Unraveling the passivation mechanisms of c-Si/SiOx/poly-Si contacts

As one of the prospective crystalline silicon (c-Si) technologies, tunnel oxide passivating contact (TOPCon) solar cells have recently attracted much attention in the photovoltaic community. However, the deficit of open-circuit voltage due to the imperfect passivation performance together with the insufficient understanding of the passivation mechanism, especially for the key structures of TOPCon devices (i.e., poly-Si/SiOx/c-Si), is a crucial factor to limit the improvement of device efficiency. To unlock the full passivation potential of this type of poly-Si/SiOx/c-Si contact, we report a systematic investigation by combining experiments and simulations. The related results can be summarized: 1) the best passivation can be obtained by optimizing SiOx quality, tailoring the in-diffusion profile, and performing hydrogenation; 2) if SiOx quality is good enough, high-level passivation can be expected even without impurity doping; 3) although the deepened in-diffusion could partially alleviate the degradation of the poor surface passivation, it would also reduce the passivation performance owing to the increased Auger recombination inherent to the high impurity doping; 4) the presence of pinholes would degrade the passivation performance, especially for the cases with the good surface passivation, a shallow in-diffusion and a low doping concentration of poly-Si, where the electric-field effect was minimized. To further confirm these conclusions, recombination distributions and the corresponding current profiles of the minority carriers were reviewed. This work clarifies the passivation mechanism of c-Si/SiOx/poly-Si contact in detail and provides effective guidance for further promoting the passivation level and the efficiencies of TOPCon devices.