On the Photo-Conductivity of Thallous Sulfide Cells

An investigation of the rise and decay of the photo-current in Tl₂S cells shows that the simple bimolecular recombination theory on which previous quantitative discussions of the photo-effect in Tl₂S were based is inadequate for the complete explanation of these curves. The dependence of the initial slopes of the response curves on light intensity is that to be expected for a homogeneous photo-conductor, but the temperature dependence of these slopes suggests the presence of barriers which influence the conductivity through the mobility. The variation of both the initial slopes and the steady-state photo-conductivity with temperature indicate an exponential dependence of the mobility upon temperature with an activation energy of 0.19 ev. These results indicate that Tl₂S conforms to neither the strictly homogeneous model nor the sharp barrier picture but possesses features of both of these models; the photo-effect corresponds to the homogeneous model, but the charge carriers move with a mobility determined by light-insensitive barriers.