Barrier Theory of the Photoconductivity of Lead Sulfide

The barrier theory of the infrared photoconductivity of PbS films is discussed, according to which the high resistance of the films arises from $n\ensuremath{-}p\ensuremath{-}n$ barriers at the surfaces between the crystallites forming the films, the barriers being formed in the oxidizing process used in preparing the films. Under the action of light, electron hole pairs are formed, these carriers become trapped in the $n$- and $p$-type regions, respectively, the resulting charge density lowers the barriers, and hence the conductivity is increased. This theory is worked out quantitatively, and compared with experimental results of Mahlman on films of the type actually used as sensitive photoconductors. The theory shows good qualitative and quantitative agreement with experiment in numerous respects, including the explanation of the dark conductivity of the films and its dependence on temperature, the photoconductivity as a function of irradiance and temperature, the time constants involved in the rise or decay of the photoconductivity, and the short-wave limit of the photoconductivity. In working out the theory of the barrier model, we use the properties of the bulk material as determined by Petritz and Scanlon, and the properties of the films are found to be consistent with our knowledge of the behavior of the bulk material.