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Process optimization of localized BSF formation for solar cells with over 20% energy conversion efficiency

The formation of a local back surface field (LBSF) via the Al-Si alloying process through small area openings on a passivated surface is a key attribute to reduce contact recombination at the local Al-Si interface. This is a much more complex process than the full area Al BSF used in a standard screen printed solar cell, and the unwanted formation of Kirkendall voids and inadequate LBSF depth are widely reported as issues. In this paper, we studied the impact of different process parameters on the local BSF thickness and Kirkendall void percentage. The process parameters investigated included the geometry of the local contact regions, the Al layer deposition thickness and the fast firing temperature profile. The details of the optimization process are discussed, and the key trends observed for each of the parameters are presented. Cells made using these optimized processes demonstrated over 20% energy conversion efficiency on solar grade p-type CZ material.