21.0%-efficient screen-printed n-PERT back-junction silicon solar cell with plasma-deposited boron diffusion source

- n-PERT BJ cells reaching 21.0% with co-diffused B emitter from PECVD BSG/SiNz.
- SEGA for n-PERT BJ cell proves high quality of PECVD BSG diffusion source.
- BSG/SiNz stacks as rear passivation reach lowest published J0 results of 40 fA/cm2.

The manufacturing process of Passivated Emitter and Rear Totally diffused (PERT) solar cells on n-type crystalline silicon is significantly simplified by applying multifunctional layer stacks acting as diffusion source, etching and diffusion barrier. We apply boron silicate glasses (BSG) capped with silicon nitride (SiNz) layers that are deposited by means of plasma enhanced chemical vapor deposition (PECVD). Optimum PECVD deposition parameters for the BSG layer such as the gas flow ratio of the precursor gases silane and diborane SiH4/B2H6=8% and the layer thickness of 40 nm result in a boron diffusion with saturation current density J0,B below 10 fA/cm2 applying an AlOx/SiNy passivation and firing. The PECVD BSG diffusion source is integrated into the n-type PERT back junction (BJ) solar cell process with screen-printed front and rear contacts. The only high temperature step is a POCl3 co-diffusion for the formation of the boron emitter from the PECVD BSG layer and for the formation of the phosphorus-doped front surface field (FSF). An independently confirmed energy conversion efficiency of 21.0% is achieved for a 156×156 mm2 large n-PERT BJ cell with this simplified process flow. This is the highest efficiency reported for a large-area co-diffused n-type PERT BJ solar cell using a PECVD BSG as diffusion source. For comparison, reference n-type PERT BJ cells with separate POCl3 and BBr3 diffusions reach an efficiency of 21.2% in our lab. A synergistic efficiency gain analysis (SEGA) for the co-diffused n-PERT BJ cell shows that the main possible efficiency gain of 1.1%abs. originates from recombination in the phosphorus-diffused front surface field while the PECVD BSG boron-doped emitter accounts for only 0.1%abs. efficiency gain. We evaluate the use of the PECVD BSG/SiNz stack as a rear side passivation as a replacement of the AlOx/SiNy stack in order to further simplify the process flow. We obtain J0,B values of 40 fA/cm2, an implied open-circuit voltage of 682 mV and a simulated n-PERT BJ cell efficiency of 21.1%.