Recent Advances in Crystalline Silicon Solar Cells

摘要

1.Introduction Grain boundaries act as recombination centers and determine the solar cell performance, and many works have been done for increasing the grain size of polyerystalline Si to improve the conversion efficiency [1-5]. Now, the larger grain size exceeding 1 cm was obtained by cast-grown method, which is large enough as compared with the minority carrier diffusion length (0.25 mm) in B-doped polycrystalline Si. However the expected high conversion efficiency has not been realized. In the region where the lifetime is relatively short, there are many defects, which appear as etch-pits by the Secco etching. The relationship between the etch-pit density and the minority carrier lifetime suggested that these defects acted as a recombination center and that they mainly determined the lifetime. In the present cast grown Si, there are many residual carbon atoms, which are mainly contaminated during the crystal growth. A larger amount of carbon atoms exist in the region where the lifetime is shorter as compared with in the region with longer lifetime [6]. These carbon atoms might be one of the reasons to create the defects. X-ray microprobe fluorescence measurement results suggested that irons were corrected at these defects, and that the defect and heavy metal complex might deteriorate the minority carrier lifetime [7]. However, the structures of these defects, the effect of a kind of metal on the trap ability, and these electrical properties are not clear yet. In the present work, we study the defect structures, the distributions of iron and its chemical state, and the electrical properties of the regions with grain boundaries and defects.