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 Δg, and the time constant τ 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. Δg and τ are given as a function of g. Δg decreases with increasing oxygenation, reaches a minimum near the minimum in g and then increases as g increases after the film becomes p-type; Δgg is a maximum, however, near the minimum in g. On the other hand, τ 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.