## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Physical Interaction
#### 5.1.1 Simplistic Synthesis Route  
The surface of the SnO\({}_{2}\) ETL can be modified using diverse SnO\({}_{2}\) facile manufacturing techniques, generating high-efficiency PSCs. This amalgamated comprehension indicates that the SnO\({}_{2}\) ETL is typically fabricated at low temperatures. Nevertheless, such synthesis routes are impotent, providing the generated SnO\({}_{2}\) film with a sufficient annealing treatment to eliminate additives and encourage strong interactions between the nanoparticles. As a result, PSCs with a SnO\({}_{2}\) layer as an ETL were utilized to contemplate the dependence of the cell performance on the SnO\({}_{2}\) annealing temperature [119]. The nanoscale optical/electronic and morphological characteristics of an SnO\({}_{2}\) ETL were investigated at various annealing temperatures (room temperature to 200 \({}^{\circ}\)C) to understand the influence of annealing on the SnO\({}_{2}\) features and confirm the components influencing the PSC performance. Understanding the effect of the annealing temperature on the layer features and ETL performance is crucial for realizing an effective PSC using a low-temperature technique, which broadens the scope of its commercial applications. The morphology of the SnO\({}_{2}\) ETL surface was investigated to understand how variations in its surface homogeneity modify the solar cell performance, which was impacted by the annealing conditions based on ITO topographies; accordingly, diverse SnO\({}_{2}\) films are depicted in Figure 9a-f. Generally, the deposited SnO\({}_{2}\) layers attain a noticeably smoother surface. Compared to the bare ITO, the SnO\({}_{2}\) ETL surface was smoother at different temperatures. Regardless of the annealing temperature, layers were successfully produced at different temperatures in 30 min. Additionally, the surface roughness was quantified using the RMS and peak-to-valley average values (Figure 9g), demonstrating that the SnO\({}_{2}\) ETL film attains a uniform and smooth surface when it is annealed at temperatures below 155 \({}^{\circ}\)C. The average RMS values of SnO\({}_{2}\) at 200 \({}^{\circ}\)C and 160 \({}^{\circ}\)C were higher than 0.8 nm, whereas those of SnO\({}_{2}\) treated at low temperatures were 20% smaller. At 160 and 200 \({}^{\circ}\)C, the peak-to-valley roughness also increased dramatically. The rate of solvent evaporation is thought to fulfill the role according to the shape of the SnO\({}_{2}\) nanoparticle layer, which changes with respect to annealing temperature. As the thickness of the deposited SnO\({}_{2}\) layer is discovered to be 20 nm, the detected roughness values of SnO\({}_{2}\) at 160 and 200 \({}^{\circ}\)C are significant and degrade the contact with the perovskite top layer by forming shunting channels similar to pinholes. It is well known that the nanoscopic morphology of the ETL and formation of shunting paths have a significant impact on the charge collection efficiency and shunt resistance of solar cells; thus, it is anticipated that the uniformity of the SnO\({}_{2}\) ETL will have an effect on both the J\({}_{\text{SC}}\) and FF of the PSC [124; 125; 126; 127]. As a result, the lower J\({}_{\text{SC}}\) and FF of the high-temperature processed SnO\({}_{2}\) are attributed to the rough surface. Although the electrical conductivity increases with higher annealing temperatures (Figure 9h), it does not seem to be a factor that affects the charge transfer within the relatively thin ETL (20 nm). An SnO\({}_{2}\)-nanoparticle-based uniform layer was systematically examined. The highest efficiency of 19.0% was obtained by annealing at 120 \({}^{\circ}\)C while adjusting the temperature from ambient temperature up to 200 \({}^{\circ}\)C. The low-temperature processing of an SnO\({}_{2}\) ETL results in a smooth and uniform SnO\({}_{2}\) surface, reduced trap density, optimal band alignment, and shifting toward the bandage, all of which contribute to creating efficient PSCs.