Characterization of POCl $_{3}$ -Based Codiffusion Processes for Bifacial N-Type Solar Cells

Bifacial n-type silicon solar cells typically feature two highly doped areas, namely, a boron-doped emitter and phosphorus-doped back-surface field (BSF). Complexity of the process sequence for forming these highly doped areas is one of the major obstacles for industrial application. This study investigates a POCl₃-based codiffusion process that allows for forming boron-doped emitter and phosphorus-doped BSF in one single high-temperature step. As a boron source, we use a borosilicate-glass (BSG) layer deposited before the diffusion process using atmospheric pressure chemical vapor deposition. We discuss the influence of the POCl₃ concentration in the process atmosphere with respect to recombination and contact formation of the BSF. By tuning the POCl₃ concentration, we achieve specific contact resistances of screen printed contacts below 5 mΩ-cm² and dark saturation current densities below 160 fA/cm² on a textured surface. We present solar cell results on 156-mm n-type Cz wafers with peak efficiencies of 19.6%.