Impact of phosphorus diffusion pre-gettering on the electrical properties of oxygen precipitates in n-type Czochralski silicon for heterojunction solar cells

The pursuit of high-efficiency silicon heterojunction solar cells (SHJ) imposes stringent requirements on the quality of Czochralski silicon (Cz-Si) wafers. Phosphorus diffusion gettering (PDG) has been widely adopted as a standard process to remove metal impurities and improve minority carrier lifetime prior to SHJ fabrication. This study reveals a critical competitive interaction between oxygen-related defects and PDG effectiveness. Specifically, swirl-distributed oxygen precipitate nuclei greatly hinder the effectiveness of PDG, leading to an absolute efficiency loss of 0.4 % compared to the baseline power conversion efficiency (PCE) of about 25.11 % for efficient SHJ solar cells. Electron beam-induced current (EBIC) results imply that the residual iron impurities are located near the oxygen precipitates after a PDG process. Deep-level transient spectroscopy (DLTS) results further identify a deep level (Ec-0.46 eV) associated with the interface states at the boundary of Fe-decorated oxygen precipitates and silicon matrix. This defect (Ec-0.46 eV) persists after the PDG, indicating the limited effectiveness of PDG in eliminating iron impurities bound to oxygen precipitates. This work provides a deep insight into the competitive gettering mechanism for iron impurities between oxygen-related defects and PDG in n-type Cz-Si, which could be instructive to optimize wafers' quality for efficient SHJ solar cell manufacturing.