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.