Analysis of Local Aluminum Rear Contacts of Bifacial PERC+ Solar Cells

A recently published industrial passivated emitter rear contact (PERC) solar cell concept called PERC+ enables bifacial applications by printing an aluminum (Al) finger grid instead of the full-area Al layer aligned to the laser contact openings on the rear side. We demonstrate that the rear contacts of these PERC+ solar cells exhibit back-surface field (BSF) depths of around 6 μm over a large range of contact linewidths, whereas PERC cells with full-area Al rear layer show a reduction of the Al-BSF depths for narrower contact lines. Using an existing analytical model for the local contact formation, we show that the measured Al-BSF depths are well described solely by the different volume of Al paste printed on the rear side. Consequently, the open-circuit voltage of PERC+ solar cells improves by up to 5 mV when reducing the contact linewidth only. In contrast, for PERC cells with full-area Al layer, the V slightly decreases with narrower contact linewidths due to the thinner Al-BSF depths. We observe a strongly reduced number of voids in the Al–Si eutectic layer for PERC+ cells, compared with PERC. As physical root cause for void formation, we propose the minimization of surface energy of the Al–Si melt.