THE EFFECTS OF GRAIN BOUNDARIES ON THE CARRIER TRANSPORT IN POLYCRYSTALLINE SILICON

The effect of grain boundaries on the minority carrier transport in polycrystalline silicon has been studied theoretically. By assuming Gaussian energy distributions of grain boundary interface states, calculations have been performed of the recombination current density and the recombination velocity at grain boundaries for different energy distributions. The distinction is also made between the minority carrier recombination velocity at the grain boundary itself, and the effective recombination velocity for the collection of these carriers by the adjacent space-charge regions. The results indicate that both velocities depend strongly on the grain boundary trap distribution and the illumination intensity, and may have a maximum value. Furthermore, grain boundary traps are likely to distribute in more than one energy level.; Grain boundaries are also characterized according to their photovoltaic and photoconductance responses, which are related to the grain boundary trap distribution. Photoconductance transient response in poycrystalline silicon has also been studied theoretically and experimentally. The theory predicts that, in principle, by measuring the transient response as a function of the illumination intensity and the temperature, various grain boundary parameters can be deduced. Based on the theory, the "laser-spot-induced photoconductance method" has been developed to measure the grain boundary parameters. This technique allows local probing of individual grain boundary in a sample containing multiple grains and is useful for studying deep level grain boundary trap states. Experimental results obtained agree closely with the theory. Some p-type polycrystalline silicon grain boundaries are found to contain donor-like states both near and above the midgap, instead of exclusively at the midgap.; The effect of the grain boundaries on the majority carrier transport in aluminum-polycrystalline-silicon (Wacker) Schottky-barrier solar cells is also studied theoretically. Calculations show that even in large-grain sample, the transport can be bulk-limited, depending on the grain boundary trap distribution. Comparison with the experimental results indicates that some grain boundaries may be represented by a fixed interface charge and a uniformly distributed interface state density. The fixed interface charge can be ascribed to the existence of donor-like states above the midgap, consistent with the photoconductance experimental observation.