Thermally stable poly-Si tunnel junctions enabling next-generation high-efficiency Si solar cells

We demonstrate thermally stable polysilicon (poly-Si)-based tunnel junctions (TJs), that is, n(+)-poly-Si/p(+)-poly-Si/SiOx and n(+)-poly-Si/SiOx/p(+)-poly-Si/SiOx, for passivating a silicon wafer, selectively extracting holes, and being compatible with the high-temperature firing process of screen-printed metal contacts. An additional interfacial oxide between the n(+)-poly-Si and p(+)-poly-Si layers enhances thermal stability and acts as a barrier layer to mitigate dopant interdiffusion between the adjacent poly-Si layers, without significantly increasing the contact resistance. The TJ's thermal stability is investigated by analysing the TJ recombination current density J(0) and effective contact resistance rho(c) after high-temperature firing (740 degrees C to 840 degrees C) of nonmetallised samples. We evaluate two different TJ applications via efficiency potential calculations: (1) When the TJ is applied on the front side of a passivated-contact Si bottom cell for 2-terminal tandem cells, a calculated add-on efficiency of 9.9% for the Si bottom-cell can be achieved with the inclusion of the interfacial oxide; (2) to enable conventional metal screen-printing on a p(+)-poly-Si layer, the TJ is applied on the rear side of a single-junction Si solar cell, giving a calculated cell efficiency potential of 23.6% at 1-Sun condition. For such a configuration, in the absence of an interfacial oxide between the adjacent poly-Si layers, the cell efficiency potential improves with peak firing temperature. In summary, we successfully develop thermally stable hole-extracting TJs for the two aforementioned applications that are fully compatible with existing industrial silicon solar cell fabrication processes.