## 3 Importance of Surface Modification of SnO\({}_{2}\) ETL
### Thickness  
The morphology of the various interfaces in the PSC determines the ways in which the charge carriers are transported; thus, the morphology of the ETL-perovskite contact can be altered to promote electron transport and achieve the required performance. With an ETL thickness of more than 100 nm, the PSC may be hampered by the longer distance that the electrons need to travel to reach the top electrode and an increase in the recombination rate, which reduces the efficiency and fill factor of the cell [75]. Owing to its high transmittance, minimal leakage current at the ETL-perovskite interface, and low recombination rate, the film exhibited superior light transmission efficiency compared to an ETL thickness of approximately 30 nm [76]. However, thin films pose a technological challenge because of their extreme unevenness, which gradually increases the number of undesired interfacial defects [77]. Consequently, a 40 nm-thick film was determined to be excellent for obtaining a high fill factor because this size was enough to achieve high FTO substrate optical transmittance properties while still ensuring the prevention of pinhole formations and efficient carrier recombination [57; 78]. The blocking of holes in perovskite layers, in addition to selective electron extraction, is another crucial role of the compact layers. The compact films should be consistent over the entire coverage of the FTO substrates to achieve an effective blocking effect. This prevents the top layers from coming into direct contact with the FTO substrates and reduces the shunt resistance of the channel. In this scenario, ensuring the blocking effect can be achieved  
Figure 4: AFM images of (**a**) SnO\({}_{2}\) and (**b**) Ni:SnO\({}_{2}\) layer. Top-view SEM images of (**c**) SnO\({}_{2}\) and (**d**) Ni:SnO\({}_{2}\) films deposited on the pure TCO substrate. Perovskite films were developed on the (**e**) SnO\({}_{2}\) or (**f**) Ni:SnO\({}_{2}\) layer. Top-view SEM images of the absorber substrates grown on the (**g**) unpolluted control layer and (**h**) doped SnO\({}_{2}\) layer. Reprinted with permission from [PERSON] et al. [71]. by increasing the thickness of the compact layers [79]. However, thin compact layers are necessary to increase electron extraction and decrease charge recombination. Causing the intermediate phase to aggregate and develop rapidly during the manufacturing of SnO\({}_{2}\) is not advisable because this would result in forming large-sized nanoparticles [80; 81; 82; 83]. Thus, to create effective devices, well-dispersed single-crystalline particles must be prepared, and the phase of the crystalline particles and surface morphology should be regulated. The content of SnO\({}_{2}\) affects the layer thickness [84]. To produce a high-performance PSC, the SnO\({}_{2}\) thickness should be approximately 64 nm with a concentration of 20%, whereas the perovskite thickness should be approximately 383 nm. This demonstrates that as the concentration of SnO\({}_{2}\) increases, the J\({}_{\rm sc}\) of the device initially increases and then decreases. The J\({}_{\rm sc}\) of the 10% cell was the lowest, whereas that of the 20% cell was the highest. The most likely explanation is that when the SnO\({}_{2}\) content changes, the film thickness increases, thereby increasing the resistance. Additionally, the film thickness affects the light transmittance. The ideal thickness to achieve high transmittance was from 40 nm to 60 nm [57]. Increasing the SnO\({}_{2}\) film thickness will increase the light scattering caused by the formation of rough surfaces, significantly reducing the optical transparency. With an increase in the SnO\({}_{2}\) concentration, the device V\({}_{\rm oc}\) value rises. The likelihood of the holes moving to the FTO substrate, which is simple for charge carriers, decreases along the thickness of the SnO\({}_{2}\) coating. Reducing the charge recombination at the interface is beneficial.