 and consists essentially ci a circular disk 8 constituted of a base metal auch as iron or nickel and having a diameter as to fit snugly within the inner surface of the c. On one side of the disk, on the lower side as shown, there is a thin layer of selenium 9 which may be coated on the disk in any suitable and well-known manner, for example, by evaporation vor by pasting. Before the selenium is applied to the disk, it is preferred that the latter he ground to make it smooth and clean, then sand` time interval will usually be between 2d and 72.

hours. The heat treatment converts the selenium into the metallic light-sensitive form.

It is necessary of course to be able to make good electrical contact withthe treated selenium layer and fo-r this reason, a metal layer is coated thereon which consistof a non-oxidizable metal such as platinum. The platinum may be 'applied in any suitable and well-known manner but it is preferred to sputter the material. instead oi coating the selenium directly withp'latinum, l have found it advantageous first to apply a thin layer I0 of a metal of the seco-nd group of elements appearing in the Mendelejeff periodic table, preferably cadmium. For this purpose, the selenium-covered iron disk may be placed in a chamber containing argon and the selenium chamber tic'ally zero.-

surface sputtered with an exceedingly thin layer of cadmium. The pressurevof the argon in the may be maintained at about 200 microns, and a sputtering current of approximately .50 milliamperes employed. During thisy sputtering process, a flask of liquid air is arranged to penetrate into the sputtering chamber so that anycondensible vapors may be immediately removed from the discharge. It is preferred that the sputtering process be' carried out in short flashes vso as to avoid heating of lthe selenium surface as much as possible. The optimum thickness of the cadmium deposited in thismanner is exceedingly small and is so thin as can just barely be seen on the surface.

The cadmiumand selenium-treated disk may then be removed from the sputtering chamber and laid aside in the air to age until the current sensitivity produced by the stimulus of the surrounding light acting on the selenium through the thin layer of cadmium, comes down to prac- This reduction in current may be indicated on a microammeter. As the current diminishes, the open circuit electromotive force as measured between the cadmium layer and the metal disk, goes up rapidly. When the current sensitivity reaches substantially zero and the generated electromotive force increases to its greatest value, the aging step is terminated. 'Ihe disk treated in this manner may then be im- `mersed in an atmosphere of argon in an evacuated chamber and a thin layer I6 of platinum sputtered over the thin layer previously laid down. Platinum has a lower specific conductivity than cadmium but its eifect is to restore conductivity to the combined selenium-cadmium surface and to make good electrical contact therewith. In case the platinum is sputtered onto the cadmium in an argon atmosphere, the pressure of the latter may be maintained at about 200 microns of mercury and a sputtering current of about 50 milliamperes employed, as in the case of the cadmium. The current is also preferably applied as short flashes in order not to heat the selenium surface unduly. The platinum layer should be extremely thin, just barely seen on the surface because as will be pointed out hereinafter, the light, in order to reach the selenium layer has to pass through both metal layers.

As stated hereinbefore. the iron or nickel disk 8 constitutes one of the terminals of the lightsensitive device and the platinum layer, the other terminal. Adequate contact can be readily made between one of the conductors I and the iron side, as oshown, but only partly over the width of the upper side, with a layer of tinfoil Il, illustrated more clearly in Figs. 2 and 3. I'he wire 'I which is provided to contact with the platinum layer may be wound between the inner edge of each slot and the inner surface of the cardboard ring, as shown in Fig. 3,. leaving a free end Il which extends over the upper surface of the ring'II for a distance of approximately the same length as the distance over which` the slots I2 extend. This wire I4 is rigidly secured to the ring II by means of the tinfoil I3 which covers the same and tends to hold it by friction against the cardboard. The other end of the wire 1 extends through one of the slots and is threaded through the internal bore of theterminal pin 6, as shown moreclear- 1y in the lower view of Fig. 2, and finally soldered to the pin at its outer end. The other pin 6 iselectrically connected to the iron disk 8 by means of a wire 1 which also passes through a central boreI in the pin 6 and is soldered thereto.

As stated hereinbefore, the tinfoil ring I3 extends over the entire width of the cardboard ring,

disk 8 by means of pressure exerted on the conductor when the light-sensitive unit is assembled in the manner described hereinafter, but

special provision must be made for lmaking' of this ring is approximately the same as that of the iron disk and the width of the ring may be approximately the thickness approximately .060". This cardboard ring is lined on the inner surface and over the entire width of the under Il only at one face of the latter (i. e. the lower face as shown) but at the upper face. extends approximately over one-half the width of the ring. IZi'his construction is desirable to avoid leakage along the peripheries of the elements constitutmg the iight-sensitive una. 1f the foil exf tended over the entire width of the upper face of the ring Il, the outer edge of the foil, at the top of the ring, might curl upwardly and bridge the 'thin platinum, cadmium and iron electrodes,

thereby short-circuiting the cell, because the foil i is atthe potential of the platinum or other exterior metal layer.

For assembly, the casing I is placed in the position shown in Fig. 2 and a transparent glass plate I5 first placed within the opening'in the casing, the diameter of the glass plate being such as to permit it to t snugly within the annular space provided. The purpose of the glass is lto protect the delicate surface of the light-sensitive layer. If desired, the glass may be entirely dispensed with and the platinum layer simply covered with a protective transparent lacquer, except' at the place where contact is to be made. 'I'lie ring Il is. then placed on top of the glass plate with the portion half covered by the tinfoil uppermost, or in case a glass window is not provided, directly within the annular space in the casing.

v. The light-sensitive unit which consists of the iron disk and the selenium and platinum layers with or without the cadmium, is next introduced into the annular space, at the same time the wires 'I are threaded through the openings in the contact pins 6 which have previously been secured in place. As stated hereinbefore, these wires are Aelectrically connected to the disk 8 and the contact ring I3 respectively.

The protective cover 5, having adiameter of approximately the same size as the outer diam'- eter of the casing, may then be secured to the latter by the screws l. 'I'he last step of course is to solder the ends of the wires 'I to their respective contact pins 6 and cut off the excess wire.

It will be apparent that, the light-sensitive member is contained wholly within the casing and is protected at the front or lower side, as shown, by the glass pla'te l5 or the lacquer layer referred to hereinbefore, and' at the .back or upper-side, as shown, by the cover 5, The con struction is extremely rugged. Adequate provision has been made for bringing out leads from ethe active members and for permitting practically the entire surface of the light-sensitive member. to be subjected to light. Light may be introduced through the glass window I5, directly onto the platinum layer, which on account of being semi-transparent, permits the radiation to pass through and alIect the under layer of cadmium or the selenium layer, or both.

While I do not desire to be limited to any particular theory of operation, it appears that when light. falls on the layers of material including f the selenium, it causes the liberation of electrons from the selenium and delivers them to the metal layers. layer I6 through the tinfoil gasket I3 which con.- nects with the wire i4 and out through the c'onductor 1 to the terminal pin 6. These electrons constitute a current which may ow through aload circuit consisting of a measuring instrument, such as a microammeter calibrated in foot candles, back to the iron plate. A cell which has These electrons ow from the vmetal been fabricated vin accordance with the method set forth hereinbefore, employing the cadmium layer, is capable of deliverin rom 100 to 300 microamperes per lumen, which readily enables the device to give an adequate reading on a microammeter when exposed to daylight or other light of practical amounts.

What I claim as new and desire to secure by Letters Patent ofthe United States, is:

1. A photo-electric cell comprising an annular casing of insulating material provided at one end on the interior, with a ange, and at the other,

end, with a cover of insulating material,` alightsensitive unit contained between said ilange and i metal coated successively with selenium, cad- 3. A photo-electric cell comprising an annularvcasing of insulating material provided at one end on the interior,with a ange, and at the other end, with a cover of insulating material, alightsensitive unit contained between said ange and cover, said unit including a metal plate coated successively with selenium, cadmium and platinum, a conductor connected to said plate, a metal ring conductor contacting with the platinum. and

a protective layer of transparent material coated on the platinum within the interior of said metal ring conductor.

4. A photo-electric cell comprising a base mium and platinum.

5. A photovoltaic cell comprising a conducting base plate, a layer of light-sensitive material con- -sistingsubstantially of selenium in crystalline form upon said base plate, a translucent intermediate layer consisting substantially of cadmium inintimate molecular -contact with the crystalline surface of said light-sensitive layer and a translucent homogeneous covering layer consisting substantially of platinum upon said intermecover, said unit including a metal plate coated successively with selenium, cadmium and platinum. 1

diate layer.

, CLARENCE W. HEWLETT.

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