Grain-boundary effects on photocurrent fluctuations in polycrystalline photoconductors

The effect of light-dependent potential barriers at the grain boundary interface has been taken into account in the derivation of the photoconductance noise in polycrystalline photoconducting films. The noise power spectrum has been calculated considering the contribution of many elementary systems constituted by two homogeneous crystalline grains separated by an adjacent photosensitive potential barrier. The noise relative to each elementary system has been studied taking into account the modulation effect on the current crossing the grains of the spontaneous fluctuation of the intergrain barrier under illumination. The theoretical results are compared with a set of experimental data concerning the photoconductance noise in polycrystalline PbS under several experimental conditions. The proposed model allows us to explain several features of the current noise both in the dark and under illumination. In particular, the origin of the 1/f noise component, arising in the power spectrum in the presence of light, and the unusual behavior of the noise spectral density vs conductance when light intensity is changed are discussed and clarified on the basis of the present theory.