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Historically, busbar pull tests have been used as a measure of metal–silicon adhesion for silicon solar cells; however, such measurements cannot be easily applied to evaluate finger adhesion and the propensity of metal fingers to peel. Finger adhesion will be increasingly important as the width of fingers decrease and busbars are effectively removed from the cell metallization. In this paper, we correlate metal-plated finger dislodgement measurements, which have been obtained using a stylus-based metallization testing tool, and busbar pull test forces with nanoindentation measurements of the Young's modulus in order to determine key determinants of strong finger adhesion. It is proposed that metal fingers with a higher Young's modulus dislodge at lower stylus impact forces because the energy associated with the impact is less easily dissipated along the fingers and consequently remains more focused on the impact location, causing not only finger dislodgement but more extensive finger peeling as well. It is shown how plating rate, chemistry, grid geometry, and postplating annealing can all contribute to plated metal finger adhesion, therefore necessitating an understanding of these factors for reliable plated metallization.

Copper plating can reduce the consumption of silver for silicon photovoltaic manufacturing, whilst also offering the potential to increase cell efficiency by way of reduced shading due to very narrow fingers and contacting silicon surfaces with low phosphorus concentrations. However, it can be challenging to plate busbars and fingers that have sufficient adhesion to the silicon. To date adhesion has typically been assessed by busbar pull tests however we propose that this measurement does not consider the properties of copper-plated fingers and may not be a good indicator of whether fingers may dislodge or peel with subsequent processing or during module fabrication. In this paper we investigate the factors of silicon interface roughness and plated copper properties using a combination of busbar pull tests and stylus-based adhesion measurements. We show that average 180° pull test forces of 2.1 N/mm can be achieved when a UV ps laser is used to ablate the silicon nitride, however ensuring strong finger adhesion is a far more complex problem with no accepted standard to determine what is “sufficient” adhesion. Although use of a ps laser to ablate the silicon nitride can result in plated metal adhering so strongly to the silicon that fragments of silicon are broken off with the finger when it is dislodged by the stylus, use of fast plating rates can result in reduced finger dislodgement forces and excessive finger peeling whereas busbar pull test forces are largely unchanged because the increased plating current is directly mostly through the fingers of the plated metal grid during light-induced plating. The plating of busbars and fingers on a cell presents challenges for uniform silicon-metal adhesion and this paper highlights the importance of finger adhesion measurement for process quality control for nickel/copper plating of p-type silicon cells in a manufacturing environment.