## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Physical Interaction
#### 5.1.1 Simplistic Synthesis Route
Because of the generation of SnO\({}_{2}\) at low temperatures and the simplicity of the manufacturing process, a solution-based technique was used. In addition, the constructed SnO\({}_{2}\) ETLs function effectively without heat annealing. However, SnO\({}_{2}\) generated by magnetron sputtering is infrequently documented, despite the great dependability, maturity, and competence of the sputtering process in both industry and laboratories [128]. Additionally, on a 10 nm SnO\({}_{2}\)-coated FTO, tiny perovskite grains tended to crystallize at the SnO\({}_{2}\)-perovskite interface, which negatively impacts the carrier transport efficiency. The volumetric expansion and ion migration that occur during the crystallization of perovskite are hampered by a rougher surface because, owing to the roughness, the materials are slowly coated onto the substrate at rates of only a few angstroms per second. However, when perovskites are created using solution methods, this issue is less significant because the ion precursors can easily travel over the substrates without having to overcome significant energy obstacles. Consequently, this issue has not been frequently explored in depth. Nevertheless, even though perovskite produced on solution-processed SnO\({}_{2}\) films attained a grain size comparable to that of perovskite developed on sputtered SnO\({}_{2}\) films, most of the grains in the former perovskite films had layered structures. This might be explained  
Figure 9: Morphological and electrical characteristics of a SnO\({}_{2}\) film. Surface topography of **(a)** a bare ITO substrate and **(b–f)** an annealed SnO\({}_{2}\) layer at various temperatures. The same color scheme is used to display the AFM images. **(g)** Roughness values measured as a function of annealing temperature based on peak-to-valley and root-mean-squared values, using bare ITO as a reference. **(h)** Electrical conductivity of thin SnO\({}_{2}\) layers. Reprinted with permission from [PERSON] et al. [119] Copyright8 2019, American Chemical Society. by the SnO\({}_{2}\) film undergoing solution processing being rougher, which yields a variable perovskite crystallization rate and degree, as well as a considerably rough surface. This may enhance the likelihood of charge recombination at the ETL-perovskite interface. As a result, compared to the sputtered SnO\({}_{2}\) film on FTO glass, which demonstrated good transparency with a transmittance close to 90% in the visible range, the solution-processed SnO\({}_{2}\) exhibited a lower transmittance of approximately 80% (Figure 10a). Additionally, the UV-vis absorption spectra of the vapor-deposited perovskite presented in Figure 10b demonstrate good absorption in the visible band. Additionally, the spectra demonstrated that sputtered SnO\({}_{2}\) attained better absorption than solution-processed SnO\({}_{2}\) for perovskite, which was explained by the latter's lower transmittance compared to that of sputtered SnO\({}_{2}\). When the perovskite film was deposited over sputtered SnO\({}_{2}\) as opposed to solution-processed SnO\({}_{2}\), the perovskite PL peaked at 788 nm, and the steady PL displayed a more pronounced quenching, which is consistent with the estimated values (Figure 10c). Room-temperature-sputtered SnO\({}_{2}\) showed an even higher transmittance compared with other high-temperature-generated spin-coated SnO\({}_{2}\) films. Using a straightforward sol-gel technique, [PERSON] et al. created homogeneous SnO2 NPs at ambient temperatures. The remaining Cl\({}^{-}\) on the surface caused the perovskite to partially decompose [129]. The hydrous SmCl\({}_{4}\) was dissolved to 0.05 M and then hydrolyzed at ambient temperature. To gather the SnO\({}_{2}\) NPs, butyl acetate was used as a precipitator. UV-ozone (UVO) treatment was used to remove the Cl\({}^{-}\) residue after spin coating the substrate to lessen the hysteresis. These ETLs did not require additional heat processing. The highest PCE for a flexible and rigid PSC were 15.27/14.74% and 19.22/18.79% (RS/FS), respectively.