## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Chemical Interaction
#### 5.2.1 Functional Groups  
ETL-perovskite interfacial passivation is a particularly challenging scientific topic because it involves using polar solvents (such as dimethyl sulfoxide (DMSO) and DMF) for perovskite solution deposition, which often destroy the bottom as-formed defect passivation layers [143]. Thus, it is highly desirable to develop a unique multifunctional ETL capable of both electron transport and the passivation of the bottom surface defects of the perovskite film. The former is caused by lattice oxygen (OL) from SnO\({}_{2}\) crystals, and the latter is caused by vacancy oxygen (OV), also known as chemisorbed oxygen or oxygen from the OH\({}^{-}\) group. One defect that might produce extra trap states near the valence band (VB) is the OH\({}^{-}\) groups on the surface of the metal oxides [144]. The PSC performance is improved, and the surface defect states are reduced by the NH\({}_{2}\)-ZnO@SnO\({}_{2}\) film's lower OV concentration ratio compared to that of a pure SnO\({}_{2}\) film. Thus, effective electron transmission and hole blocking are aided by the correct energy-level alignment. Additionally, the inclusion of NH\({}_{2}\)-ZnO NCs resulted in a decrease in the diffraction peak intensity of the perovskite crystal, thereby enhancing the crystallinity of the perovskite film. Through spin coating, NPC60-OH was successfully incorporated into Pero-SCs to change the SnO\({}_{2}\) ETL [145]. Because the OH\({}^{-}\) group of phenol is connected to NPC60-OH, it is possible to consider this compound as a Lewis base. As a result, this weak interaction might be created by the fullerene derivative pyrrolidine-C60-substituted phenol (NPC60-OH) donating electrons to the undercoordinated Sn, which results from an oxygen vacancy in SnO\({}_{2}\). The NPC60-OH's functional phenolic hydroxyl group and its ability to donate electrons may have caused the development of dipoles on the SnO\({}_{2}\) surface, which most likely caused the lower work function (WF) of the SnO\({}_{2}\). The ETL was followed by the deposition of the Spiro-OMeTAD HTL, which had a thickness of 190 nm, and then the perovskite active layer, which had a thickness of 360 nm, as shown in Figure 13a. Figure 13b shows the PSC level diagrams and chemical structure of NPC60-OH. According to calculations, the WF measurements for SnO\({}_{2}\) and SnO\({}_{2}\)/NPC60-OH were 4.43 eV and 4.14 eV, respectively, indicating that including the NPC60-OH layer successfully lowered the WF of the SnO\({}_{2}\) layer. The development of interface dipoles may be responsible for the decrease in the WF of SnO\({}_{2}\) following the alteration of the NPC60-OH layer [146; 147; 148]. Charge transport may be facilitated, and charge recombination at the interface may be decreased as a result of the smaller energy band gap of the ETL-perovskite interface. In addition, adding a NPC60-OH layer increased the grain size of the perovskite film, indicating improved perovskite film quality that was consistent with the improved PSC performance. When the NPC60-OH layer was added to modify the SnO\({}_{2}\) ETL, the trap-state density of the perovskite film decreased by approximately 22%, indicating that NPC60-OH could drastically lower and passivate the trap-state density in the perovskite film. The electrons flowed from the perovskite active layer to the ETL more efficiently, and there was less recombination inside the perovskite layer after SnO\({}_{2}\) was modified with the NPC60-OH layer. The improved Jsc and FF values were produced by the reduced recombination and facilitated charge transfer. Based on this, chemical interactions occurred with under-coordinated Sn. The oxygen vacancies on the SnO\({}_{2}\) surface were filled by the Lewis base hydroxyl groups inserted at the end of the long chain in 9-(1-(6-(3,5-bis(hydroxymethyl)phenyloxy)-1-hexyl)-1H-1,2,3-triazole-4-yl)-1-nonyl [60] fullerene (C9) [149]. Thus, the C9-modifying layer can efficiently passivate oxygen-vacancy-related defects on the surface of the SnO\({}_{2}\) ETL.