Enhanced Stability of Exposed PECVD Grown Thin n+ Poly-Si/SiOx Passivating Contacts With Al2O3 Capping Layer During High Temperature Firing

Carrier selective poly-Si/SiOx contacts have become a very strong contender for next generation high-efficiency Si solar cells as well as Si-based tandem cells. A thin unmetallized poly-Si/SiOx passivated contact on the top surface of the bottom Si cell provides an excellent opportunity for higher efficiency two-terminal Si-based tandem cells. However, this article shows that when the manufacturable low-cost screen-printed contacts are applied on the rear side of the Si cell, the exposed poly-Si/SiOx contact on the front undergoes appreciable degradation in J0 during high temperature contact firing without a capping layer on the front poly-Si. In this article, unmetallized n-TOPCon structures with ~10-nm poly-Si showed significant degradation after a typical simulated firing process (~750 °C), resulting in decreased implied Voc from 728 to 708 mV and increased J0 from 5 to 15 fA/cm
²
. We found that a ~7-nm Al
₂
O
₃
capping layer on poly-Si provides excellent protection with no degradation and can also be easily removed after firing for subsequent processing. For comparison, PECVD grown ~20 nm SiNx and SiOx layers were also investigated for firing stability. The SiNx capping layer was also found to be effective in preserving stability, but the SiOx failed due to lack of hydrogen supply. Large area n-type front junction Si solar cells were fabricated with a boron diffused screen-printed emitter on the front and Ag evaporated thin n-TOPCon on the rear to demonstrate ~0.3% higher absolute efficiency when the rear n-TOPCon was capped with Al
₂
O
₃
during the firing of the front screen-printed contacts.