Nov. 8, 1966 J. G. VAN SANTEN ET AL 3,284,235 METHOD OF MANUFACTURING PHOTO-CONDUCTIVE LAYERS

Filed Feb. 4 , 1965

FORM ACTIVATED Cd 3 POWDER I MIX C d S POWDER FORM A ND CdC/2 SOLUTION ado/2 SOLUTION

H3’ PO 4 “SIXOLUTION WITHQ FORM TO FORM Cd S DISPERSION H3 PO" 8 O LU TION I S PRA Y Cd S DOIISIPERSION SUBSTRATE TO FORM LAYER I DRY LAYER + I HEAT LAYER TO FORM H PHO TOCONDUCTOR

F/G/

INVENTORS JOHANNES G. VAN SANTEN HENDRIK ESVELT HENDRIK J.M. JOORMANN BYZEWL 73 AGENT { 3,284,235 United States Patent 0 CC Patented Nov. 8, 1966

1 2 and preferably at a temperature of at least 450° C. and 3,284,235 METHOD OF MANUFACTURING PHOTO at most 700° C. The duration of the heating treatment CONDUCTIVE LAYERS is not critical and may be longer or shorter as the heating Johannes Gerrit van Santen, Hendrik Esvelt, and Hen temperature is low or high. It is preferably long enough drik Jacobus Maria Joormann, Emmasingel, Eindhoven, for the cadmium halide to be substantially evaporated. Netherlands, assiguors to North American Philips Com The grain size of the powder used preferably is at most pany, Inc, New York, N.Y., a corporation of Delaware 10011.. For many uses, for example, for pho-tocells having Filed Feb. 4, 1963, Ser. No. 255,837 a small electrode spacing, generally a smaller grain size Claims priority, applicatgzlr‘l1 Netherlands, Feb. 14, 1962, is used, preferably of at most 50p. . ,s 1 6 11 Claims. (Cl. 117_201) 10 The powder used is preferably activated previously, for example, by sintering in the presence of activators. The The invention relates to a method of manufacturing ‘a activators used preferably are gallium and copper in sub photoconductive layer employing cadmium sulphide, cad stantially equal atomic amounts. Other suitable activators mium selenide or mixed crystals of said compounds, and are silver, which may for instance partly or entirely be to a ph-oltoconductive layer manufactured by such a meth substituted for copper, and indium, which may for instance od. Such photo-conductive layers are used, for exam partly or entirely be substituted for gallium. The amount ple, in photocells and image intensi?e-rs. An example of of each of these activators preferably is at least 5 X 10"“ a known method consists in that powdered activated cad and at most 5><10_2 atomic percent of the cadmium in mium sulphide together with a binder is ‘applied to a sup— the powder. port. The activated cadmium sulphide powder was ob~ By the use of the method in accordance with the inven tained, for example, by sin-tering cadmium sulphide to tion not only layers of small surface area, which are gen which activators such as, for example, copper, silver and erally used in photo-cells, ‘but also highly photosensitive chlorine, had been added and subsequently grinding the layers of comparatively large size, for example, of several sintered material to powder. Binders which have been square decimetres, can be made. Such layers are suited, suggested include fats, waxes, cellulose lacquer and vapor for example, for use in image intensi?ers. izable organic binders, such as vaporizable ketones, for In order ‘that the invention may readily be carried out, example, acetone, to which waterglass may be added. The an example thereof will now be described with reference photosensitivity of the resulting layers, however, was to the accompanying diagrammatic drawings, in which comparatively low, which is due to a poorly conducting FIG. 1 is a block diagram of the sequence of opera contact between the grains of the powder. 30 tions and stages of the manufacture of a photo-sensitive According to another lmown method, powdered cad layer on a support, and mium sulphide, which was dispersed in a solution of FIG. 2 is a plan view of a photocell provided with cadmium chloride, cadmium bromide or cadmium iodide such a photosensitive layer. and to which activators in the form of salts may be added, The manufacture of a photo-sensitive layer employing was applied to a support and, after evaporating the sol 35 cadmium sulphide will now he described, by way of vent, heated so that the cadmium chloride is the ?rst to example, with reference to FIG. 1. melt and dissolves a small portion of the cadmium sul The initial powder, A, consists of cadmium sulphide phide, after which the cadmium chloride evaporates with containing as activators 2><10"4 gram-atom of copper the deposition of cadmium sulphide activated by chlorine and 2><10-4 gram-atom of gallium per gramanolecule of and any further activators which may have been added. 40 cadmium sulphide (FIG. 1), for instance the amount of Although the sensitivity can be slightly improved, the each of these activators, copper and gallium, being cells were particularly slow, that is to say, after illumina 5><1O—2 atomic percent of the cadmium in the cadmium tion the resistance rose only slowly to the dark resistance. sulphide. This powder is obtained, for example, by sinter Further, in many cases the coherence of the grains Was ing to compactness powdered pure cadmium sulphide to found to be unsatisfactory. 45 gether with the required amounts of copper nitrate and The present invention which relates especially to a gallium nitrate at a temperature of, for example, 900° C. method of manufacturing photo-conductive layers in which in a hydrogen sulphide atmosphere for 2 hours, after a powder consisting of cadmium sulphide, cadmium sele which the sintered product is ground and the resulting nide or mixed crystals of said compounds and a solution powder is strained to obtain a maximum particle size of of cadmium chloride, bromide and/ or -iodide are applied 50 37p. (400~mesh). 12 grams of a previously prepared solu to a support, after which the solvent is evaporated and the tion of 25% by weight of cadmium chloride in 96% assembly is heated, has for its object to improve this alcohol, B, were added to 500 grams of said powder. Fur method. It was surprising to ?nd that the use of oxidized ther, enough alcohol was added to ‘provide after mixing phosphorus provided not only satisfactory coherence be 55 -a sprayable mixture C. 1.8 cc. of a 25% solution of tween »the powder ‘grain-s, but also a material decrease of orth-ophosphoric acid (H3PO4) in Water, D, was added the slowness and a high photo-sensitivity. to said mixture and the assembly was again mixed to According to the invention, phosphorus in oxidized produce a mixture ‘E. Immediately thereupon, that is to form was added to the solution. The phosphorus was say, about 2 minutes after the addition of the phosphoric preferably added in the pentavalent form, for example, in acid, the mixture E is sprayed as va thin layer on a su - the form of phosphorus pent-oxide or another compound port. If the mixture B would be used a long time after which on heating leaves no other residue than oxidized its manufacture the phosphoric acid may have attacked phosphorus, for example, phosphoric acid or ammonium the cadmium sulphide to an undesirable extent. phosphate. The amount of the phosphorus added in The layer F provided on the support is dried at a tem oxidized form may be comparatively small and prefer 65 perature of 70° C. The support together with the dried ably is at least .05 atomic percent and at most 0.5 atomic layer, G, is heated to a temperature of 550° C. in air at percent of the amount of cadmium in the powder. The atmospheric pressure for half an hour with. the formation amount of cadmium halide used preferably is at least 0.7 of a photo-conductive layer H on the support. This mol. percent and at most 7 mol. percent of the amount of duration of the heat-treatment is suf?cient to enable the cadmium cha-lcogenide in the powder. 70 cadmium chloride to be evaporated. ‘Heating after evaporation of the solvent is preferably In spraying the mixture on the support, attention should effected in an oxygen atmosphere, for example, in air, be paid to the fact that a sufficient amount is used to form 3,284,235 3 4 a continuous layer, which is readily perceived with the 2. A method as set forth in claim 1 wherein the pow eye. After the heat-treatment, a layer having a thickness der has a grain size less than 1001i. of from about 100m to 200p is obtained. 3. A method as set forth in claim 1 wherein the pow In this manner, a photocell was made in which a photo der has a grain size less than 50a. conductive layer 2 consisting of activated cadmium sul 4. A method as set forth in claim 1 wherein the pow phide was applied to a glass support 1 having a length of der is ?rst added to the cadmium halide solution, and then 5 cms. and a'width of 3 cms. (FIG. 2). Two electrodes the phosphorus is added to the solution. 3 and 4 were provided on the photo-conductive layer 2 5. A method of manufacturing a photoconductor de by the deposition of aluminum from the vapour state with vice comprising a layer of cadmium chalcogenide photo the use of a mask. The electrodes each had a length of 10 conductive material selected from the group consisting 2 cms. and a width of 1 cm. and they were spaced by a of cadmium sulphide, cadmium selenide and mixed crys narrow gap 1/2 mm. wide and 2 cms. long. With the aid tals thereof, comprising the steps, dispersing said photo of silver paste 5, 6 nickel contact leads 7, 8 were pro conductive material in powdered form in a solution con vided on the electrodes 3, 4 respectively. The silver sisting essentially of phosphorus in oxidized form and at paste consisted of powdered silver dispersed in ethoxylin 15 least one cadmium halide selected from the group con resin and was hardened at a temperature of 120° C. after sisting of cadmium chloride, cadmium bromide and the provision of the nickel leads. cadmium iodide, the phosphorus content of said solution After the photoconductive layer had been mechanically lying between 0.05 and 0.5 atomic percent of the cadmium removed from the edges of the glass support, the re content of the powder, the cadmium ‘halide content of mainder of this layer together with the contacts was em 20 said solution lying between 0.7 and 7 mole percent of the bedded in ethoxylin resin (not shown). cadmium chalcogenide content of the powder, depositing The resulting photocell was then tested and the results a portion of said solution onto a support to form a layer, of the test are given in the following table. The voltage and thereafter removing the solvent and heating said layer applied across the photocell was 20 v. and the intensity of for a period of time su?icient for the cadmium halide to the illumination used was 1.6 lux. The light was pro 25 evaporate and form a coherent layer. duced by a tungsten lamp (temperature 2,600” C.). In 6. A method as set forth in claim 5 wherein the phos the table, in indicates the dark current, iL the current after phorus is added in the pentavalent form. illumination and 7'0_1 the decay time in seconds after the 7. A method as set forth in claim 6 wherein the phos elapse of which the current strength is reduced on inter phorus is phosphorus pentoxide. ruption of the exposure to 0.1 part of the value at the il 30 8. A method as set forth in claim 6 wherein the phos lumination used. phorus is added as phosphoric acid or ammonium phos TAB LE phate. 9. A method of manufacturing a photo conductor de Voltage in, FA. in, ,uA. 7'1/10v vice comprising a layer of activated cadmium chalcoge seconds nide photoconductive material selected from the group consisting of cadmium sulphide, cadmium selenide and 20V ______"i 0.05 ' 48 0. 4: mixed crystals thereof, comprising the steps, dispersing said activated photoconductive material in powdered form The table shows that the photocell has a comparatively in a solution consisting essentially of phosphorus in oxi high sensitivity and a short decay time on exposure to a 40 dized form and at least one cadmium halide selected from comparatively weak illumination. the group consisting of cadmium chloride, cadmium The invention is not restricted to the sizes and the con bromide and cadmium iodide, the phosphorus content of struction of the electrodes mentioned hereinbefore. The said solution being between 0.05 and 0.5 atomic percent electrodes may be shaped in another form, for example, of the cadmium content of the powder, the cadmium they may be of a known comb-shape. Further the in halide content of said solution being between 0.7 and 7 vention is not restricted to use in photocells. The inven mole percent of the cadmium chalcogenide content of the tion is also suitable for other purposes, particularly those powder, depositing a portion of said solution onto a sup in which photoconductive layers of large size have to be port to form a layer, and thereafter removing the solvent used, for example, in image intensi?ers. By the method and heating said layer in an oxygen-containing atmos ‘according to the invention photo-conductive layers sev 50 phere at a temperature between about 450° and 700° C. eral decimetres long and wide may be manufactured for a period of time su?icient for the cadmium halide to which have satisfactory coherence and uniformity and a evaporate and form a coherent layer. high photo-sensitivity. 10. A method as set forth in claim 9 wherein the photo . Further, the invention is not restricted to the use of conductive powder contains gallium and copper as the cadmium sulphide, but may also be employed with pow 55 activators. wered cadmium selenide or with powders consisting of 11. A method as set forth in claim 10 wherein the con mixed crystals of cadmium sulphide and cadmium sele tent of said activators lies between about 5><10~3 and nide, as the case may be, with added activators. 5><10—2 atomic percent of the cadmium content of the What is claimed is: powder. 1. A method of manufacturing a photoconductor de 60 References Cited by the Examiner vice comprising a layer of photoconductive material se lected from the group consisting of cadmium sulphide, UNITED STATES PATENTS cadmium selenide and mixed crystals thereof, comprising 2,765,385 10/ 1956 Thomsen ______117—201 X the steps, dispersing said photoconductive material in 3,133,888 5/1964 Oikawa ______252—-501 powdered form in a solution consisting essentially of a FOREIGN PATENTS minor amount of phosphorus in oxidized form and at least one cadmium halide selected from the group con 217,301 9/1958 Australia. sisting of cadmium chloride, cadmium bromide and 1,055,725 4/1959 Germany. cadmium iodide, depositing a portion of said solution onto a support to form a layer, and thereafter heating said de 70 ALFRED L. LEAVITT, Primary Examiner. posit to produce the photoconductive material. WILLIAM L. JARVIS, Examiner.