Theory of grain boundary recombination and carrier transport in polycrystalline silicon under optical illumination

The physics controlling the recombination of minority carriers at grain boundaries in polycrystalline silicon under optical illumination is described theoretically, and a model for the grain boundary space-charge potential barrier height is presented. The model is based on the assumption of a Gaussian energy distribution of grain boundary interface states. Attention is also focused on the electrical conduction in this material under illumination. The dependence of space-charge potential barrier height and the effective recombination velocity on the illumination level, the grain size, and the bulk diffusion length of minority carriers (L_b) is investigated. Computations show that if the illumination level is high, the sensitivity of effective recombination velocity to grain size (d) in the intermediate grain size range (i.e. d≈L_b) is much higher than that in the small and large grain size ranges. It is found that the resistivity of polysilicon decreases on increasing illumination level. The dependence of polysilicon resistivity on grain size under optical illumination is found to be much higher than that under dark conditions