Large area industrial solar cells on low cost 100% Mc SoG Si substrates: Efficiencies exceeding 16%

Abstract

The focus of this work is the development of large area industrial type screen printed silicon solar cells made of 100% solar grade silicon (SoG-Si). The feedstock under investigation is produced by 6N Silicon Inc. from metallurgical grade silicon with a lower cost low energy refining technique compared to the Siemens process. Solar cells were processed at ISC Konstanz with industrial type process equipment. This work was conducted over a one year period, where the material quality was improved and individual process steps were enhanced or adapted to the properties of the SoG-Si. These efforts made on both sides of the production chain resulted in solar cells that exceed 16% efficiency on standard 156×156 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> sized substrates made of 100% 6N Silicon material. For the purpose of this study, the 6N Silicon feedstock was processed into multicrystalline silicon ingots. These ingots achieved bulk resistivities between 0.5-2.5 Ωcm. The 6N purification approach uniquely applies an Al-Si melt as the primary purification step. The aluminium content of the feedstock end product, however, was observed to be lower than 0.05 ppmw. The overall contamination levels are sufficiently low to meet the needs of a solar cell producer. With a stable baseline process at ISC Konstanz, the improvements of 6N Silicon material were tested against a single polysilicon reference brick. In addition, process enhancements leading to higher efficiencies were also investigated. The same average efficiency level as the polysilicon reference was reached with the 6N material by applying the baseline process. An advanced process was then defined that still satisfies industrial production standards. This advanced process includes a novel isotexture, an adapted shallow POCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> diffusion, new metallisation paste and a new front side grid design. Running this advanced process achieved average efficiency of 16.04% with 100% 6N Silicon feedstock, and a best cell having 16.33% efficiency.