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%.