Elucidating and engineering recombination-active metal-rich precipitates in n-type multicrystalline silicon

Solar cells based on n-type upgraded metallurgical grade multicrystalline silicon (mc-Si) substrates may be a promising path for reducing the cost per watt of photovoltaics. The detrimental effect of metal point defects in both n- and p-type silicon is known, but the recombination activity of metal-silicide precipitates, especially in n-type mc-Si, is still not well established, impeding modeling and process optimization efforts. In this contribution, we provide a rationale for why metal-rich precipitates may limit minority-carrier lifetime in n-type mc-Si, in contrast to as-grown p-type mc-Si, which is dominated by metal point defects. Using μ-XRF, we identify metal-rich precipitates along a recombination active grain boundary in the low-lifetime “red zone” region of n-type wafers from a corner brick. To reduce the concentration of precipitated metals, we phosphorus-diffuse the wafers. Grain boundaries remain recombination active, which may be attributed to incomplete gettering of point defects and dissolution of recombination-active metal-rich precipitates.