Effect of impurity gettering on the efficiency of metallurgical-grade silicon solar cells

Abstract

The efficiency of thin film epitaxial solar cells deposited onto low cost metallurgical-grade silicon (MGSi) was improved by the impurity gettering of the MGSi substrates. The gettering techniques investigated include (i) work damage on the back side of the wafers, (ii) intrinsic gettering using a one- and two-step thermal anneal and (iii) phosphorus gettering. The effectiveness of the various gettering techniques is evaluated from the electrical characteristics of solar cells. A one-step thermal anneal improves the solar cell efficiency from 7.9% (air mass 1) to 11.2% for 50 μm epitaxial cells. A two-step anneal further improves the efficiency to 11.8%. The phosphorus gettering gives the same efficiency as a two-step anneal but with a shorter annealing time. The improvement in cell efficiency is attributed primarily to the increase in the fill factor and short-circuit current density J sc. Work damage in addition to a two-step anneal did not improve the overall electrical characteristics. It was also found that the cell efficiency is improved on an increase in epitaxial layer thickness. MGSi and semiconductor-grade silicon (SGSi) substrates on which are deposited epitaxial layers of the same thickness give comparable electrical results. The annealing of SGSi considerably degrades the cell performance. Computer modeling is used to analyze the current-voltage curves for the components of current losses. The major current losses for the MGSi solar cells were due to bulk recombination.