Silicon solar cell process development, fabrication and analysis. Phase II. Annual report, 1 July 1979-30 June 1980

Abstract

Solar cells were fabricated from EFG (RH) ribbons from multiple dies, silicon on ceramic (SOC), dendritic web, cast silicon by HEM, and semi-continuous CZ from both VARIAN and HAMCO. Baseline and improved solar cells were made from the sheets. Baseline solar cells processed in both Phase I and Phase II, involving cells from EFG, SOC, dendritic web, and HEM, indicated that no significant improvement in silicon sheet quality has been achieved in Phase II. Solar cells from semi-continuous CZ showed cell performance close to the conventional CZ control cells, although the cells from the semi-continuous CZ have shown wider performance range because of variation in crystalline perfection. Generally, process upgrading provided improvement in cell performance, the improvement depending on the process used and the quality of the sheet silicon. Study of the effect of grain size on solar cell performance suggested that the minimum grain size to make solar cells of 10% AMO efficiency is about 500 ..mu..m, which is expected to provide minimum module efficiency of 10% AMI. If other harmful impurities are added in the process of sheet growth, the minimum grain size must be increased. The BSF study showed that the higher the resistivity of the starting substrates, the greater the relative improvement in cell performance, probably because of greater shift in Fermi levels at the back L/H junction (pp+) and also because of the higher initial values of minority carrier diffusion length. This study also suggested that proper control of the back-surface field (BSF) process could minimize the junction shunting problems often introduced by the BSF processing.