## 4 Interface Design Rules for Efficient SnO2 ETL
### Surface Trap  
The defect density within the ETL-perovskite interface has a significant impact on PSC performance. The optoelectronic properties are destroyed by electron or hole entrapment and recombination caused by the defect levels inside the bandgap [105; 106; 107; 108]. The perovskite absorber surface and grain boundaries are located where deep-level flaws are most frequently present [109; 110; 111]. Thus, interface engineering is crucial for preventing non-radiative procedures that convert electricity into thermal power by transforming \"active\" interface flaws into \"inert\" defects. The high density of defect states in a SnO\({}_{2}\) film presents another difficult problem. It has been suggested that the SnO\({}_{2}\) surface contains a significant number of Sn dangling bonds, which can absorb O\({}_{2}\) and H\({}_{2}\)O from the atmosphere, trap electrons in the conduction band for recombination, and create potential barriers to block electron transition [112]. Many Sn dangling bonds are present in the SnO\({}_{2}\) ETL layer. Sn atoms saturate the surface Sn dangling bonds after (NH\({}_{4}\))\({}_{2}\)S is added to SnO\({}_{2}\) colloids, forming S-Sn bonds [113]. As a result, surface oxygen vacancies may be eliminated by the creation of S-Sn bonds. Additionally, as shown in Figure 8a-c, the interface Sn-S bonds can prevent moisture from penetrating the cell. Moreover, the interfacial S-Sn bonds create a bond with the Pb atoms for S-Sn-Pb formation in the perovskite crystalline procedure. The efficiency of electron extraction is increased by the direct connection of perovskite to the electron transport channel. By lowering the defect density on the ETL's surface, this method lowers the barriers to electron transport and boosts electron mobility, conductivity, and stability. In addition, the interfacial trap defects can be effectively reduced through the creation of I-Sn bonds at the surface, and Cl\({}^{-}\) diffusion in the perovskite layer positively affects the film crystallinity [114]. Although the high-frequency capacitances presented in Figure 8d are nearly undetectable at frequencies above 100 Hz, the low-frequency capacitances clearly differ depending on the annealing process of SnO\({}_{2}\). As the preparation processes for the examined devices, with the exception of the SnO\({}_{2}\) annealing temperature, were the same, the variations in the low-frequency capacitances were solely due to the contributions of the ETL-perovskite interfaces instead of other components, such as HTLs and perovskite layers. Through repetitive charging and discharging, the cell capacitance increases in response to low-frequency perturbations caused by the deep-level traps at the interfaces [115; 116; 117]. The defect density state, E\({}_{\text{w}}\) vs. n\({}_{\text{t}}\) (=E \(-\) E\({}_{\text{bandedge}}\)), was calculated from the capacitance-frequency diagram [118]. The deep-trap distribution noticeably changes when n\({}_{\text{t}}\) values under various temperatures are examined, as shown in Figure 8e, demonstrating that the SnO\({}_{2}\)-annealing temperature affects the defect generation at the ETL-perovskite interface. The following equation can be used to compute the trap density (\(n_{tmp}\)) [120; 121]:  
\[N_{defect}=\frac{2\epsilon_{0}\epsilon_{r}V_{TFL}}{eL^{2}}\,\]  
where \(L\) is the thickness of the perovskite films, \(e\) (1.6 \(\times\) 10\({}^{-19}\) C) is the element charge, \(\epsilon_{0}\) (8.85 \(\times\) 10\({}^{-14}\) F cm\({}^{-1}\)) is the vacuum permittivity, \(\epsilon_{r}\) is the relative dielectric constant, and \(V_{TFL}\) is the trap-filled limit. The effective defect density as a function of f is described as follows [122; 123]:  
\[n_{0}(f)=\frac{N_{0}}{1+\left(2\pi f\right)^{2}\tau_{dam}^{2}},\]  
Figure 8: Modification of the interface mechanism. (**a**) H\({}_{2}\)O and O\({}_{2}\) enter the SnO\({}_{2}\) surface, (**b**) S-O and S-Sn bonds form to stop H\({}_{2}\)O and O\({}_{2}\) from entering, and (**c**) Sn-S-Pb anchors form to lessen the interfacial trap states.