## 3 Importance of Surface Modification of SnO\({}_{2}\) ETL
### Surface Roughness  
\"Roughness\" is a term that refers to the surface morphology of a material. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to assess the surface morphology of the coated films, where a smooth and dense surface is attributed to a good substrate with a low root-mean-square (RMS) roughness value. Currently, the SnO\({}_{2}\) ETL surface and the perovskite surface coated on the SnO\({}_{2}\) ETL are characterized as rough surfaces; however, the AFM of perovskite is beyond the scope of this review.  
Typically, the SnO\({}_{2}\) ETL modification procedure results in a decrease in the material surface roughness, resulting in a smooth and dense surface. According to [PERSON] et al., the application of 2,2,2-Trifluoroethanol (T-SnO\({}_{2}\)) improves the smoothness of the SnO\({}_{2}\) film surface [70]. This improvement was demonstrated via AFM analysis, which indicated that the roughness of the T-SnO\({}_{2}\) ETL was 1.67 nm while that of the pure SnO\({}_{2}\) ETL was 1.13 nm. However, there remain a few exceptions. With or without Ni\({}^{+}\) doping, compact SnO\({}_{2}\) films were spin-coated onto fluorine-doped tin oxide (FTO) substrates using a SnO\({}_{2}\) colloidal dispersion solution [71]. Subsequently, both ETL films showed a flat surface, but the Ni:SnO\({}_{2}\) film exhibited a rougher surface (23.15 nm vs. 22.63 nm) (Figure 4a,b); this is consistent with the surface morphology shown in the SEM image (Figure 4c,d). The Ni:SnO\({}_{2}\) aggregates produced large particles when the mass concentration of Ni\({}^{+}\) was greater than 1 mg mL\({}^{-1}\), making it impossible to form an ETL layer that is smooth and dense. In other words, increasing the particle size changed the morphology of the SnO\({}_{2}\) film, making the film thicker and rougher. Therefore, adjusting the concentration along with a suitable SnO\({}_{2}\) particle size was essential to achieving effective PSC performance. Unexpectedly, the AFM on SnO\({}_{2}\) and Ni:SnO\({}_{2}\) perovskite films attained roughness values of 31.8 and 23.25 nm, respectively (Figure 4e,f), indicating that the SnO\({}_{2}\)-modified perovskite was smoother, as shown in Figure 4g,h. In addition, a 20 mm thick SnO\({}_{2}\) film was prepared using spin coating and spraying on the FTO substrate, respectively, to evaluate the variations in surface morphology with the two approaches [72]. The film fabricated by applying spin coating yielded an RMS roughness of 14.1 nm and maximum variation of 92.2 nm, whereas that produced by employing spraying yielded an RMS roughness of 5.79 nm and maximum variation of 41.2 nm. Thus, the ultrasonic spraying technique exhibited significant development potential for flexible perovskite cells because it is well-suited for creating a SnO\({}_{2}\) ETL on a large-area substrate. In general, modifying the surface of the SnO\({}_{2}\) ETL results in a more uniform and smoother morphology than that of pristine SnO\({}_{2}\) through AFM reduction, including synthetic and impurity incorporation, leading to a decrease in interfacial charge recombination. The smooth surface contributes to reducing the formation of large clusters on the surface and reducing pinholes, whereas rough surfaces are responsible for reducing surface wettability, not achieving coverage over the entire surface, and lowering device performance. In addition, the smooth surface of SnO\({}_{2}\) contributes to improving electron mobility, enhancing electron transport in the SnO\({}_{2}\) film [73; 74].