GRAIN-BOUNDARY BARRIER HEIGHTS AND RECOMBINATION VELOCITIES IN POLYSILICON UNDER OPTICAL ILLUMINATION

A new model of recombination of carriers at grain boundaries in polycrystalline silicon under optical illumination is presented by considering the monoenergetic density of grain boundary states. Calculations have been performed on the grain boundary barrier heights (V(g)), and on the interface and effective recombination velocities of minority carriers as a function of illumination level, grain size (d) and bulk diffusion length of the minority carriers (L(b)). These computations show that if the grain size lies in the range W(g) < d < L(b) (where W(g) is the depletion width) and the illumination level is high, the dependence of V(g) on grain size and illumination level will be much higher than that in the small and large grain size ranges. It is also found that, in the small grain size range, the dependence of interface and effective recombination velocities on the grain size is quite different, especially when illumination level is low. The dependence of the bending of the minority-carrier quasi-Fermi level in the grain boundary space-charge and quasi-neutral regions on the grain size and on the illumination level is studied. Calculations show that the validity of the quasi-equilibrium assumption decreases as grain size decreases and V(g) increases. This model predicts that the existing experimental studies cannot be used to calculate the effective recombination velocity of the minority carriers for polysilicon with small grain sizes. A number of experimental results have been fitted well on the basis of the theory.