Adhesion of Al-metallization in Ultra-sonic Soldering on the Al-rear Side of Solar Cells

Today's Photovoltaic module production is driven by cost. Due to its high conductivity and solderability the precious metal silver is used in standard solar cell production as base material for solder pads. The need for replacing the costly silver emerges in various PV metallization techniques On the rear side, these solder pads (busbars) locally prevent the formation of a back-surface-field (BSF) leading to a decrease in open-circuit voltage. We therefor aim at reducing the silver content in solar cells while maintaining the solderability. To face this challenge we developed an interconnection method based on ultra-sonic (US) soldering to contact the aluminum on standard crystalline silicon (c-Si) solar cells. The method consists of a two-step process where first a tin alloy is employed directly on the aluminum and second conventional interconnectors are soldered onto the applied tin alloy. We investigate the adhesion of five different metallization pastes on the solar cells and compare it to the adhesion of soldered interconnector ribbons with our two-step US-soldering method on these metallizations. We find the adhesion of the solder joints to be independent of the metallization/cell adhesion. Metallographic analyses reveal that the tin alloy forms a direct contact to the Al-Si-eutectic underneath the metallization during our US process. The measured adhesive forces range from 0.5N/mm to 16N/mm of the metallization/cell interface and from 1N/mm to 2N/mm for the US-solder joints. Additionally we manufactured a 16-cell module consisting of cells with US-soldered contacts on the Al-rear side and compare it to a conventional module. The IV-characteristics show less loss in FF from cell to module (CTM) for the US-soldered module.