Figure 5. (a) Al 2p core level spectrum of ZnTiO3 capped with Al electrodes before and after post-annealing treatment; (b) Secondary electron cutoff energy of as-fabricated and 300℃ post-annealed ZnTiO3 films. The Al layers were removed via a wet chemical etch prior to XPS and UPS characterization.
Based on the material characterization, we can illustrate the energy band diagram to understand the performance-enhanced effect of the perovskite/silicon heterojunction solar cells through post-metallization annealing. If there is no SiO2 film between the c-Si surface and the ZnTiO3 layer, lots of defects (dangling bonds) are likely to appear at the Si/ZnTiO3 interface. In this case, Fermi energy pinning effect may also exist, which will affect the downward band bending of c-Si. As a result, a large number of electrons and holes will recombine through interface states, as shown in Figure 6a, and thus leading to a low PCE (see Figure 3c). When a layer of SiO2 is added between the c-Si and ZnTiO3, the interface states is greatly reduced due to chemical passivation effect of SiO2,45and thus charge carrier recombination is also decreased, as schematically shown in Figure 6b. For the as-fabricated solar cells with SiO2/ZnTiO3/Al as electron-selective contacts, both the energy bands in Figure 6a, b co-exist due to that the rear sides of c-Si are not fully coated by SiO2 layers. However, for the solar cells after post-metallization annealing, the rear sides of c-Si are fully coated by SiO2 layers. Moreover, the WF of ZnTiO3 film is lower due to more Al doping. This leads to a more downward bending of the energy band, resulting in a stronger depletion of holes but a stronger cumulation of electrons, thus lowering carrier recombination (See Figure 6c). Such behavior is well known as field-effect passivation. Therefore, combining the chemical passivation of SiO2 layer and field-effect passivation from Al doped ZnTiO3, carrier recombination is greatly reduced compared to that in the as-fabricated devices. Additionally, more electrons but less holes at the rear side of c-Si incurs larger ratio of electron conductivity to hole conductivity, which is beneficial for improving the electron selectivity.3Consequently, the cell performance is significantly enhanced by post-metallization annealing.