Photoelectric effects at semiconductor electrodes

Abstract

Useful electrical work can be derived from photoelectric cells containing zinc oxide electrodes in contact with oxygen-free aqueous solutions containing organic compounds, or from irradiated cadmium sulfide electrodes. Study of these cells has shown that, with electron acceptors at the dark electrode, continuous current results from photolysis of the semiconductor electrode material. Steady photovoltages of 1.8 volt, open circuit, can be maintained over long periods of illumination; steady photocurrents of 1.5 ma can be delivered through 100 ohm external resistance. Zinc oxide electrodes are sensitive to the entire ultraviolet region of the spectrum. The actual operating photovoltages and/or photocurrents obtained deepnd on: (1) the presence of an oxidizable substrate at the irradiated zinc oxide surface, and (2) the nature and concentration of the electron acceptor at the dark electrode, as well as on the light intensity and the external load. Factors which promote the creation of positive holes in zinc oxide, such as doping with copper or silver, favor rapid response to the light and reproducible accumulation of optimum photoelectric effects. Many substances which are efficient electron acceptors at the dark electrode can be re-oxidized by atmospheric oxygen and are thus reversible. Reactions occurring at the irradiated zinc oxide or cadmium sulfide electrodes are not reversible. Formamide and alcohols are suitable substrates for oxidation at the irradiated zinc oxide electrode. At irradiated cadmium sulfide electrodes organic oxidizable materials have no effect; the sulfide itself undergoes photo-oxidation. Ceric ions, in acid solution at the dark, reference, platinum electrode, give the highest photovoltages and photocurrents so far sustained.