Discussion of recombination current mechanisms at grain boundaries in the bulk: a simplified model

The disruption of the periodic crystal lattice at the boundaries of crystal grains in polycrystalline silicon causes a high density of states within the 'band gap' and hence generally leads to bending of the electronic bands. In this paper we present a simplified model for recombination current mechanisms at such grain boundaries in the bulk, faraway from the p-n junction. We show that even if the Fermi level at the grain boundary is pinned in the middle of the bandgap the ideality factor of the associated recombination current is not necessarily equal to '2'. Only at higher voltages, when the minority carrier quasi Fermi level is approximately constant across the grain, the total recombination increases with voltage according to an ideality factor m = 2. At lower voltages the minority carrier flow to the recombination active grain boundaries is limited by diffusion and thus the total recombination current is described by m =1. We further show that the influence of the grain boundary recombination decreases with increasing doping. Consequently, doping can cause effects describable as grain boundary passivation.