US2866878A

US2866878A - Photoconducting devices

Info

Publication number: US2866878A
Application number: US504911A
Authority: US
Inventors: George S Briggs; Ralph W Christensen
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1955-04-29
Filing date: 1955-04-29
Publication date: 1958-12-30
Anticipated expiration: 1975-12-30

Description

De- 30, 1958 G. s. BRIGGS ET AL 2,866,878

PHOTOCONDUCTING DEVICES Filed April 29. 1955 faz (d5-,450% (die 25% am 75% M2 /aa/Z @die INVENTORS lafms'zaas wp BYEAJPH 144' CME/:22mm

irraiA/ir PHOTOCONDUCTHNG DEVICES George S. Briggs and Ralph W. Christensen, Lancaster',

Pa., assignors to Radio Corporation of America, a corporation of Delaware Application April 29, 1955, Serial No. 504,?11

6 Claims. (Cl. 201--63) This invention relates to improved photoconducting compositions which are particularly useful in gap type and area type photocells and in other devices'utilizing photoconductive bodies. The invention includes improved methods for preparing the improved photoconducting compositions and improved devices utilizing the improved photoconducting compositions of the invention. i

A photoconductive device is one which displays a reduced resistance to electric current flow when irradiated, for example, with light. In its simplest form, a photoconductive device comprises a photoconductive body and a pair of electrodes attached thereto. With a voltage applied to the electrodes, the device passes an increased amount of electric 'current when there is .an increase in the intensity of light irradiating the device.

Ideally, a photoconductive device is a perfect insulator when light to which it is sensitive is absent, and is a perfect conductor when a maximum intensity of light to which it is sensitive is present. Actually, a photoconductive device behaves as a higher resistance conductor when light to which it is sensitive is absent, and behaves as a lower resistance conductor when light to which it is, sensitive is incident upon the photoconductive body.

The change in conduction produced by a unit variation of light intensity is referred to as the photosensitivity of the device. The measure of photosensitivity is in terms of photocurrent under standard conditions. The current passed by the device in darkness is referred to as the dark current; the current passed when the device is `irradiated is referred to as the light current; and the difference between the light current and the dark current is referred to asV the photocurrent.

Ice

cells of a single composition exhibit a change in photoconductivity over periods of time. ln some cases the photoconductivity increases, while in other cases the photoconductivity decreases. It is desirable that such photocells maintain substantially constant photoconductivity so that they may be incorporated into apparatus without compensating networks and devices.

An object of the inventionl is to provide improved photoconducting compositions, improved bodies of said photoconducting compositions and improved photoconducting devices.

Another object is to provide improved photoconducting compositions, bodies and devices that are responsive to a broad band of frequencies in the electromagnetic spec.-

trum.

Another object is to provideimproved photoconducting compositions, bodies and devices that maintain substantially constant photoconducting properties with respect to time.

A further object is to provide improved methods for .preparing the improved photoconducting compositions of the invention.

. .Photoconducting compositions according to the invention comprise an intimate physical mixture of timely-diyided crystal particles of at least two different photoconducting materials, preferably dispersed in an electricall 1y-insulating, film-forming vehicle. An intimate physical e fl One type of photoconductive device comprises a single crystal of a photoconductive material and electrodes attached to the crystal. Such single crystal photocells exhibit large photocurrents and high ratios of light current to dark current. However, the crystals are small in size and consequently the total current passed by a single crystal is small. When greater currents are passed through the crystal, the crystal heats up and the photosensitivity of the crystal is reduced, either temporarily or permanently. Furthermore, single photoconductive crystals are difficult to grow and are fragile. Thus, the expense of manufacture and maintenance often prohibits the use of such single crystal photocells.

Another type of photoconductive device comprises a body including finely-divided photoconducting powder particles of a single composition and electrodes attached to said body. The body may include, for example, an unbonded powder or a powder mixed with a binder such as a-synthetic resin.' Such powder-type photocells may be made in any desired size, shape` or currentrcarrying capacity. Although such powder-type photocells often respond` to a 'broader band of frequencies than crystal, photocells, it is frequently desirable to provide photocellsl of this'type that respond to still broader bands of frequencies.

It has been observed that previous powder-ty ev photo-` mixture of at least two photoconducting materials has been found to provide a broader spectral response and J better electrical properties than finely-divided crystal par- `t'icles of a single photoconducting material. Photo-conducting devices comprising bodies of the improved compositions of the invention, are effectively more photosensitive due to the broader spectral response and are more stable with respect to time.

A method of the invention includes intimately mixing finely-divided crystal of at least two different photoconducting materials preferably by dispersing in an electrically-insulating, film-forming vehicle.

The devices of the invention include a body having a Ycomposition.according to the invention and at least one electrode attached to saidbody.

The invention will be more fully described in the following detailed description when read in conjunction with the drawings herein:

Figure 1 is a perspective view of a first photocell ac-l cording to the invention,

Figure 2 is a family of curves illustrating the change in photosensitivity with: respect to time of compositions according to the invention and of the component photoconductors of said photocouducting composition, and

Figure 3 is a perspective view of a second photocell cording to the invention.

Similar reference characters are used for similar elements throughout the drawings. u

A preferred method for preparing a photoconducting composition according to the invention follows.

Fifty 'parts by weight of photoconducting cadmium sulphide and fifty parts by'weight of photoconducting cadmium selenide are dispersed in fty parts by weight of a 1% solution of ethyl cellulose in amyl alcohol. The mixture is then coated on a substrate and the amyl alcohol removed byY evaporation.V The composition of the resulting body calculated from'the initial mixture is 50 parts photoconducting cadmium sulphide, 50 parts photoconducting cadmium selenide and 0.5 parts ethyl cellulose (all parts by weight).

`(,an'mnfm sulphide photocomuctz'ng powder.-A preferred photoconducting cadmium sulphide powder is prepared as follows: An intimate mixture of 100 grams of precipitated cadmium-sulphide, l0 grams of cadmium chloridefly gramof ammonium chloride, 1 .7millilitersl Qi 0.1. Mcuprecbloride. andv 250 millilitersv of water is, prepared. This mixture may be prepared in a blender such as is used for mixing powders with water. The yellow; viscous slurryis dried; at; about 15.0" C. for about l5 hours.

The dried cakeis then brokenzupdntopea-size lumps and; packed; into a 12 inch test tube to. aV depth of about seven;inchestThetubeis provided with. a. stopper having an inlet tube therethrough.` for the purpose of maintaining a substantially stagnant atmosphere in the test tube Whileamaintainingatmospheric pressure through the subsequent liring steps; The testY tubevfilled with'. the dried mixture isfired atlaboutk 600'. C; for about 2.01 minutes. The fired product is then remo-ved from the test tube andallowedi to. soak inwater-until it disintegrates. This ordinarily takes about 2O` minutes. Thel product is Washedonia fine, sintered, glassfilter, dispersing the cake once or twice in water until the washings contain less than-:01 M cadmium chloride.

The product of the first fir-ing is brown in colorand of av relativelyv fine particle, size. During the rst firing, there is present in the charge about cadmium chloride which is a solvent ux for cadmium-sulphide. The smalll particles of: cadmium sulphide partially or completely dissolved in the cadmium chloride and are recrystallized into small crystals.- which are o f the order of- 0.3 mill in size and havey copper andl halide incorporatedv therein. At this stage, theproduct is photoconductmg.

The washed product of` the first tiring is moistened with aY solution containing equal parts of 0.14 M aqueous cad mium chloride and 1.0y M` ammonium chloride. The excess solution is removed by suction. After drying, the powder is passed through a 325 mesh sieve andthe tailings discarded.

The dry,A sieved powder is placed in a test tube to a depth not greater than 4.5 inches. and ired for about minutes at 600 C. in a stagnant atmosphere. The tired mass is removedl from the furnace andpermitted to cool. During this second firing, the powder sinters to a stick, which is then grated through a mesh sieve. During the second tiring there is present only a controlled trace ofsuperticialchloride. The` slightly sinteredV stick is easily broken up into a powder which exhibits a low dark resistivity. and high. dark current.

Aboutf0.2 gram of sulphur is placed in thek bottom of a test tube and the sieved brown: powder from the second firing` is placed on, top of the sulphur to. a depth of about 4.5 inches. The powder in the test tube is tired at about 500 C., for about 10 minutes ina stagnant atmosphere and then, while still in the thirdy tiring, a vacuum is applied to the powder by. means of anV inlet tube and the firing, continued for about ltl-minutes.V with the vacuum applied. The test tube is removed from the furnace, cooled and the product passedthrough a 325 mesh. screen.. During the third tiring, the sulphur vaporizesV and passes through the mass of brownA powder. Thel product of the third firing exhibits ay high dark resistivity, a low dark current anda high. photosensitivity. Typical measurements, indicate the ratio of light current to dark current of about; 10S- and a high speed of" response.

. The cadmium sulphide photoconducting powder which is, the product of; thethirdV firing istan to dark brown in Color,` the color darkening with increases in either the proportions of copper or increasesof the first tiring tem# perature. The. average particle size varies according to the first tiring temperature, being of the order Yof 0.34 mil for 600 C. and ofthe order of 0.7 milA for 650' C. The powders exhibit a panchromatic absorption, although the spectral response is peaked in red and is practically nil 'in the blue region of the spectrum. TheV powder is non-luminescent, has substantially uniform particle size audis freediowing.

Since. Cadmium. sulphide isu partially. soluble, in4 molten cadmium chloride, it is believed that the presence of about 10% cadmium chloride in the mix during the rst firing permits the growth of discrete uniformly sized crystals bonded by the Water soluble cadmium chloride. The tired lump disintegrates readily in water. Thus, the content -of cadmium chloride isnot critical, its purpose being principally to provide a crystallizing me-dium for the cadmium sulphide host crystal. Although cadmium chloride is preferred, any crystallizing medium for the host crystal which does not otherwise adversely eiect the product, may be used in place of cadmium chloride. Similar crystallizing media are used for other host crystals.

Ammonium chloride is introduced into the mix to (1) convert to cadmium chloride any cadmium oxide which may be present in the mix, and (2) to provide a tiring atmosphere that prevents oxidation. An activator proportion of chloride is incorporated into the host crystal during the lirst.` firing.v This. amount is extremely. small and may come from either the cadmium chloride or the ammonium chloride.

Copper is introduced into thernix in a proportion equivalent to..100 partsper million of copper with respect to. cadmium. sulphide. The amount of copper is not critical; however, it is preferred to use between 50 and 300y parts. per million of copper. In place of copper, other monovalentl cations may be incorporated into the cadmium. sulphide host crystal. For example, 200 parts permillion; of' silver in place ofcopper produces an orange-colored powder having an intermediate photosensitivity andi a low rate of decay.

The firing temperature during the first ring is somewhat-critical. The firing temperature shouldhbe above the melting point of cadmium chloride which is about 580 C. Belowl this temperature practically no crystal growthy occurs and the copper does not diffuse into the host crystal. Higher temperatures during the first firing produces a powder which has a darker color, larger V particle size, lower dark resistivity and higher dark current. The preferred temperature is the lowest temperaturerthat insures prompt melting of the solvent material, produces a small particle size and a high dark resistivity inthe final product. A temperature of about 600 C. ispreferred.

The second firing sinters the powder into a stickand increasesthe conductivity and photosensitivity of the material. Small particles are probably sintered onto the surface of the largerones, thus, reducing the number of particle-to-particle contacts. Again, aV tiring temperature ofthe order of 600-'C. is, preferred as the lowest ternperat-ure which insuresv the prompt melting of cadmium chloride.

During thethird firing, the sulphur vapor which passes through the mass of photoconducting powder reduces the darkicurrent of the powder, kpresumably by diminishing the chloride in the powder toa value substantially equivalent to the amount of copper present in` the product. At 500 C., the powder does not sinter and the photosensitivity` of the powder isY only slightly affected. At higher temperatures, the loss in` photosensitivity is greater. f A In eachof 'the tiring steps enough. time should herallowed to b ri1, 1 g the` entire charge tothe furnace temperature. Eortubes about oneinch in` diameter, about 2,0 minutes. is, required. Longer tirings up to one hour make no noticeable difference in the powder. Similarly, the speedY with ,which the productis` cooled makes little or n.0 difference. in the unal, product.

VGrindirlg theinishelpowder progresively reduces its photosensitivity, similar to,V the observations on single crystals. Grinding of,n an. intermediate product isunde. sirable because it produces all particle sizes and shapes, and. because srindinsis inherently uncontrollable. lt is best, therefore, to avoidV grinding at any stage. The

powder from the first tiring is putthrough a 325 mesh sieve to eliminate the few lumps which may have formed in earlier steps in the process and also to establish an upper limit (such as 1.7 mils) to the particle size. After the second tiring, the 50 mesh sieve is used to achieve a more uniform disintegration of the sintered stick and to avoid crushing. After thethird firing the final product is passed through a 325 .mesh sieve to eliminate any aggregates over 1.7 mils. .Less than 5% is lost at this stage. In passingmaterial through a'sieve no hard rubbing is used. Y

The electrical properties of the nal product are influenced by the amount of chloride present during the second firing. With too much chloride, the final product has a high dark current; with too little chloride, the final product has a low sensitivity.

Cadmium selenide photoconducting powder.-A preferred photoconducting cadmium selenide is prepared by the general method described for the cadmium sulphide photoconducting powder except that cadmium selenide is substituted for cadmium sulphide in the starting mixture. Thus, the starting mixture comprises 100 grams of cadmium selenide, grams of cadmium chloride, 1 gram of ammonium chloride, 1.7 milliliters of 0.1 M copper chloride and 250 milliliters of water.

AReferring to Figure 1, a photocell according to the invention comprises a pair of spaced electrodes 53 attached to a photoconducting body 55 of the invention. The device may be prepared according to the following procedure. A glass plate 51 is provided with -a strip of conducting material 200 mils wide and having a 20 `mil gap running across and at right angles to the strip forming two electrodes 53. Such a strip may be produced by masking the plate 51 and then spraying with a silverresin composition. Other methods may be used, for example, painting or silk screening. f l

A mixture is prepared comprising about 100 parts by weight of photoconducting powder `and parts by weight of a solution containing one weight percent of ethyl cellulose resin dissolved in amyl alcohol. A drop of this mixture is placed on the gap and allowed to dry. When the drop 55 is dry, the photocell is ready for use. The photoconducting body 55 comprises 100 weight parts of'photoconductor and 0.5 Weight parts of ethyl cellulose.

While the photocell of Figure 1 utilizes a photoconducting body 55 comprising a resin-bonded powder, an unbonded powder may also be used. A photoconducting body bonded with an electrically-insulating, film-forming Vehicle is preferred. It is preferred to use 0 to l weight parts of a binder to 100 weight parts of the photoconducting powder. In the above described example, between zero and 100 parts of the 1% ethyl cellulose solution may be used with 100 parts of photoconducting powder. Increased amounts of ethyl cellulose increases the resistivity of the photocells although the above range of constituents is not critical. Other electrically-insulating, film-forming vehicles may be used as a binder, for example, a silicone resin, araldite resin or acryloid resin. In addition to photoconducting Cadmium sulphide and photoconducting cadmium selenide, other photoconductors such las photoconducting cadmium sulfoselenide and cadmium telluride may be used. The photoconducting compositions of the invention comprise generally mixtures of finely-divided crystal particles of at least two different photoconductors which mixtures may be bonded orunbonded. The photoconducting cadmium sulphide and cadmium selenide may be mixed in any proportion, each mixture having its owndistinctive electrical properties.

Referring to Figure 2, the photocouductivity of the photocells of Figure 1 comprising only the preferred photoconducting cadmium sulphide dispersed in ethyl cellulose increase with time as shown by the curve 31. The photoconductivity of the photocells of Figure l comprising only the preferred. cadmium .selenide dispersed in ethyl cellulose-decreases with time as shown by the curve 33. In either case, these photocells change with time, making it extremely diicult to design the photocells into circuits with other components. The stability of these photocells is improved by utilizing the compositions of the invention, preferably a mixture com'- prising 50%A byv weight of photoconducting cadmium sulphide and 50% by weight of photoconductingycad mium selenide bondedpwithethyl cellulose. As shown by the curve 37, therphotoconductivity of the mixture remains substantially constant with respect to time. t

There is also shown characteristics of photocells comprising mixtures of 25% by Weight photoconducting cadmium sulphide and 75% by weight photoconducting v cadmium selenide 'bonded with .ethyl cellulose by the curve'35, and 75 by weightnof photoconducting cadmium sulphide and .2,5%V by weight of photoconducting cadmium selenide bonded with ethylljcellulose by the curve 39. In each case there is shown an improvement instability -with respect to time overphotocells made with only one constituent photoconducting powder.

Table I gives comparative'V data of the operation of typical photocells constructed according to Figure l, when irradiated with 0.1 lumen*.(73`'footcandles) of light from an incandescent source. Y'I `he 'spectral response of photocells comprising only the Apreferred photoconducting cadmium sulphide powderA bonded with ethyl cellulose is peaked at about 7500-A. as shown in column 5. The spectral responseof photocells comprising only the preferred photoconductingcadmiumV selenide powder bonded with ethyl cellulose is'peake'd at'about 9250A. as shown in column 1. As shown in column 3, the spectral response lof photocells comprising a mixture of equal parts of photoconducting cadmium sulphide and photoconducting cadmium selenide bonded with ethyl cellulose is peaked at about 7500 A. and is more sensitive over a broaderrange ofthe spectrum than photocells comprising only one of the constituent photoconducting materials. Further, a photocell comprising one of the mixtures of the invention appears to have its own distinctive characteristics rather than having characteristics which is the meanl of the characteristics o-f the constituents. As shown in columns 2 and 4 photocells cornprising other mixtures of the component photoconducting powders bonded with ethyl cellulose exhibit an increased photoconductive response over a broader range of the spectrum than either component photoconductor, and, each mixture exhibits its own distinctive peak spectral response. A similar effect is obtained with photocells comprising mixtures of photoconducting powders without a binder.

Referring now to Figure 3, another type of photocell is the area. type photocell which may comprise a subtrate 61, such as porcelain, mica, Bakelite, but preferably glass, upon which has been formed a pair of interdigitated electrodes 63. A layer 65 comprising a photoconducting composition according to the invention is coated over the gap area of the electrodes. A typical device has a gap width of 20 mils and a gap length of 1000 mils. The electrodes 63 are then connected to a voltage source 67 external to the device.

The compositions of the invention may be utilized to prepare photoconductive devices and elements that are useful in meters, relays, picture converters, picture intensitiers, pickup devices, switches and so forth. The devices comprise 4a body of a photoconducting composition of the There has also been describedV4 novel methods for preparing the improved' photoconducting, compositions of the in vention, and photoconducting bodies andphotoconducting devices comprising the 'improved phtoconducting compositions of the invention. The improved photoconductingY compositions of' the invention provide a bro-ader spectral response. and better electricalA properties than finely-divided crystal particles of a single photo- .conductingA material;

TABLE'I Relative spectral. response (normalized) .of powder-type photocells in, arbitrary urritsj Compositions,

CdSE, percent 100 7,5, 50k 25y v 0 CdS, percent A 25 50 75 100 ethyl celluloso.....r.. 0.*5 r 0. 5- 0. 5 0; 5 0. 5

0. 0 0 0 0 0 3. 16 31 5 0 11 91 200 46 0, 45. 430 1,100 460 0 100 700 1,'500 800 0 140 1,000. 2,000 1, 400 30 320 1,300 2,7400 2,200 120 500 1, 500 2,1300 2, 600 200` 640 z 1,450. 2,000 2, 400 2504 740 1,300l l 1,500 1,700 290l 770 1,000 1,000 1, 000 280 680 700 '500 `300 220 500 400 100, 60

Irradlatlon= 0.1 lumen from an incandescent source.

Gap=20 x 200 mils.

Applied vo1tage=90 voltsD. C.

What is claimed;

l. A 25 to 75 weight percent photoconductive. body consisting essentially ofk aA physical mixture. of nely- Idivided crystal particles of photocouducting cadmium sulphide and 25: to 75 weight percent photoconducting cadmium selenidedispersed in an electrically-insulating, lm-forming vehicle.

2. A photoconductive body consistingessentially of a physical mixture of iinely-divided crystal particles of photoconducting cadmium` sulphide and photoconducting cadmium selenidedisperseds in a binder, said mixture comprising about parts by Weight of photoconducting cadmium sulphide7 about 50` partsby weight of photoconductin,7 cadmium selenide and about 0.5 part by Weight of ethyl. cellulose.

3'. A photoconductive device comprisingv a body ofa physical mixture of finely-divided crystal particles of photoconducting. cadmium sulphide and photoconducting cadmium s-elenide dispersed in an electricallyainsulating, film-forming, vehicle, said mixture consisting essentially of 50 parts by weight of photoconducting cadmium` sulphide, 50 parts vby Weight off photoconducting cadmium 'selenide and` 0.5 part by weight of ethyl cellulose, and at least one electrode in contact with said body.

4. A photoconductive body consisting essentially. of a physical mixture of finely-divided crystal particles olif 25 .to Weight percent photoconducting cadmium sulphide and 25 to 75I weight percent photoconducting cadmium Iselenide dispersed in up to, one weight percent ethyl cellulose.`

5. A photoconductive body consisting. essentially. of a physical mixture of finely-divided crystal particles of photoconducting cadmium sulphide and photoconducting cadmium selenidedispersed in` a binder, said mixture comprising about 25 to 75 parts` by weight photoconducting7 cadmium sulphide, about 25 to 75 parts by Weight photoconducting cadmium selenide and about 0.5 part by weight of an electrically-insulating, nlm-forming vehicle;v the total parts cadmium sulphide, and cadmiuml selenide 'be- .ing 100,

6. A photoconductive device comprising a body, con,- sisting essentially of ay physical mixture of finelydivided crystal particles of 25; to 75 weight percent photoconducting*v cadmium sulphide and 25 to 75 lWeight percent photoconducting cadmium selenide dispersed` in an. electricallyinsulating, film-forming vehicle, and, at least one electrode in contact with said body.

References Cited` in the file ofthis patent UNITED STATES, PATENTS 2,552,626 Fisher et al. May. 1,5, 1951 2,647,066 Homer V- V V .f.. Iuly 2S, 1953 2,651.700 Gans V A s Sept. 8, 1953 2,710,813, Forgue --.p a June 14, 1955 2,756,385 Thomsen Oct. 2, 19,56

UNITED STATES PATENT QEETCE CERTIFICATE OF CORRECTION Patent No. @86657878 December BO5 1958 George 5 Briggs et alo It is hereb'r certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column '7, line 18, Table I, first column thereof, for "GdSE" read CdSe linev 37, strike out "25 to '75 Weight p'ere'entY; line 39, before "photoeondueting" insert 25 to '75 Weight percent al1-n,

Signed and sealed this: 21st day of July 195% (SEAL) Attest:

KARL Hhl AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION December '50, 1958 Patent Not 2oy878 George 5,. Briggs et alf,

It is herebY certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7, line 18, Table I, first column thereof, for "GdSE" read GdSe line 37, strike out "25 to '75 weight pereentf'; line 39, before njplflotooonchioting insert m- 25 to '75 Weight percent a Signed and sealed this' 21st day of July 19591,.

(SEAL) Attest:

KARL Ht AXLINE Attesting OHcer ROBERT C. WATSON Commissioner of Patents

Claims

6. A PHOTOCONDUCTIVE DEVICE COMPRISING A BODY CONSISTING ESSENTIALLY OF A PHYSICAL MIXTURE OF FINELY-DIVIDED CRYSTAL PARTICLES OF 25 TO 75 WEIGHT PERCENT PHOTOCONDUCTING CADMIUM SULPHIDE AND 25 TO 75 WEIGHT PERCENT PHOTOCONDUCTING CADMIUM SELENIDE DISPERSED IN AN ELECTRICALLYINSULATING, FILM-FORMING VEHICLE, AND AT LEAST ONE ELECTRODE IN CONTACT WITH SAID BODY.