A study of impurities in poly-Si wafers prepared by a vacuum casting method

Abstract

A vacuum casting method was attempted to produce cheap poly-Si wafers ready for solar cell fabrication without any cutting or slicing. However, the 5 x 5 cm(2) lab scale cast poly-Si wafers contained high carrier concentrations, above 3 X 10(17)/cm(3), and crystalline defects like grain boundaries and gas porosity. We carried out a set of vacuum casting experiment in order to identify the main sources of impurities during vacuum casting and investigated the impurities in the cast poly-Si wafers. The efficiency of the test solar cells fabricated on the cast poly-Si wafers was below 1.0% whereas the efficiency of the solar cell fabricated on the wafer sliced from starting Si was 5.5%. The low efficiency of the test cells was attributed to high carrier concentrations and gas porosity inside the cast poly-Si wafer.