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.