The mechanism of photoconductivity in polycrystalline cadmium sulphide layers

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Abstract
The mechanism of photoconductivity in polycrystalline CdS has been studied over the temperature range 100–300 K using Hall‐effect and conductivity measurements in the dark and under white light illumination. Samples were prepared in thin film form by spray pyrolysis and as power‐binder mixtures. Dark conductivities covered the range 10−9–101 Ω−1 cm−1. Dark conductivity is interpreted in terms of a two‐dimensional version of the grain‐boundary barrier model developed by Seto for polycrystalline Si. Except at very low carrier densities, Hall mobilities are found to be thermally activated, and intergrain barrier heights φb are derived for spray pyrolysis layers with doping levels covering the range N = 1014–1018 cm−3. A maximum barrier φbmax ≊0.2 Ev is found at a corresponding doping level, Nmax ≊2×1016 cm−3, which represents the situation where the barrier depletion layers just extend through the whole grain. From this we derive a mean grain diameter of 0.3 μm in good agreement with the result of transmission electron microscopy. For samples having N<Nmax the average free carrier density n̄≪N and photoconductivity occurs entirely through an increase in n̄ up to the point where the depletion regions begin to contract away from the center of the grains. For samples having N≳Nmax both μ and n̄ increase. The detailed variations of μ and n̄ are interpreted in terms of the Seto model with the added hypothesis that photogenerated holes are all trapped at grain boundaries. Hall‐effect measurements are interpreted on the assumption that the Hall coefficient R measures the average carrier density in the grain, i.e., R = (n̄e)−1, and we note that n̄ may differ significantly from the doping level N, even when N⩾Nmax. Photo‐Hall results provide evidence in support of this hypothesis.