Using silicon injection phenomenon during fire-through contact formation to improve process control and performance of screen-printed multicrystalline-silicon solar cells

High-performing screen printed, fire-through Si solar cells require a uniform, about 10-μm-thick, back surface field (BSF), with a peak Al concentration of about 10¹⁸ cm^-3. This entails forming an ~30-μm-thick Si-Al melt, which initiates sporadically and tends to agglomerate and ball up, thus producing a non-uniform BSF. We have developed a method to stabilize the Si-Al melt by deploying Si injection (at about 550°C for 2-5 s) during ramp-up, which initiates a thin, uniform melt along the entire Si-Al interface. This promotes adhesion between the Si surface and the growing Al-Si melt. We have previously shown that this process produces an excellent back contact, but the quality of the front contact and overall cell efficiency were not evaluated. The primary concern was whether molten glass frit would rapidly react with SiN:H and etch away a significant thickness of N⁺ silicon. Here, we report that high-efficiency cells with a uniform BSF, an open-circuit voltage >620 mV, and short-circuit current density >32 mA/cm² can be produced by a T-t firing profile that includes Si injection.