From Current Literature

Application of Antimony-Cesium Cathodes in Photocells

Antimony-cesium photocathodes ^{1,2}, as is known, are distinguished by a whole series of very unusual properties that open up entirely new possibilities for the design of photocells and other photoelectric devices. These possibilities have by no means yet been fully exploited, but there already exist certain designs of photocells whose realization with other cathodes would have been either completely impossible or extremely difficult and disadvantageous with respect to the sensitivity of the device.

The most important of the properties of antimony-cesium cathodes that are being used at the present time are: a) their transparency and high sensitivity to light incident from the side opposite to the emitting surface, b) the fact that the dependence of the cathode sensitivity on the processing time is represented not by a curve with a maximum, as is usually the case, but by a curve with saturation, and c) the presence of a sharp selective maximum in the blue part of the spectrum. To these should be added one further property of these cathodes which may be an advantage but is as yet rather their shortcoming. This is the very high specific resistance of the Sb—Cs layer, which exceeds the specific resistance of antimony by \(10^5\)—\(10^7\) times.

Underlying all these, as well as other, properties, as was pointed out earlier ^{1,2}, is the circumstance that these layers are a chemical compound of Sb and Cs, which, as was established by direct experiment ^3, has the formula \(\mathrm{SbCs}_3\).

The greatest interest in the use of the transparency of Sb—Cs cathodes is presented by the manufacture of photocells with a large area of light-sensitive surface, intended for the most complete utilization of scattered light, since in the case of transparent cathodes the working surface of the photocell (the cathode is located on the inner surface of the bulb) is at least twice as large as for an opaque cathode. Taking into account that the sensitivity of a transparent oxygen-silver-cesium cathode never exceeds 20 \(\mu\mathrm{A}/\mathrm{Lm}\), whereas for Sb—Cs cathodes sensitivities of 50 \(\mu\mathrm{A}/\mathrm{Lm}\) and higher are easily obtained, the advantages of the new cathodes become quite obvious. These advantages, however, are not limited to sensitivity, since owing to the absence of fatigue in these cathodes they permit, without harm to the photocell, the removal of high photocurrent densities. For a good cathode, densities of up to 0.1 \(\mathrm{mA}/\mathrm{cm}^2\) are entirely permissible. However, in order to make full use of the sensitivity of these cathodes, it is necessary to take measures to eliminate the insufficient replenishment of electrons due to the high resistance of the layer. For this purpose one may either provide the cathode with several separate leads or, restricting oneself to a single lead, connect it with a system of conducting strokes applied to the inner surface of the bulb ^3.

The above-mentioned character of the dependence of the cathode sensitivity on the processing time made it possible, in particular, to manufacture photocells

of unusually small dimensions (down to \(5 \times 15\) mm, as against \(26 \times 60\) mm for the smallest existing ones to date), while at the same time having high cathode sensitivity (30–50 \(\mu\text{A}/\text{lm}\)) and all its other advantageous properties. Such photocells are of great interest in those cases where it is necessary to strive for the utmost reduction in the dimensions of the apparatus. In particular, such photocells (if needed) can, with the same ease as the miniature lamps now used, be inclined, inserted into tweezers, soldered in, etc.

The features of the spectral distribution of the sensitivity of Sb—Cs cathodes have proved extremely advantageous in application to phototelegraphy. Since the product of the spectral-sensitivity curve by the energy distribution in the spectrum of the light sources used in phototelegraphic apparatus (incandescent lamps) comes very close to the eye-sensitivity curve, these photocells make it possible correctly to transmit the densities of colored objects in single-color reproduction\(^4\).

N. Khlebnikov, Moscow

Literature

1. N. S. Khlebnikov and N. S. Zaitsev, Uspekhi fizich. nauk, 19, 278, 1938.
2. N. S. Khlebnikov and N. S. Zaitsev, Zhurnal tekhnich. fiziki, 9, 44, 1939.
3. N. S. Zaitsev, Zhurnal tekhnich. fiziki (in press).
4. A. M. Gurevich and I. G. Kesaev, Tekhnika svyazi, 1938.