Figure 3. (a) J -V curves of the solar cells with
Si/SiO2/ZnTiO3/Al contacts before and
after film-annealing; J -V curves of the solar cells with
(b) Si/SiO2/Al contact and (c)
Si/ZnTiO3/Al contact at the back side at three
post-metallization annealing temperatures.
In order to get further insight into the mechanism of enhanced
passivation effect by post-metallization annealing, we have studied the
microstructure of ZnTiO3-based contact before and after
annealing by the HAADF-STEM images, together with the EDX mapping.
Figure 4a shows the HAADF-STEM image of the
c-Si/SiO2/ZnTiO3/Al interface, which is
annealed at 300 °C for 30min. There are distinct contrast differences
between each layer, indicating a clear hierarchical structure. Figure 4b
further reveals that Al has diffused into the entire
ZnTiO3 layer. Although such behavior is also observed in
the sample without annealing treatment, shown in our previous
study,40 the Al content is higher. Moreover, annealing
is expected to be beneficial for Al diffusing deeper into the
ZnTiO3 layer, which should be more obviously observed in
a relatively thick ZnTiO3 film. The Al element occurred
in the ZnTiO3 film has a substantial influence on the
material property, as will presented in the following. From HAADF
images, presented in Figure 4c,d, an obvious ultrathin
(~1 nm) SiO2 layer is observed between
c-Si and ZnTiO3 interface for both the samples with and
without post-annealing. However, SiO2 layer is
discontinuous at the interface between c-Si and ZnTiO3for the unannealed sample. After post-annealing treatment, the
SiO2 layer becomes uniform and continuous, which can
contribute to the improvement of passivation quality.