## 3 Results and Discussion
A new finding is that the \(J_{01}\) of an Al-BSF solar cell increases quickly with the decreasing injection level compared with a PERC silicon solar cell. These results reveal that the output performance of an Al-BSF solar cell decreases rapidly when the injection level is getting low compared with a PERC silicon solar cell. Figure 6 shows the comparison of \(J_{02}\) for these two kinds of solar cells at 25 \({}^{\circ}\)C and AM1.5G. From Figure 6, it is found that the \(J_{02}\) of Al-BSF and PERC solar cells increase with the increasing injection level. This is because the effective minority carrier lifetime in the depletion region decreases sharply in high injection levels [45; 46]. Meanwhile, owing to rear passivation, the \(J_{02}\) of PERC solar cell is always lower than that of Al-BSF solar cells under the same injection level. It is also observed that the \(J_{02}\) variation of a PERC solar cell is significantly smaller than that of an Al-BSF solar cell as the injection level is varied from 200 W/m\({}^{2}\) to 1200 W/m\({}^{2}\). Specifically, when the injection level increases from 900 W/m\({}^{2}\) to 1200 W/m\({}^{2}\), the \(J_{02}\) of a PERC solar cell increases from \(2.94\times 10^{-9}\) A/cm\({}^{2}\) to \(9.86\times 10^{-9}\) A/cm\({}^{2}\), but the \(J_{02}\) of an Al-BSF solar cell increases rapidly from \(9.12\times 10^{-9}\) A/cm\({}^{2}\) to \(2.77\times 10^{-8}\) A/cm\({}^{2}\). The obtained result agrees with other publications [47; 48]. \[J_{d1}=J_{01}\bigg{(}\exp\frac{V+JR_{s}}{n_{1}V_{T}}-1\bigg{)} \tag{2}\]2 \(J_{01}\) versus injection level for an Al-BSF solar cell and a PERC solar cell at 25 \({}^{\circ}\)C and AM1.5G. Figure 7 describes the variation in the \(J_{01}\)/\(J_{02}\) ratio of an Al-BSF solar cell and a PERC solar cell as a function of the injection level at 25 \({}^{\circ}\)C and AM1.5G. As can be seen from Figure 7, it is found that both the \(J_{01}\)/\(J_{02}\) ratio of the Al-BSF solar cell and the \(J_{01}\)/\(J_{02}\) ratio of the PERC solar cell increase with the decreasing injection level. This is because the low injection level leads to a small \(J_{02}\) value. These results suggest that \(J_{02}\) could be neglected only at a low injection level. Whether it is under high injection or under low injection, the \(J_{01}\)/\(J_{02}\) ratio of a PERC solar cell is always smaller than that of an Al-BSF solar cell under the same injection level. Moreover, the \(J_{01}\)/\(J_{02}\) ratio of a PERC solar cell increases slowly with the decreasing injection level compared with an Al-BSF solar cell. These results show that a PERC solar cell performs well when the injection level varies from low to high compared with an Al-BSF solar cell [46]. \[J_{d1}=J_{01}\bigg{(}\exp\frac{V+JR_{s}}{n_{1}V_{T}}-1\bigg{)} \tag{2}\]3 \(J_{02}\) of an Al-BSF solar cell and a PERC solar cell as a function of the injection level at 25 \({}^{\circ}\)C and AM1.5G. \[J_{d1}=J_{01}\bigg{(}\exp\frac{V+JR_{s}}{n_{1}V_{T}}-1\bigg{)} \tag{2}\]4 Variation in \(J_{01}\)/\(J_{02}\) of an Al-BSF solar cell and a PERC solar cell as a function of injection level at 25 \({}^{\circ}\)C and AM1.5G. Effect of Jo/Jo\({}_{02}\) on Electrical Parameters of PERC Silicon Solar Cellsand Al-BSF Silicon Solar Cellsunder Different Injection Levels  
The relative magnitude of the recombination parameter of the diffusion region and the recombination parameter of the depletion region can be expressed by the \(J_{01}\)/\(J_{02}\) ratio. Figure 8 shows the variation in open-circuit voltage with the \(J_{01}\)/\(J_{02}\) ratio for PERC and Al-BSF silicon solar cells under different injection levels. From Figure 8, it is clearly observed that the open-circuit voltages of PERC and Al-BSF silicon solar cells are strongly influenced by the \(J_{01}\)/\(J_{02}\) ratio, i.e., the open-circuit voltages of these two silicon solar cells increase with the decreasing \(J_{01}\)/\(J_{02}\) ratio.