Impact of Iron Precipitation on Phosphorus-Implanted Silicon Solar Cells

Ion implantation is a promising method to implement a high-performance emitter for crystalline silicon solar cells. However, an implanted emitter redistributes and mitigates harmful metal impurities to a different degree than a diffused one. This paper quantitatively assesses the effect of iron contamination level on the bulk diffusion length and open-circuit voltage of phosphorus-implanted solar cells manufactured with varying gettering parameters. By synchrotron-based micro-X-ray fluorescence measurements, we directly observe a process-dependent iron precipitate size distribution in the implanted emitters. We show that controlling the iron precipitate size distribution is important when optimizing final cell performance and discover a tradeoff between large shunting precipitates in the emitter and a high density of recombination active small precipitates in the wafer bulk. We present a heterogeneous iron precipitation model capable of reproducing the experimentally measured size distributions. We use the model to show that the dominant gettering mechanism in our samples is precipitation and that implanted emitters with surface phosphorus concentrations around 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> induce little-to-no segregation-based gettering. Based on this finding, we discuss optimal gettering strategies for industrial silicon solar cells with implanted emitters.