Figure 2. (a) EQE spectra of the solar cells at each annealing step; (b) the PL mappings of the as-fabricated sample and the sample annealed in air ambient at 300 °C for 30 min; (c) minority carrier lifetime of ZnTiO₃-based solar cell at each annealing step, which is obtained in Suns-V _OCmeasurement mode; (d) illuminated J -V curves and the corresponding electrical parameters of the champion device.

According to the discussion above, the improved passivation effect by post-metallization annealing is the cause of the enhancement of cell performances. To understand the origin of the enhanced passivation (or cell performance), we have conducted several experiments, in which the fabrication of solar cells deviates from the standard process of the perovskite/silicon heterojunction solar cells mentioned above. One of them is that the SiO₂/ZnTiO₃ layers were annealed at 250 ℃ for 30 minutes before metallization (i.e.annealed without Al capping layer), named film-annealing-250 ℃, as indicated in the inset of Figure 3a. As can be observed, the annealing of SiO₂/ZnTiO₃ stack layers before Al metallization has no significant effect on the J -V curves of solar cells, and thus there is no improvement in PCE. Comparing this result with Figure 1, we can infer that the Al capping layer plays an important role in annealing, which contributes to the improvement of cell performances. Figure 3b, c shows the J -V curves of the solar cells with either Si/SiO₂/Al contact or Si/ZnTiO₃/Al contact at the back side, with three different post-annealing treatments. For the Si/SiO₂/Al contact, the V _OC decreases with increasing annealing temperature, resulting in the degradation of PCE. For the Si/ZnTiO₃/Al contact, the V _OCincreases with increasing annealing temperature, leading to the enhancement of PCE. Therefore, it seems that the improvement of cell performance mainly relates to the ZnTiO₃/Al stacks. However, the PCE of the solar cells with ZnTiO₃/Al electron-selective contacts are much lower than that of the counterpart (with SiO₂/ZnTiO₃/Al electron-selective contacts), indicating that SiO₂ also plays an important role in the high-performance solar cells with ZnTiO₃-based electron-selective contacts. Indeed, SiO₂ layer is widely demonstrated to have good passivation effect in c-Si solar cells.^21,45Therefore, SiO₂, ZnTiO₃ and Al, each layer has a unique effect in improving the passivation and thus theV _OC, J _SC and PCE of solar cells by the post-metallization annealing.