Gettering of metallic impurities in photovoltaic silicon

This work addresses the issue of structural defect-metallic impurity interactions in photovoltaic silicon and their effect on minority carrier diffusion length values. Aluminium and phosphorus segregation gettering studies were performed on photovoltaic silicon in order to gain insight into these interactions and quantify the effect of gettering on solar cell performance. Integrated circuit grade silicon was also studied for comparative purposes. Additionally, a novel rapid thermal annealing technique, designed to dissolve metallic impurity precipitates, and Deep Level Transient Spectroscopy were utilized to determine the as-grown impurity concentration in both grades of materials. Significant differences in gettering responses between the two grades of silicon are observed. Gettering treatments greatly improve I.C. grade silicon with a specific gettering temperature providing the optimal response. Photovoltaic grade silicon does not respond as well to the gettering treatments and, in some cases, the material degrades at higher gettering temperatures. The degradation is primarily observed in dislocated regions of multicrystalline photovoltaic silicon. Additionally, these dislocated regions were found to possess the highest as-grown metallic impurity concentration of all the materials studied. The dislocation-free photovoltaic silicon has a higher diffusion length relative to dislocated silicon but could not be improved by the gettering methods employed in this study. A model is presented to describe these phenomena where the high concentration of metallic impurities at dislocations produce relatively low minority carrier diffusion lengths as well as the degrading response with higher gettering temperatures while microdefects create an upper limit to the photovoltaic grade material's diffusion length.