## 5 Surface Engineering of SnO\({}_{2}\) ETL
### Interfacial Physical Interaction
#### 5.1.2 Interface Modifier
The Cs+, on the other hand, prefers to stay on the SnO\({}_{2}\) film to increase the electrical conductivity of the ETL and to improve the energy band alignment with the perovskite layer in the device, which primarily improves the fill factor of the device. However, due to their heterogeneity and undesirable aggregation, LiF and NaF are not suited for the post-treatment of SnO2 films. It might be because LiF has a weak crystallization characteristic in aqueous solutions, which causes microcrystallites to form on the film [138]. Therefore, the aggregation of free nuclei during the crystallization of NaF is most likely the cause of the uneven morphology of film [139]. Researchers have sought after suitable passivation materials that can align with the energy band and reduce the defect density. Halide substances, such as KCl and NH\({}_{4}\)Cl, are potential solutions to these issues. The mechanism of halide ions at the interface must be clarified, and the preparation procedure must be investigated [140]. The impacts of SnO\({}_{2}\) surface halogenation on the performance of PSCs were thoroughly examined. Tetrabutylammonium chloride (TBAC), tetrabutylammonium bromide (TBAB), and tetrabutylammonium (TBAI) were used to passivate the SnO\({}_{2}\) surface, and the concentration gradient of the passivation solution was investigated. The favorable effects of SnO\({}_{2}\) surface halogenation on the SnO\({}_{2}\)-perovskite interface characteristics were clearly observed through the extensive characterization of perovskite layers and PSC devices. Charge carrier dynamics were examined to investigate the reasons behind the enhanced performance of interface-designed devices. Theoretical studies and experimental results demonstrated that TBAC might be the best passivation material for SnO\({}_{2}\) surfaces. The concentration of the solution also increased the visibility of the passivation effect. TBAC might encourage the  
Figure 11: Top-view SEM images of SnO\({}_{2}\) deposited on FTO substrate are shown in (**a**) bare-FTO, (**b**) pristine SnO\({}_{2}\), (**c**) SnO\({}_{2}\)-Cl1 (SnO\({}_{2}\): 0.003 M NH\({}_{4}\)Cl), (**d**) SnO\({}_{2}\)-Cl2 (SnO\({}_{2}\): 0.008 M NH\({}_{4}\)Cl), (**e**) SnO\({}_{2}\)-Cl3 (SnO\({}_{2}\): 0.013 M NH\({}_{4}\)Cl), and (**f**) SnO\({}_{2}\)-Cl4 (SnO\({}_{2}\): 0.02 M NH\({}_{4}\)Cl) (non-uniform regions are indicated by the yellow dotted circles). Reprinted with permission from [PERSON] et al. [135]. Copyrights 2021. Elsevier B.V. (**g**) Diagrammatic representation illustrating the mechanics of various SnO\({}_{2}\) film forms from water and SnO\({}_{2}\) precursors that have been diluted with ethanol. SEM images of the SnO\({}_{2}\) colloidal dispersion layers that were formed after being diluted with (**h**) water, (**i**) ethanol, and (**j**) DMF. (**d–f**) Different SnO\({}_{2}\) films’ water contact angles for (**k–m**), respectively. Reprinted with permission from [PERSON] et al. [136] Copyrights 2020 International Solar Energy Society. Published by Elsevier Ltd.  
development of perovskite crystals, reduce interface flaws, and boost the internal recombination resistance. It was evident that the size of the perovskite particles increased after the addition of TBAC, and the defect density between the particles also decreased to some extent. This directly lowered the recombination center, encouraged charge transfer, lowered carrier recombination at the interface, and increased device stability. On the other hand, the addition of TBAB also resulted in a larger particle size, but there were still more holes, and TBAI had little effect on the growth of perovskite crystals. As a result, Cl\({}^{-}\) was the best halogen anion for SnO\({}_{2}\) surface passivation and might efficiently promote the development of a perovskite layer. Consequently, the PV performance of PSCs was enhanced. Halide ions may interact with Sn atoms on the SnO\({}_{2}\) surface, increasing the charge density and enabling a high-efficiency charge extraction.