Epitaxial silicon technology for low-cost solar cells. Interim report no. 1, 1 Apr--31 Jul 1975

The initial phase of this program suggests the use of epitaxial structures as a means of improving the quality of solar cells fabricated from EFG ribbon material. In fact, graded-base epitaxial solar cells have been made using material that did not yield solar cells by direct diffusion. Comparison of cells and diodes made by epitaxial growth and by direct diffusion have shown that the minority-carrier lifetime is generally higher in the epitaxial devices and the saturation current density is lower (by orders of magnitude in some cases). While defect studies are continuing, present evidence shows that the dislocation density is reduced in the epitaxial layer compared with that in the substrate. Furthermore, the data also suggest that the detrimental effects of grain and twin boundaries on the device performance is reduced. The best cells made by epitaxy on EFG substrates (provided by Mobil-Tyco) have efficiences of about 9% (compared with 6% by diffusion). However, the cells studied so far are relatively small (0.75 sq cm), and no attempt was made to optimize AR coatings or metallization grids; they are intended for diagnostic work rather than for state-of-the-art performance. (GRA)