Bulk lifetime and efficiency enhancement due to gettering and hydrogenation of defects during cast multicrystalline silicon solar cell fabrication

摘要

Cast multicrystalline silicon (mc-Si) shows a significant variation in quality depending on the location of the brick in the ingot and the location of the wafer in the brick. Variation also occurs in ingots from different suppliers, which is attributed to the difference in the cleanliness of the crucible used for growth and the quality of the silicon feedstock used. Process-induced lifetime investigation conducted in this paper showed that wafers from the top region of mc-Si ingot grown by Heat Exchanger Method (HEM) benefited most from the gettering step during phosphorus diffusion to form the n~+ junction. Wafers from the bottom of the ingot, however, benefited most from the hydrogenation taking place from the SiN_x. film during the co-firing cycle used to form simultaneous front and back contacts and aluminum back surface field. Wafers from the middle region benefited from both, the diffusion-gettering, and the SiN_x-hydrogenation. Un-textured, 4 cm~2, screen-printed, best solar cell efficiencies of 15.9% and above were achieved on wafers from top, middle, and bottom regions of most of the ingots used in this study because the bulk lifetime exceeded 100 μs after gettering and hydrogenation. Lifetimes in excess of 300 μs were achieved from the middle region of some mc-Si ingots. Solar cell efficiencies of 16.7% were attained from the middle regions of two out of the three ingots investigated in this study. Device modeling was performed to provide guidelines to reduce the efficiency variation across different regions of the ingots and to obtain the highest possible efficiency with a given bulk lifetime and device structure.