Dopant diffusion through ultrathin AlOx and AlOx/SiOx tunnel layer in TOPCon structure and its impact on the passivation quality on c-Si solar cells

A comparison study of ultrathin atomic layer deposited AlOx, wet-chemically oxidized SiOx, and their combination as the tunnel layer in TOPCon structure with both B-doped and P-doped poly-Si contact layers on n-type c-Si wafers was carried out. The passivation quality with the three types of tunnel layers was examined as a function of thickness and annealing temperature. Ideally, the high density of negative fixed charge in the AlOx is expected to provide a positive benefit on the passivation quality with p-type poly-Si as the contact layer, however, it is surprisingly observed that the AlOx and AlOx/SiOx do not yield a better passivation than the SiOx. Searching for the mechanisms behind, an interesting phenomenon is observed that the AlOx, especially the AlOx/SiOx bi-layer, significantly enhances B diffusion and suppresses P diffusion. It is also found a remarkable accumulation of B in the AlOx and AlOx/SiOx region, forming a reservoir for B diffusion. Furthermore, the free carrier assistant extrinsic diffusion is an additional factor for the enhanced B and retarded P diffusions by the AlOx/SiOx. The enhanced B diffusion causes extra Auger recombination as well as recombination through B–O pair defects and degrades the p-TOPCon passivation quality; additionally, a quantity of Al diffuse into the c-Si wafer could be another potential factor degrading passivation quality, because Al forms deep-level defects and induces a significant SRH recombination.