Electronic processes at grain boundaries in polycrystalline semiconductors under optical illumination

The dependence of minority carrier lifetime (τ) on the doping concentration Nd, grain sizedand interface state density Nisat the grain boundaries in (n-type) polycrystalline semiconductors has been calculated analytically. The recombination velocity at grain boundaries is enhanced by the diffusion potential Vdadjacent to the boundaries, and ranges fromsimeq 10^{2}to 10⁶cm . s^-1depending on Nisand Nd. Under illumination, the population of the interface states is altered considerably from its dark level and as a result, Vddecreases to that value which maximizes recombination (equal concentrations of electrons and holes at the boundary). This causes τ to decrease with increasing Nd. Sample calculations for polycrystalline silicon show that for low angle boundaries with interface state densities ofsimeq 10^{11}cm^-2eV^-1, τ decreases from 10^-6to 10^-10s as the grain size is reduced from 1000 to 0.1 µm (forN_{d} = 10^{16}cm^-3). For a constant grain size, τ decreases with increasing Nd. The open-circuit voltage of p-n junction solar cells decreases fortau leq 10^{-7}s, whereas that for Schottky barrier cells remains at its maximum value untiltau lsim 10^{-8}s.