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.