Dopant-grading proposal for polysilicon passivating contact in crystalline silicon solar cells

Polycrystalline silicon/crystalline silicon (poly-Si/c-Si) passivating contacts are an attractive option for improving the efficiency of c-Si-based solar cells. The heavy doping required for a poly-Si layer to ensure high-quality passivation of the contact can cause substantial dopant diffusion into the c-Si during the high thermal annealing process, as well as internal defects; this reduces the passivation quality. We propose a simple dopant grading of the poly-Si layer to suppress dopant diffusion and defects. The grading is implemented by varying the dopant concentration through poly-Si preparation. Accordingly, a high dopant concentration is distributed far from the c-Si interface, whereas a low one is an inverse. The grading improves passivation quality significantly, allowing for a high implied open-circuit voltage (iV_oc) of 730 mV and a low recombination current density (J_o) of 5.1 fA/cm². Owing to the grading, the solar cell devices exhibit enhancements of 10 mV, 0.5%, and 0.6% in the open-circuit voltage, fill factor, and efficiency, respectively. A silicon heterojunction solar cell using a back-side-graded poly-Si layer achieves a conversion efficiency of 22%. The proposed grading technique exhibits considerable potential as a simple and efficient technology for c-Si solar cells.