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

• Published in: Solar energy materials and solar cells 2021-05, Vol.223, p.110970, Article 110970
• Main Authors: Lu, Linna, Zeng, Yuheng, Liao, Mingdun, Zheng, Jingming, Lin, Yiran, Feng, Mengmeng, Zhi, Yuyan, He, Haiyan, Ding, Waner, Shou, Chunhui, Qin, Ganghua, Yan, Baojie, Ye, Jichun
• Format: Article
• Language: English
• Subjects: Aluminum; Atomic layer deposition; Augers; Defects; Diffusion; Diffusion layers; Impurity diffusion; Interface passivation; Passivity; PECVD; Photovoltaic cells; Polysilicon; Recombination; Silicon; Solar cells; TOPCon solar cell
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2021.110970

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. [Display omitted] •The passivation quality in p-TOPCon with ALD-AlOx and SiOx as the tunnel layer was studied.•The AlOx and AlOx/SiOx bi-layer do not provide a better passivation than the SiOx.•The AlOx and AlOx/SiOx bi-layer significantly enhance B diffusion and suppress P diffusion.•The B accumulation in the AlOx and AlOx/SiOx forms a reservoir for the enhanced B diffusion.•The free carrier assistant diffusion is another factor for the enhanced B diffusion by the AlOx/SiOx.