A single-layer MoS2 (n)/a-Si(i)/c-Si (p)/μc-Si (p+) heterojunction solar cell structure was designed, and the back field′s band gap, doping concentration and defect density of the layers on open circuit voltage, short circuit current, fill factor and conversion efficiency was simulated using AFORS-HET software. The results show that the back-surface field band gap is between 1.5-1.7 eV, and the back-surface field doping concentration is greater than 1×1018 cm-3, the solar cell of this structure has a relatively stable performance. As the defect density increases, the solar cell efficiency decreases linearly with the logarithm of the defect density. When the defect density is controlled below 1011 cm-3, a conversion efficiency greater than 24.10% can be obtained, and when the defect density is 109 cm-3, the highest conversion efficiency 29.08% can be obtained. Finally, the effect of the back-surface field layer on the structure of the solar cell is studied. The results show that when the defect density is effectively controlled, the addition of the back-surface field improves the cell efficiency significantly.