US1827016A - Photoelectric cell - Google Patents

Description

PHOTOELECTRIC CELL Filed July 2, 1927 Patented Oct. 13, 1931 TBS ABRAHAM J'OFFE, 0F LENINGRAD, BUSSTA, ASSIGN'OR TO INDUSTRIAL RESEARCH COM- PATENT OFFICE PANY, OF CAMBRIDGE, MASSACHUSETTS, A CORPORATION OF MASSACHUSETTS PHOTOELECTRIC CELL Application filed July 2,

substance is subjected to considerable elec trical stress, the medium through which the ion travels at high velocity gives rise to an augmentation of the number of charged particles. The accumulative action efiectsa general movement of ions toward one of the electrodes and results in a greatly magnified space current with abrupt reduction of impedance to produce amplification of the impulse originally exciting the single ion. The original impulse may be energy derived from any physical phenomenon such as light, heat, eipctron bombardment or other electrical e cats.

The primary object of my invention is to translate energy impulses into electrical undulations. Another object is to provide an impedance, the magnitude of which changes in response to variations of an exciting force. Other objects and features will be'ap-' parent as the specification is perused in connection with the accompanying drawing which illustrates a preferred embodiment.

In the figure, the impedance in question comprises a sheet or layer 1 of suitable material interposed between electrodes 2-2. The material may comprise a coating of linseed oil, solidified by baking or a thin layer of bakelite or other phenolic condensation product having thickness preferably below 10 mu or .01 mm. In case ionization is initiated by light impinging'onthe surface of the solidified mass, the latter should preferably include light sensitive material as sulphur in its natural state or salt previously activatedby X-rays.v This ingredient may be incorporated by way of admixture or condensation and may be uniformly distributed throughout the body. In order that the sensitive product may be presented to the direct 1927. Serial No. 203,023.

action of the rays, one of the electrodes, the v upper in the figure, preferably should be permeable to light, e. g. by beating gold, silver, or similar ductilemetal very thin or as a coat formed integral with the dielectric by any of the well-known sputtering or bombardment processes. The ionizing source is indicated at 5.

The operation of the device described above Wlll be apparent from a consideration of the following discussion. In practically all substances a number of free ions exist. These ions are also in constant motion. The average distance through which an ion travels before it collides with an atom or molecule of the substance in which it occurs is called the mean free path of the ions in that substance. If a suflicient potential gradient exists through that substance an ion in falling through that potential gradient for a distance equal to its mean free path acquires sufficient kinetic energy to ionize the atom or molecule with which it collides. If, however, the potential gradient is not suiliciently large no ionizing collisions will occur. When a sufliciently large potential is impressed across a substance so that ionizing collisions do occur the number of charged particles liberated by accumulative ionization within the body due to each original free ion is in the ratio of 2 to the exponent D /)t where D is the thickness of the material between the point at which the free ion originates and the surface of the material to which it is attracted by the ionizing potential and A is the mean free path of the ion in the materiah Thus it is apparent that if D is very large as compared with the number of charged particles created is quite enormous and would result in the passage of very large currents through the substance. If however D is merely several times larger than )t, the number of charged particles created bears a direct relation to the number of original free ions but is still within a reasonable value. The number of free ions in the substances of low conductivity such as those with which the applicant is dealing is so small as to be practicably negligible. If, however, in accordance with the description above, D is very large with respect to a, the number of 8 is 'impressed'on that su stance.

M) impressed across them. If, however, the

thickness is of the order of several times that of A, the number of charged particles liberated by accumulative ionization due to the passage of an originally free ion'is of such a value that the current passing through the substance due to these particles is within a reasonable value.

From the foregoin it will be evident that when a wave of light frequency passes through the translucent electrode and impinges on the sensitive layer, there is created an action within the layer setting free one or more charged articles which are electrostatica'lly propelle toward the other electrodeunder the influence of the voltage gradient established by high potential source 3 (e. g. 3000 to 5000 volts). For every ion dislodged in this manner, there is liberated by accumulative ionization within the bod a large number of charged particles in the ratio of 2% (two to the exponent DA) where D is the thickness of dielectric, and A is the mean free path of ions in the dielectric or material of low specific conductivity. Since the number of free ions within the dielectric itself is negligible, it will be seen that the number of 7 charged articles originating at the light the ordinary gas-tube cell.

sensitive ayer are considerably greater than any free electrons which may exist within the dielectric. It is apparent that as the intensity of light is varied, the magnitude of space current undergoes a chan commensurate with said ratio, hence amp ification of the light wave variation is obtained in terms of current undulations :which' may be transferred through transformer 4 to a load cir-v cuit for any desired purpose.

Obviously this invention ofiers marked advantages over photo-electric cells of the gas tube type. The device is much more rugged and far less subject tobreaka The ight is not required to pass throu a glass wall and is therefore not subjecte to the absorptive action of lass upon the light radiation, articularly o the shorter wave-lengths. The ight waves (or electrons) are incident directl u on the electrode and may even permeate t e electric material at least through apart of its thickness thereby giving rise to cumulative ionization which is far more efiective to produce amplification than. in the case of Various ways of forming the thin of dielectric or poorly conducting material are described in my copending applications Serial No. 65,262, filed October 28 1925; Serial Nos. 109,213 and 109,214, filed Ma 15, 1926; Serial Nos. 160,771, 160,7 72, filed January 12, 1927 and Serial Nos. 168,448, 168,449 and 168,450, filed February 15, 1927 which are incorporated herein by reference.

1 claim:

1. A light sensitive electrical device comprising an insulating layer having a thickness not greater than 10 mu, a photo-electrically active substance distributed through metallic sheets, at least one of said sheets being transparent to light, and a layer of insulating material having a thickness not greater than 10 mu and impregnated with photoelectric substances.

5. A light sensitive electrical device, comprising an insulating layer having a thickness not greater than the order of several times the mean free path of ions in that layer, and means associatedwith said insulating layer to liberate electrons by photo-electric action.

6. In an electrical device, two electrodes spaced by an insulating medium whose thickness is not greater than the order of several times that of the mean free path of ions in N that medium, means associated with said insulating medium to liberate electrons by photoelectric action, and a source of potential 1m ressed across said electrodes, said potentia bein 'sufiiciently great to cause said electrons to iave ionizing collisions with atoms of said medium.

7 A photo-cell, comprising a pair of electrically conducting terminal layers having extended surfaces disposed adjacent to each other and separated from each other with an insulating junction layer of negligible thickness, said insulating unction layer embodying substance adapte to pass electrons from one conducting layer to the other under photoelectric action, one of said conducting layers being transparent.

8. A photo-cell, comprisin a pair of electricall conducting termina layers having exten ed surfaces disposed adjacent to each other and separated from each other with an insulatin junction layer of a thickness not greater 10 mu,'said insulating junc-- tion layer embodying substance adapted to pass electrons from one conducting layer to the other under photo-electric action, one of said conducting ayers being transparent.

9. A photo-cell, comprisin a pair of electricall conducting termina layers having extencl ed surfaces disposed adjacent to each other and separated from each other with an. I insulating junction layer of negligible thickness, one of said conducting la ers bein transparent, and means associate with sai insulating junction layer to liberate electrons by photo-electric action and pass a dischlzlarge from one conducting layer to the ot er.

Signed by me at Cambridge, Massachusetts this 7th day of June, 1927.

ABRAHAM JOFFE.

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