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The overall objective of this program is to define the effects of impurities, various thermochemical processes, and any impurity-process interactions upon the performance of terrestrial solar cells. The results of the study form a basis for silicon producers, wafer manufacturers, and cell fabricators to develop appropriate cost-benefit relationships for the use of less pure, less costly solar grade silicon. Fifteen 4 ohm-cm p-type ingots were grown during this quarter, both single crystal and polycrystal, to initiate the Phase IV effort. These ingots were evaluated through electrical measurements, spark source mass spectroscopy, and atomic absorption spectroscopy of the melt. Carbon and oxygen concentrations were measured by ir spectroscopy and found to be within normal limits. Cells made from Mo-doped poly material exhibited very low efficiencies. Cells made from vanadium-contaminated polycrystalline Si also showed very low efficiency due to junction shorting effects; however, a few of the cels with lower inclusion densities could be measured. Projections of effective lfe of solar cells containing some specific impurities were made. On the basis of data derived from elevated temperature stressing of solar cell material, it is concluded that cells containing Mo or Ti as impurities will not degrade appreciably during a twenty year period due to impurity aging effects. However, initial data indicate that cells containing Cr, Nb, or Ag degrade more rapidly than Mo or Ti-doped cells, and may not retain useful efficiencies over a twenty year period. (WHK).
The overall objective of this program is to define the effects of impurities, various thermochemical processes, and any impurity-process interactions upon the performance of terrestrial solar cells. The results of the study form a basis for silicon producers, wafer manufacturers, and cell fabricators to develop appropriate cost-benefit relationships for the use of less pure, less costly solar grade silicon. Cr is highly mobile in silicon even at temperatures as low as 600°C. Contrasting with earlier data for Mo, Ti, and V, Cr concentrations vary from place to place in polycrystalline silicon wafers and the electrically-active Cr concentration in the polysilicon is more than an order of magnitude smaller than would be projected from single crystal impurity data. We hypothesize that Cr diffuses during ingot cooldown after groth, preferentially segregates to grain boundaries and becomes electrically deactivated. Both Al and Au introduce deep levels when grown into silicon crystals. Accelerated aging data from Ni-contaminated silicon imply that no significant impurity-induced cell performance reduction should be expected over a twenty-year device lifetime. Combined electrical bias and thermal stressing of silicon solar cells containing Nb, Fe, Cu, Ti, Cr, and Ag, respectively produces no performance loss after 100 hour exposures up to 225°C. Ti and V, but not Mo, can be gettered from polycrystalline silicon by POCl3 or HCl at temperatures of 1000 and 1100°C.