Mechanism of Photoconductivity in Chemically Deposited Lead Sulfide Layers

Many of the classical problems involved in the mechanism of photoconductivity in chemically deposited PbS layers have been resolved by a combination of (a) materials control, (b) measurements of conductivity, photoconductivity, Hall effect, thermoelectric effect, and oxygen sorption, and (c) analysis of an inhomogeneous model. The dark conductivity process, characterized by a small thermally activated mobility, is the same for samples without added oxidant in the deposition and for samples with added oxidant, and is dominated by intergrain barriers to free-hole transport. Carrier densities derived from Hall measurements are equal to those derived from thermoelectric power measurement and are therefore interpreted to be equal to the carrier density in the grains of the PbS layer. Layers prepared without added oxidant are only very weakly photosensitive, corresponding to sensitizing centers lying 0.13 eV below the conduction band, whereas layers prepared with added oxidant are strongly photosensitive, corresponding to sensitizing centers lying 0.22 eV below the conduction band. In all cases photoconductivity results primarily from an increase in free hole density, with some small contribution from an increase in hole mobility. Increases in hole mobility caused either by photoexcitation or by photoadsorption of oxygen are attributed to an increase in hole tunneling through intergrain barriers as the results of decreased depletion layer widths. Oxygen adsorbed on the free surface of the layer increases the free-hole density without appreciably affecting the mobility; oxygen which is adsorbed on the surface and then diffuses to intergrain barriers increases the free-hole density, and also to a lesser extent the hole mobility and the hole lifetime. In several ways the results following from photoexcitation and photoadsorption of oxygen are similar, and they may both be described in terms of the same inhomogeneous model.