A study on Impurities in Poly-Si Wafers Obtained from Vacuum Casting and their Effect on Solar Cell Efficiency

Abstract

A vacuum casting method was tried 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 concentration, above 3 x 10(17)/cm(3), and crystalline defects like grain boundary 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 concentration and gas porosity inside the cast poly-Si wafer as well as the surface roughness occurred during etching in solar cell fabrication process.