A Comparison of Bulk Lifetime, Efficiency, and Light-Induced Degradation in Boron- and Gallium-Doped Cast mc-Si Solar Cells

High-efficiency boron- and gallium-doped multicrystalline silicon (mc-Si) cells were fabricated and compared in this paper. The quality of three different boron-doped mc-Si ingots and one gallium-doped mc-Si ingot was investigated and compared by means of lifetime measurements and solar cell efficiencies. Untextured screen printed 4-cm² cell efficiencies in excess of 16% were achieved in this paper when the lifetime after gettering and hydrogenation exceeded 100 mus. This was true for most wafers from top, middle, and bottom regions of the boron-doped ingots. Lifetimes in excess of 300 mus were achieved from the middle region of some boron- and gallium-doped mc-Si ingots. High efficiencies in excess of 16.7% were attained from the middle region of most ingots investigated in this paper regardless of gallium or boron dopant. Light-induced degradation in efficiency (2%-3% relative) was observed in some of the boron-doped mc-Si wafers in which oxygen concentration was high (15 ppm). In contrast, gallium-doped solar cells were found to be very stable under illumination irrespective of their location in the ingot. Device characterization and modeling were performed to show that the combined effect of large variation in resistivity and lifetime along the gallium-doped mc-Si ingots results in variation in the cell efficiency from different regions of the gallium-doped ingots. Design rules were established to determine the optimum thickness of the solar cell for extracting maximum efficiency when the bulk lifetime and resistivity vary along the length of the ingot for a better utilization of the whole ingot