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Polysilicon passivating contact layer for crystalline silicon solar cells: a dopant-grading approach

In polycrystalline silicon/crystalline silicon passivating contact (hereafter poly-Si/c-Si), the heavy doping requirement for a poly-Si layer to provide high-quality contact passivation may promote dopant diffusion into c-Si. A dopant-grading approach is proposed to constrain the dopant diffusion from poly-Si into c-Si during the high thermal annealing. The approach is implemented by adjusting the dopant concentration appropriately throughout the thickness of the poly-Si layer. Accordingly, a dopant-grading with a high dopant concentration distributed far from the c-Si interface enhances passivation quality, enabling a 730 mV implied open-circuit voltage and a 5.1 fA/cm2 recombination current density. The grading improves the open-circuit voltage, fill factor, and efficiency of the device by 10 mV, 0.5%, and 0.6%, respectively. A heterojunction cell device including graded-poly-Si at the back side achieves an efficiency of 22%.