Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, September, 1929 Light-Sensitive Cells Practical Cells of Various Types By John P. Arnold I. Photo-Conductive Cells ANY brief discussion of cells of the photo-conductive ■ class bfrom the practical standpoint must be confined mainly to those employing annealed selenium for the light-sensitive material. It is true that some research has been made in recent years with compounds of antimony, bismuth, copper, lead, molybdenum, silver, thallium, etc., but the results of such investigations have not led to the development of light-sensitive devices which are greatly superior to the selenium cell, excepting possibly the thallium cell to be described below. The literature in reference to selenium is voluminous, and the reader who 1. A sketch of a typical selenium cell-form. wishes to study the subject thoroughly should consult the bibliography compiled by M. F. Doty, "Selenium: A List of References, 1817-1925," published by the New York Public Library, 1927. Of particular interest are the references to its physical and chemical constants, its electrical and optical properties, the cells and their uses, and also the early patents relating to visual communication, sound recording, etc. Light action in selenium was first reported fully in 1873 by Willoughby Smith who used bars of this element for high resistances. When sunlight fell upon them, it was observed that their conductivity increased. While useless for the purpose for which they were intended, the curious phenomenon exhibited soon led to the development of the first practical instrument for controlling an electrical current by means of light. Essentially a selenium cell comprises two metallic electrodes between which annealed selenium is deposited. As the specific electrical resistance of selenium is quite high — 30 to 2500 megohms per cubic cm. (Bidwell) — the cells should be so designed that only a short path of relatively large cross-section is traversed by the current flowing between the electrodes when these are connected to a source of potential. Furthermore, the selenium should be spread in a thin film so that a relatively large area, with respect to its volume, is affected by the light. The films should not be thicker than 0.0014 cm., as Brown1 has shown that this is the effective depth for the penetration of light into the surface of the element. To obtain the greatest ratio between the light-dark current (sensitivity) and to lessen inertia (the lagging of the electrical response behind instantaneous changes of illumination), this latter requirement should always be fulfilled in preparing the cells. At one time it was thought that the heat treatment or annealing process was a very important factor, but Piersol, in a paper referred to later, states that neither the length of time nor the temperature (within certain limits) is at all critical. Before discussing a few practical cells, it is necessary to point out the materials which are used for their construction. We have three components : the conducting electrodes, the insulating base, and the selenium film. As a number of metals will oxidize at the annealing temperature and combine with the selenium as a selenide, platinum, gold and nickel are the best materials to use, but due to the expense of gold and platinum, such metals or alloys as copper, aluminum, zinc, brass, Constantin, German silver, and also graphite and carbon may be employed. Unfortunately, however, selenium films seem to adhere better to those metals which oxidize than to those which do not. For insulation, quartz, glass, porcelain, slate, mica, soapstone, and bakelite are most satisfactory in preventing leakage currents across the electrodes of the cells. Of the various allotropic forms of selenium, the grey crystalline metallic variety alone has the suitable lightsensitive property, and the object of the annealing process is to convert the commercial forms of selenium to this variety. Various cell-forms have been suggested, but perhaps the most satisfactory is the grid arrangement shown in Fig. 1. Here a thin film of metal is deposited either by electrolysis, cathode sputtering, photograving processes, or some similar method. Mc Mahon and Brown2 prefer gold and platinum paints consisting of colloidal solutions of the metals in essential oils. The base (b) may be quartz or sandblasted glass, and the metallic film is divided into two electrically conducting portions, as shown. The selenium film bridges the gaps between the two electrodes or, by the method of the investigators mentioned above, large crystals of selenium are employed. Wires leading to the binding p'osts of the cell are attached to the electrodes (a, a) by means of Wood's metal. After a film is deposited on the form, it is then necessary to convert the selenium, usually from an amorphous variety, to its light-sensitive form. Piersol3 describes a process of annealing by means of which "several hundred selenium cells have been made with identical characteristics." He anneals the cells at 180 deg. C. for five minutes. An electric oven is often used for this purpose when accurate temperature control is desired, although a Bunsen burner is satisfactory in most cases. An even simpler construction than the foregoing has been proposed by Martin* made by heating powdered vitreous selenium to 216 deg. C. in a test tube and allowing it to cool slowly. By turning the test tube nearly upside down, long threads of selenium can be poured out, their diameters depending on the temperature. Threads about 2 cm. long and 0.1 cm. in diameter were found to be most satisfactory. These threads were annealed and then strapped down on a microscope slide by wires at either end. Cells so constructed are said to have a resistance of about V2 megohm. It is advantageous to place a selenium cell in a vacuum to prevent the absorption of moisture. Ruhmer5 suggested this improvement in design. He cut double parallel threads in soapstone or unglazed porcelain tubing and into these grooves wound No. 40 B&S gauge wires, which were secured at one end to the base of an incandescent lamp. The soapstone or porcelain tubing was divided in two pieces in order that wedges could be inserted to take up the expansion of the wires during the annealing of the cell. The bulb was then evacuated. Such cylindrical cells may be placed at the focus of a parabolic reflector and illuminated from all sides. 1 Pliys. Rev., Vol. 34, p. 201: 1912. 2 Jour. Opt. Soc. Am., Vol. 11, p. 223' 1925. 3 Phys. Rev., Vol. 30, p. 664 ; 1927. * Jour. Opt. Soc. Am., Apr., 1928. f'Das Selen," p. 11; Berlin, 1902.