Low-temperature soldering for the interconnection of silicon heterojunction solar cells

Interconnecting silicon heterojunction (SHJ) solar cells by low-temperature ribbon soldering allows the use of standard stringing equipment and might therefore be the cheapest and most straightforward implementation in existing fabrication lines. However, solder joints on low-temperature metallization pastes of SHJ cells are known for a weak adhesion to the cell surface. This work is dedicated to a better understanding of the interaction between solder and low-temperature metallization on SHJ solar cells. We evaluate soldering during a lamination process with standard copper ribbons featuring six different low-temperature solder alloys. The mechanical adhesion of the solder joints and microstructural changes in the metallization pastes during the soldering process are analyzed. Our study includes three metallization pastes, two of them show poor adhesion on the wafer surface after soldering (up to 0.03 N/mm) and one paste performs slightly better (up to 0.28 N/mm). We find this difference to be caused by an interaction of the pastes and the liquid solder, neither depending on the composition of the solder alloy nor on the soldering time. A fine grain structure of the Ag-particles of such pastes correlates with a higher penetration of solder components (Sn and Bi) resulting in dissolution of the metallization and detachment from the cell surface.