Development of Bifacial n-Type Front-and-Back Contact Cells with Phosphorus Back Surface Field via Mask-Free Approaches

Abstract

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Industrial bifacial n-type front-and-back contact (nFAB) solar cells consist of a boron-doped p+ emitter and a phosphorus-doped n+ back surface field (BSF). A conventional BSF formation method with tube-based POCl3 diffusion typically requires the use of a masking layer to protect the front emitter against cross doping or two wet-chemical etching steps. Herein, two alternative mask-free BSF formation approaches are investigated, either via phosphorus ion implantation or atmospheric pressure chemical vapor deposition (APCVD) of phosphosilicate glass (PSG). The fabricated cells indicate comparable efficiencies achievable by mask-free methods, as compared with conventional tube-based POCl3 diffusion. In addition, the APCVD process is further improved by optimizing the phosphorus contents in the PSG layer. Cell performances with different phosphorus contents and thus different BSF sheet resistances are studied. A lower phosphorus content (higher BSF's sheet resistance) improves cells' Jsc, whereas a higher phosphorous content results in high fill factor (FF) due to better rear contact resistance and low BSF sheet resistance. The optimized condition results in nFAB cell with a peak efficiency of 21.4% and FF values over 81.0%.