Al fire through contacts for p-type bifacial PERC devices

The authors present their work on Aluminum fire through (Al-FT) contacts on the rear side of p-type bifacial PERC devices. On test structures, only exhibiting a rear screen-printed Al electrode, the authors demonstrate that widening the temperature profile in the peak firing zone has a beneficial impact on the implied V_oc leading to an increase from 646 to 652 mV. The Photoluminescence based analysis is accompanied by scanning electron microscope analysis at finger cross-sections showing randomly distributed inverted-pyramid-shaped Al-Si contacts exhibiting a 0.5 to 2.5 µm thick back surface field. On cell level, the approach is not yet mature and a peak efficiency of 20.5 % could be achieved, which is in clear distance to the 21.3 % of the reference cell featuring additional laser openings and an Al non-fire through (Al-NFT) rear electrode. The Al-FT’s BSF seems not to exhibit the same quality compared to its NFT pendant, because V_oc decreases by 8 mV at a similar effective BSF coverage. The need for narrow Al fingers and, simultaneously, a high specific resistance of the Al-FT paste of ≈ 40µΩcm, leads to a limited FF reaching 80.5 % compared to 81.3 % for the Al-NFT.