Photosensitization of PbS Films

The effect of O₂ on the photoconductive response of evaporated PbS films has been experimentally investigated. The dark conductance g, the photoconductance, and the time constant for the photoconductive response of PbS evaporated films vary when the films are exposed to oxygen. Initially g decreases several orders of magnitude when an outgassed film is exposed to oxygen; it reaches a minimum and then increases. The thermoelectric power changes from negative to positive at the minimum in conductance. and are given as a function of g. decreases with increasing oxygenation, reaches a minimum near the minimum in g and then increases as g increases after the film becomes p-type; g is a maximum, however, near the minimum in g. On the other hand, increases steadily as the film changes from n- to p-type. These results are attributed to the effect of two oxygen surface states. A low-lying O^- state alters the Fermi level of the originally n-type film. The surface charge thus formed lowers the effective carrier mobility. A higher energy O^-- state, when empty, traps photoelectrons and hence enhances the hole photoconductivity.