## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Chemical Interaction
#### 5.2.1 Functional Groups
The long alkyl chains in C9 also facilitated an orderly self-assembly. The hydrophobicity of fullerene with long alkyl chains prevents the C9-modified SnO\({}_{2}\) surface from wetting via polar solvents, such as DMF, during the deposition of solution-processed perovskite films. This inhibits heterogeneous nucleation and promotes grain-boundary migration. The C9-modifying layer can enhance the quality of perovskite films by reducing the grain boundary, increasing the grain size, and enhancing the crystallinity. This can also enable interfacial charge extraction and reduce trap-assisted charge recombination. A cutting-edge method was used to significantly increase the capacity of graphene for dispersion in water (Figure 13d) [150]. Owing to the high cost of organic solvents, their high boiling point, and high environmental contamination, it is better to exfoliate graphite in an aqueous solution with a surfactant to produce large quantities of graphene [151]. An aqueous solution has a better chance of evaporating completely in a SnO\({}_{2}\) layer that has undergone low-temperature processing. After interface engineering with graphene, the perovskite electrons are driven more towards graphene, indicating the existence of a contact between them. Hydrophobic carbonaceous naphthalene diimide (NDI)-graphene was initially incorporated into the SnO\({}_{2}\) nanocrystal film to fix octahedral [Pb\({}_{6}\)]\({}_{4}\) via the van der Waals interactions at the perovskite-ETL interface. A 3-aminopropyl triethoxysilane (APTES) self-assembled monolayer (SAM) was employed as an interfacial layer to alter the SnO\({}_{2}\) ESL-perovskite layer interface, which could improve the interfacial contact and  
Figure 13: (a) Cross-sectional SEM image of the Pero-SC with the NPC60-OH layer, (b) chemical structure of NPC60-OH, and (c) energy-level diagrams of the associated device materials. Reprinted with permission from [PERSON] et al. [145] Copyright 2019. American Chemical Society. (d) Planar PSC construction. The detailed diagram displays the NDI’s chemical constitution and the relationship between NDI-graphene and perovskite films. Reprinted with permission from [PERSON] et al. [150] Copyright 2018. American Chemical Society. the form and crystallinity of the perovskite films [146]. A functionalized APTES SAM can also induce dipole formation to modify the band energy alignment and increase the built-in potential. The charge extraction is accelerated because of the increased pushing force for photogenerated carrier separation. These alkyl chains can also act as electrically insulating barriers, slowing down the recombination processes and return of electrons from the electrodes to the perovskite layer. This is possible because APTES SAMs are multifunctional. Additionally, by passivating these trap states at the perovskite surface via hydrogen-bonding interactions, the terminal groups can reduce the charge accumulation and recombination caused by these trap states. To reduce the anomalous hysteresis behavior and stabilize the PV performance of planar PSCs, we further regulated and optimized the grain size of the perovskite films. To boost the effectiveness of collecting electrons from the surface, phosphoric acid was used to remove any surface dangling bonds from SnO\({}_{2}\). More than 47.9% of the Sn dangling bonds were removed by the chained phosphate groups that make up most of the phosphorus at the borders. The reduced electron transport barriers caused by the decrease in surface trap states led to an increase in electron mobility by nearly three times when the concentration of phosphoric acid was optimized at 7.4 atom% in the SnO\({}_{2}\) precursor. Additionally, as the concentration was increased, the additional perovskite layer to the phosphate-passivated SnO\({}_{2}\) (P-SnO\({}_{2}\)) ETL slowly improved the stability. Because of the improved electron collection efficiency, P-SnO\({}_{2}\) ETLs can significantly improve the PCE of PSCs [152].