Rapid Photo-Assisted Forming Gas Anneal (FGA) for High Quality Screen-Printed Contacts for Silicon Solar Cells
Hilali, Mohamed M.
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Formation of low-cost high-quality contacts is the key to cost-effective silicon solar cells. Screen-printing is widely used in Industry because it is simple, low-cost and rapid. However, cost and throughput gains are attained at the expense of performance. Fill factors of most commercial cells are in the range of 0.68-75 for single crystalline material. This paper shows that a rapid 400°C/0.5-3 min photo-assisted anneal in a forming gas ambient can raise the fill factor (FF) of screen-printed (SP) single and multicrystalline (mc) Si cells from - 0.70 to - 0.77 and 0.76, respectively. Dark I-V analysis showed that this results from a decrease in series resistance by a factor of 2 to 4. Thus initial belt firing conditions can be tailored (~ 700DC) to first prevent the junction shunting, which generally results in high series resistance (Rs), and then the rapid photo-assisted anneal in forming gas ambient can be used to reduce the resistive losses for achieving high FF without much junction shunting. The LBIC analysis on multicrystalline silicon shows that a 30-second forming gas anneal in RTP not only reduces the glass frit at the silicon/silver interface but also enhances hydrogenation of bulk defects.