Photosensitization of PbS Films

Abstract

The effect of ${\mathrm{O}}_{2}$ on the photoconductive response of evaporated PbS films has been experimentally investigated. The dark conductance $g$, the photoconductance $\ensuremath{\Delta}g$, and the time constant $\ensuremath{\tau}$ 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. $\ensuremath{\Delta}g$ and $\ensuremath{\tau}$ are given as a function of $g$. $\ensuremath{\Delta}g$ decreases with increasing oxygenation, reaches a minimum near the minimum in $g$ and then increases as $g$ increases after the film becomes $p$-type; $\frac{\ensuremath{\Delta}g}{g}$ is a maximum, however, near the minimum in $g$. On the other hand, $\ensuremath{\tau}$ 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 ${\mathrm{O}}^{\ensuremath{-}}$ state alters the Fermi level of the originally $n$-type film. The surface charge thus formed lowers the effective carrier mobility. A higher energy ${\mathrm{O}}^{\ensuremath{-}\ensuremath{-}}$ state, when empty, traps photoelectrons and hence enhances the hole photoconductivity.