Constitution and mechanism of the selenium rectifier photocell

This paper describes experiments to elucidate the exact physical and chemical structure of the selenium rectifier photocell, especially that of the thin surface film. A technique is described for sputtering films of cadmium oxide which, though transparent in the thickness required for a cell, have an electrical conductivity exceeding that of graphite. The thickness of the films can be closely controlled. With such films, on pure crystalline selenium, cells were produced with white-light sensitivities of over 700 µA per lumen, open-circuit voltages up to 0.5 V under high illumination, and maximum quantum efficiencies up to 70 %. The optical properties of the films are described, and the way in which the technique may be used to produce other non-metallic films is indicated. The cadmium oxide is found to have a negative Hall coefficient, and is therefore an N type semi-conductor. Further experiments with single films of gold, and double films of zinc oxide and gold, illustrate the behaviour of these, in intimate contact with selenium. The metal-selenium contact yields a poor photocell, the metal-zinc oxide-selenium contact one whose properties are critically dependent on the thickness of the intermediate oxide layer, and the N type cadmium oxide-selenium contact one for which the efficiency is high, and the thickness of cadmium oxide not critical. It is suggested therefore that in the practical photocell, the essential mechanism is a contact between two suitable semi-conductors of dissimilar types, any extra metal film when present serving simply to raise the lateral conductivity of the intermediate semi-conducting film when this is not high enough to eliminate undesirable effects of a high internal resistance in the finished cell.