Impact of pre-fabrication treatments on n-type UMG wafers for 21% efficient silicon heterojunction solar cells

Silicon heterojunction solar cells achieve high conversion efficiency due to the excellent surface passivation provided by the hydrogenated intrinsic amorphous silicon films. However, they require a high-quality wafer as a starting material because their low-temperature processing does not allow for gettering. Czochralski-grown upgraded metallurgical-grade (UMG-Cz) silicon is a low-cost alternative to electronic-grade silicon for silicon solar cells, but is often limited in lifetime by grown-in defects. We have previously shown that pre-fabrication treatments, namely tabula rasa, phosphorus diffusion gettering, and hydrogenation, can significantly improve the bulk quality of UMG-Cz wafers. These help to mitigate the impact of grown-in oxygen precipitate nuclei and metallic impurities. In this work, we fabricate rear-junction silicon heterojunction solar cells on both as-grown and pre-treated UMG-Cz and electronic-grade wafers. We show that pre-fabrication treatments have a marked impact on solar cell efficiencies. With pre-fabrication treatment, the efficiency improves from 18.0% to 21.2% for the UMG-Cz cells and 21.2%-22.7% for the electronic-grade cells. Comparison of the open-circuit voltages of the as-grown and pre-treated UMG-Cz and electronic-grade cells using Quokka simulations reveals that the bulk lifetime remains the primary limiting factor for the UMG-Cz wafers.