## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Physical Interaction
#### 5.1.2 Interface Modifier  
The unique aspect of this study is that, unlike previous studies that attempted to passivate SnO\({}_{2}\) with chlorine, NH\({}_{4}\)Cl is used here without a doping procedure and placed on the SnO\({}_{2}\) layer as a modifier [135]. Furthermore, the precursor for SnO\({}_{2}\) is an inexpensive salt of SnCl\({}_{4}\).5H\({}_{2}\)O. In general, the doping-free surface treatment is concentrated. The surface roughness of SnO\({}_{2}\) can be successfully reduced using NH\({}_{4}\)Cl as a modifier, leading to a smooth surface for the ETL. To produce PSCs based on the ETL, a smooth ETL surface is crucial because a rough ETL may produce numerous pinholes that act as electron-hole recombination sites between the ETL and the perovskite layer, increasing the series resistance (Rs) and deteriorating the PSC performance. Additionally, a smooth ETL surface can provide better physical contact with the perovskite layer, boosting shunt resistance (Rsh) and improving the performance of planar PSCs. Furthermore, NH4 Cl surface treatment can increase the surface coverage and shunt resistance; however, as the concentration of NH4 Cl increases with respect to that of SnO2-Cl3, the surface uniformity declines, which can increase the number of undesirable pinholes known as recombination centers. According to Figure 11a-f, the surface morphology of the unaltered and modified SnO2 exhibits uniform grains, complete FTO coverage, and a smooth, pinhole-free compact morphology. Naturally, NH4 Cl-SnO2 films become less uniform at higher NH4 Cl concentrations (0.02 M), as indicated by the yellow dotted circles in Figure 11f, which is due to the residual solution on the surface. When subsequent layers are deposited over these regions, pinholes may occur, reducing the performance of the PSC. The existence of Cl upon transformation is clearly visible, indicating that NH4 Cl was successfully incorporated into SnO2 without a significant negative impact on the bulk structure of SnO2. By incorporating ethanol into the commercially available SnO2 precursor, the SnO2 ETL surface can be successfully regulated (Figure 11g) [136]. This method increased the stability and wettability of the SnO2 precursor on the glass substrate. Thus, using ethanol helped create a SnO2 film with a uniform morphology that was devoid of aggregation, which also improved the perovskite crystal quality. Compared to the other samples, dimethylformamide (DMF), a polar organic solvent, may have a long-lasting effect on the surfactants in the SnO2 colloid dispersion because the DMF-SnO2 film exhibits a clearly non-uniform film shape and large aggregations (Figure 11h-j). The properties of the ETL-perovskite interface are typically substantially affected by the shape of the SnO2 film, which in turn affects the internal resistance of the perovskite. To produce high-performance PSCs, the ethanol-SnO2 film surface must be flat. Additionally, the water contact angles of the various SnO2 film types show how the wettability of the dripping solution differs (as shown in Figure 11k-m). Figure 11: _Cont._  
It has been confirmed that treating SnO\({}_{2}\) with alkali salts enhances the performance of PSCs, which is attributed to fewer surface defects [112]. As a result, the impact of various alkali fluorides has been researched [137]. The incorporation of SnO\({}_{2}\) with KF, RbF, and CsF can accelerate charge in PSCs, reduce defect density, and improve the electrical properties of SnO\({}_{2}\). The type of cation will determine the function of the alkali cation. Particularly, the smaller cations, such as the Rb\({}^{+}\) and K\({}^{+}\) cations, can migrate into the upper perovskite layer, which reduces defects and, consequently, limits recombination in the perovskite layer. This results in a much improved V\({}_{\infty}\) of the corresponding device.